drift of zooplankton, benthos, and fish...appendixes242-261.macrophyte,benthos,fishlarvae,and...

DRIFT OF ZOOPLANKTON, BENTHOS, AND LARVAL FISH

AND DISTRIBUTION OF MACROPHYTES AND LARVAL FISH

IN THE ST. MARYS RIVER, MICHIGAN,

DURING WINTER AND SUMMER, 1985

David J, Jude, Michael Winnell, Marlene S, Evans,

Frank J. Tesar, and Richard Futyma

Prepared under Contract DACW 35-85-C-0005

for U.S. Army Corps of Engineers, Detroit District,

Detroit, Michigan 48231

Great Lakes Research Division

Special Report No. 124

The University of Michigan

Ann Arbor, Michigan 48109

1988

ACKNOWLEDGMENTS

We are indebted to the many colleagues who assisted in thecollection of data for this study under many times hazardouswinter and cold spring conditions. Winter drift sampling wasmade tolerable by our resident ombudsman Glen Tompkins, whopurchased equipment, scouted resting sites, and whose knowledgeof the area facilitated sampling immensely. Others performingyeoman's duty during winter included: Curt Ritter, Heang Tin,and Jim Wojcik. Spring larval fish sampling was conducted withthe able assistance of Phil Schneeberger , Phil Hirt, Heang Tin,Mary Sweeney, and Pam Mansfield. Summer drift collections weremade with help from Mary Sweeney and Phil Hirt. Chuck Elzingaassisted with macrophyte collections. Heang Tin processed mostlarval fish samples. Benthos samples were processed by ConnieAchtenburg, Jim Wojcik, Roger LaDronka, Chuck Elzinga, and HeangTin. Zooplankton samples were counted by Mohammed Omair andbiomass data generated by Phil Hirt and Mary Sweeney. We grate-fully acknowledge the data entry and computer work provided byConnie Achtenburg.

We thank Tom Edsall of the Great Lakes Fishery Laboratoryfor permission to use some of their sampling gear and Tom Poe,Jarl Hiltunen, and Chuck Hatcher for good advice on how to sampleand what conditions to expect. We thank H.K. Soo of the GreatLakes Environmental Research Lab for use of their current meterduring winter and Jules Gelderhoos, Dearborn Campus, Universityof Michigan, for use of their current meter during summer. AlBallert of the Great Lakes Commission provided information forour report and Captain Taylor of the U.S. Coast Guard in SaultSte. Marie provided advice and radio contact for safety purposes.The University of Michigan Biological Station provided the use oftheir Boston Whaler and vehicles for macrophyte sampling duringsummer and various pieces of equipment for our winter activities.We thank Claire Schelske for use of the photometer for our summermeasurements.

Mary Sweeney, Heang Tin, and Scott DeBoe provided figuresfor the report, and Bev McClellan entered corrections, new text,and some tables. We gratefully acknowledge the contribution thatPam Mansfield made in putting together and producing the finalreport. Steve Schneider provided very useful editorialcontributions. We thank Walt Duffy, Jim Diana, and Jim Bowersfor critical comments. This project was funded by the U.S. ArmyCorps of Engineers, Detroit District.

11

TABLE OF CONTENTS

ACKNOWLEDGMENTS i i

LIST OF APPENDIXES v

INTRODUCTION . . • 1

METHODS 3

Macrophytes • . . .

.

3

Benthos 3

Zooplankton 22Identifications 22Biomass Determinations 23

Fish Larvae 25Study Area 25Sampl ing Procedures 28Sampling Schedule 34Sample Processing 34

RESULTS 35Macrophytes 35

Introduction 35Results 35

Drift: Benthos, Fish Larvae, Fish Eggs,and Macrophytes 37

Introduction 37Mesh-Size Comparisons 37Seasonal and Transect Comparisons of Drift Densities... 51Seasonal and Transect Comparisons of Drift Rates

and Drift Intensities 56Diel Comparisons of Drift Densities 63Depth Strata Comparisons of Drift Densities 68Station Density Comparisons Within Transects 74Weather Effects on Drift Densities 82Ship Passage Drift Studies 84

Zooplankton 94Zooplankton Abundance and Community Structure 94Biomass Studies 105

Fish Larvae 115Lake Herring 115Lake Whitef ish 125Burbot 130Rainbow Smelt 133Yellow Perch 134Other Species 136Fish Eggs 136Summer Drift and Biomass of Larval Fish 136

iii

DISCUSSION 140Macrophytes 140Benthic Drift 141

Comparison with Previous Benthic Studieson the St. Marys River ' 141

Comparison with Previous Drift Studieson the St . Marys River 145

Some Factors Influencing Drift 146The Fate of Drifting Benthos and the Effect of

Weather and Vessel Traffic on Benthos 153Zooplankton 159

Zooplankton Abundance and Community Structure 159Biomass 161

Fish Larvae 163Lake Herring 163Lake Whitef ish 164Impacts of Winter Navigation on Fish 166

LITERATURE CITED 168

APPENDIXES (on microfiche cards inside back cover)

iv

LIST OF APPENDIXES

PAGE

Appendixes 1-6. Lengt±i of time sampled, current velocity,and total volume of water filtered for drift samples. A - 1 to A -15

Appendix 7. Winter and summer water temperature (C) for

drift samples. A -16 to A -20

Appendixes 8-27. Mean densities, standard error, andpercent of total benthos for Frechette Point winter driftsamples. A -21 to A -40

Appendixes 28-47. Mean densities, standard error, andpercent of total benthos for Frechette Point summer driftsamples. A -41 to A -73

Appendixes 48-73. Mean densities, standard error, andpercent of total benthos for Lake Nicole t winter driftsamples. A -74 to A -99

Appendixes 74-101. Mean densities, standard error, andpercent of total benthos for Lake Nicole t summer driftsamples. A-lOO to A-128

Appendixes 102-114. Mean densities, standard error, andpercent of total benthos for Point aux Frenes winter driftsamples. A-129 to A-141

Appendixes 115-134. Mean densities, standard error, andpercent of total benthos for Point aux Frenes summer driftsamples. A-142 to A-161

Appendixes 135-154. Macrophyte, benthos, fish larvae, andseston dry weight biomass estimates for Frechette Pointwinter drift samples. A-162 to A-181

Appendixes 155-174. Macrophyte, benthos, fish larvae, andseston dry weight biomass estimates for Frechette Pointsummer drift samples. A-182 to A-201

Appendixes 175-200. Macrophyte, benthos, fish larvae, andseston dry weight biomass estimates for Lake Nicole t winterdrift samples. A-202 to A-227

Appendixes 201-228. Macrophyte, benthos, fish larvae, andseston dry weight biomass estimates for Lake Nicole t summerdrift samples. A-228 to A-255

Appendixes 229-241. Macrophyte, benthos, fish larvae, andseston dry weight biomass estimates for Point aux Freneswinter drift samples. A-256 to A-268

Appendixes 242-261. Macrophyte, benthos, fish larvae, and

seston dry weight biomass estimates for Point aux Frenes

sununer drift samples. A-269 to A-288

Appendixes 262-266. Light measurements: energy flux and

extinction coefficients for Izaak Walton Bay, Lake Nicolet,

western Lake Munuscong, eastern Lake Munuscong,

and Raber Bay. A-289 to A-304

Appendix 267. Zooplankton abundance, composition, and

statistics for various dates, stations, mesh sizes, and

sampling periods on the St. Marys River. A-305 to A-327

vi

INTRODUCTION

The St. Marys River, the connecting channel between LakeSuperior and Lakes Huron and Michigan, is part of the vastsurface waters of the Great Lakes and therefore bears aconsiderable amount of recreational boat and conunercial shiptraffic. Several questions have been raised regarding the impactof large ship passage on the indigenous fish and wildlife. Inaddition, extended winter operation of the lock complex at SaultSte. Marie, Michigan has been proposed to 31 January ± 2 wk.This contract (DACW 35-85-C-0005) was prepared by the U.S. ArmyCorps of Engineers, Detroit District, to obtain data to allowpredictions regarding the impact of winter shipping to as late as15 February on the St. Marys River. The results will beincorporated into a supplemental environmental impact statementconcerning extension of the operating season of the lockfacilities at Sault Ste. Marie to 31 January ± 2 wk. Because ofthe surge and currents that ships can generate, they may havesubstantial effects on the spawning grounds of such importantfish as lake whitefish (Coregonus clupeajforinis) , lake herring (C.artedll) , and other recreationally important species. There isalso concern about the effects of ship passage on benthicinvertebrate communities, and how sediment resuspension, possiblycaused by ship traffic, may impact the primary producers in thesystem, especially the aquatic macrophytes.

The fauna of the St. Marys River is a combination of animalsdrifting or moving in from Lakes Superior, Huron, and Michiganand those produced within the system. The river includesbackwater areas; riverine channels, embayments, and lakes;scattered wetlands; and other estuarine habitat. A recent report(Duffy et al. 1987) summarizes a considerable amount of physical,chemical, and biological information, including ours, on theSt. Marys River ecosystem.

To examine some of the impacts of winter and summernavigation on the indigenous fauna and flora, we studied threecomponents of the St. Marys River ecosystem: the macrophytecommunity, the drifting benthos and zooplankton community, andthe larval fish community. The macrophyte studies were aimed atdetermining how far the natural boundary of plant growth extendedinto the river. We also collected a series of spatially andtemporally spaced light measurements in the river to obtain someinformation on light extinction coefficients, therebyestablishing what turbidity currently exists in the St. MarysRiver and how that might affect or limit plant growth.

The second thrust of the study was aimed at documentinginvertebrate drift in the St. Marys River during winter ice coverand during summer. For this purpose, sampling stations wereestablished along three transects across the river. Several

depth strata were sampled at each of these transects. Driftsamples were collected with two different mesh sizes toefficiently sample both the benthos and zooplankton. Samplingwas performed during the day and night so no major periods ofpeak drift would be missed. All invertebrates were removed fromthe sample as well as plant fragments, detritus, and fish larvae.All organisms from each of these groups were then weighed andbiomass determined.

Lastly, we conducted larval fish sampling in the spring toattempt to document lake herring and lake whitefish spawninggrounds in the St. Marys River. Transects were located from thepower canal in Sault harbor to the southern end of NeebishIsland. Fish larvae were sampled at night to reduce netavoidance. At each transect, three stations were established;one was at 1 m, one at 2 m, and one was in the channel. Onestation was located near the head of the St. Marys River whereLake Superior enters, to establish whether fish larvae weredrifting in from Lake Superior or Izaak Walton Bay. Results fromthese studies were used to draw conclusions regarding the impactship passage might have on the respective components of theaquatic ecosystem.

METHODS

MACROPHYTES

All sampling was done from a 5-m Boston Whaler motor boatequipped with a Datamarine electronic depth finder. To locatethe maximum depth to which submerged macrophyte beds extended ata site^ samples of bottom-growing plants were taken with agrappling hook along several transects perpendicular to the depthcontours. This estimate was refined by using a Ponar grab-sampler to take a number of bottom samples, each coveringapproximately 620 cm* of substrate, at depths within about ±0.6 mof the original estimate. After assigning a depth to the outerboundary of macrophyte beds, the boat was moved over that depthcontour for a distance of 1 km, and Ponar grab-samples were takenat intervals of approximately 30 m, for a minimum of 30 samples.In practice, sampling was done over a depth range within 0.5 m ofthe estimated boundary depth. Plants retrieved in each samplewere placed in a labeled plastic bag and were later identified inthe laboratory. Specimens were identified with the aid ofmanuals by Fassett (1957), Voss (1972), and Prescott (1962), andthe herbarium of the University of Michigan Biological Station,Pel Iston, Michigan.

During the same day that bottom sampling at the outermacrophyte boundary was done, and on four other occasions, lightpenetration measurements were taken over plant beds in threelocations in the vicinity of the 1-km transect in each of thefive portions of the river (Fig. 1). A LI-COR model LI-185quantum meter was used to measure energy flux atphotosynthetically active wavelengths (400-700 nm) . Measurementswere taken at the water surface, at 1-m depth intervals, and atthe maximum depth (which depended on site, ca. 20 cm above thebottom) to which the light sensor could be lowered. The datafrom each light-measurement location were used to calculatevertical light extinction coefficients by the least square-methods (Lind 1979)

.

Five sites along the length of the river were chosen forstudy, one in each of the following parts of the river: IzaakWalton Bay (Mosquito Bay), Lake Nicolet, western Lake Munuscong,eastern Lake Munuscong, and the Raber Bay—Maud Bay area (seeFig. 2). Plant sampling and light measurements were done duringJuly and August, 1985.

BENTHOS

Samples of the macrophytic, zooplanktonic, benthic, larvalfish, and fish egg drift were collected during a period of ice

V ONTARIO

WEST LAK£ MUNUSC0N6THANSCCT

EAST LAKE MUNUSC0N6TRANSECT

RABER BAYTRANSECT

Figure 1. Location of the five transects (dark bar) where thedepth boundary of submerged macrophytes and light penetrationwere measured in the St. Marys River, 1985.

Figure 2. Detailed location maps for the five transectsestablished to sample macrophytes on the St. Marys River,1985. Taken from NOAA Chart 14884.

^'i m--EMo'rf

Figure 2. Continued.

'"-^^r-^fil^^t ,zi

a;

cou

UD

' ",„ o„ T\\vA,.•

- -

. ""-^^^'iv'fe .lu \ - :^ ,

r'w.r;S<?

0)

en

Figure 2. Concluded.

cover (23 February - 7 March 1985) and a period free of ice (2-14June 1985) from three transects located at Frechette Point, lowerLake Nicolet, and Point aux Frenes (Lake Munuscong) (Figs. 3,4).At the Frechette Point and Pt. aux Frenes transects, samplestations were established at 1-, 2-, and 3-m depths on eitherside of the navigation channel for a total of six stations ateach transect. A seventh station at each site was located in thenavigation channel at a water depth of 9-10 m. In lower LakeNicolet, the nxjmber of stations was increased to nine because thenavigation channel is divided into upbound and downboundchannels. Like the two previous transects, stations were locatedat 1-, 2-, and 3-m depths on the shoreward side of both channels,and a station was established in each navigation channel. Afinal station was located at 2 m in the central area of the lakebetween the two channels (see Fig. 4). Although 988 driftsamples were scheduled to be collected, the final number ofsamples collected was 964.

The vertical water column was sampled for drift at sub-surface, mid-depth, and near-bottom locations, depending uponstation depth. At 1-m stations, drift samples were collectednear bottom. At 2- and 3-m stations, samples were collected atmid-depth and near bottom. Finally, at all navigation channelstations, samples were collected from all three water depthlocations.

Two simultaneous replicates were collected four times ateach depth stratiom at each station during approximate 12-h timeperiods corresponding to Day 1, Night 1, Day 2, and Night 2. Daysampling generally occurred between 1000 and 1700 h EST, whilewinter night sampling occurred between 1900 and 2300 h and summernight sampling between 2200 and 0200 h.

The drift at all stations was sampled with nets that were1 m in length with a 29-cm diameter mouth; mesh size was 355 Mm.In addition, the drift at all 3-m and navigation stations duringDay 1 and Night 1 time periods was sampled using a similarlysized drift net but of finer mesh size (153 Mm). At allstations, except navigation channel sites, nets of similar meshsize were attached at the top and bottom of the net ring tocrossrods by slipping the crossrods through the rope whichsecured the net to the ring (Fig. 5). Each net was clipped tothe center pole such that it was attached at three locations.Crossrods were fastened to a sleeve such that the wholeapparatus, including nets, rotated about a central, 16-mm, T6160aluminum support rod which followed the design of Poe and Edsall(1982). Depending on the vertical water depth required, set-screw stops were affixed above and below the crossrod apparatusto prevent vertical movement. In winter, 0.3- by 1-m rectangularholes were bored into the ice using a gas-powered auger. Thesampling unit with nets attached was then lowered into the water

10

ONTARIO

^T. AUX PI

PT. AUX FRENESTRANSECT

Figure 3. Location of the three transects (dark bar) wherebenthic drift was sampled in the St. Marys River during winterand summer, 1985.

11

Figure 4. Detailed location maps for the three transectsestablished to sample drift on the St. Marys River, winter andsummer, 1985. Taken from NOAA Chart 14883.

12

M*

' *^ "^^^'l-' ^F^^~/gSaT^— -.11,JL.^ ^^ 2L

®K

||l! nyy^g*^^"^ ^•M^^^rs^

'ji^Yi,n^^^r^'^:^m<L^'^2^

13

Figure 4. Concluded.

14

WATEROR ICE

SURFACE

CENTRAL (lernm x 366 cm

)

SUPPORTr25—

I

. mm >

NET, 355 OR I53prn

-r.- nSET''• ^ SCREW

Figure 5. Apparatus used to sample drifting organisms in theSt. Marys River. Shown is dual net arrangement mounted on a PVCsleeve which allowed rotation according to currentdirection. Nets were set without bottom support during the winterand with cross support during the summer.

15

and pushed 10-20 cm into the river bottom. Surface ice acted assupport so the pole with nets attached remained upright. Duringthe summer sample period, a 1- by 1-m base was constructedthrough which the central, aluminum support rod extended 10-20cm. When lowered into the water the weight and size of the baseas well as the sinking of the rod into the river bed adequatelyanchored the whole apparatus. At rocky sites, the support wasextended 5-10 cm and the base was wedged among the rocks, therebyanchoring the sample device even in rough weather conditions.At the top of each support rod and protruding out of the waterwas a float with fluorescent markings which functioned to shortennighttime searches and to warn unsuspecting boaters.

In the navigation channel, the apparatus to which nets wereaffixed consisted of a 2-cm by 15-m nylon rope, a crossrod, twoclips for each pair of replicate nets, and lead weights.Crossrods were stabilized by nylon string supports leading up thenylon rope at 45 • angles from each end of the rod. Each net wasclipped at the top and just below center at an angle of 120".With ice cover, these sets were tied off at the surface to rodslaid over the ice hole. In the summer, these sets were tied offto the sides of the 5-m Boston Whaler which was anchored in thechannel.

At each sampling location, temperature and water currentwere determined (see Appendixes 1-7 for raw data). Water currentwas measured with a Marsh-McBirney Model No. 527 current meter inthe winter. However, due to adverse weather conditions whichcaused the probe to freeze, current was determined during Day 1

sampling periods for all transects except at Frechette Pointwhere several measurements taken indicated little variation inwinter current speed. During the summer sample period, adifferent Marsh-McBirney current meter failed to operate and wasreplaced by a Gurley-Fisher current meter.

Adverse weather conditions affected sampling schedulesduring both winter and summer periods making it impractical toadhere strictly to the original sampling scheme at Point auxFrenes and Lake Nicolet (Table 1). Extremely poor weatherconditions and rough brash ice in the navigation channelprevented collection of 64 winter samples at stations 5 (24samples), 6 (16 samples), and 7 (8 samples) at Point aux Frenesand at station 5 (16 samples) in lower Lake Nicolet.Additionally, these same weather conditions necessitated doublesetting nets, i.e., setting Day 1 and Day 2 nets on the same dayor night period, at stations 1 (Day) and 2 (Night) at Point auxFrenes. During the summer sampling period, high winds and0.6- to 1.2-m waves, especially at Point aux Frenes, required thedouble setting of day and night nets during the same day or nightat all stations.

16

Table 1. Benthic drift sampling dates for transect stationsduring winter and stammer periods on the St. Marys River. FP «Frechette Point, LN = Lake Nicolet, PAF » Pt. aux Frenes, Surf» sub-surface, Mid = mid-depth, Bott « near-bottom, Trans. =

Transect, St a. = Station, m = meters. A dash means no samplecollected.

FP

FP

FP

FP

FP

FP

FP

LN

LN

LN

LN

Depthstrata

Mesh Day1

Night1

:)epth(m)

Day2

Night2

1.0 Bott 355 23 Feb 23 Feb 24 Feb 24 Feb

1.02.0

MidBott

355355

23 Feb23 Feb

23 Feb23 Feb

2424

FebFeb

2424

FebFeb

1.53.01.53.0

MidBottMidBott

355355153153

23 Feb23 Feb23 Feb23 Feb


2424

FebFeb

2424

FebFeb

0.54.08.20.54.08.2

SurfMidBottSurfMidBott

355355355153153153

23 Feb23 Feb23 Feb23 Feb23 Feb23 Feb


242424

FebFebFeb

242424

FebFebFeb

1.53.01.53.0

MidBottMidBott

355355153153



2424

FebFeb

2424

FebFeb

1.02.0

MidBott

355355

23 Feb23 Feb

23 Feb23 Feb

2424

FebFeb

2424

FebFeb

1.0 Bott 355 23 Feb 23 Feb 24 Feb 24 Feb

1.0 Bott 355 3 Mar 3 Mar 2 Mar 5 Mar

1.02.0

MidBott

355355

3 Mar3 Mar

3 Mar3 Mar

2

2

MarMar

5

5

MarMar

1.53.01.53.0

MidBottMidBott

355355153153

3 Mar3 Mar3 Mar3 Mar


2

2

MarMar

5cMarMar

0.54.6

SurfMid

355355

3 Mar3 Mar

3 Mar3 Mar

2

2

MarMar

5

5

MarMar

17

Table 1. (continued)

Trans, Sta.Depth(m)

Depthstrata

Mesh(/xm)

Day1

Night1

Day2

Night2

(LN)4

9.20.54.69.2

BottSurfMidBott

355153153153


3

3

3

3

MarMarMarMar

2 Mar 5 Mar

LN 5 1.02.0

MidBott

355355

- - -—

LN 6

6

0.54.08.20.54.08.2


355355355153153153

6 Mar6 Mar6 Mar6 Mar6 Mar6 Mar

6

666

66

MarMarMarMarMarMar

7 Mar7 Mar7 Mar

7 Mar7 Mar7 Mar

LN 7

7

1.53.01.53.0 .

MidBottMidBott

355355153153


6

6

66

MarMarMarMar

7 Mar7 Mar

7 Mar7 Mar

LN 8 1.02.0

MidBott

355355

6 Mar6 Mar

6

6

MarMar

7 Mar7- Mar

7 Mar7 Mar

LN 9 1.0 Bott 355 6 Mar 6 Mar 7 Mar 7 Mar

PAF 1 1.0 Bott 355 28 Feb 26 Feb 28 Feb 27 Feb

PAF 2 1.02.0

MidBott

355355

28 Feb28 Feb

2828

FebFeb

27 Feb27 Feb

28 Feb28 Feb

PAF 3

3

1.53.01.53.0

MidBottMidBott

355355153153


26262626

FebFebFebFeb

27 Feb27 Feb

27 Feb27 Feb

PAF 4

4

0.54.99.80.54.99.8


355355355153153153


262626262626

FebFebFebFebFebFeb

28 Feb28 Feb28 Feb

27 Feb27 Feb27 Feb

FP 1 1.0 Bott 355 2 Jun 2 Jun 3 Jun 3 Jun

18

Table 1. (continued)

Depth Depth Mesh Day Night Day NightTrans. Sta. (m) strata ium) 1 1 2 2

FP 2 1.0 Mid 355 2 Jun 2 Jun 3 Jun 3 Jun2.0 Bott 355 2 Jun 2 Jun 3 Jun 3 Jun


3 1.5 Mid 153 2 Jun 2 Jun - -

3.0 Bott 153 2 Jun 3 Jun - -

FP 4 0.5 Surf 355 2 Jun 2 Jun 3 Jun 3 Jun4.6 Mid 355 2 Jun 2 Jun 3 Jun 3 Jun9.2 Bott 355 2 Jun 2 Jun 3 Jun 3 Jun

4 0.5 Surf 153 2 Jun 2 Jun - -

4.6 Mid 153 2 Jun 2 Jun - -

9.2 Bott 153 2 Jun 2 Jun — —


5 1.5 Mid 153 2 Jun 2 Jun - -

3.0 Bott 153 2 Jun 2 Jun — —


FP 7 1.0 Bott 355 2 Jun 2 Jun 3 Jun 3 Jun

LN 1 1.0 Bott 355 5 Jun 5 Jun 6 Jun 6 Jun

LN 2 1.0 Mid 355 5 Jun 5 Jun 6 Jun 6 Jun2.0 Bott 355 5 Jun 5 Jun 6 Jun 6 Jun


3 1.5 Mid 153 5 Jun 5 Jun - -

3.0 Bott 153 5 Jun 5 Jun - -

LN 4 0.5 Surf 355 5 Jun 5 Jun 6 Jun 6 Jun4.6 Mid 355 5 Jun 5 Jun 6 Jun 6 Jun9.2 Bott 355 5 Jun 5 Jun 6 Jun 6 Jun

4 0.5 Surf 153 5 Jun 5 Jun - -

4.6 Mid 153 5 Jun 5 Jun - -

9.2 Bott 153 5 Jun 5 Jun - -


19

Table !• Concluded.

Depth Depth Mesh Day Night Day NightTrans. Sta. (m) strata im) 1 1 2 2

LN 6 0.5 Surf 355 6 Jun 6 Jun 7 Jun 7 Jun4.6 Mid 355 6 Jun 6 Jun 7 Jun 7 Jun9.2 Bott 355 6 Jun 6 -Jun 7 Jun 7 Jun

6 0.5 Surf 153 6 Jun 6 Jun - -

4.6 Mid 153 6 Jun 6 Jun - -

9.2 Bott 153 6 Jun 6 Jun — —


7 1.5 Mid 153 7 Jun 7 Jun - -

3.0 Bott 153 7 Jun 7 Jun — —


LN 9 1.0 Mid 355 7 Jun 7 Jun 8 Jun 8 Jun

PAF 1 1.0 Bott 355 11 Jun 11 Jun 12 Jun 11 Jun

PAF 2 1.0 Mid 355 11 Jun 11 Jun 12 Jun 11 Jun2.0 Bott 355 11 Jun 11 Jun 12 Jun 11 Jun


3 1.5 Mid 153 11 Jun 11 Jun - -

3.0 Bott 153 11 Jun 11 Jun — —

PAF 4 0.5 Surf 355 13 Jun 14 Jun 13 Jun 14 Jun4.6 M\d 355 13 Jun 14 Jun 13 Jun 14 Jun9.2 Bott 355 13 Jun 14 Jun 13 Jun 14 Jun

4 0.5 Surf 153 13 Jun 14 Jun - -

4.6 Mid 153 13 Jun 14 Jun - -

9.2 Bott 153 13 Jun 14 Jun — —


5 1.5 Mid 153 12 Jun 13 Jun - -

3.0 Bott 153 12 Jun 13 Jun — —


PAF 7 1.0 Bott 355 12 Jun 13 Jun 12 Jun 13 Jun

20

To compensate for the uncollected winter samples, a specialsummer study was devised which required the collection of 40drift samples. The purpose of the study was to estimate theimpact on drift of upbound and downbound passage of ships in

excess of 213 m in length. This study was conducted duringdaylight hours on 10 June 1985 at station 3 at Frechette Point.Sampling was conducted in the same fashion as all previous driftsampling at Frechette Point except sampling was centered aroundpassage of a ship using 153-Mm mesh nets only. Replicate sampleswere collected at mid-depth and near-bottom during five periodscorresponding to before ship passage, during ship passage, 5 minafter ship passage, 10 min after ship passage, and 15 min aftership passage. Each period was approximately 5 min in duration.Between periods there was a 2-3 min lag during which nets wereretrieved and reset. Current speed was monitored for the"before" period and continuously in 1-min intervals for the"during" period, but no current measurements were taken duringthe "after" periods. Additionally, for each ship, length, width,and draft were noted. Ship speed was estimated by timing shippassage from bow to stern past a fixed point directly oppositestation 3. Utilizing this scheme, 20 samples were collected forone upbound and one downbound ship passage.

All net contents were washed into labeled, 0.5-L Mason jars,preserved in 4% formaldehyde, and returned to the BenthosLaboratory. The collected drift was examined at 3-30X usingstereo-zoom dissecting microscopes. Benthos, fish larvae, andfish eggs in the drift were identified and enumerated, with eachcomponent placed in labeled vials. All macrophytic material wasremoved and placed in labeled vials for identification in theBotany Laboratory. Remaining contents of each drift sample wereagain placed in 4% formaldehyde and taken to the ZooplanktonLaboratory (Great Lakes Research Division) where final processingoccurred. All benthos, fish larvae and eggs, and macrophyticvials were taken to the Zooplankton Laboratory for dry weight andash-free dry weight analyses.

All data initially recorded on raw data sheets stored in theBenthos Laboratory were coded and entered into the University ofMichigan's AMDAHL computer. All analyses were conducted usingthe Michigan Interactive Data Analysis System (MIDAS) which is asystem of statistical analysis programs supported by theStatistical Research Laboratory at the University of Michigan.Comparisons of drift density differences were based on lognQ(x+l)transformations of numbers per 1,000 m^ density estimates tobetter approximate normality. Scheffe multiple comparisons (a =

0.05) (Sokal and Rohlf 1969) were used to evaluate benthic andlarval fish drift density differences among net types, transects,stations, depth strata, diel periods, and components of the shippassage study.

21

Drift rates were calculated for day and night periods duringwinter and summer at Frechette Point and Lake Nicolet. At Pointaux Frenes, estimates were made only for the summer, becausesamples were not collected for the entire transect during winter.At each transect, the river cross-section was divided vertically(depth strata) and horizontally (stations). The area of eachresultant subsection was estimated by station distance from shoreand station depth. The area between adjacent stations wasdivided equally. The upper 0.5 m of the river cross-section wasconsidered the surface depth stratum. The mid-depth stratumextended from 0.5m deep to midway between the mid-depth net andthe bottom net location. The remaining vertical portion of theriver was assigned to the bottom depth stratum. Based on theseareal approximations and by calculating an average currentvelocity and drift density for day and night periods at eachriver subsection during a given season, we estimated seasonaldrift rate passing each of our transects (number of individuals/24 hr) by summing day and night estimates derived from thefollowing equation:

Drift rate =

(cm/s) (3600 s/h) (h/d) (m^)(m/100 cm) X density/1,000 mM

During winter the daylight period was assumed to be 11 h and thenight time was 13 h. During the summer the daylight period was15 h and the night was 9 h in duration. Our discharge estimate(m^/24 h) was calculated by removing the last term in theequation (X density/1,000 mO. Total drift rate and dischargepassing each transect was calculated by summing appropriatevalues across all subsections and time periods within eachseason. Based on these sums, drift intensity was calculated fromthe total number of individuals passing per 24-h period dividedby discharge expressed as mVs (Waters 1972).

ZOOPLANKTON

Identifications

Zooplankton in samples were identified to either genus orspecies. At most stations, zooplankton were identified to nohigher than genus level. However, species identifications wereconducted at station 3 at all three sampling transects.

For those collections where zooplankton were identified tothe genus level, each sample was subdivided as many times asnecessary in a Folsom plankton splitter to yield two subsamplesof 200-250 animals each. For collections where zooplankton wereidentified to species, each sample was subdivided to yield twosubsamples of 100-150 animals each. Both subsamples wereexamined.

22

Zooplankton were examined in a circular counting dish undera Wild Stereozoom M8 microscope. Immature copepodites andcladocerans were identified to genus, while nauplii were combinedas a group. Adult copepods and cladocerans were identified tospecies for station 3 samples collected during day 1 and night 1;sex determinations were made for adult copepods. At all otherstations, identifications were to genus level. With theexception of Asplanchna, rotifers were not enumerated oridentified. All data were entered on coding sheets.

Biomass Determinations

Samples received in the Zooplankton Laboratory were splitinto three groups: plants, benthos, and fish larvae and fisheggs. Organisms were sorted and placed in separate labelledvials within the original sample jar. No fish larvae werecollected during winter.

Dry weights and ash-free weights were determined forbenthos, plants, and seston. The basic procedures were asfollows.

Benthos

—

For most benthic samples, organisms were removed from thevial and washed several times in a petri dish of distilled water,All work was done under a Zeiss binocular microscope. Animalswere enumerated and placed in preweighed 4-mm x 13-mm aluminumboats manufactured by Hewlett Packard. Boats were individuallyplaced on numbered 26-mm diameter aluminum discs and dried in adesiccator (containing silica gel) over 2 days at roomtemperature. After 2 days, boats were reweighed on a Cahnelectrobalance. The difference between the original and finalweigh provided an estimate of the weight of benthic organisms inthe original sample.

The boats and numbered discs were then placed in a mufflefurnace and burned for 2 h at 450-500*»C. After cooling, theboats and numbered discs were removed from the furnace andreweighed on the electrobalance. The difference in weight afterdrying in the desiccator and burning in the furnace provided anestimate of the ash-free weight of benthos in the originalsample.

On occasion, samples contained large n\jmbers of benthos.In this case, weighing procedures for seston (see below) werefollowed.

23

Aquatic Plants

—

When small amounts of plant material were present, vialcontents were filtered on a prewashed and preashed 2.4-cm Whatmanglass fiber filter. The sample was then washed several timeswith distilled water, placed on a numbered 2.6-cm diameteralxominum disc, and dried for 2 days at 55 ^^C in an oven. Thefilter was then reweighed on a Cahn electrobalance, placed backon its numbered disc, and burned in the muffle furnace. Aftercooling, the ashed sample was reweighed.

In practice, many plant samples contained small fragments ofplant material which would better be designated as detritus.Thus, we defined plant material as larger, more intact plants andfragments.

In some cases, large amounts of plant material were presentin samples. In these cases, plant material was removed, washed,and placed directly on preweighed, 5.0-cm diameter, numberedaluminum discs. Samples were desiccated for 2 days in the dryingoven, reweighed, and then ashed in the muffle furnace for 2 h andreweighed. Most weighings were done on a Mettler balance.

Seston

—

Due to the large amount of material present in mostsamples, only subsamples of seston were weighed. During thesubsampling procedure for zooplankton identifications, onesubsample containing 600-1,000 animals was retained and placed ina labelled vial. This seston subsample was subsequently filteredwith a 2.4-cm prewashed and preashed glass fiber filter. Thesample was washed several times with distilled water and placedon a numbered 3.2-cm diameter aluminum disc. It was dried at55*^0 for 2 days in an oven, then the filter reweighed. Thefilter was placed back on its numbered disc and burned for 2

hours in the muffle furnace prior to final reweighing.

Zooplankton

—

Individual zooplankton dry weights were determined usingthe following procedure. Numerically dominant taxa wereidentified and samples selected for biomass determinations.Approximately 10-50 animals of each taxon were removed from asample, washed, and placed on preweighed, circular 5-mm diameteraluminum boats. The boats were placed on numbered 26-mm diameteraluminum discs, dried at room temperature for 2 days in adesiccator, and reweighed. Ash-free weights were based onliterature conversion values.

24

Weather conditions were coded into 1 = calm, 2 « windy, and3 = very windy. These values were then averaged to give aquantitative estimate of weather conditions to relate to driftresults.

FISH LARVAE

Study Area

Seven larval fish sampling stations were established on theSt. Marys River. Station 1 was located at the railroad bridge onthe Sault Edison Hydropower Canal in Sault Ste. Marie (Table 2,Fig. 6). We sampled on the east side of the bridge between thefirst and second support columns.

Station 2 was near buoy 86A off Six Mile Point on the eastside of the river. Substrate was sand, gravel, and cobble.

Station 3 (buoy 53) was located in the west (downbound)channel where the channel splits around the northern tip ofNeebish Island. The channel was 1.1 km from the mainland, andonshore cattail stands predominated. Substrate was mostly firm,though occasionally soft, and appeared to be organic material; norocks were noted.

Station 4 (buoy 72) was located in the east channel upstreamfrom Neebish Island. At this station the channel was about 1.3km from Sugar Island. Substrate was mostly soft, and includedorganic material and sand. Cattails predominated along theshoreline.

Station 5 (buoy 62) was located off the southwestern tip ofSugar Island. It was the most unique station, being very rockywith large boulders common alongshore. This station had anundulating contour with large boulders projecting from the waterup to 100 m from shore.

Station 6 (buoy 33) was located in the upbound channel onthe east side of Neebish Island. The bottom was sand, with theadjacent shoreline comprised of an extensive wooded swamp.Scattered organic debris (sticks, tree branches) was sometimesencountered.

Station 7 was at buoy 13. This station was at the southerntip of Neebish Island, east of Winter Point, near a dump site(rocks, dredge spoils) which forms a spit in the river and a baybehind the dump site. The shore was wooded and the bottom firmand sandy with some organic material. The entire shorelineappeared to be similar from Winter Point to the spit. Atstations 2 through 7, channel sampling was conducted between the

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PT. AUX PI

Figure 6. Location of stations (1 to 7) where fish larvae and\eggs were sampled in the St. Marys River, 1985. Channel buoy \

niombers are in circles. STA » station.

27

9- and 11-m contours (in the shipping channel), while nearshoresamples were taken at the 1- and 2-m depth contours, along theshore adjacent to the buoy marking the station.

Sampling Procedures

Bridge Samples

—

At the railroad bridge we collected duplicate night sampleswith a 0.5-m diameter, 363-Mni mesh net. The net was secured tothe bridge with rope and sampled <1 m below the surface for 10min. Surface water temperature and current velocity weremeasured (Figs. 7 and 8). During the first week, samples werecollected with a 1-m diameter net, but because of the difficultyin retrieving the net against the strong current, we switched toa 0.5-m diameter net for all remaining samples.

Shore Pull Net Samples

—

A specially designed pull net (Fig. 9), equipped withhandles and a 363-Mm mesh net mounted on a rectangular frame (20X 57.5 cm), was used at night for collection of shore samples ateach station. The net was equipped with a flowmeter and waspulled by two people walking in about 1 m of water. The netusually was in mid-depth and the two people were about 4 m apart.They walked upstream as fast as possible for 66 paces orapproximately 61 m. Duplicate, non-overlapping samples werecollected. Water temperature was measured at each station forall samples.

Push Net Samples

—

Push net samples (Fig. 10) were collected using a 0.5-mdiameter, 363-Mm mesh net rigidly mounted in front of a 4.9-mBoston Whaler; duplicate tows were performed at about 0.5 m belowthe surface of the water. We ran the boat engine (70-hpEvinrude) at 1,000 RPM upriver for 5 min at the 2-m depthcontour.

Channel Samples

—

Channel samples were collected with a 1-m diameter, 363-Mm-mesh net; duplicate, stepped oblique tows were performed(Fig. 11). We standardized the tows so that the net fished atfour depths (surface, 3m, 5m, and 7 m) for 1 min each, with15 s between each depth as the net was retrieved. The engine wasrun at 1,500 RPM.

28

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Figure 7. Mean and range (vertical lines) of bottom watertemperatures measured at six stations in the St. Marys River,1985. X's designated surface water temperature in the EdisonHydropower Canal - station 1.

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Sampling Schedule

Sampling was performed weekly (6 wk) from 26 April to 30 May1985 (Table 3 shows sampling schedule). A total of 228 sampleswas collected, all at night.

Sample Processing

All fish larvae samples were preserved immediately in 10%formaldehyde solution. Samples were sorted using binocularmicroscopes. Larval fish identifications were based on taxonomicdescriptions by Auer (1982) and Cucin and Faber (1985). All fishlarvae were measured to the nearest 0.1 mm (lake whitefish andlake herring) or 0.5 mm total length (larvae of other species)and enumerated. Rainbow smelt eggs, recognized by the presenceof a stalk, were also counted.

Table 3. Dates when night larval fish sampling on theSt. Marys River was conducted during 1985. a shoredm), b « 2 m, c channel.

StationsWeek No. Sampled Dates

1 24 Apr2,3 26 Apr4-7 27-28 Apr

5-7 29-30 Apr2-4 30 Apr-1 May1 1 May

1 6 May5-7 7-8 May2-4 8-9 May

2-4 13-14 May5-7 14-15 May1 15 May

1 22 May5-7 23-24 May2-4 24-25 May

1 27 May2c, 3,4 28-29 May

2a, 2b, 5-7 29-30 May

34

RESULTS

MACROPHYTES

Introduction

Studies were undertaken to describe the relationship betweenthe degree of light penetration through the water column andmaximum depth of occurrence of macrophytes. This was partlyaimed at confirming the findings of Listen et al. (1986) andListen and McNabb (1986), who showed that in the clear waters ofthe upper reaches of the St. Marys River plants grow at greaterdepths than in the more turbid waters of the downstream portions.

Results

The frequencies of plants occurring in grab samples takenalong the 1-km transects at the five sampling sites (Table 4)showed that members of the Characeae, a family of green algae,were the most abundant plants at all transects. In most samplesthe charophytes retrieved did not have reproductive structures,which are necessary for proper identification. Therefore, theyare listed as Nltella sp. indeterminable or Chara sp.indeterminable. Most of the specimens of Nltella probablybelonged either to the species Nltella flexllis or N. opaca.Only one species of Chara was identified, Chara fragilis, but oneor two other species were probably also in the samples. Innearly all samples Nltella was present in much greater quantitiesthan Chara.

Vascular plants were important at two sites, Izaak WaltonBay and Raber Bay. These were also the only two sites where allgrab samples contained plants. In Raber Bay, samples were takenat depths which were probably 0.3-0.6 m shallower than the actualmacrophyte bed boundary. This was the first site sampled and oursubsequent experience showed that vascular macrophytes tended notto grow at depths as great as did the charophytes. Izaak WaltonBay is relatively shallow throughout (<5 m) and no place wasfound where plants did not grow on the bottom; the grab-sampletransect was located in the central deepest portion of the bav.Therefore, the data for Izaak Walton Bay are not from a truemacrophyte boundary.

Fifteen sets of light readings were taken at each of thefive river localities (see Appendixes 262-266). Each set ofreadings was used to calculate a vertical light extinctioncoefficient and all 15 were used to calculate a mean extinctioncoefficient for each site. A 95% confidence interval for themean extinction coefficient was derived using Student's t-

35

Table 4. Plant frequency along 1-km transects over the outerdepth boundary of submerged macrophyte beds in five locations in

the St. Marys River ^ August 1985. Frequencies are given in termsof the percentage of grab-samples in which the plant occurred.

IzaakWalton Lake West Lake East Lake Raber

Plants Bay Nicolet Munuscong Munuscong Bay

70.0 53.1 77.4

6.7 9.4 25.8

36.7 18.8 71.0

3.2

Nitella sp. indeterm. 33,3 86.7

Nltella flexllls 56.7

Chara sp. indeterminable 50.0 6.7

Chara gloJbularls 20.0

Pot^mogeton sp.indeterm. 6.7

Pota;nogetoi3 praeJongus 13.3 3.2

Potamogeton gramlneus 23.3 3.2

Potamogeton rlchardsonll 3.2

Elodea canadensis 3.3 6.7

Vallisneria amerlcana 6.5

Isoetes macTospora

No plants in sample

Sampling depth (m)

Number of samples

10.0

13.3 13.3 34.4

4.5 8.2 5.2 4.9 4.0

30 30 30 32 31

36

statistic (Mendenhall 1971). The mean extinction coefficients,with their confidence intervals, are plotted against the depth ofthe macrophyte boundary (Fig, 12). Also plotted are tworegression lines; the data from Izaak Walton Bay were omittedfrom the regression calculations because they do not correspondto the true maximum depth of macrophyte occurrence. Theextinction coefficients from the 15 sets of light readings fromeach of the four other sites were used in calculating line A, andline B was fitted to the mean extinction coefficients from thefour sites. Line B shows a much stronger relationship betweenmacrophyte boundary depth and light extinction coefficient (r=-0.976) than line A (r=-0.655). All correlation coefficientscited are significantly different from zero, p<0.02. This is dueto the great variability in the light extinction measurements,particularly those from the three downstream stations.

In their investigations. Listen et al. (1986) foundmacrophytes growing on the bottom of the navigation channel northof Izaak Walton Bay, at depths greater than 10 m. If we assumethat the depth of the macrophyte boundary would lie at 10.5 m inthe bay, and use the light extinction coefficient data from thatsite and the remaining four in regression calculations, we obtainthe results shown in Figure 13. Curve C, obtained by polynomialregression of all 75 extinction coefficient measurements from thefive sites, shows a moderately strong relationship between depthand extinction coefficients (r=-0.897). An even strongercorrelation (r=-0.968) is shown by line D, which is derived bylinear regression of the five mean light extinction coefficients.

DRIFT: BENTHOS, FISH LARVAE, FISH EGGS, AND MACROPHYTES

Introduction

Benthic drift collected by coarse- (355 Aim) and fine- (153/Ltm) mesh nets during winter (ice-cover conditions) and summer(ice-free conditions) in the St. Marys River was represented by71 benthic taxa, 5 larval fish taxa, and 12 macrophytic taxa(Table 5). Due to study design, all stations at each transectwere sampled by coarse-mesh nets only; therefore, resultspresented here will consider only those estimates derived fromcoarse-mesh nets unless otherwise stated.

Mesh-Size Comparisons

In nearly all comparisons, total benthic and larval fishdrift densities estimated by fine-mesh nets exceeded that ofconcurrently set coarse-mesh nets. Only in about half of thesecomparisons was the difference statistically significant, withtotal benthic and larval fish densities in fine-mesh nets always

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exceeding coarse-mesh net densities. At Frechette Point, netcomparisons for stations 3-5 indicated the difference in totalbenthic drift density was significant only at station 5 in winterand only at station 4 during summer. When averaged over allstations, there was no significant difference in total benthicdrift density between mesh sizes during winter. However, duringsummer, total benthic drift density averaged over all stationswas significantly greater in fine- (1,562/1,000 mM thancoarse- (1,153/1,000 mM mesh nets (Tables 6 and 7).

At stations 3 and 4, summer larval fish drift density wassignificantly greater in fine-mesh nets (129/1,000 m^ and 233/1,000 mS respectively) when compared with coarse-mesh nets (59/1,000 m' and 77/1,000 m\ respectively). There were no mesh-size-related density differences at station 5. When averagedover all samples at stations 3-5, fine-mesh nets retainedsignificantly greater numbers of fish larvae than did coarse-meshnets (183/1,000 m^ vs. 87/1,000 m').

At Lake Nicolet, fine-mesh net estimates of total benthicdrift density were significantly greater only at stations 6 and 7during winter; whereas, in summer this was the case only atstation 4. However, when averaged over all stations, fine-netestimates of total benthic drift density exceeded those of coarsenets during both winter (102/1,000 m^ vs. 18/1,000 mM and summer(1,383/1,000 m^ vs. 670/1,000 mM (Tables 6 and 7).

Mesh-size-related differences in drifting fish larvaedensities were significant only at navigation channel stations 4and 6 in Lake Nicolet. At station 4, the number of fish larvaeretained by fine-mesh nets (1,972/1,000 m^) was significantlygreater than that retained by coarse-mesh nets (661/1,000 m^).Similarly, in the upbound channel, the fine-mesh nets (778/1,000m^) had a significantly greater number of fish larvae than didcoarse-mesh nets (323/1,000 mO . When averaged over all samplesat stations 3, 4, 6, and 7 having concurrently set fine- andcoarse-mesh nets, larval fish drift density was significantlygreater in fine-mesh nets (1095/1,000 mM than coarse-mesh nets(501/1,000 mO.

At Point aux Frenes, the difference between total benthicdrift density catches based on mesh size was significant at allstations except station 3 during both seasons. When averagedover all stations, estimates of total benthic drift density weresignificantly greater in fine when compared with coarse-mesh netsduring both winter (150/1,000 m^ vs. 25/1,000 m^) and summer(764/1,000 m^ vs. 183/1,000 m') (Tables 6 and 7).

Ratios of fine to coarse-mesh net densities suggested avariety of relative catch efficiencies dependent upon taxon,season, and transect (Tables 6 and 7). However, when comparisons

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50

of log densities for each major component of the drifting benthoswere made for densities averaged over all coarse and fine-meshnets, respectively, significant density differences were notedonly for Naididae, Chironomidae, total benthos without Hydra,total benthos, and Mysis relicta. In all but the lattercomparison, i.e., for M. relicta, densities in fine-mesh netssignificantly exceeded those in coarse-mesh nets by a ratioranging from 1.52 to 2.80. For M. relicta , the coarse-netdensity estimate (5.5/1,000 m^) was greater than that of thefine-mesh net (2.7/1,000 mO by a ratio of 2.06.

At Point aux Frenes, there were no significant larval fishdrift density differences among mesh sizes at stations 3, 4, and5 or at stations 3-5 combined. However, in all comparisons,densities in fine-mesh nets exceeded those in coarse-mesh nets.

At stations and times where nets of both sizes were setconcurrently, 59 benthic taxa were collected. While the numberof taxa retained by coarse nets (50 taxa) was greater than thatof fine nets (40 taxa), this trend was not generally evident.Within each station pooled over all depths at each transect andduring each season, differences in the number of taxa retained byeach mesh size never exceeded three. Among those comparisonswhere the differences were at least three, the number of taxa infine-mesh nets exceeded those in coarse-mesh nets. Only atFrechette Point during summer were there any substantivedifferences in the number of taxa retained by the two mesh sizes.At Frechette Point, coarse-mesh nets consistently collected threeto nine more taxa than did fine-mesh nets.

When comparing percent occurrence of macrophytes for thefine- and coarse-mesh sizes over all transects and stations,percent occurrence in fine-mesh nets was greater in winter (61%)and summer (64%) than in coarse-mesh nets (39% and 48%,respectively). However, there was no difference in dominantplant taxa for coarse- and fine-mesh nets between seasons or forwinter and summer seasons between mesh sizes. In all casesexcept fine-mesh nets in summer, the dominant macrophytes wereNitella, Potamogeton , and Elodea in order of decreasing percentoccurrence. Among fine-mesh nets in the summer, the order of thelast two taxa was reversed.

Seasonal and Transect Comparisons of Drift Densities

General

—

Using 355-Mm-mesh nets, the benthic drift was comprised of67 taxa which averaged 562/1,000 m^ when pooled over all samplescollected (Table 8). The dominant forms were Chironomidae (183/1,000 mO, Hydra (170/1,000 mM , and Ephemeroptera (73/1,000 mM

.

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Seasonally^ both the nxomber of taxa and average benthic driftestimates were lower in the winter (37 taxa and 195/1,000 m^)than during the summer (59 taxa and 871/1,000 m^) period.Although no fish larvae or eggs were caught during the winter,fish larval fish species were collected during the summer.Average total larval fish drift was 383/1,000 m^. The nxjmber offish eggs retained by coarse-mesh nets in the summer averaged 2/1,000 m^ (Table 8).

Benthos

—

Average winter total benthic drift density was significantlygreater at Frechette Point when compared with those at LakeNicolet or Point aux Frenes, which were not significantlydifferent. Benthic drift pooled over all winter samples atFrechette Point averaged 482/1,000 m\ while at Lake Nicolet andPoint aux Frenes similar averages were 49/1,000 m^ and 21/1,000m\ respectively (Table 9; Appendixes 8-27, 48-73, 102-114).Additionally, a greater number of taxa was collected at FrechettePoint (36 taxa using both mesh sizes; 355 /xm = 34, 153 fxm = 17)than at Lake Nicolet (14 taxa using both mesh sizes; 355 /xm = 12,153 Mm = 8) or Point aux Frenes (10 taxa using both mesh sizes;355 Mm = 9, 153 Mm = 6). Chironomidae was the dominant taxon atall transects. At Frechette Point Chironomidae made up 81% ofaverage benthic drift density; at Lake Nicolet it was 52%, and atPoint aux Frenes it was 82%. Remaining components of the driftcommunity structure varied at each transect. At Frechette Point,Hydra was the second-most dominant taxon making up 12% of benthicdrift followed by Naididae (4%). At Lake Nicolet, Ephemeroptera(largely Leptophlebla) was the second-most dominant (24%)followed by Mysis relicta (17%, 8/1,000 m^). At Point auxFrenes, Corixidae (6%), Amphipoda (5%, largely flyaleila azteca),and Mysis relicta (3%) were the only remaining taxa, other thanChironomidae, making up a significant portion of benthic drift.

As during winter, average summer total benthic drift densityat Frechette Point (1614/1,000 mM was significantly greater thanthat at Lake Nicolet (597/1,000 mM or Point aux Frenes (502/1,000 m') (Table 9; Appendixes 28-47, 74-101, 115-134), whichwere not significantly different. The total number of taxacollected decreased from 55 taxa at Frechette Point to 35 taxa atLake Nicolet and 22 taxa at Point aux Frenes. Chironomidae hadthe highest density of taxa at Lake Nicolet where it averaged252/1,000 m' (42% of benthic drift). Ephemeroptera at LakeNicolet averaged 222/1,000 m^ [largely Hexagenia (155/1,000 mOand Caenis (53/1,000 m^)] and made up 37% of summer benthicdrift. At Frechette Point, Chironomidae drift density decreasedto 258/1,000 m^ (16% of benthic drift) relative to that ofwinter. While drift densities for practically all taxa increasedfrom winter to summer at Frechette Point as at all transects.

53

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54

most notable increases at Frechette Point were for Hydra (975/1,000 m\ 60% of benthic drift), Naididae (174/1,000 m\ 11%),Trichoptera [66/1,000 m\ 4%; largely Oecetis (51/1,000 m^)],Chaoboridae (57/1,000 m\ 3.5%), and Ephemeroptera [35/1,000 m\2.2%; largely Caenis (16/1,000 m^].

At Point aux Frenes as at Frechette Point, the percentage ofChironomidae in the benthic drift decreased from 82% (winter) to19% (summer) even though average density increased from 17/1,000m^ to 96/1,000 m^. The dominant taxon in the summer benthicdrift at Point aux Frenes was Hydracarina which averaged 138/1,000 m^ and made up 28% of benthic drift. Remaining abundanttaxa in the benthic drift included Ephemeroptera [94/1,000 m^(19% of benthic drift); largely Caenis (52/1,000 mM andHexagenla (38/1,000 m^)], Trichoptera [40/1,000 m^ (8% of benthicdrift); largely Oecetis (37/1,000 m^)], and Naididae (29/1,000m\ 6% of benthic drift).

Fish Larvae

—

Average larval fish drift density was significantly greaterat Lake Nicolet (872/1,000 mM than at Frechette Point (77/1,000m^) or Point aux Frenes (12/1,000 mM , with the density atFrechette Point being significantly greater than at Point auxFrenes (Table 9). In all cases, rainbow smelt larvae weredominant, making up 70-85% of total larval fish densities. Whenaveraged over all samples, rainbow smelt constituted 82%, burbot0.6%, lake herring 0.1%, yellow perch and deepwater sculpin<0.1%, and damaged, unidentified larvae 18% of average larvalfish density. All identified eggs (84% of total eggs) were thoseof rainbow smelt. Fish eggs were only collected at FrechettePoint (Table 8)

.

Macrophytes

—

Based on all samples collected during winter and siommer with355-Mm-mesh nets, macrophytes occurred in 50% of all samples,with the most frequently occurring taxa being Nitella (20% of allsamples), Potamogeton (18%), and Elodea (12%) (Table 8). Allremaining plant taxa occurred in <4% of samples. There were nostrong seasonal differences in percent occurrence of macrophytes(winter = 48% of all samples, summer = 57% of all samples). Thethree plant taxa previously noted were the dominant forms duringboth seasons. However, only 6 macrophytic taxa were collected inwinter compared with 10 in the summer.

Notable percent occurrence differences for macrophytes wereobserved among transects. At Frechette Point, macrophytescomprised of nine taxa occurred in 83% of samples, while they

55

were less frequent at Lake Nicolet (27%, six taxa) and Point auxFrenes (46%, five taxa). Additionally, dominant forms anddiversity varied among transects. At Frechette Point,Potamogeton (50%), Elodea (37%), and Nitella (22%) were the mostfrequently encountered macrophytes. At Lake Nicolet, Nitella(14%), Potamogeton (4%), and Tgpha (1%) occurred in greatestfrequency. At Point aux Frenes, macrophytes occurring most oftenwere Nitella (28%), Lemna (9%), and Potamogeton (3%).

Seasonal comparisons at Frechette Point indicatedmacrophytes occurred in greater frequency and diversity in thesummer (89%, nine taxa) than in winter (77%, five taxa). Inaddition, the dominant plants changed seasonally from Potamogeton(42%), Elodea (22%), and Chara (4%) in winter to Potamogeton(58%), Elodea (53%), and Nitella (41%) in summer. The mostnotable seasonal change for a given taxon was for Nitella (2%vs. 41%) (Table 8).

At Lake Nicolet, macrophytes were encountered in similarfrequencies during each season (27% vs. 26%). Four plant taxawere collected during each season, but the dominant plant taxadid not change appreciably between seasons, with Nitellaoccurring most frequently.

Macrophytes occurred in slightly greater frequency anddiversity in summer (49%, five taxa) when compared with winter(41%, three taxa) at Point aux Frenes. During both seasons, themost frequently occurring plant was Nitella, although in winterpercent occurrence of Lemna (22%) was quite similar to that ofNitella (27%). Occurrence among samples for Lemna decreasedconsiderably to 1% in the summer.

Seasonal and Transect Comparisons of Drift Ratesand Drift Intensities

Frechette Point

—

The input of water from Lake Superior to the head of theSt. Marys River is apportioned so that 76% flows down the westernside of Sugar Island and 24% flows into Lake George (Listen etal. 1986). The portion going into Lake George flows southwardabove St. Joseph Island, then eastward around the island, therebynot returning to the original 76% flowing west of Sugar Island(Don Williams, personal communication, U.S. Army Corps ofEngineers, Detroit District). During the 23-24 February 1985period when drift samples were collected at Frechette Point, flowat the head of the river was 1,940 mVs. Based on a 76%diversion, 1,490 mVs were expected to have passed our FrechettePoint transect during this period. However, based on our currentvelocity and areal approximations for subsections of a cross-

56

section of the river, the discharge rate was 1,030 m^/s. Due tothis difference, two estimates for drift rate were determined.The first is the "calculated*' drift rate which is based on ourmeasurements and will be referred to in the ensuing text simplyas drift rate(s). The second is the "adjusted" drift rate whichis our calculated values adjusted upward or downward to reflectthe difference between expected and calculated discharge at agiven transect and season. When referring to the latter driftrate, it will be referred to specifically as "adjusted" driftrate(s)

.

Based on our winter discharge values, 49 m^/s (4.8% oftotal) was discharged along the western shoreline (stations 1-3)at Frechette Point, 69 mVs (6.7% of total) along the easternshoreline (stations 5-7), and 913 mVs (88.6% of total) in thenavigation channel (station 4) (Table 10). The winter benthicdrift rate for the entire transect was 49.0 x 10V24 h("adjusted" = 70.9 x 10V24 h). Along the western shoreline, thebenthic drift rate was 2.84 x 10V24 h (5.8% of total). Thebenthic drift rate was lowest along the eastern shoreline (0.19 x10V24 h, 0.4% of total). Greatest drift rate occurred in thenavigation channel (46.0 x 10V24 h, 93.9% of total).

Drift intensity [see METHODS and Waters (1972) for details]was similar along the western shoreline (57,959) and thenavigation channel (50,383) but was very low on the easternshoreline (2,754). Overall, winter drift intensity at FrechettePoint was 47,573.

As only an average monthly discharge value was available tous for June (1,917 mVs), 76% of this value (1,457 mVs) was usedas the expected discharge at Frechette Point as well as at LakeNicolet and Point aux Frenes. Our calculated discharge estimateduring June at Frechette Point was 1,217 mVs. The percentage ofdischarge apportioned to the river subsections during summer atFrechette Point was similar to that of winter (Table 10). Thebenthic drift rate for the entire transect was 151.3 x 10V24 h("adjusted" = 181.4 x 10V24 h), while the drift rate for larvalfish was 1.87 x 10V24 h ("adjusted" = 2.71 x 10V24 h)

.

Greatest seasonal changes in benthic drift rates atFrechette Point occurred along the eastern shoreline where thedrift rate of 18.2 x 10V24 h (12% of total) represented a 96-fold increase when compared with winter (Table 10). Along thewestern shoreline, the benthic drift rate was 8.13 x 10V24 h(5.4% of total), while that for larval fish was 0.35 x 10V24 h(18.7% of total). In the navigation channel, benthic drift rate(124.9 X 10V24 h, 82.6% of total) and larval fish drift rate(1.43 X 10V24 h, 76.5% of total) were greater than thoseobserved along either shoreline.

57

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Benthic drift intensity was greatest on the eastern side ofthe river (193,617), but larval fish drift rate (0.09 x 10V24 h,4.8% of total) and drift intensity (957) were the lowest amongthe three subsections of the river at Frechette Point. Whilebenthic drift intensity along the western shoreline (127,031) wassimilar to that of the navigation channel (117,941), larval fishdrift intensity was much greater on the western shoreline (5,469)when compared with the navigation channel (1,350) and the easternshoreline. Overall, larval fish drift intensity for the transectwas 1,537, while that for benthos was 124,322. For the benthos,this drift intensity represented a 261% increase from winter tosummer (Table 10).

Lake Nicolet

—

Discharge was 2,005 m^/s at the head of the river during the2-7 March 1985 period when drift samples were collected at thelake Nicolet transect. Based on a 76% apportionment of flow, weexpected a discharge of 1,524 m^/s at our Lake Nicolet transect.Our discharge calculation was 1,578 mVs. Percent dischargeattributable to each river subsection was fairly similar (Table11). The benthic drift rate for the entire transect duringwinter was 2.83 x 10V24 h ("adjusted" = 2.73 x 10V24 h).Greatest benthic drift rates were calculated for the eastern(stations 7-9) (0.93 x 10V24 h, 32.9% of total) and middle(station 5) (0.82 x 10V24 h, 29.0% of total) subsections of theriver. The lowest benthic drift rate was calculated for theupbound channel (0.21 x 10V24 h, 7.4% of total). Benthic driftrates were 0.32 x 10V24 h (11.3% of total) on the western shoreand 0.55 x 10V24 h (19.4% of total) in the downbound channel(Table 11).

Greatest benthic drift intensity occurred on the easternshoreline (3,310) and least in the upbound channel (857).Benthic drift intensities in remaining subsections of the riverwere similar, ranging from 1,441 to 1,687. Overall, benthicdrift intensity for the transect during winter was 1,793 (Table11).

As at Frechette Point, expected discharge at our LakeNicolet transect during June was 1,457 mVs. Based on ourmeasurements, we calculated a discharge of 1,282 mVs. Percentdistribution of the water mass attributable to each subsection ofthe river varied little seasonally. The benthic drift rate forthe entire Lake Nicolet transect during June was 29.0 x 10V24 h("adjusted" = 33.0 x 10V24 h). Relative to the winter, benthicdrift rate increased by a factor of 10.2 in summer (Table 11).In all comparisons of summer and winter benthic drift rateswithin each river subsection, those in summer were greater.Summer benthic drift rates were lowest among the shorelines

59

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(2.90-3.70 X 10V24 h) and were similarly high in remainingsubsections (6.01-8.40 x 10V24 h). Most notable increases inthe percentage of total benthic drift rate were for the twonavigation channels. In the downbound channel, the percentage oftotal drifting benthos increased from 19.4% in winter to 29.0% inSlammer. In the upbound channel, the increase was from 7.4% inwinter to 20.7% in summer. Consequently, during winter 26.8% ofdrifting benthos was in the two channels, while during summer theportion of drifting benthos in the two channels increased to49.7%. The most notable decrease occurred along the easternshoreline where the percentage of the total benthic drift ratedecreased from 32.9% in winter to 12.8% in summer. Regardless ofbenthic drift rate differences among river subsections, benthicdrift intensities were very similar in all subsections (13,810-20,904) except in the downbound channel where benthic driftintensity was 40,500. The benthic drift intensity for the entireLake Nicolet transect was 22,621 which represented an increase bya factor of 12.6 compared with winter.

Larval fish drift rate during summer was 175.9 x 10V24 h("adjusted" = 199.9 x 10V24 h). Seventy-two percent (126.4 x

10V24 h) of the total larval fish drift rate originated in themiddle portion of Lake Nicolet (station 5). Larval fish driftrates in other subsections of the river were similar and muchlower than in the middle subsection ranging from 8.76 x 10V24 hto 15.9 X 10V24 h (5-9% of total).

For the transect as a whole, larval fish drift intensity was137,207. However, all values except that for the middle portionof the river were much lower than that for the whole transectranging from 42,319 to 89,831. Larval fish drift intensity forthe middle portion of the river was very large (316,792) (Table11).

Point aux Frenes

—

Winter drift rates and intensities were not calculated atPoint aux Frenes since only stations 1-4 could be sampled.During summer, the expected discharge at Point aux Frenes was thesame as that used at Frechette Point and Lake Nicolet (1,457 mVs). Based on our measurements and an average navigation channeldepth of 10 m, we calculated a discharge of 2,248 mVs, with67.7% (1,509 mVs) flowing down the navigation channel, 17.2%(384 mVs) along the western shoreline, and 15.4% (345 mVs)along the eastern shoreline. The benthic drift rate at Point auxFrenes was 2.39 x 10V24 h ("adjusted" = 15.9 x 10V24 h). Withrespect to river subsections, benthic drift rates decreasedeastward, with 9.67 x 10V24 h (40.4% of total) on the westernshore, 8.22 x 10V24 h (34.4% of total) in the channel, and 6.02X 10V24 h (25.2% of total) on the eastern shore (Table 12).

61

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62

Benthic drift intensity for the entire transect was 10,679.Highest benthic drift intensity occurred on the eastern shore(25,182) and lowest in the channel (5,447) (Table 12).

Larval fish drift rate for the entire transect was 4.60 x10V24 h ("adjusted" = 2.99 x 10V24 h). Maximum larval fishdrift rate and drift intensity occurred in the navigation channel[4.34 X 10V24 h (94.4% of total) and 2,876, respectively].Although benthic drift rate was greatest on the western shore,larval fish drift rate was lowest (0.027 x 10V24 h, 0.6% oftotal). In addition, larval fish drift intensity was very low(70). Although higher than along the western shore, larval fishdrift rate and drift intensity were still both low on the easternshore [0.23 x 10V24 h (5.0% of total) and 667, respectively](Table 12).

Diel Comparisons of Drift Densities

Benthos

—

Comparisons of diel benthic drift densities during bothwinter and summer at each transect indicated night abundanceswere significantly greater than daytime densities for allcomparisons except at Frechette Point in summer. In this lattercomparison, daytime benthic density [2,488/1,000 m^; largelyHydra (74%)] was significantly greater than the night benthicdensity estimate (750/1,000 mO. Among comparisons havingsignificantly greater night densities, abundances at nightincreased over day densities from a minimum of 882% to a maximumof 2,198%. At each transect, the number of taxa collected wasgreatest in night samples regardless of season.

Daytime benthic drift density at Frechette Point wasdistinguished from that at Lake Nicolet and Point aux Frenesduring both winter and summer due to a large componentattributable to Hydra (Table 13). Daytime winter drift atFrechette Point averaged 97/1,000 m^ and was comprised primarilyof Hydra (54/1,000 mM, Chironomidae (26/1,000 mM , and Naididae(13/1,000 mM. Similar estimates at night averaged 866/1,000 m^with Chironomidae (756/1,000 m"). Hydra (64/1,000 m^), andNaididae (28/1,000 m^) being the most numerous drifting benthos.During summer, daytime benthic drift averaged 2,488/1,000 m^ andwas dominated by Hydra (1,836/1,000 mM , Naididae (303/1,000 mM ,

and Chironomidae (244/1,000 mM . At night, average benthic driftdensity (750/1,000 m^) decreased with Chironomidae (272/1,000mM, Trichoptera [126/1,000 m' ; largely Oecetis (102/1,000 mM],Hydra (144/1,000 m^), and Chaoboridae (87/1,000 mM the dominanttaxa.

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66

At Lake Nicolet, average daytime winter benthic density waslow (4.3/1,000 m^), with the dominant taxa being Chironomidae(2.6/1,000 m^), Hydra (1.0/1,000 mM, and Ephemeroptera (0.5/1,000 mM. At night, benthic drift density increased to 95/1,000m^. While Chironomidae remained the dominant taxon (54/1,000mM, Ephemeroptera made up 24% and Mysis rellcta 17% of benthicdrift (Table 13).

Lake Nicolet siommer daytime benthic drift abundance averaged84/1,000 m^ and was dominated by Chironomidae (41/1,000 mM andHydra (18/1,000 m^). Average summer benthic drift increasedconsiderably at night (1,110/1,000 mM, with Chironomidae (464/1,000 mM, Ephemeroptera (442/1,000 mO , and Trichoptera (104/1,000 m^) being the most abundant components. For mayflies insummer night drift, Hexagenia was the most numerous averaging310/1,000 m^. Among caddisflies, Oecetis was the most abundantaveraging 93/1,000 m^.

At Point aux Frenes, average winter benthic drift abundanceincreased from 3/1,000 m^ in the daytime to 40/1,000 m^ at night.A similar diel trend was observed during the summer where averagebenthic drift increased from 102/1,000 m^ during the day to 901/1,000 m^ at night. Chironomidae and Corixidae were the mostabundant taxa (1/1,000 m^) in daytime winter benthic driftsamples; whereas, the night estimate was dominated byChironomidae (34/1,000 mO (Table 13).

During summer at Point aux Frenes for both day and night,Chironomidae made up only 19% of benthic drift which averaged102/1,000 mVs during the day and 901/1,000 mVs at night (Table13). The most abundant drifting taxa during the day wereHydracarina (31/1,000 mM , Naididae (24/1,000 mM , and Corixidae(12/1,000 m^). At night, Hydracarina remained the most numerousdrifting taxon collected averaging 244/1,000 m^. Other majorcomponents included Ephemeroptera [85/1,000 m%* largely Caenis(102/1,000 m') and Hexagenia (76/1,000 m')], Corixidae (92/1,000m^), Amphipoda (82/1,000 mM , and Trichoptera [76/1,000 m\-largely Oecetis (75/1,000 m^)].

Fish Larvae

—

Diel larval fish drift densities were significantlydifferent only at Lake Nicolet where densities during the night(1,269/1,000 m^) exceeded day densities (475/1,000 mM by afactor or 2.67 (Table 13). At Frechette Point, average daylarval fish drift density (88/1,000 mO exceeded night density(67/1,000 mM, but the difference was not significant. At Pointaux Frenes, night density of larval fish drift (17/1,000 mO wastwice that of daytime density, but the difference was notsignificant.

67

Macrophytes

—

When examining diel differences pooled over all transectsand stations, percent occurrence of macrophytes was low.Macrophytes occurred in 54% of day samples and 46% of nightsamples. Dominant plant taxa were Potamogeton (23%), Nitella(22%), and fiiodea (14%) in day samples; whereas, at nightdominant forms were Nitella (19%), Potamogeton (14%), and Elodea(10%). The number of taxa collected during day and night sampleswas the same (nine taxa), with two taxa encountered only duringthe day and another two only at night.

Depth Strata Comparisons of Drift Densities

Frechette Point

—

Nearly all estimates of benthic drift density at alltransects, regardless of mesh size, had lowest abundances at thesurface and greatest at the bottom. However, during winter atFrechette Point, benthic drift was greatest near bottom (630/1,000 mO and least at mid-depth (289/1,000 mO. In spite ofthis deviation from the general trend, there were no significantdensity differences among the three depth strata estimates oftotal benthic drift density during winter.

Regardless of water column depth during the winter atFrechette Point, benthic drift was dominated by Chironomidae (81-85%) (Table 14). Remaining taxa contributing substantively tothe drift were Naididae and Hydra which cumulatively made up 12-17% of the drifting benthos.

Summer benthic drift density estimates among depth stratasampled were significantly different. The surface estimate oftotal benthic drift density (135/1,000 mO was significantlylower than either mid-depth (490/1,000 mO or bottom (827/1,000m^) estimates, which were not significantly different.

At all depth strata, Hydra was the dominant taxon duringsummer, making up 57-92% of the drifting benthos. The percentageof total drifting benthos attributable to Hydra decreased fromsurface to bottom even though density increased. As during thewinter only two other taxa contributed moderate numbers to thedrifting benthos; Naididae and Chironomidae. These two taxacombined made up 17-28% of total benthic drift density, withdensities increasing from surface to bottom. During winter andsummer, greatest densities of Mysis xelicta occurred in surfacewaters and decreased from surface to bottom. Finally, duringboth winter and summer, the number of taxa collected was least atthe surface (7 taxa, winter) and greatest at the bottom (51 taxa,summer)

.

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71

There were two general trends with respect to driftingbenthos and depth strata. First, benthic drift density wasdominated by Chironomidae in winter and Hydra in summer,regardless of depth strata. Second, density and diversity of thedrifting benthos were greatest near bottom during both seasons.

There were no significant larval fish drift densitydifferences among depth strata sampled. Greatest numbers of fishlarvae were collected at mid-depth (96/1,000 mM; the leastoccurred at the surface (50/1,000 mM

.

Lake Nicolet

—

Benthic drift density and diversity at Lake Nicolet wereconsiderably reduced at all depths when compared with those atFrechette Point. However, both parameters had similar trends tothose at Frechette Point with increasing abundance and diversityfrom surface to bottom regardless of season (Table 14). However,only during winter were density estimates of total benthic driftsignificantly different, with the bottom (83/1,000 mM andsurface (4/1,000 m^) estimates being different. The mid-depthdensity estimate (20/1,000 mO was not significantly differentfrom surface or bottom estimates.

The winter surface estimate of drifting benthos wasdominated by M. relicta (63%, but only 2.6/1,000 mM andChironomidae. At mid-depth, while abundance of M. relictaincreased to 6.5/1,000 m% its percentage of drifting benthosdecreased to 33% due to increases in other components of thebenthos, largely Chironomidae and Ephemeroptera. At bottom, M.relicta increased to a maximum of 10.8/1,000 m\ but itspercentage of drifting benthos continued to decrease to 13%; onceagain due to increases in chironomids and ephemeropterans

.

During summer, density and diversity estimates wereconsiderably greater than during winter. However, regardless ofdepth, the chironomids and ephemeropterans made up 75-85% oftotal benthic drift density. Mysls relicta occurred in densitiessimilar to those of winter, ranging from 5.6/1,000 m^ to 6.7/1,000 m^ (Table 14).

The general pattern of drifting benthos with respect todepth at Lake Nicolet was one of increasing density and diversityfrom surface to bottom depth strata. Winter estimates ofdrifting benthos were dominated by chironomids and mysids at thesurface and these two taxa plus ephemeropterans at remainingdepths. Summer estimates of these two parameters were greaterthan winter estimates and were dominated by chironomids andephemeropterans, with densities of mysids being similar at alldepth strata.

72

Comparisons of larval fish drift at Lake Nicolet indicatedmid-depth (1,357/1,000 mM and surface (667/1,000 mM densitieswere significantly greater than at the bottom (540/1,000 m^).There was no significant difference between mid-depth and surfacedensity estimates of larval fish drift.

Point aux Frenes

—

As with Lake Nicolet and Frechette Point, benthic densityand diversity increased with increasing depth strata and overallwere greater in sxjmmer when compared with winter. Winter benthicdensity and diversity estimates at Point aux Frenes were mostsimilar to those at Lake Nicolet. Comparisons of depth strataestimates for total benthic drift density were non-significant.Chironomidae was the dominant taxon, making up 67-98% of totalbenthic drift density (Table 14). Although M. relicta comprised22% of the surface total benthic drift density, average densitywas only 1.4/1,000 m% with even lower abundances at remainingdepth strata. Other components of the benthos, mostly Corixidae(9.3% of benthic drift) and Amphipoda (7.8%), made up moderateamounts of benthic drift only at the bottom depth stratum.

During summer, total benthic drift densities on the bottomand at mid-depth were significantly greater than at the surface.Hydracarina was the most numerous taxon encountered in thedrifting benthos at each depth strata. Chironomidae andEphemeroptera were the second- and third-most abundant taxa inthe drifting benthos at mid-depth and bottom. No mysids werecollected during the summer at Point aux Frenes.

The general trend for drifting benthos at Point aux Freneswas for density and diversity to increase from surface to bottomdepth strata during both seasons. Winter benthic drift wasdominated by Chironomidae at all strata, while in summer driftingbenthos, Hydracarina, Chironomidae, and Ephemeroptera were themost numerous benthic forms.

At Point aux Frenes, larval fish drift density wassignificantly greater at the surface (41/1,000 m^ when comparedwith bottom (11/1,000 mM and mid-depth (9/1,000 m^). Thedensity difference between the latter two depth strata was non-significant.

When depth strata pooled were pooled over all transects andstations, percent occurrence of macrophytes was similar atsurface (48%), mid-depth (44%), and bottom (54%) depth strata.All three depth strata were dominated by the same macrophytes;Nitella, Potamogeton , and Elodea. Surface samples werecharacterized by only five taxa; whereas, remaining depth strataeach had nine plant taxa.

73

station Density Comparisons Within Transects

Frechette Point

—

Comparison of total benthic drift density during the winterindicated stations 1-4 had significantly greater abundances ofdrifting benthos (535-1,216/1,000 mM than did stations 5-7 (20-38/1,000 mO (Table 15). In addition, greatest diversity waspresent at stations 2-4 (14-21 taxa), with remaining stationshaving :^8 taxa present. Although densities of Hydra did notstrongly differ across stations (4-212/1,000 mM , it made up aconsiderably greater portion of the drifting benthos at stations5 and 6 (65-70%) than at remaining stations (^20%). At theselatter stations, Chironomidae was the dominant form (83-99%).Mysis relicta occurred in similar densities at all stations (0-7/1,000 m^) but was generally greatest in abundance at stations 1-

4.

There were no significant station differences for totalbenthic drift density during summer. However, all majorcomponents of the benthic drift except Ephemeroptera andHydracarina had maximal densities at stations 1-3. Theproportion of total benthic drift density excluding Hydradecreased from station 1 (75%) to station 7 (32%). Excludingstation 1 (20 taxa), the number of taxa collected at each stationduring the summer declined in the same manner that total benthicdrift density without Hydra did, with greatest diversity atstation 2 (33 taxa) and least at station 7 (21 taxa).

Greatest numbers of fish larvae were captured at stations 4,5, and 6 (85-139/1,000 mO , with those at station 5 being thehighest (Table 15). Remaining station density estimates rangedfrom 41/1,000 m^ to 56/1,000 m^. However, there were nosignificant differences in larval fish drift densities amongstations.

The only macrophyte occurring at all stations, regardless ofseason, was Potamogeton (Table 15). During winter, Elodeaoccurred only at stations 1-4, and stations 5-7 had onlyPotamogeton present (except station 7 which had two samples withChara) . During summer, Potamogeton , Elodea, and Nitella occurredat nearly all stations. Remaining plant taxa occurred randomlyamong stations, except Bryum which was notably frequent atstation 7. During both seasons, station 4 had the highestdiversity of macrophytes.

Lake Nicolet

—

No significant differences in total benthic drift densityamong stations were evident during either winter or summer. In

74

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76

addition, at most stations the nuunber of taxa collected wassimilar (2-8) (Table 16). During winter, the most notable trendsamong major components of the benthic drift were: (1) atstations 1-4, mysids were most numerous, and (2) at station 9,Chironomidae and Ephemeroptera were the only taxa encountered andthey were very abundant relative to other stations.

During summer, benthic drift at stations 1-3 and 8-9 waslargely Ephemeroptera (36-63%), with the remaining, more mid-lakestations 4-7 dominated by Chironomidae (51-70%). Remaining majorbenthic drift components were most abundant at depths ^3 m.

Even though very high larval fish drift abundances wereencountered at station 5 in the central portion of Lake Nicolet(up to 15,600/1,000 m% see Appendixes 87 and 88), averagestation density estimates for stations 1-8 ranging from 101/1,000m^ to 347/1,000 m^ were non-significant. However, all estimatesof larval fish drift were significantly greater than at station 9

which averaged only 21/1,000 m^.

During winter at Lake Nicolet, macrophytes were mostfrequently encountered at stations 3 and 4. None or very fewplants occurred at stations 2 and 7-9. The dominant plantoccurring in Lake Nicolet drift samples was Nitella. Duringsummer, macrophytes were collected at stations 1-4 and 6, whereinNitella was dominant at all except station 1 where Utriculariaoccurred in equal frequency with Nitella (13%) (Table 16).

Point aux Frenes

—

During winter total benthic drift density was dominated bychironomidae; there were no significant station differences.However, during sxjmmer, the two stations nearest shore (1 and 7)had significantly greater total benthic drift density (1,712/1,000 m^ and 1,328/1,000 m\ respectively) than did thenavigation channel, station 4 (88/1,000 mO , but these twostations were not significantly different from stations 2-3 and5-6 (206-582/1,000 m^) (Table 17). The drifting benthos at allstations was dominated by Ephemeroptera, Hydracarina, Corixidae,Chironomidae, and Amphipoda. The number of benthic taxacollected at each station was similar (8-11) except at station 7

where 16 taxa were collected.

No fish larvae were collected at stations 1, 2, and 6.Maximum numbers were collected in the drift at stations 4 (37/1,000 mM and 7 (28/1,000 mM. Drifting fish larvae estimatesranged from 2/1,000 m^ at station 3 to 10/1,000 m^ at station 5.The latter two stations had significantly fewer fish larvae thandid station 4, but neither was significantly different from

77

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station 7. Density differences between stations 4 and 7 werenon-s ign i f i cant

.

Nitella and Lemna were the most frequently occurringmacrophytes at stations 2-4 during winter at Point aux Frenes.No plants were collected at station 1. During summer, plantswere collected in greatest frequency at stations 7-9. However,regardless of how frequently macrophytes were encountered at eachstation, Nitella occurred most often or shared equal dominancewith other plants at all stations.

Weather Effects on Drift Densities

Lake Nicolet

—

During the two sxjmmer day-periods at Lake Nicolet, two verydifferent weather conditions were encountered. On Day 1 (6 June1985) at stations 1-4, weather conditions changed from a shortperiod of calm winds and rain to sunny and windy resulting in0.3-0.6-m waves. On Day 2 (7 June 1985), the river in thevicinity of stations 1-4 had no waves and was essentially calm.Day 1 current velocities at the shallower stations (1-3) averaged27 cm/s and were considerably greater than those measured on Day2 under calm conditions (8 cm/s). Little variation in currentvelocity was noted at station 4 (21 vs. 20 cm/s). No shipspassed downbound on the windy day, however, the Canadian Olympicand the Kupa passed downbound on the calm day while nets were setat station 1.

Based on estimates pooled over stations 1-4, samples fromthe windy day had significantly greater numbers of Chironomidae(42/1,000 m^ vs. 19/1,000 m') and total benthic drift (78/1,000m^ vs. 41/1,000 mM when compared with samples from the calm day(Table 18). While no significant density differences were notedfor other major benthic drift components or for total fish larvaldrift, for nearly all, greatest densities occurred on the windyday.

Point aux Frenes

—

A similar wind event occurred at Point aux Frenes. On Day 1

(11 June 1985) wave height was 0.3 m to 0.6 m at stations 1-3,but on Day 2 (12 June 1985) it was reduced to <0.3 m. Only thecomparison for Chironomidae was significant, with greaterdensities observed on the windy day (29/1,000 m^ when comparedwith the calm day (7/1,000 mM . However, in many comparisons,though non-significant, drift densities were greatest on thewindy day (Table 18). No fish larvae were collected at stations1-3 during the day.

82

Table 18. Mean density (X, no. /I, 000 ra») and percentageof average total density (%T) for benthos, fish larvae,and fish eggs pooled over stations 1-4 at Lake Nicoletand over stations 1-3 at Point Aux Frenes during thesummer under conditions of calm and windy weather forcomponents of the drift collected with 355-iim mesh netsin the St. Marys River, 1985. Included at the end of %Tcolumn are the number of benthic taxa and the number ofsamples (N) collected. Densities rounded to nearestwhole number. Percentages based on five significantfigures and rounded to nearest tenth. Column stimmationssubject to round-off errors.

Lake Nicolet Poi nt aux Frenes

Calm WiIndy Calm Windy

Taxon X %T X %T X %T X %T

Hydra 8 15.6 48 38.0 5 4.8Naididae 16 13.0 5 7.0 3 2.4Mysis relicta 1 1.2Amphipoda 3 5.4 6 4.4Hydracarina 5 10.6 4 2.9 38 54.4 63 58.3Ephemeroptera 3 5.3 3 2.4Corixidae 15 21.1 3 2.7Trichoptera 9 18.8 3 2.0 5 7.0 3 2.4ChaoboridaeChironomidae 19 39.0 42 33.8 7 10.5 29 26.9Total benthos

w/o Hydra 41 84.4 78 62.0 69 100 102 95.1Total benthos 48 - 125 - 69 — 108 —

No. of taxa 8 10 5 7

Rainbow smelt 227 75.3 253 77.5Burbot 11 3.2Damaged larvae 74 24.7 63 19.3Total fish larvae 301 — 327 — - —

N 16 16 10 10

83

Frechette Point

—

Weather comparisons at Frechette Point were confounded bydiel differences, because weather conditions on Days 1 and 2 werewindy (0.3- to 0.6-m waves), while night weather conditions werecalm with no waves. As previously noted total benthic driftdensity estimates pooled over all stations indicated the windyday densities (2,488/1,000 mM significantly exceeded calm nightdensities (750/1,000 m^) by a factor of 3.32. The number of fishlarvae caught during the day (88/1,000 mM exceeded, but notsignificantly, the number retained at night (67/1,000 m^) by afactor of 1.31. Additionally, the tug, Canonie, and a barge itwas guiding passed upbound during the day. At night the RogerM. Keys, Belle River, St. Clair, and Fred E. White Jr. passeddownbound while a variety of station nets were set. Regardlessof the higher number of ship passages during the night, daytimedensities for both benthos and fish larvae were greater thannight densities.

Ship Passage Drift Studies

Introduction

—

Upbound and downbound ship passage studies were conducted on10 June 1985 during daylight. Weather for both studies was sunnyand windy. Wave height at Frechette Point was 0.3 m to 1.0 m,with wind speeds ranging from 26 km/h to 44 km/h. The upboundportion of the study was based on the 1031 EDT passage of theComeaudoc (length = 223 m, width = 23 m, draft = 4.6 m) . Thespeed of the Comeaudoc was 13.2 km/h. The downbound portion ofthe study was based on the 1437 EDT passage of the V.W. Scully(length = 223 m, width = 23 m, draft = 7.6 m) . The speed of theV.W. Scully was 13.5 km/h. The passage of both vessels waswithin 150 m of station 3 (3 m deep) where 153-Mm nets were setto collect drift in five approximate 5-min periods correspondingto Before-, During-, After 1- (+5 min). After 2- (+10 min), andAfter 3- (+15 min) ship passage periods.

Passage of the upbound ship had a much greater visibleeffect on the physical appearance of the sampling device than didthe downbound vessel. When placed in the water with netsattached, the m^in support rod of the sample device was normallydeflected 10-15 from the vertical at Frechette Point. However,during the 2-min period when the Comeaudoc pas§ed nearest thenets, the main support rod was displaced 20-25 from vertical.Additionally, mid-depth current velocity increased from 41-43 cm/s to 54-56 cm/s. In the following minute, current velocitydecreased to 44 cm/s, and the main support rod returned to itsnormal deflection in the current.

84

with passage of the downbound vessel, there was noobservable deflection of the main support rod from its normalposition in the 8 min representing the During-ship-passageperiod. During this 8-min period, in the 5 min prior to theV.W. Scully's passage, mid-depth current velocity ranged from 46cm/s to 54 cm/s. However, in the 2-min period during which thevessel passed closest to the nets, current velocity decreased to43 cm/s and 41 cm/s, respectively* In the ensuing minute justafter immediate passage, current velocity decreased further to 40cm/s

.

Estimates of downbound benthic and ichthyoplankton driftcatches required correction, because at least one of the tworeplicate nets located at the bottom during the second 5-minperiod after passage (After 2) of the downbound V.W. Scullyapparently tangled around a portion of the sample device and didnot sample properly. As one replicate had no benthos and anotherhad only a low number relative to the mid-depth net catch,estimates of the drift near bottom in the After 2 period were notconsidered in statistical comparisons. However, estimates fromthese two nets do appear in density averages in tables where theymay be part of a pooled average (Tables 19 and 29).

Comparisons of Upbound and Downbound Pooled Drift

—

When comparing upbound (N = 20) and downbound (N = 18) meandrift density pooled over all respective samples, drift wassignificantly greater for Naididae (4,623/1,000 m^ vs. 1,292/1,000 mM, Chironomidae (2,329/1,000 m^ vs. 1,536/1,000 mM , andtotal benthic drift (9,608/1,000 m^ vs. 5,416/1,000 mO inupbound samples (Tables 19-20). Upbound and downbound densityestimates for all other major components of the benthic drift andfor larval fish drift were statistically non-significant. WhileHydra r Naididae, and Chironomidae were the dominant benthic taxain all cases, Naididae was the most numerous of the 53 benthictaxa (Table 1) collected in the upbound vessel study (48% oftotal benthic drift), and Hydra was predominant among the 11 taxacollected in the downbound vessel study (45%). In both studies,rainbow smelt was the dominant larval fish, making up >90% oftotal fish larvae. The only other larval fish encountered wasburbot which made up <3% of total fish larvae.

Comparisons of Mid-depth and Bottom Pooled Driftin the Upbound Study

—

All comparisons of benthic drift densities pooled over mid-depth and bottom depth strata (N = 10) in the upbound vesselstudy were non-significant except for Naididae. Naidid driftdensity was significantly greater at bottom (6,319/1,000 m^) than

85

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at mid-depth (2,926/1,000 mM. Total larval fish drift densitywas significantly higher at mid-depth (617/1,000 mM whencompared with bottom (313/1,000 mM

.

Comparisons of Mid-depth and Bottom Pooled Drift in the DownboundStudy

—

Data on drift from mid-depth and bottom were pooled in thesame manner as was done for the upbound study. We found nostatistically significant drift density differences betweenmid- (N = 10) and bottom (N = 8) depth strata for major benthiccomponents of the drift. However, as in the upbound vesselstudy, total larval fish drift density was significantly greaterat mid-depth (578/1,000 mO when compared with the bottom strata(184/1,000 mO.

Comparisons of the Five Ship Passage Periods at Each DepthStratum and Combined Depth Strata for Respective Upbound andDownbound Studies

—

The only significant density difference among either upboundor downbound ship passage periods at mid-depth (N = 2), bottom (N=2), and for both strata combined (N = 4) was for total larvalfish drift density at mid-depth in the upbound study. In thiscomparison, total larval fish drift density in the After 1 period(369/1,000 m^) was significantly lower than in all remainingperiods for which density differences were non-significant (517-1,108/1,000 m^) (Tables 21-25).

While density trends between depth strata and between thetwo studies were apparent, there were no clear trends from one 5-

min period to another within the upbound or downbound vesselstudies. Total benthic drift density in the upbound vessel studywas consistently greater near bottom when compared with mid-depthduring all sample periods except the After 3 period. However, inthe downbound study, most mid-depth density estimates of thebenthos were greater than bottom estimates. When pooling datafrom both depth strata, the total benthic drift density estimatesduring the upbound vessel passage exceeded downbound estimatesduring all 5-min sampling periods. When examining benthic driftdensity trends successively across the five sample periods, therewas no consistent trend at any combination of depth strata ineither study (Tables 21-25).

Similar comparisons of total larval fish drift density forupbound and downbound studies indicated mid-depth densitiesexceeded bottom estimates during each 5-min sampling period.When pooling data from both depth strata, total larval fish driftdensity in the downbound vessel study was similar during all 5-

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min periods (Tables 26-30). During the upbound vessel passage,there was a decline in total larval fish drift from 435/1,000 m^(Before) to 365/1,000 m^ (During), to 229/1,000 m^ (After 1).There was a sharp increase in larval fish drift density duringthe After 2 period to 863/1,000 m^ (Tables 21-26). During theAfter 3 period, density was 435/1,000 m^ as larval fish driftdensity returned to the level estimated during the Before period.This trend occurred at both depth strata.

ZOOPLANKTON

Zooplankton Abundance and Community Structure

Winter

—

Zooplankton abundances, as determined by No. 2-mesh drift netcollections, were extremely low during the winter study period(late February to early March, 1985) averaging 313/m^ ±229/m^ atFrechette Point, 376/m^ ±207/m' at Lake Nicolet, and 187/m^ ±152/m^ at Point aux Frenes (Table 31, Appendix 267). Differences inzooplankton abundances among the three locations werestatistically significant (p = 0.05; Kruskal-Wallis test). Meancurrent velocity decreased along the course of the riveraveraging 25.8 cm/s at Frechette Point, 18.9 cm/s at LakeNicolet, and 4.0 cm/s at Point aux Frenes; differences also werestatistically significant (p = 0.05; Kruskal-Wallis test).

The zooplankton community was dominated by adult Diaptomussicilis (mean individual dry weight 10.4 /xg) and by adultLimnocalanus macrurus (mean individual dry weight of 37.6 /xg).Nauplii, immature copepodites, and cladocerans were extremelyrare. There were no differences in the average individual dryweights of zooplankton among the three locations during winter;average individual dry weights ranged from 10.8 tig at LakeNicolet to 11.0 Mg at Frechette Point and Point aux Frenes.

Zooplankton distributions were not uniform in the watercolumn. Abundances (based on No. 2-mesh net collections) weresignificantly different (p = 0.05; Mann-Whitney U-test) betweenchannel stations and shallower stations (Table 32). In allinstances, zooplankton densities were greater in the more rapidlyflowing and deeper channel stations than in more shallow waters.There was no apparent difference in mean zooplankton size betweenshallow and channel stations (11.2 ng versus 10.8 Mg at FrechettePoint; 10.8 Mg versus 10.8 Mg at Lake Nicolet; 10.6 versus 11.0Mg at Point aux Frenes). This suggests that while zooplanktonabundances varied between channel and shallow stations, communitystructure did not. Current velocities (Table 32) weresignificantly (p = 0.05; Mann-Whitney U-test) higher at channelstations than at shallow stations.

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99

Table 31. Mean zooplankton abundance (No./m^) and standard deviation In par-

entheses at Frechette Point, Lake Nlcolet, and Point aux Frenes, 26 February-3 March 1985 and 2-13 June 1985. All estimates based on No. 2-inesh net col-

lections.

Frechette Point

Feb-MarJun

313.8172.2

(228.9)

(119.6)

Lake Nicole

t

376.389.4

(206.6)(113.9)

Polnteaux Frenes

187.3

21.1(151.5)

(49.0)

Table 32. Mean zooplankton abundance (No./m^), current velocity (cm/s), andweather conditions at channel and shallow stations at Frechette Point, LakeNicole t, and Point aux Frenes, 26 February-3 March 1985 and 2-13 June 1985.

All estimates based on No. 2-mesh net collections. Weather conditions: 1 »

calm, 2 « windy, 3 » very windy.

Frechette Point Lake 1Nicole

t

Point aux FrenesParameter Shallow Channel Shallow Channel Shallow Channel

Feb-MarZooplankton 264.0 476.8 321.1 468.3 152.5 245.2Current 20.5 43.5 13.8 27.3 2.8 6.0Weather - - - - - -

JunZooplankton 149.5 247.7 87.3 93.7 25.4 6.7Current 29.6 49.3 11.0 19.4 14.8 24.9Weather 1.5 1.0 1.5 1.1 2.1 2.0

Zooplankton abundances in No. 2-mesh net collections variedsignificantly among depth strata (i.e., surface, mid-depth, andbottom)- at the three locations. Overall, there was a strongtendency for zooplankton abundances to decrease with decreasingdepth; current velocity also decreased with depth (Table 33). AtFrechette Point, mean abundances decreased from 750/m^ in surfacesamples, to 316/m^ in mid-depth samples, to 248/m^ in bottomsamples. These differences were statistically significant (p =

0.05; Kruskal-Wallis test) as were differences in zooplanktonabundances between mid-depth and bottom samples (p « 0.05; Mann-Whitney U-test). The decrease in current velocity with depthalso was statistically significant for both three-depth (surface,mid-depth, and bottom) and two-depth (mid-depth and bottom)strata comparisons. At Lake Nicolet, decreases in zooplanktonabundance and current velocity with depth strata werestatistically significant for three-depth and two-depthcomparisons. At Point aux Frenes, zooplankton abundances variedsignificantly with depth strata both for two-depth and three-depth comparisons. Differences in current velocity with depthstrata were significant only for the three-depth comparisons.

100

Table 33. Mean zooplankton abundance (No./m^) by depth strata and averagecurrent velocity (cm/s) at Frechette Point, Lake Nlcolet, and Point auxFrenes, 26 February-3 March 1985 and 2-13 June 1985. All estimates based onNo. 2-mesh net collections.

Surface Mld-De^Zoo-

thCur-

BottomParameter Zoo- Cur- Zoo- Cur-

plankton rent plankton rent plankton rent

Feb-MarFrechette Pt. 750.3 40.0 316.6 28.9 248.2 21.5Lk. Nicole

t

611.3 24.6 423.8 20.7 281.9 16.1Pt. aux Frenes 204.8 5.7 253.9 4.3 133.0 3.3

JunFrechette Pt. 238.6 52.2 197.7 36.4 144.4 29.9Lk. Nicole

t

116.0 20.7 129.4 14.8 52.4 11.6Pt. aux Frenes 5.9 26.2 14.2 17.3 28.2 15.2

There were no apparent differences in the mean individual sizesof zooplankton collected from the three depth strata at FrechettePoint (10.7 ixg, 11.2 ^g, and 11.1 ug) , Lake Nicolet (10.8 ^g,10.9 MQr and 10.7 ^g) , and Point aux Frenes (10.9 ^g, 11.0 ug,and 10.9 ^g). This suggests that there were only minor changesin zooplankton community structure with depth strata.

Zooplankton did not exhibit significant (p = 0.05; MannWhitney U-test) variations in abundance between day and nightNo. 2-mesh net collections (Table 34) at any of the threelocations. Mean animal size also was similar during day andnight collections at Frechette Point (10.9 ^g and 11.2 ag), LakeNicolet (10.9 ug and 10.8 Mg), and Point aux Frenes (11.0 ug and11.0 Mg) suggesting that zooplankton community structure wassimilar in day and night collections. Differences in currentvelocities (Table 34) between day and night collections were notstatistically significant.

Finally, zooplankton abundances were compared in matchedNo. 10~mesh and No. 2-mesh net collections. Zooplanktonabundances were significantly (0 « 0.05; Mann-Whitney U-test)higher in No. 10-mesh than in No. 2-mesh net collections atFrechette Point (9.56/m* versus 326/mM and Lake Nicolet (769/m^versus 404/mM. However, such differences were not statisticallysignificant at Point aux Frenes (183/m^ versus 210/mM. The meansize of zooplankton collected by the No. 10-mesh net tended to besmaller than for No. 2-mesh net collections, averagingrespectively 9.3 ug and 10.7 ^g at Frechette Point, 9.3 ^g and10.8 ug at Lake Nicolet, and 8.0 Mg and 11.2 ^g at Point auxFrenes.

101

Table 34. Mean zooplankton abundance (No./m^), current velocity (cm/s), and

weather conditions during day and night collections at Frechette Point, Lake

Nicole t, and Point aux Frenes, 26 February-3 March 1985 and 2-13 June 1985.

All estimates based on No. 2-mesh net collections. Weather conditions: 1 «

calm, 2 « windy, 3 « very windy.

ParameterFrechetteDay

! PointNight

LakeDay

Nicole

t

NightPoint aux

DayFrenesNight

Feb-MarZooplanktonCurrentWeather

JunZooplanktonCurrentWeather

329.426.4

228.438.42.0

296.825.2

118.130.11.0

421.118.7

107.615.51.3

331.619.1

71.712.21.0

160.64.0

3.1

18.92.2

213.94.0

39.115.42.0

Spring

—

The second sampling was in early June. Zooplanktonabundances as determined by No. 2-mesh net collections (Table 31)were significantly lower in June (p « 0.05; Mann-Whitney U-test)at each of the three stations than in February. Abundances inJune averaged 172/m^ at Frechette Point, 89.4/m^ at Lake Nicolet,and 21.1/m^ at Lake Munuscong. Differences in zooplanktonabundance among the three locations also were statisticallysignificant (p = 0.05; Kruskal-Wallis test). Average flowvelocity decreased from Frechette Point (34.2 cm/s) to LakeNicolet (13.9 cm/s) and then increased at Point aux Frenes (17.1cm/s). Differences in flow velocity also were statisticallysignificant (p = 0.05; Kruskal-Wallis test) among the threestations.

The zooplankton community was more diverse in early Junethan in February. At Frechette Point and Lake Nicolet, adultDiaptomus slcilis (mean individual dry weight of 12.6 Mg)continued to be a dominant species. Immature LimnocalanusmacruTus copepodites (mean individual dry weight of 11.4 ug) alsowere abundant at these two stations. Other abundant taxaincluded nauplii, immature Cyclops (mean individual dry weight of1.7 ug) and Diaptomus spp. (0.9 ^g) copepodites. ImmatureCyclops spp. copepodites generally were the numerically dominanttaxon at Point aux Frenes. Mesocyclops sp. and cladocerans suchas Daphnia, Bosmina longlrostris, and Eubosmlna coregonl alsowere abundant at some stations. Small numbers of the Daphniamorph, D. mlnnehaha, were present at station 3 at FrechettePoint. It probably entered the site from a local stream or smallriver rather than from Lake Superior; the morph has not beenrecorded from the Great Lakes. Small numbers of epibenthicspecies were present in the various collections suggesting that

102

the littoral zooplankton community was well developed. Meanindividual zooplankton dry weight averaged 11.9 Mg at FrechettePoint, 11.7 Mg at Lake Nicolet, but only 1.9 /xg at Point auxFrenes. Thus, zooplankton were not only less abundant at Pointaux Frenes than at the two upriver stations, but were smaller.This suggests that the downriver decrease in zooplankton abun-dance was most strongly associated with the larger-bodied taxa.

As in winter, differences in zooplankton abundance (based onNo. 2-mesh net collections) between channel and shallow stationswere significant (p = 0.05; Mann-Whitney U-test). At FrechettePoint and Lake Nicolet, zooplankton were more abundant at channelstations than at stations located in more nearshore, shallowregions (Table 32). There was no apparent difference in meananimal size between channel and shallow stations both atFrechette Point (12.1 /xg and 11.9 Mg) and Lake Nicolet (11.7 Mgand 11.7 Mg); this suggests that zooplankton community structurewas similar in channel and shallow areas. Current velocitieswere significantly higher at channel stations than shallowstations (Table 32). At Point aux Frenes, abundances weresignificantly greater in shallow (25/mM than channel (7/m^)stations. Furthermore, zooplankton tended to be smaller atshallow stations (1.7 Mg) than in the channel (4.0 Mg). Thissuggests that there were large differences in zooplanktoncommunity structure between shallow and channel stations. As atthe other two upriver locations, current velocity wassignificantly higher in the channel than in the shallow regionsof the river.

Differences in zooplankton abundances (No. 2-mesh net col-lections) with sample depth were significant (p = 0.05; Kruskal-Wallis test for surface, mid-depth, and bottom comparisons; Mann-Whitney U-test for mid-depth versus bottom comparisons). AtFrechette Point, mean abundances decreased with sample depth asdid current velocities (Table 33); differences were statisticallysignificant for all comparisons. There was little change in meanzooplankton size with increasing depth strata (12.3 Mg^ 11.9 ng,and 11.9 Mg, respectively). Differences in zooplankton abundanceand current velocity with depth strata were statisticallysignificant for all comparisons at Lake Nicolet; lowestzooplankton abundances and current vel-ocities were observed nearthe river bottom. Mean zooplankton size showed little changewith increasing depth strata (from 11.7 Mg to 11.9 Mg to 11.1Mg) . Differences in zooplankton abundance with depth strata werenot statistically significant at Point aux Frenes; differences incurrent velocity were significant among the three depth stratabut not for mid-depth versus bottom com-parisons. There was astrong trend for mean zooplankton size to decrease with depthstrata, i.e., from 4.7 Mg to 2.2 Mg to 1.7 Mg.

103

In contrast with winter zooplankton, summer zooplanktonexhibited significant (p = 0.05; Mann-Whitney U-test) differencesin abundance with sampling time based on No, 2-mesh netcollections. At Frechette Pointy zooplankton were more abundantin day (228/m^) than night (118/mO collections (Table 34).Greatest abundances during daylight hours were not anticipated.Mean animal size was 12.0 ^g during day collections and 11.9 fxg

during night collections suggesting that there were little dielchanges in zooplankton community structure. Diel differences inzooplankton abundance may have been related to the higher currentflow (38.4 cm/s versus 30.1 cm/s) and generally more windycollection periods (2.0 versus 1.0, where 1 = calm, 2 = windy,and 3 = very windy) associated with day versus night collections(Table 34); these differences were statistically significant.Similarly, zooplankton were significantly more abundant in day(421/mM than night (332/m^) collections made at Lake Nicolet(Table 34); mean animal size averaged 11.7 Mg during day andnight collections. Again, average current flow was significantlyhigher during day (15.5 cm/s) than night collections (12.2 cm/s);the weather was also significantly more windy during daycollections (weather conditions of 1.3 and 1.0 respectively forday and night sampling). Only at Point aux Frenes, where thezooplankton community was dominated by small taxa, werezooplankton significantly more abundant in night (39.1/mM thanday (3.1/m^) collections (Table 34). Animals tended to besmaller in night collections averaging 1.7 ug versus 4.0 Mgduring the day. Although the average current flow wassignificantly higher during day (18.9 cm/s) than night (15.4 cm/s) collections (Table 34), there was no significant difference inweather conditions. The weather was windy to very windy duringthe day (average weather condition of 2.2) and was windy (averageweather condition of 2.0) at night.

Zooplankton densities were significantly (Mann-Whitney li-

test) more abundant in No. 10-mesh net collections than in No. 2-mesh net collections, averaging respectively 542/m^ and 241/m^ atFrechette Point, 284/m^ and 73/m^ at Lake Nicolet, and 439/m^ and9/m^ at Point aux Frenes (Table 35). At Frechette Point, wherethere was only a 2.3-fold difference in zooplankton abundancebetween the two collection methods, the mean size of animals waslarge averaging 12.1 Mg for the No. 2-mesh net and 7.0 Mg for theNo. 10-mesh net. However, at Point aux Frenes, where there was a50-fold difference in abundance estimates between the two nets,zooplankton were considerably smaller, averaging only 2.6 Mg inthe No. 2-mesh-net collections and 1.3 Mg in the No. 10-mesh net.

104

Table 35. Mean zooplankton density (No./m3) and mean animal dry weight (jig)

in No. 2-me8h and No. 10-me«h net collections made at!"<^5«"f

Z^^^*^' ,^5^Nicolet, and Point aux Frenes, 26 February-3 March 1985 and 2-13 June 1985.

Frechette Point Lake Nicolet Point aux Prwies

Net Size Density—Weight Density Weight Density Weight

ls^=r^ 362.6 10.7 403.9 10.8 210.3 11.2

S::?0 956:2 9.3 769.1 9.3 183.0 8.0

JunNo. 2

No. 10

240.7 12.1 72.8 11.7 8.8 2.6

541.6 7.0 283.7 3.7 438.7 1.3^

Biomass Studies

Overview

—

The general purpose of the biomass investigations was tocharacterize the drift or load of particulates carried by theSt. Marys River. Load was estimated in terms of dry weight andash-free weight. Dry weight and ash-free weight were estimatedby direct weighing of the appropriate samples (seston, plants,benthos, mysids) or from estimates based on taxa abundance andseparately determined dry weight of selected individuals(zooplankton and fish). An average ash-free weight conversionfactor of 0.047 (determined in the laboratory) was used for fishand a factor of 0.045 (the mean value determined for mysids inthe laboratory) was used for zooplankton. Detritus dry weightwas estimated by subtracting estimated zooplankton dry weightfrom seston dry weight. Although this estimate generally wasgood, some negative values were calculated. This sampling erroroccurred when the seston was strongly dominated by zooplankton.Since different subsamples were used for zooplankton abundanceestimates and seston determinations, small variations insxibsample weights resulted in some negative detritus dry-weightestimates. These sampling errors were further magnified whendetritus ash-free weight was estimated.

In the following paragraphs, biomass data are presented asdry weight for zooplankton, plants, benthos (including mysids),seston, and detritus. The percentage of dry weight that was ash-free weight is also given for seston ( %AFW: seston ) . Ash-freeweights are not presented for other components of the river loadbecause a constant conversion factor was used for zooplankton (adominant component of river load) and fish, and because thedetritus ash-free estimates were not sufficiently accurate towarrant discussion.

105

General Composition of River Load

—

The average load carried by the St. Marys River was 5,406mg/1,000 m* in February and 11,129 mg/1,000 m* in early June(Table 36, Appendixes 135-261 )• Seston was the major componentof the load during both sampling seasons with zooplankton (63.2%)predominating in February and detritus (86 .9%) in June. Asexpected, %AFW: seston was lower in winter (4.8%) than in spring(25.6%). Plants were minor contributors to total river load withthe absolute and relative contribution increasing in spring.Benthos also was a minor contributor to river load both in winterand spring. Mysids were more abundant in winter than in spring,contributing to a greater percentage of winter (31.7%) thanspring (5.1%) benthic biomass. Fish larvae were not present inwinter collections and were minor components (0.04%) of springcollections. Differences in all components of river load werestatistically significant (p « 0.05; Mann-Whitney U-test) betweenwinter and spring.

River Load During Winter

—

There were statistically significant (p « 0.05; Kruskal-Wallis test) differences in dry weight for all components (exceptmysids) of river load among the three stations; comparisons werebased on No. 2-mesh net collections. Average total river loadwas 6,835 mg/1,000 m* at Frechette Point, 5,619 mg/1,000 m^ atLake Nicolet, and 2,687 mg/1,000 m^ at Point aux Frenes (Table37; Appendixes 135-154, 175-200, 229-241). The major component

Table 36. Mean concentration (mg dry weight/1,000 w?) and percent composition

of the various components of river drift, seston percent (of dry weight), ash-

free weight (AFW), and current velocity (cm/s) during 26 February-3 March 1985

and 2-13 June 1985. All estimates based on No. 2-mesh net collections. Mysis

weights are included in the total benthos weight. Detritus weights are the

difference between seston and zooplankton weights.

Parameter

ZooplanktonPlantsTotal benthosMysisFishSes tonDetritusTotal

Z AFW: ses tonCurrent

WinterConcentra tion Percen t

3,416.322.645.114.30.0

5,338.91,922.65,406.6

4.818.1

63.20.40.80.30.0

98.835.6

j££iB&.Concentration Percent

1,047.8313.192.54.74.8

10,719.39,671.511,129.7

25.620.8

106

Table 37. Mean concentration (mg dry weight/1,000 m^) and percent compositionof the various components of river drift, seston percent (of dry weight), ash-free weight (AFW), current velocity (cm/s), and weather conditions by locationduring 26 February-3 March 1985 and 2-13 June 1985. All estimates based onNo. 2-mesh net collections. Weather conditions: 1 « calm, 2 « windy, 3 « verywindy. Mysis weights are Included In the total benthos weight. Detritusweights are the difference between seston and zooplankton weights

.

Frechette Point

Concentra tion %

Lake Nicole

t

Point aux FrenesParameter Concentration Z Concentration %

Feb-MarZooplankton 3,449.4 50.5 4,069.6 72.4 2,056.2 76.5Plants 73.0 1.1 3.0 0.1 5.1 0.2Benthos 87.4 1,3 30.6 0.5 6.3 0.2Mysls 8.2 0.1 25.4 0.5 2.0 0.1Fish 0.0 0.0 0.0 0.0 0.0 0.0Seston 6,674.8 97.7 5,585.1 99.4 2,675.8 99.6Detritus 3,225.4 47.2 1,515.5 27.0 619.6 23.1Total 6,835.2 5,618.7 2,687.2

X AFW: seston 6.4 3.8 4.5Current 25.8 18.9 4.0

JunZooplankton 2,056.6 8.9 1,047.1 51.7 40.2 0.3Plants 984.3 4.3 22.2 1.1 33.8 0.3Benthos 67.2 0.3 122.4 6.1 76.8 0.7Mysis 6.6 0.0 6.6 0.3 0.0 0.0Fish 1.7 0.0 10.4 0.5 0.1 0.0Ses ton 22,094.0 95.5 1,868.7 92.3 11,618.2 99.1Detritus 20,037.4 86.6 821.6 40.6 11,578.0 98.7Total 23,147.2 2,023.7 11,728.9

% AFW: ses ton 26.7 24.6 26.9Current 34.2 13.9 17.1Weather 1.5 1.1 2.1

107

of river load was zooplankton with the percentage increasingdownriver as detritus concentrations decreased. Plant andbenthic biomass also decreased downriver. Average currentvelocity decreased from 25.8 cm/s at Frechette Point, to 18.9 cm/s at Lake Nicolet, to 4.0 cm/s at Point aux Frenes. Althoughaverage current velocity was considerably lower at Point auxFrenes than at Lake Nicolet, river load composition was similarwith detritus accounting for approximately 23% to 27% of theload.

Total river load was higher in the channels than in theshallows (Table 38); such differences were significant (p = 0.05;Mann-Whitney U-test) at all three locations. Since zooplanktonwas the major component of river load (and was more abundant atchannel stations. Table 32), the greater river load in channelthan shallow stations was expected. Detritus concentrations alsowere significantly higher in the channels as were currentvelocities. Differences in plant, benthos, and mysid densitiesbetween channel and shallow stations were not statisticallysignificant (p = 0.05; Mann-Whitney U-test and median test).

River load concentrations varied with depth strata (Table39). Total river load varied significantly (p = 0.05) with depth(Kruskal-Wallis test for surface versus mid-depth versus bottomcomparisons; Mann-Whitney U-test for mid-depth versus bottomcomparisons). Total river load decreased with depth, reflectingthe decrease in zooplankton density and biomass with depth.Current velocity also decreased with depth, with differencesstatistically significant at all three stations. Detritusconcentrations were significantly different for the three depth-strata comparisons but not for mid-depth versus bottomcomparisons. Since surface samples were collected only in thechannels, significant differences based on three-way but not two-way comparisons probably reflect channel effects. Differences inbenthos and mysid densities with depth strata were notstatistically significant. Thus, there was no apparentrelationship between current velocity among various regions ofthe water column and benthic drift from the sediments.Differences in plant biomass with depth strata were notstatistically significant (p = 0.05; Mann-Whitney U-test andmedian test). Median test results were used for thosestatistical comparisons where a large number of samples did notcontain measurable plant biomass.

River load differed between day and night sampling (Table40). Day versus night comparisons (p = 0.05; Mann-Whitney U-test) were statistically significant only for benthos and mysidswith biomass increasing at night at all three stations. Currentvelocities were not significantly different between day and nightsampling. This suggests that the greater biomass of benthos and

108

Table 38. Mean concentration (mg/1,000 m^) of the various components of river

drift, seston percent (of dry weight), ash-free weight (AFV), current velocity(cm/s), and weather between channel and shallow stations 26 February-3 March1985 and 2-13 June 1985. All estimates based on No. 2-mesh net collections.Weather conditions: 1 " calm, 2 » windy, 3 * very windy. Mysls weights areIncluded In the total benthos weight. Detritus weights are the differencebetween seston and zooplankton weights.

Frechette Point Lake Nicole

t

Point aux FrenesParameter Shallow Channel Shallow Channel Shallow Channel

Feb-MarZooplankton 2,960.4 5,079.2 3,428.6 5,047.9 1,670.8 2,698.5Plants 80.4 58.3 0.1 7.7 8.0 0.4Benthos 87.2 87.9 40.5 14.2 6.7 5.6Mysls 8.0 8.8 33.6 11.7 0.6 4.4Fish 0.0 0.0 0.0 0.0 0.0 0.0Seston 5,597.3 10,266.7 4,841.8 6,823.9 1,976.7 3,841.0Detritus 2,636.9 5,187.5 1,359.2 1,760.9 305.9 1,142.5Total 5,764.9 10,412.9 4,916.0 6,845.8 1,991.4 3,847.0

% AFU: seston 6.4 6.2 3.7 3.9 5.6 2.7Current 20.5 43.5 13.8 27.3 2.8 6.0

JunZooplankton 1,776.1 2,991.3 1,022.7 1,095.8 44.3 26.7Plants 1,254.7 82.9 23.8 18.9 39.5 14.4Benthos 79.1 27.7 127.4 112.5 93.3 20.1Mysls 6.9 5.9 3.1 13.7 0.0 0.0Fish 0.9 4.3 12.4 6.5 0.1 0.4Seston 24,914.1 12,691.8 2,011.1 1,584.0 15,080.5 219.6Detritus 23,138.4 9,700.5 988.4 488.2 15,041.0 192.9Total 26,225.7 12,806.7 2,174.7 1,721.9 15,213.4 254.5

% AFU: seston 28.1 22.0 25.1 23.6 26.9 26.8Current 29.6 49.3 11.0 19.4 14.8 24.9Weather 1.5 1.0 1.5 1.1 2.1 2.0

109

Table 39. Mean concentration (mg dry weight/1,000 m^) of the various componentsof river drift, seston percent (of dry weight), ash-free weight (AFW), and cur-rent velocity (cm/s) by depth strata and location, 26 February-3 March 1985 and2-13 June 1985. All estimates based on No. 2-mesh net collections. Mysisweights are Included In the total benthos weight. Detritus weights are thedifference between seston and zooplankton weights.

February-March JuneParameter Surface Mid-depth Bottom Surface Mid-depth :Bottom

FrechetteZooplankton 7,741.4 3,558.8 2,758.1 2,941.7 2,357.7 1 ,715.0Plants 72.8 51.3 86.1 37.9 2,298.2 181.0Benthos 66.2 54.6 114.2 20.1 50.4 85.6Mysls 18.0 8.5 6.5 10.8 4.3 7.7Fish 0.0 0.0 0.0 0.6 1.5 2.2Seston 16,378.0 6,549.7 5,378.0 4,378.5 9,551.8 33 ,583.0Detritus 8,636.6 2,290.9 2,619.9 1,436.8 7,194.1 31 ,868.0Total 16,517.0 6,655.6 5,578.3 4,437.1 11,901.9 33 ,868.0

% AFW: seston 7.7 4.0 7.8 16.1 25.8 28.9Current 40.0 28.9 21.5 52.2 36.4 29.9

Nicole

t

Zooplankton 6,579.6 4,634.2 3,018.6 1,361.6 1,534.4 598.2Plants 13.3 0.0 2.6 5.2 15.3 31.1Benthos 6.7 23.8 41.7 45.4 183.3 92.1Mysls 6.1 20.0 34.3 0.4 8.0 6.9Fish 0.0 0.0 0.0 8.0 16.2 6.5Seston 8,908.9 6,134.4 4,342.1 1,685.9 1,926.0 1 ,864.9Detritus 2,329.3 1,500.2 1,323.5 324.3 391.7 1 ,266.7Total 8,928.9 6,158.2 4,386.4 1,744.4 2,140.8 1 ,994.6


Point aux FrenesZooplankton 2,228.4 2,802.5 1,453.4 27.9 30.9 48.6Plants 0.0 0.0 10.3 24.2 13.1 49.9Benthos 8.4 2.3 8.7 2.2 53.5 104.2Mysls 8.2 1.3 1.0 0.0 0.0 0.0Fish 0.0 0.0 0.0 0.5 0.1 0.1Ses ton 3,228.8 3,487.7 1,930.9 171.1 2,,334.7 20,,034.4Detritus 1,000.4 685.2 477.5 143.2 2,,303.8 19,,984.4Total 3,245.4 3,490.0 1,949.9 198.0 2,,401.4 20,,187.2


110

Table 40. Mean concentration (mg/1,000 m^) of the various components of river

drift, seston percent (of dry weight), ash-free weight (AFU), current veloci^(cm/s), and weather during the day and night, 26 February-3 March 1985 and2-13 June 1985. All estimates based on No. 2-me8h net collections. Weatherconditions: 1 - calm, 2 > windy, 3 " very windy. Mysis weights are Includedin the total benthos weight. Detritus weights are the difference between ses-ton and zooplankton weights.


t

Point aux FrenesParameter Day Night Day Night

Feb-MarZooplankton 3,579.9 3,320.9 4,568.8 3,570.6Plants 42.6 97.6 5.5 0.4Benthos 7.9 168.4 11.4 49.8Mysis 0.0 16.5 8.3 42.5Fish 0.0 0.0 0.0 0.0Seston 6,538.8 6,810.9 5,868.5 5,301.6Detritus 2,960.9 3,490.0 1,999.8 1,731.0Total 6,589.3 7,076.9 5,885.4 5,351.9

X AFW:se8ton 6.0 6.7 3.5 4.1Current 26.4 25.2 18.7 19.1

JunZooplankton 2,729.7 1,408.8 1,260.1 839.8Plants 1,968.0 37.8 23.1 21.2Benthos 66.3 68.0 5.3 231.4Mysis 2.1 11.0 0.0 13.0Fish 1.5 1.9 5.6 15.1Seston 38,846.7 5,973.2 2,549.2 1,207.0Detritus 36,117.0 4,564.4 1,289.1 367.2Total 40,882.5 6,080.9 2,583.2 1,474.7

% AFV:seston 32.5 21.1 25.9 23.3Current 38.4 30.1 15.5 12.2Weather 2.0 1.0 1.3 1.0

Day Night

1,761.8 2,350.60.0 10.35.3 7.20.0 4.20.0 0.0

2,270.9 3,080.7509.1 730.9

2,276.2 3,098.2

6.0 3.04.0 4.0

15.2 65.227.3 40.19.6 141.40.0 0.00.1 0.2

7,742.1 15,419.27,726.3 15,354.07,763.3 15,600.9

25.2 28.518.9 15.42.2 2.0

111

mysids in night over day collections was associated with verticalmigration from the river floor.

River load concentration varied between No. 2-mesh (355 m)and No. 10-mesh (153 /xm) net collections (Table 41). Zooplanktonand seston concentrations were higher in No. 10-mesh and No. 2-

mesh-net collections at Frechette Point and Lake Nicolet; totalriver load concentrations also were higher in the No. 10-mesh netcollections. These differences were statistically significant (p= 0.05; Mann-Whitney U-test). At Point aux Frenes, zooplanktondry-weight concentrations (but not abundance) were significantlygreater in the coarser No. 2-mesh net collections; mean size ofzooplankton also was larger in the No. 2-mesh net collectionsaveraging 11.2 ng versus 8.0 /xg for the No. 10-mesh net. Sestonand total river load concentrations were significantly higher inthe No. 2-mesh than the No. 10-mesh net collections at thislocation. The reason for this difference has no readyexplanation. Differences in detritus, plant, benthos, and mysidconcentrations between No. 2-mesh and No. 10-mesh net collectionswere not statistically significant among sites. This suggeststhat these particulates were sufficiently large to be efficientlycaptured by the No. 2-mesh net. Although %AFW: seston tended tobe higher in No. 10-mesh than No. 2-mesh net collections,differences were not statistically significant.

River Load During Spring

—

With the exception of mysids and %AFW:seston, differences inJune river load composition were significantly (p = 0.05;Kruskal-Wallis test) different among Frechette Point, LakeNicolet, and Point aux Frenes. Total load (dry weight. No. 2-

mesh net) averaged 23,147 mg/1,000 m' at Frechette Point, 2,024mg/1,000 m^ at Lake Nicolet, and 11,729 mg/1,000 m^ at Point auxFrenes (Table 37; Appendixes 155-174, 201-228, 242-261). Sestonwas the major component of the river load at all three stations.

The river apparently carried a greater mineral load in Junethan in February-March (Tables 36, 37) as %AFW: seston wasconsiderably higher in June (25.6%) than in February-March(4.8%). This increase may have been associated with increasedinflows from small streams and rivers. It may also have beenassociated with the disappearance of the protective ice coverwhich had served to limit wind-induced wave activity and sedimentresuspension.

At Frechette Point, zooplankton was a minor component of theJune river load averaging 8.9% versus 50.5% in February-March(Table 37). This decrease in percent composition was due mainlyto the large increase in detritus between winter (3,225 mg/1,000mM and spring (20,037 mg/1,000 m^) sampling. Plants were more

112

Table 41. Mean concentration (mg/dry weight/1,000 nP) of the various components

of river drift in No. 2-mesh and No. 10-mesh net collections, 26 February-3 March 1985 and 2-13 June 1985. Also given is the percentage of dry weightthat is ash-free dry weight (AFW). Mysis weights are included in the totalbenthos weight. Detritus weights are the difference between seston andzooplankton weights.


t

Point aux FrenesParameter No. 2 No. 10 No. 2 No. 10 No. 2 No. 10

Feb-MarZooplankton 3,863.0 8,856.9 4,375.5 7,170.1 2,345.0 1,458.5Plants 55.2 89.6 3.3 3.4 1.2 0.6Benthos 86.1 44.7 22.9 18.3 4.3 4.0Mysls 6.7 0.4 20.2 14.9 2.8 0.7Fish 0.0 0.0 0.0 0.0 0.0 0.0Seston 8,353.4 13,034.9 6,631.5 10,530.5 3,439.6 1,888.7Detritus 4,490.4 4,178.0 2,238.0 3,360.9 1,094.6 430.2Total 8,494.7 13,168.3 6,657.7 10,552.2 3,445.0 1,893.3

% AFW: seston 7.3 14.2 4.4 4.9 5.0 10.2

JunZooplankton 2,905.9 3,770.4 849.2 1,035.5 23.1 585.0Plants 2,705.8 79.3 53.3 69.0 32.3 43.3Benthos 76.4 53.0 146.6 47.2 9.3 28.5Mysis 3.9 0.1 20.8 2.1 0.0 0.4Fish 3.8 2.1 6.1 13.1 0.2 0.6Seston 18,722.7 12,857.6 1,258.6 3,023.1 1,,299.8 37 ,625.0Detritus 15,816.8 8,883.2 400.4 1,987.6 1,,276.7 37 ,040.0Total 21,508.7 12,992.0 1,464.6 3,152.4 1,,341.6 37 ,697.4

% AFW: seston 26.4 43.2 22.8 45.0 27.9 23.7

113

abundant in June (984 mg/1,000 mO than February (73 mg/ljOOOm^), averaging 22.2% of the total river load versus 1.1% inwinter; as in February-March, the greatest biomass of plants wasobserved at Frechette Point. Benthos and fish, while moreabundant in spring than in winter, continued to be minorcomponents of river load. February-June statistical comparisonsMann-Whitney U-test) were significant (p = 0.05) for allcategories except mysids, seston, and total river load.

At Lake Nicolet, zooplankton was a smaller contributor tototal river load in June (51.8%) than February (72.4%) (Table37). Detritus accounted for an average of 40.6% of the riverload in June versus 27.0% in February; however, detritusconcentrations were lower in June than in February. Plants,benthos (excluding mysids), and fish all increased in biomassbetween February and June but were relatively minor components ofriver load. All winter-spring comparisons were statisticallysignificant (p = 0.05; Mann-Whitney U-test).

At Point aux Frenes, total river load concentrationincreased markedly between February-March (2,687 mg/1,000 mM andJune (11,728 mg/1,000 mM (Table 37), although these differenceswere not statistically significant (p = 0.05; Mann-Whitney li-

test). Zooplankton accounted for a very small component of theriver load, averaging 0.3% in June versus 76.5% in winter. Thislow contribution was due to a marked decline in zooplanktonabundance and mean size between winter and June while detritusconcentrations increased 18-fold. Biomass of plants, benthos,and fish increased between winter and spring. With the exceptionof total river load, all winter-spring statistical comparisonswere significant (p = 0.05; Mann-Whitney U-test).

As in February-March, there were differences in river loadconcentration and composition between channel and shallowstations (Table 38). At Frechette Point, zooplankton and mysidbiomass was significantly (p = 0.05; Mann-Whitney U-test) greaterin channel station samples, while benthic biomass wassignificantly greater in shallow station samples; differences in%AFW: seston also were significant. At Lake Nicolet, zooplanktonbiomass was significantly greater in channel station samples,while seston biomass was greater at shallow stations. Althoughdetritus concentrations were higher at shallow stations,differences were not significant. At Point aux Frenes,zooplankton, detritus, and seston biomass was greater in shallowstation samples, while fish were more abundant in the channel.

Although total river load concentration varied with depthstrata at Frechette Point (Table 39), such differences were notsignificant (p = 0.05; Kruskal-Wallis test for three stratacomparisons and Mann-Whitney U-test for two-depth comparisons).Differences in zooplankton and detritus biomass (which increased

114

with depth) and benthic biomass (which decreased with depth) weresignificant. At Lake Nicolet^ zooplankton, seston, fish, andtotal river load biomass varied significantly with depth strata;maximum biomass was observed at mid-depth strata. At Point auxFrenes, benthos, seston, fish, detritus, and total biomass variedsignificantly among the three depth strata with lowest biomassobserved in surface strata. Mid-depth versus bottom comparisonswere significant only for benthos.

There were significant differences in river loadconcentration and composition between day and night collections.At Frechette Point, zooplankton, plant, seston, detritus, andtotal biomass densities were higher during day collections thannight collections (Table 40); these differences were significant(p = 0.05; Mann-Whitney U-test). Current velocities also weresignificantly higher during the day (38.4 cm/s versus 30.1 cm/s).Day-night differences in weather conditions were statisticallysignificant. The weather was windy during the day but calm atnight. Differences in benthos, mysid, and fish biomass betweenday and night sampling were not significant.

At Lake Nicolet, differences in zooplankton, seston,detritus, and total biomass between day and night sampling (Table40) were significant (p = 0.05; Mann-Whitney U-test). In allinstances, biomass was higher during the day than during thenight. Such differences may have been related to physicalfactors such as current velocity and resuspension. Currentvelocities were higher during the day (15.5 cm/s versus 12.2 cm/s) and the weather was slightly more windy (1.3 versus 1.0);these differences were significant. Benthos, including mysids,and fish were significantly more abundant at night. Thissuggests that, in contrast to Frechette Point, some components ofthe benthic community made diel migrations between the riverfloor and water column.

At Point aux Frenes, zooplankton and benthos were moreabundant at night than during the day, and differences weresignificant (p = 0.05; Mann-Whitney U-test). While the meansizes of zooplankton collected during day and night collectionswere similar at Frechette Point (12.0 ng and 11.9 ixg) and LakeNicolet (11.7 ug and 11.7 ng) , there were large differences inthe mean size of zooplankton in day (4.9 /zg) and night (1.7 /xg)

collections at Point aux Frenes (Table 35). This suggests thatthe increased biomass of zooplankton at night at Point aux Freneswas associated with the vertical migration of small epibenthiczooplankton, i.e., immature Cyclops spp. and Mesocyclops edax.As zooplankton and benthos were minor components of the riverload at Point aux Frenes, day and night differences in totalriver load were not significant. Detritus concentrations werenot significantly different between day and night collectionsdespite higher current velocities during daylight hours (18.9 cm/

115

s versus 15.4 cm/s). Although it was windier during the day thannight (2.2 versus 2.0), these differences were not significant.This suggests that during calm weather (e.g., at FrechettePoint), current velocity has a significant effect on river load.However, as the weather becomes more windy, small differences incurrent velocity may not have as major an effect on river load asincreased wave activity resulting from the windier conditions.

River load estimates differed between No. 2-mesh and No. 10-mesh net collections (Table 41). At Frechette Point, differenceswere significant only for fish, total biomass, and %AFW:seston;estimates were higher in No. 2-mesh than No. 10-mesh net samples.There is no ready explanation for these higher biomass estimatesfrom the No. 2-mesh net.

At Lake Nicolet, net comparisons were significant forseston, fish, detritus, total biomass, and %AFW:seston; theNo. 10-mesh net collected the greater biomass (Table 41).Although zooplankton were significantly more abundant in No. 10-mesh net collections than No. 2-mesh net collections, biomass wasnot significantly different. This is because, although smallzooplankton were collected in greater numbers in the No. 10-meshnet collection, they apparently did not contribute substantiallyto biomass.

At Point aux Frenes, net comparisons were significant onlyfor zooplankton, seston, and total biomass; the No. 10-mesh netcollected the greater biomass (Table 41). Although detritusconcentrations were higher in the No. 10-mesh net collections,these differences were not statistically significant (p = 0.05;Mann-Whitney U-test).

FISH LARVAE

Lake Herring

Lake herring (Coregonus artedll) larvae (total numbercollected = 281) were collected during all 6 wk of the study andat all seven stations. The total number collected per week (sameeffort) (Fig. 14) started at 52, peaked at 80, and then declinedto 14 larvae in the last week.

Densities of larvae were generally greatest during the first3 wk and tapered off during the last 3 wk of May (Fig. 15, Table42). Mean densities varied from to 1,451 larvae/1,000 m'.Maximum densities usually occurred at the 1-m depth, butoccasionally densities were greater at 2 m. Neither depth had aconsistently greater density of larval lake herring over the 6

wk. In the channel, densities were very low with a maximum of

116

80

70-

oS 60uUJ

o

o

50

40

30

20

10

O

\.-'' ^-v

12 3 4 5 6

SAMPLING PERIOD

Figure 14. Total nijmber (NO.) of lake herring (LH) and lakewhitefish (LW) larvae collected over 6 wk (24 April to 30 May) inthe St. Marys River, 1985.

117

fO

OOO

HI<><

o

ffi

3

1000

600T

200T

100-

0--

200

100

]

lOO-i

0-.

100 -

-.

1400-

1300.

1200-

200.

100-

300 J

l-M DEPTH STATION 2

NORTH L. NICOLET

^^2-M DEPTH

STATION 3

MIDDLE L. NICOLET

STATION 4J50UTH L.

NICOLET

STATION 5

NORTH NEEBISH ISLAND

STATION 6EAST NEEBISH ISLAND

STATION 7NORTH L. MUNUSCONG

26-28 29-5 6-9APRIL APR MAY MAY

3-14 23-25 28-30MAY MAY MA"Y

DATEFigure 15. Density of lake herring larvae collected from 26April to 30 May at six stations in the St. Marys River.

118

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only 17 larvae/1,000 m^. Only 9% of the larvae were collected in

the channel.

Although larval lake herring were collected at all sevenstations, occurrences and densities varied considerably. Onlytwo larvae were collected in the Edison Hydropower Canal -

station 1. At the other six stations, lowest densities and mostsporadic occurrences were at stations 3, 4, and 5 (Fig. 15).During the first 3 sampling wk, larval fish densities wereespecially high at the 1-m depth at the north Lake Nicolet andeast Neebish Island stations. Lower densities occurred atstation 7 - north Lake Munuscong, but some larval lake herringwere present throughout the 6 wk. Among stations, densities oflarval fish were generally highest at the 2-m depth.

Average size of larval lake herring increased from April tothe end of May (Fig. 16). Since lake herring hatch at 8.5 to12.8 mm (Auer 1982), the presence of a few 10.2- to 12.0-mmlarvae suggests some hatching occurred during the fourth week ofMay. Based on larval fish sizes, hatching was completed by thelast week of May.

The length-frequency distribution of lake herring larvae atall stations below the Sault Power Canal (Fig. 17) showed larvaewe collected ranged from about 8 to 25 mm. Modes were observedat about 11 and 17 mm.

The percentage of the lake herring collected that were non-yolk-sac larvae increased almost linearly over the course of thestudy (Fig. 18) from to 2% in the first 2 wk, up to 100% duringthe last week. Therefore, the decline in the total number oflarvae that we collected, from 52 in week 1 to 14 in week 6, ispartially attributable to net avoidance as well as naturalmortality as the larvae grow older. Overall, 61% of all lakeherring larvae collected were yolk-sac larvae and 39% were non-yolk-sac larvae.

Lake Whitefish

Lake whitefish iCoregonus clupeaformis) larvae werecollected during all 6 wk of the study and at six of the sevenstations. Larvae were not collected in the Edison HydropowerCanal. Sixty-two larvae were captured, making them less abundantthan lake herring. The total number of lake whitefish larvaecollected per week (Fig. 14) increased gradually from 6-7 inweeks 1 and 2 to a peak of 19 at week 4, and then a gradualdecline to 6 in the last week of May.

Densities were generally greatest during the first 2 wk ofMay (Fig. 19). Maximum densities occurred at 1-m depths and

125

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Figure 16. Mean total length and standard error (vertical lines)

of five larval fish species collected in the St. Marys River,1985. Total number collected is in parentheses. Larvae collectedat Station 1 (Edison Hydropower Canal) are not included.

126

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2 3 4 5 6

SAMPLING PERIOD

Figure 18. Percentage of non-yolk-sac lake herring (LH) and lakewhitefish (LW) larvae collected over 6 wk (24 April to 30 May) inthe St. Marys River, 1985.

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Figure 19. Density of lake whitefish larvae collected from 26April to 30 May at six stations in the St. Marys River.

129

ranged from about 600 to 1,000 larvae/1,000 m*. Few lakewhitefish larvae were captured at 2 m, and none were caught inthe channel. Although present at the six stations, occurrencesand densities varied considerably among stations. The middle andsouth Lake Nicolet stations produced the most larvae.

Average sizes of lake whitefish larvae increased from Aprilto the end of May (Fig. 16). Hatching appeared to be over bymid-May as few smaller larvae were collected thereafter. Lakewhitefish larvae collected ranged from about 12 to 23 mm(Fig. 20). There were modes at around 14 mm and 22 mm; thelatter group was collected during the last week of the study.Lake whitefish hatch at sizes of 8.8-15 mm (Auer 1982). Thecurve for the percentage of lake whitefish larvae that were non-yolk-sac (Fig. 18) followed the same pattern observed for lakeherring. Percentages were low in weeks 1 and 2 and thenincreased dramatically to 100% in weeks 5 and 6. Overall, unlikelake herring where the pattern was reversed, 29% of the lakewhitefish larvae were yolk-sac larvae, while 71% were non-yolk-sac larvae.

Burbot

Burbot (iota lota) larvae were collected during all 6 wk ofthe study and at all seven stations. We collected 1,893 larvae.

Larval burbot densities peaked during the first week of Mayat the two stations farthest downstream (Table 42). These twostations also produced the highest densities recorded (1,126 and584 larvae/1,000 mO and were overall the most productive forburbot larvae. Peak densities at the other five stationsoccurred from mid-May to the end of May. Listen et al. (1986)collected burbot larvae from April to July with peak numbers inMay.

Burbot larvae were most consistently present in channelsamples, however, occurrences were similar between the channeland the 2-m depth. At 1 m, occurrences were sporadic, but peakdensities occurred there at three stations. These patterns fromthe three sampling gear used in 1985 are very similar to thefindings of Listen et al. (1986) for 1982 and 1983.

Mean size of burbot larvae changed only slightly over the 6sampling wk, suggesting continual hatching and recruitmentthroughout late April and all of May (Figs. 16, 21). Listen etal. (1986) found little change in larval burbot sizes until lateJune-early July. These results suggest an extended hatchingperiod for burbot in the St. Marys River.

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In summary, burbot larvae occur throughout the St. MarysRiver system* This suggests that burbot spawn throughout thesystem, but transport from localized areas cannot be dismissed.Burbot larvae are passively dispersed by currents shortly afterhatching (Mansfield et al. 1983). The area around Neebish Islandappears to be the most productive for burbot. Overall the riveris moderate in burbot production, as the peak density of almost1,500 larvae/1,000 m^ (Table 43) was 16 times less than the24,000 larvae/1,000 m^ found in Green Bay near Escanaba, Michigan(Mansfield et al. 1983).

Rainbow Smelt

Rainbow smelt iOsmerus mordax) larvae were not collecteduntil 13 May (Table 42). Listen et al. (1983, 1986) also foundinitial occurrences of rainbow smelt in mid-May during 1981 and1982, however in 1983 initial occurrence was on 5 May, suggestingearlier spawning. We found peak densities at most stationsduring the last week of May; however, these may not be seasonalpeaks as Listen et al. (1986) found peak densities during thefirst 2 wk of June. Listen et al. (1986) also recorded a peakdensity of just over 20,000 larvae/1,000 m^ which was more than 7

times greater than the peak density of just over 2,600 larvae/1,000 m^ that we found (Table 43).

We collected rainbow smelt larvae at all seven stations. Ingeneral, densities were greatest at the three stations aroundNeebish Island. Listen et al. (1986) also found greatestdensities near the island. We found occurrences of larvae to besimilar in the channel and at the 2-m depth. Occurrences werevery sporadic at the 1-m depth. Greatest densities usuallyoccurred in the channel, although at two stations maximumdensities occurred at the 2-m depth. Overall these results agreewith those of Listen et al. (1986) regarding the three depthzones.

Mean sizes of rainbow smelt larvae changed only slightlyover the last 3 wk of May (Fig. 16). Lengths ranged from 4.5 to10 mm with a mode at 5.5 mm (Fig. 22). This suggests continualhatching of fish larvae during this period. Listen et al. (1986)found larval rainbow smelt sizes did not increase noticeablyuntil the end of June.

In siOTmary, rainbow smelt production is moderate to high inthe St. Marys River. The waters around Neebish Island appear tobe the most productive.

133

Table 43. Maximum larval fish densities (number/1,000 m') ofvarious fish species sampled in the St. Harys River.

Liston et al. (1983, 1986) PresentListon and McNabb 1986) study

Species 1981 1982 1983 1985

Lake herringCoregonus artedii 340 4,970 2,030 1,450

Lake vhitefishCoregonus cIupea/oriDls 2,380 770 4,300 603

BurbotLota iota 260 1,490 280 1,130

Rainbow smeltOsmerus mordax 15,400 2,590 20,100 2,670

Yellow perchPerca flavescens 14,500 35,400 14,500 10,800

Yellow Perch

Yellow perch {Perca flavescens) larvae, like rainbow smelt,were not collected until mid-May (Table 42). Densities peakedduring the last 2 weeks in May. A peak density of almost 11,000larvae/1,000 m* was recorded (Table 43). This may be a seasonalpeak as Liston et al. (1986) found peak densities at the end ofMay.

We collected yellow perch larvae at five of the sevenstations. Larvae were absent from the Edison Hydropower Canaland at the middle Lake Nicolet stations. Lack of larvae in thecanal may be due to later spawning in the Lake Superior area, asListon et al. (1986) did not collect larvae there until late Juneand early July. Greatest densities occurred at the north LakeMunuscong station, although the overall peak density occurred atthe north Lake Nicolet station. Liston et al. (1986) also foundgreatest densities in Lake Munuscong. Peak densities of 11,000to 35,000 larvae/1,000 m* (Table 43) indicate that the St. MarysRiver system, especially Lake Munuscong, is very productive foryellow perch.

Overall we found greatest densities at 1 m, although thesecond highest density occurred at the 2-m depth. Densities werelow and occurrences sporadic in the channel. These results agreewith those of Liston et al. (1986) regarding depth zones.

134

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Mean sizes of yellow perch changed slightly over the 3

sampling vk. Larvae ranged from 5.5 to 10 mm with one mode at7.5 mm (Fig. 23). This suggests continual hatching over thisperiod. Sizes of larval yellow perch recorded by Liston etal. (1986) did not increase substantially until the beginning ofJune.

In summary^ the St. Marys River, especially Lake Munuscong,is very productive for yellow perch larvae. Spawning must occurin late April to mid-May and larval fish densities peak in lateMay.

Other Species

Three emerald shiner (Wotropis atiierlnoldes) larvae from20.5 to 22.5 mm were collected during the first week of samplingat stations 6 and 7. Since we arbitrarily defined larvae as anyfish ^25.4 mm, we sometimes, as in this case, collect fish whichwere hatched the year before. A similar case occurred forspottail shiner. The fish collected were also yearlings producedin 1984. They ranged in length from 19 to 25 mm and in densityfrom 6 to 1,283/1,000 m^. Spottail shiners were present duringall weeks of sampling except the first, and occurred at four(stations 4-7) of the seven stations sampled.

There were four deepwater sculpin {Myoxocephalus thompsoni)collected during the study; they ranged from 10.0 to 17.1 mm.They were collected at the Edison Hydropower Canal (week 3), inthe channel at stations 3 and 4 (week 4), and in the channel atstation 7 (week 5). We concluded from these data that theselarvae probably originated from Lake Superior, as they wereexclusively collected in the channel where passive transportwould be most probable. Also, deepwater sculpin are not known tospawn in connecting river systems, but are believed always tospawn in deep waters of the Great Lakes (Scott and Grossman 1973;Mansfield et al. 1983). Because of their large size, the larvaewe collected had probably hatched some months before. Auer(1982) reported that newly hatched deepwater sculpin have notbeen described, but that larvae 8.2 mm (not newly hatched) havebeen described. Some of our larvae were twice that size.

Fish Eggs

We collected fish eggs in many of the samples collected inthe latter weeks of the study. None of these were lake herringor lake whitefish eggs, as they all were less than 2 mm, theminimum size of the eggs of these species. Many had distinctivestalks, a unique characteristic of rainbow smelt eggs. Rainbowsmelt eggs are about 1 mm in diameter, which is the size of theeggs we collected.

136

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137

Summer Drift and Biomass of Larval Fish

Larval fish drift was measured during the winter (nonecaught) and summer, when large numbers of mostly rainbow smeltwere collected, A detailed discussion of larval fish drift canbe found in the section entitled: RESULTS - DRIFT: BENTHOS, FISHLARVAE, FISH EGGS, AND MACROPHYTES - Seasonal and TransectComparisons of Drift Densities - Fish Larvae. A brief discussionwill ensue here. Rainbow smelt (82%), burbot (0.6%), lakeherring (0.1%), yellow perch and deepwater sculpin (<0.1%), anddamaged and unidentified larvae (18%) made up the speciescollected during drift sampling. Rainbow smelt were mostly newlyhatched, as larvae 4.5-6 mm made up over 90% of the catch(Fig. 24). Burbot larvae collected remained as small as thosecollected during our spring larval fish survey, about 4 mm(Fig. 24). However, a few larger burbot up to 6 mm werecollected. Damaged larvae were also small, ranging from 3 to 6mm (Fig. 24). We believe most of these larvae were damagedrainbow smelt.

138

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139

DISCUSSION

MACROPHYTES

Macrophyte data show a strong relationship between themaximum depth at which these plants occur and the degree ofpenetration of light through the water, which is in agreementwith the findings of Listen et al. (1986) • The stations in thelower part of the river, such as Lake Munuscong and Raber Bay,have greater water turbidity and, therefore, shallower outermacrophyte boundaries. Not only is the water more turbid atthese places than at upstream sites such as Izaak Walton Bay andLake Nicolet, but also more variable in turbidity, as shown bythe greater width of the confidence intervals for mean lightextinction coefficient (Fig. 7). This point is also apparentfrom casual observation during our studies in the river, for muchmore spatial and temporal variations in water clarity were seenin the downstream portions than upstream. On one occasion, inthe course of several hours while we sampled the plant transectat Raber Bay, a mass of turbid water gradually approached andengulfed the site. The increasing variability in water turbiditywith distance downstream is probably related to greatercontributions by tributary streams and rivers, which add greatvolumes of turbid water, particularly during periods of highrunoff. There are very few major tributary streams emptying intothe St. Marys River between Izaak Walton Bay and Lake Nicolet,but a number of important tributaries downstream.

Although water clarity is a determinant of the maximum depthat which aquatic macrophytes will grow, it is not the soledeterminant of the location of the boundaries of plant beds.Sampling of plants along the 1-km transects showed that substratecharacteristics also affected plant distribution. Plants wereabundant on clay substrates and much less so on sand or gravelbottoms. The large number of grab samples containing no plantsin the east Lake Munuscong transect (Table 4) is due to thevariable nature of the substrate in that area, which containedsome large patches of sand.

Other, less obvious factors may also control the maximumdepth of plant occurrence. For instance, in the course ofsearching for a site for the 1-km transect in Lake Nicolet, weinvestigated a deep "trench" in the area 2-4 km south of WasigBay. Despite the clay substrate and relatively transparentwaters at this location, plants were not found growing at depthsas great as at the place where the transect was eventuallylocated, some 6 km to the south. The macrophyte beds also tendedto be more patchy at the former site than at the latter. Thereason for this difference is not readily apparent; furtherinvestigations may be warranted.

140

These observations suggest that any estimates of possibleeffects on submerged macrophyte beds in the St. Marys Riverresulting from natural or man-made perturbations must take intoaccount not only possible changes in water turbidity, but alsochanges in the river bottom substrate. It would also benecessary to have more detailed data on the location, coverage,and depth of macrophyte beds along the length of the river priorto such perturbations to assess their effects.

BENTHIC DRIFT

Comparison with Previous Benthic Studies on the St. Marys River

Whole River Comparisons

—

Direct comparison of whole river trend results of thepresent drift study with those of a survey of benthic density anddiversity (Listen et al. 1983, 1986) are tenuous because of thetime differential, locational differences within the river, andlevel of taxonomic detail. Nonetheless, some comparisons ofwhole river trends for percent composition of the benthos anddrifting benthos are possible. The number of benthic taxacollected in Liston's benthos survey (162 taxa) was considerablygreater than the 71 drifting benthic taxa. Much of thisdifference is due to generic identification of the Chironomidaein the benthos survey not afforded to the drift study. Of 14drifting benthic taxa not encountered in the benthic surveys,most notable were Mysis relicta, Pontoporela hoyl, Chaoboridae,and Psycodidae. Poe et al. (1980) also collected mysids andchaoborids in drift samples but not in benthic samples in theSt. Marys River. With great likelihood, M. rellcta and P. hoyicollected in the present study originate from Lake Superior andare only temporarily residents in the slower flowing, deeperportions of the river. Seagle et al. (1982) reported apreponderance of chaoborids in drift samples which were notpresent in the same proportion in benthic samples of the IllinoisRiver. In this latter study, chaoborids were thought tooriginate from slow, backwater areas connected to the IllinoisRiver, or from the river itself in areas of slow current.Chaoboridae, being collected only at Frechette Point, possiblyoccurs only in the upper, deeper, slow flowing portions of theriver. However, there remains the strong possibility that themajor reason chaoborids occurred only in Frechette Point driftsamples is that chaoborids migrated up into the water column inslower, upstream water, possibly even backwater portions of theriver, and were drawn or flushed into the fast flowing waterabove Frechette Point by currents and by their own nocturnalactivity. Farther downstream where current velocities areconsiderably slower than at Frechette Point they likely resettle.This being the case, chaoborids probably inhabit the slower.

141

deeper portions of the entire river proper as well as connectingbackwaters.

Psycodidae occurred only in a limited number of winterFrechette Point drift samples. Psycodids are known to inhabitquiet, very organically enriched waters (Hilsenhoff 1975).Absence of psycodids in benthic samples, coupled with a limitedpresence in drift samples, suggested the source of theiroccurrence may be external and not part of a sustained riversupported population. A possible source may be an upstreamsewage plant or factory input. This is suspected becauseconcurrent with occurrence of psycodids, but not limited tosamples in which psycodids occurred, was a clear, entangled,fibrous material. This material was the dominant component ofall winter drift samples at Frechette Point and decreasedsubstantially in a downstream direction.

The general lack of drift density differences acrossstations within each transect differed from benthic distributionpatterns. Listen et al. (1986) found consistently greaterbenthic densities along the western, leeward side of the riverthan along the eastern, windward side. They attributed thedifference to wave action scouring the bottom on the eastern sideof the river. This trend was observed only at Frechette Pointduring the winter. None of the remaining transect station driftdensities during either season exhibited this windward/leewardtrend.

On a river-wide basis. Listen et al. (1986) reported onlypercent occurrence among samples collected. Agreement betweenpercent occurrences for dominant taxa and between order ofdominance within each survey was moderate at best. Percentoccurrence for each taxon in benthic samples was greater than in

drift samples (Table 44). While Chironomidae occurred ingreatest frequency in both surveys, remaining orders of dominancediffered considerably. Notable high percent occurrences inbenthos samples but not in drift samples were recorded forCeratopogonidae, Hexagenia, Ephemera, Caenis, Polycentropus

,

Polychaeta {Manayunkia speclosa) , and Mystacides. Among presentdrift samples, notable high percent occurrences not observedamong Listen's benthic samples included those for Hydra,Hydracarina, and Mysis rellcta. Although percent occurrencediffered between the two surveys, all major benthic survey taxaoccurred in drift samples.

Comparisons at Frechette Point

—

Based on Ponar grab samples from Frechette Point and SixMile Point by Poe et al. (1980), estimated benthic density was14,126/m^; of 75 benthic taxa collected, 25% was Chironomidae,

142

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143

23% Oligochaeta, 20% Gastropoda, 11% Pelecypoda, 7% Polychaeta,3% Amphipoda, 1% each for Ephemeroptera and Trichoptera, and 10%miscellaneous. While mollusks were seldom collected in driftsamples, percent composition of the 64 drifting benthic taxa wasfairly similar to that of the bottom population. Chironomidaemade up 31%, Naididae 9%, Trichoptera 3%, and Ephemeroptera 2% ofthe drifting benthos (Table 9). However, there remained aconsiderable difference between the benthos of the two studies,because in our drift study Hydra made up 49% and Chaoboridae 3%of the drift at Frechette Point.

Comparisons at Lake Nicolet

—

Station II of Listen et al. (1986) corresponded to LakeNicolet stations 1-5 in the present drift study. In Listen'sstudy, the Chironomidae dominated (33-85%) benthic densitieswhich ranged from 4,800/m* to 34,700/m*, excluding the navigationchannel (294-1, 750/m^ ) . Oligochaeta made up from <1% to 64% ofbenthic density. Hyalella azteca made up 10% of benthicabundance. Although Trichoptera and Ephemeroptera wererepresented by several taxa, neither was numerically important,except Caenis which at maximum made up 18% of benthic density.Polychaeta was occasionally numerically important, making up to17% of benthic abundance.

By comparison, in our drift study, Chironomidae made up 42%of average summer benthic drift at Lake Nicolet. The second- andthird-most abundant drifting benthos, Ephemeroptera (37%) andTrichoptera (9%) (Table 9), as well as presence of M. xelictadiffered considerably from benthic composition in Listen's study.Nonetheless, dominant, frequently occurring genera common to thebenthos of each study were often the most frequently collectedtaxa in our drift study.

Comparisons at Point aux Frenes

—

Station VII of Listen et al. (1986) corresponded with thePoint aux Frenes transect in the present drift study. Benthicdensities ranged from 1,500/m* to 29,900/m^ outside thenavigation channel and 700/m* to 2,300/m^ within the channel.Chironomidae was the dominant benthic taxon (22-85% of benthicdensity), followed by Oligochaeta (6-80%). Listen et al. (1986)noted that benthic diversity was lowest at Point aux Frenes (55taxa). This corresponded with results from our drift study inwhich a study minimum of only 25 benthic taxa was collected whichwere made up mostly of Hydracarina, Chironomidae, Ephemeroptera,and Cerixidae in order of decreasing numerical importance. Thisorder differed considerably from the order found by Listen etal. (1986) in the benthos.

144

Comparison with Previous Drift Studies on the St, Marys River

Direct comparison of station and transect benthic driftdensities between the present study and those of Poe etal. (1980) and Poe and Edsall (1982) is limited to FrechettePoint. The Lake Nicolet and West Neebish transects of Poe andEdsall (1982) are not comparable to the present Lake Nicolettransect, because the former was located considerably northwardnear Six Mile Point at the upper end of Lake Nicolet and thelatter was located somewhat southward in the West NeebishChannel. The present Lake Nicolet transect was located in themain body of water in lower Lake Nicolet between Poe's transects.For comparisons between the two studies, Poe's winter benthicdrift densities were averaged for each station (1-3) at FrechettePoint by pooling across the 31 January 1982 through 9 March 1982samples periods during which the river was under ice cover.Poe's drift density during ice-free conditions was represented bythe 29 April 1982 to 1 May 1982 sample period for stations 1-3and for the navigation channel (station 4; only daytime samplescollected). Average transect drift density was estimated bypooling across all station and sample dates for ice-covered andice-free conditions, respectively, in the study of Poe and Edsall(1982). In our study, comparison of benthic drift density withthat of Poe et al. (1980) and Poe and Edsall (1982) was made bypooling in the same manner as above for stations 1-3 and for thenavigation channel (station 4; Day 1 and Day 2 averaged) atFrechette Point. Comparisons between the two studies was basedon drift collection by Poe using 571-Mm-mesh nets and in ourstudy, 355-Mm-mesh nets.

Poe's 1982 winter average benthic drift density for stations1-3 ranged from 43/1,000 m^ to 63/1,000 m\ with a transect meanof 47/1,000 m^. In 1985, similarly pooled estimates for stations1-3 ranged from 680/1,000 m^ to 1,216/1,000 m\ with a transectmean of 898/1,000 m^. In summer, Poe's station drift densitiesranged from 42/1,000 m^ to 603/1,000 m^ and averaged 64/1,000 m^in 1982. During our study in summer 1985, similar estimatesranged from 1,456/1,000 m^ to 1,884/1,000 m' and averaged 1,659/1,000 m^. In Poe's 1982 navigation channel samples, daytimebenthic drift density averaged 183/1,000 m^ and decreased fromsurface (236/1,000 mO to mid-depth (177/1,000 mM to bottom(135/1,000 mO . In our 1985 navigation channel samples, daytimebenthic drift density averaged 1844/1,000 m^ and increased fromsurface (721/1,000 mM to mid-depth (1,630/1,000 mM to bottom(3,183/1,000 mM. When the 1985 to 1982 ratios of pooledtransect average (stations 1-3) densities were calculated, the1985 winter, under-ice benthic drift densities we obtained were1,911% greater than in 1982; under ice-free conditions in 1985,

145

our estimates were 2,592% greater than 1982 estimates. The sameratio for ice-free navigation channel estimates indicated 1985surface (306%), mid-depth (921%), bottom (2,358%), when averagedover all three water depth strata (1,008%), were considerablygreater than the 1982 estimates. While some of these differencesmay be attributable to annual variation and slight variations inseasonal sampling times, the primary factor influencing thesedifferences is the mesh size employed.

The reversal of depth of maximiun drift density in the watercolumn between the present study and that of Poe and Edsall(1982) is unexplainable. However, similar studies on theMississippi River have been inconclusive as well. Matter andHopwood (1980) found a drift density difference between depthstrata which was related to specific taxa. In their study,ephemeropterans were most numerous in the upper nets at nightwhile trichopterans were most abundant in lower nets regardlessof the diel period. Seagle et al. (1982) observed a similartrend for ephemeropterans and Chaoborus which were most numerousnear the surface and for hydropsychid trichopterans which weremost numerous near the bottom in both the Mississippi River andIllinois River. In another study on the Mississippi River,Eckblad et al. (1983) noted depth of maximum drift density variedwith season. During June, drift was greatest near the bottom andwas comprised largely by Hydra . However, during July, drift,largely Corixidae, was maximal near the surface. At Point auxFrenes in the present study, corixids increased in drift densityin the summer but were most numerous near bottom (Table 7). Infact, nearly all taxa including Ephemeroptera and Chaoboridaeoccurred in greatest densities in bottom nets. The taxa leastlimited by water column depth was M. relicta which being a goodswimmer, active migrater, and emigrating form from Lake Superior,is not limited by the shallow depths of the river. While resultsfrom the present study strongly indicated most drifting benthosoccurred near bottom, seasonal factors may alter the observedpattern.

Some Factors Influencing Drift

Benthic Density and Production

—

Several studies have reported no direct relationship betweendrift and benthic population composition, standing crop, orproduction (Elliott 1967; Morris et al. 1968; Bishop and Hynes1969; Waters 1972). Nonetheless, some studies have shown thatdrift was density dependent (Muller 1954a; Waters 1961, 1962a;Pearson and Franklin 1968) . Additionally, Waters (1972)speculated that drift may reflect excess production. Ghetti andRavanetti (1984) reported a positive correlation between driftdensity and productivity, noting drift increased around emergence

146

periods. Although Pearson (1970) was able to demonstrate a goodcorrelation between drift and production for a caddisfly, nonecould be shown for a co-occurring mayfly.

The discussion offered by Bishop and Hynes (1969) regardingdrift, carrying capacity, and production perhaps best relates theinteraction of these concepts. Density-dependent drift is leastevident in streams which are subject to spates and severedisruption. In examples of density-dependent drift, amphipodsand mult ivoltine ephemeropterans dominated the benthos. As aconsequence, drift was influenced by the density of the theseinvertebrates and their excess productivity. However, in streamsexperiencing spates or disruptions, carrying capacity is notattained. Subsequent drift results from behavioral activity andnot from excess production. Regardless of these arguments.Bishop and Hynes (1969) concluded that the relationship betweendrift and production is "obscure."

In the St. Marys River, little is known empirically aboutits benthic carrying capacity. The only estimates of biomass andproduction are for 1-m and 3-m deep stations in lower LakeNicolet and Lake George during 1981 (Listen et al. 1983). At the1-m station, average biomass was 6,387 mg/m*. Total annualproduction for common benthic taxa was 11,319 mg/mVyr.Production estimates in excess of 2,000 mg/mVyr included thosefor Chironomidae and Isopoda. At the 3-m station, average annualbiomass in 1981 was 4,550 mg/m*. Total annual production was10,704 mg/mVyr, with production estimates in excess of 2,000 mg/mVyr for Chironoidae and Amphipoda.

Biomass and production estimates from the St. Marys Riverprovide some useful information but given the lack of clarityregarding the effect of biomass and production on drift, are ofminimal use in explaining our data. Nonetheless, several of themore numerous and productive invertebrates did appear frequentlyin drift samples suggesting these factors should not be ignored.In addition to biomass and production, several other factors mayexert a controlling influence on drift. Based on ourunderstanding of these factors, they are interdependent to somedegree and require simultaneous consideration.

Benthic forms which are most inclined to drift include inorder of quantitative importance: Ephemeroptera, Simuliidae,Trichoptera, and Plecoptera (Waters 1972). The likelihood ofeach drifting is dependent upon life stage, current velocity,total discharge, temperature, light intensity (Elliott 1967;Bishop and Hynes 1969; Waters 1972), and in the present study,vessel traffic, and weather conditions. The latter two factorswill be considered in a subsequent section.

147

Life Stage and Current Velocity

—

Aquatic insects in later, larger life stages are the mostlikely individuals to drift (Waters 1972). One mechanisminfluencing drift of these individuals is a need for increasedforaging areas, thereby subjecting them to increased exposure tocurrents, jostling of one another causing one or more to becomedislodged, or movement into the water column for emergence(Mundie 1956; Bishop and Hynes 1969; Waters 1972; Bailey 1981;Morgan and Waddel 1981). Any or all of these life stage factorswill be species specific with subsequent drift composition notadequately reflecting either the pre- or post-benthic populationstructure. Moderate agreement between drift and benthiccompositions must certainly reflect these life stage factors andaccount for a portion of the difference observed.

A most notable disagreement between our drift study andthose of Poe et al. (1980) and Listen et al. (1986) was veryfrequent occurrence of Hydra and Mysis relicta in drift samplesbut not in benthos samples. However, Hydra did make up a largepercentage of total benthic drift density in the high impact areaat Frechette Point where Poe et al. (1980) sampled. Comparativeoccurrences of these two invertebrates suggest 1) even thoughmysids are not easily caught by Ponars, they are very likely notlong-term residents of the river, but rather originate in LakeSuperior and are either transported to Lake Huron, consumed byfish, or die of natural causes before passing out of the river,and 2) that Hydra occurred in substantive quantities upstream ofFrechette Point, possibly on rocks and boulders, plants, andpilings and other man-made structures that were not or could notbe sampled by usual benthic sampling devices. Greater driftdensities of Hydra at Frechette Point relative to Lake Nicoletand Point aux Frenes suggested favorable, upstream currentconditions enabling them to establish a large population whichquite possibly preys heavily on zooplankton originating in LakeSuperior.

Occurrence of large numbers of Hydra in the drift may be dueto a force, such as current, causing dislodgement. Winter driftdensities were lower than those in summer, and winter Hydra werelarger sized (3-5 mm) than those in summer (<2 mm, many <1 mm).Large summer densities of small Hydra strongly indicatedreproductive activity and dispersal of young to new areas ofattachment. We conclude that occurrence of Hydra in the drift inthe present survey was induced primarily by current in winter butlargely by reproductive activity in summer over and above someresidual level attributable to current.

With respect to remaining drifting benthic taxa, currentvelocity appeared to influence drift density. In contrast toHydra, remaining taxa are mobile and subject to contact with

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currents and subsequent entrance into the drift through their ownmovement. Consistently greater drift densities at FrechettePoint relative to Lake Nicolet and Point aux Frenes were likelyrelated to higher current velocities.

Total Discharge

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Little can be stated regarding the effect of total dischargeon drift density in the St. Marys River as discharge iscontrolled at the locks and varied little over the course of thetwo time periods sampled. In smaller streams where most driftstudies have been conducted, total discharge is often anoverriding factor influencing increased drift due to flooding.While flooding was not a factor in our study, drift rates andintensities are related to discharge. Calculated driftintensities in our study are all well below maximal valuesreported by Waters (1972) who cited maximal drift intensitiesranging from 2 x 10* to 3 x 10* in studies where all species wereconsidered. Moreover, by substituting in the linear regressionequation relating total discharge to invertebrate drift ofElliott (1970), we calculated a drift intensity of 27,681. If arange of ±50% about the 27,681-value is calculated (13,841-41,522), 37% of our values fall within this range, 21% werehigher, and 42% were lower. To our knowledge, drift intensitieshave not been calculated for other large rivers, and givenagreement between our values and those of Elliott (1970), eventhough his were based on streams, we feel assumptions inherent inour calculations and results obtained were reasonable. However,we acknowledge the differences between expected and calculateddischarges and offer that more frequent measures would enhancefuture estimates of drift rates and drift intensities. At best,our estimates may be thought of as a first approximation.

When examining benthic drift density, drift rate, and driftintensity seasonal ratio differences among transects, the degreeof seasonal change was greatest at Lake Nicolet when comparedwith Frechette Point (Tables 10-11). In addition, the degree ofdifference between these two transects decreased from winter tosummer (Table 45). Common to Frechette Point and Lake Nicolettransects is a similar seasonal reproductive and behavioralactivity of the benthic organisms. Differing between thetransects is the physical structure of the river basin at eachtransect location which in turn affects current velocity. Wespeculate that while reproductive and behavioral activity of thebenthos at Frechette Point increased during summer when comparedwith winter, the high current velocities observed during bothseasons dampen seasonal differences to a greater degree atFrechette Point than at Lake Nicolet. At Lake Nicolet whereseasonal ratio differences were considerably greater than atFrechette Point, we suspect that reproductive and behavioral

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Table 45. The ratio between pairs of transects of benthicand larval fish densities, rates, and intensities in the St.Marys River, 1985. FP = Frechette Point, LN - LakeNicolet, PAF = Point aux Frenes. See METHODS and Waters(1972) for definition of drift intensity.

Winter Summer

Transect Drift Drift Driftratio density rate intensity

Drift Drift Driftdensity rate intensity

BenthosFP/LNFP/PAFLN/PAF

9.842.302.33

17.3 26.5 2.713.231.19

5.226.331.21

5.5011.62.12

FishLN/FPLN/PAFPAF/FP

11.372.70.16

94.138.22.46

89.366.81.34

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activity may be a more important factor influencing seasonalratio differences than the effect of current velocities whichwere slow relative to those at Frechette Point. To express theserelationships numerically, we found that at Frechette Point, theconstant, eroding effect of higher current velocities on thebenthos allowed a three-fold increase due to seasonalreproductive and behavioral activity. At Lake Nicolet where theeroding effect of currents is less than at Frechette Point, therewas a ten-fold increase in the drift ratios. We speculate mostof the increase is due to activity of the organisms. However,some seasonal difference may be related to the type of organismsin the drift which in turn are influenced by the physical natureof the basins they encounter while drifting. A good comparisonof this effect is between Hydra and Mxjsis relicta. While bothanimals drift through Frechette Point, the former was caught inour drift nets with much greater frequency at Frechette Pointthan the latter. However, at Lake Nicolet, mysids were retainedin greater relative frequency than were Hydra. This differenceis related to animal mobility and the influence of these twosegments of the river on the animals. Neither animal is expectedto be able to resettle in the high current velocity at FrechettePoint. However, upon reaching the slower currents in LakeNicolet, Hydra, being sessile animals, resettle slowly, andmysids, being highly mobile animals, resettle quickly.Regardless of time of day, but most particularly at night,mysids, which are active migraters, reenter the drift but Hydrado not unless disturbed or unless they are in a reproductivephase of their life cycle. Consequently, while both animals passthrough Frechette Point and presumably accumulate in LakeNicolet, mysids were more evident in drift nets than were Hydra.The effect of mysids accumulating in Lake Nicolet would beexpected to be more evident in drift samples due to their activemigratory nature than for Hydra due to slower currents. The sameeffect may occur at Point aux Frenes but evidently to a lesserdegree based on our data. Accumulation of mysids in LakeMunuscong may be less intense owing to increased predationeffects due to increased time spent in the river, siltation,temperature changes, or natural mortality. Additionally, thelarge, slow-moving water mass in Lake Nicolet apparently providesan excellent nursery for fish which is not present at FrechettePoint. We suspect the physical difference among transectscontributes to benthic and larval fish drift differences amongtransects as they interact with the behavioral characteristics ofa given taxon.

Water Temperature

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The effect of water temperature on drift within respectiveseasons was minimal, because water temperature differences amongstations within transects and among transects were negligible.

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However, water temperature acting in concert with other factorsconsidered in this section likely had a positive effect onseasonal drift differences. Water temperature directly affectsdevelopment rates and physical conditions in the river, such asice conditions. Waters (1962b) and Pearson and Franklin (1968)observed increased drift with increased water temperature.Muller (1954b) noted a decrease in drift at lower temperatures,and Waters (1962a, 1966, 1981) found low drift in winter. Lowerbenthic densities in winter, decreased water temperature, andslower developmental rates in winter with their converse insummer, coupled with input of young, newly recruited individualsinto the system, likely accounted for a substantial portion ofseasonal differences observed in the present study.

Results of studies evaluating the effect of anchor-ice ondrift vary. Anchor ice increased the amount of drift in thestudy of Bishop and Hynes (1969). Logan (1963) concluded thatsurface ice cover had no effect on benthic density except nearthe edges of the stream. With spring break-up, floating surfaceice did not increase drift (Logan 1963). This conclusion agreedwith that drawn by Poe and Edsall (1982) that floe ice did notincrease drift density. Nevertheless, we postulate that thereremains a possibility that ship passage under conditions of icebreak-up may increase drift when there is considerable anchorice. This effect will most probably be more intense in slowerflowing portions of the river where thicker ice develops andpossibly, movement of nearshore ice blocks to offshore positionstakes longer than in narrow portions of the river with highercurrent velocities. The surging of water and ice chunks inshallow water due to ship passage may be analogous to surging ofice blocks along the shoreline of Lake Michigan during ice break-up (Seibel et al. 1975). If so, there exists a strongpossibility that nearshore benthic drift, where ice blocks mightbe expected to interact most with the bottom, will be increasedabove that of natural ice break-up. ' However, since wind exerteda dominant impact on drift during summer, the effect of wind onmovement of ice blocks with subsequent nearshore scour may be afar more important force influencing additional drift than shippassage. Additionally, since shipping normally begins with icebreak-up, neither the effect of shipping nor wind seem to havebeen detrimental to benthic populations. But in terms of factorspotentially affecting drift, the scour of disturbed ice blocks onthe bottom, regardless of the nature of the disturbance, is afactor which may increase drift of benthos.

Light Intensity

—

The fact that benthic drift density is strongly dependent onlight intensity has been reported by many investigators (Elliott1967; Bishop and Hynes 1969; Waters 1972). Present results

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agreed well with the diel nature of drift activity, i.e., minimaldrift during the day and maximal drift nocturnally. Elliott(1967) reported greatest drift just after sunset; the time whennight samples were collected in the present study. Whilegreatest diel drift density is species specific (Elliott 1970),barring catastrophic drift due to flooding or pollution, mostdrift results from behavioral activities (Bishop and Hynes 1969;Waters 1972). This brings the discussion full-circle to lifestage, developmental factors. Increased drift at night is highlyrelated to increased activity of invertebrates and to a lesserextent upon the carrying capacity of a particular stretch ofriver (Waters 1972).

The Fate of Drifting Benthos and theEffect of Weather and Vessel Traffic on Benthos

Fate of Drifting Benthos

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There remains the question of the fate of drifting benthos.As with mysids, drifting benthos inherent to the river may driftout of the river, be consumed by a predator, die of naturalcauses, or resettle downstream. Natural factors influencingdownstream drift distance include current velocity, length ofnocturnal period, activity of the individual [size, shape,density, center of gravity, swimming ability (McLay 1970)], watertemperature, substrate type, unoccupied downstream areas, andavailability of slow water areas (Bishop and Hynes 1969; McLay1970; Elliott 1971; Waters 1972).

Poe et al. (1980) speculated that macroinvertebrates settleout of the water column quickly relative to the more buoyant andpassive macrophytic material. Bishop and Hynes (1969) noted thatdrift distance of invertebrates is not very long due tothigmotactic and rheotactic responses. Waters (1972) observedthat aquatic insects characteristic of swift streams reattachrapidly when released into the water column and concluded itseemed likely that they do not swim freely or drift for longdistances.

McLay (1970) observed a range of downstream drift distancesfrom 0.5 m to 19.3 m (X = 11 m) . Elliott (1970) found the rateof return of drifting benthos to substrates was a decreasingexponential function which decreased with increased distance fromthe point of origin of an individual into the drift. Averagedrift distance was 20 m. Additionally, Elliott (1970) noted thatChironomidae, the most numerous drifting benthic form in thepresent study, resettled no more quickly than dead invertebrates.Nonetheless, 99% of them had resettled within 15 m in a currentof 10 cm/s and within 91 m in a current of 60 cm/s (Elliott1970). Although application of Elliott's findings to the

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St. Marys River which is a much larger and deeper river than thestream studied by Elliott (16-40 cm deep, modal depthapproximately 20 cm) may be erroneous, no other empirical methodsfor estimating drift distance exist. As a consequence, we havecalculated a minimal resettling distance for Chironomidae in theSt. Marys River using Elliott's equation values and assumptions.Due to the differences in river depths, we have also calculatedan "adjusted drift distance."

At Frechette Point, average mid-depth navigation channelcurrent velocity was 51 cm/s, which when substituted intoElliott's equation indicates 99% resettlement of driftingchironomids within 78 m in a stream of modal depth 20 cm. Basedon these measures, the resettling rate for the average benthoslocated at mid-depth (10 cm) in Elliott's stream would be 0.13cm/m (= 10 cm/78 m) . This assumes a linear rate of descentthrough a column of water travelling at a constant currentvelocity and direction. This resettling rate is used tocalculate the "adjusted drift distance" for the averagechironomid located at mid-depth in the navigation channel atFrechette Point (4.5 m) . For chironomids located at this depthin the navigation channel at Frechette Point, 99% resettlementwould be expected to occur within 3,462 m (= 450 cm/0.13 cm/m).The same organism located at mid-depth at the shallowest depthsampled (station 1 = 1 m) would have a 99% resettlement within192 m based on an average current velocity of 26 cm/s and aresettlement rate of 0.26 cm/m. In a similar fashion, at LakeNicolet the "adjusted drift distance" at mid-depth in the upboundnavigation channel is 1,440 m, while that of the downboundchannel is 1,710 m. At Point aux Frenes, the "adjusted driftdistance" at mid-depth in the navigation channel is 1,350 m. Atthe shallowest stations sampled nearest these three channellocations, the "adjusted drift distances" are 35 m, 75 m, and 60m, respectively. From these calculations, we project 99%resettlement of the main component of mid-depth, driftingChironomidae within 60 m in the nearshore to approximately 3,500m in the navigation channel. Occurrence of faster or slowercurrent velocities downstream will enhance or impede driftdistance. However, based on these estimates, drift originatingat mid-depth or bottom at any transect would likely resettledownstream prior to attaining the "adjusted drift distance" dueto occurrence of slower water below each transect.

The exception to this may be those individuals occurring atthe surface of the navigation channel. Expected drift distancesfor these individuals are 6,000 m at Frechette Point, 3,103 m inthe downbound channel at Lake Nicolet, 2,727 m in the upboundchannel at Lake Nicolet, and 2,647 m at Point aux Frenes. Inthese cases, as with those above, drift distances are anticipatedto be shorter than calculated due to current variations.

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The more passive portions of the drift, such as macrophyticmaterial, are expected to resettle in slower, downstream portionsof the river. Although Poe et al. (1980) and Poe and Edsall(1982) speculated this material may be removed from the systemupon being disturbed, we suspect it may be transported downstreamto resettle in slow water areas (particularly Lake Nicolet andLake Munuscong), and thereby, reassume its role in the river'sproduction ecology cycle. While some may be lost from the lowerreaches of the river, we suspect this is insignificant relativeto the whole river and would not anticipate this altering benthicproduction negatively.

Once suspended in the water column organisms become easierprey for fish. The degree to which drifting benthos are utilizedas a food by fish varies with the size and species of fish(Waters 1972). Selective feeding on drifting benthos occurs,particularly among young salmonids. However, this changesconsiderably as these fish get larger (Waters 1972). Waters(1972) concluded in general that fish are "opportunists" whichoften, but not always, consume drifting benthos.

Our speculation regarding the fate of drifting benthos inthe St. Marys River is that a great proportion of themacroinvertebrates resettle within a short distance and that asmall fraction is consumed or destroyed by drifting activity.Mysids may drift through the entire river system, but few otherbenthos would be expected to do the same. The majority ofdrifting Hydra probably perish or fare poorly when drifting fromthe northern portions of the river into Lake Nicolet unless byhappenstance they resettle in suitable areas. Most otherdrifting benthos are capable of movement and reenter the drift tofind suitable habitat.

Effects of Weather and Vessel Traffic on Drift

—

Studies on the effect of ship passage on benthic drift arefew and results are contradictory and seem to be dependent uponcircumstances particular to each study. Herricks (1981) observedincreased benthic drift following barge passage during low flowbut not during high flow periods in the Mississippi River.Seagle and Zumwalt (1981) observed no increases in drift densitydue to "tow" passage in the Mississippi River, but attributedthis to the confounding effect of high flow. Eckblad (1981) alsodemonstrated no significant increase in drift due to "tow"passage in the Mississippi River. However, northbound (upbound)"tows" in the Mississippi River increased drift at a 3-m samplingdepth (Eckblad 1981). In addition, Eckblad (1981) noted that 9%of the benthos attached to rock surfaces were dislodged withincreased current velocity. However, Seagle and Zumwalt (1981)observed no such trend.

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The inability of the two ship passage studies in the presentstudy to demonstrate detectable increases in the density ofdrifting benthos or fish larvae was probably related to theoverriding effect of windy weather conditions. Based on visualobservations, greatest impact on drifting benthic densities areexpected to occur during upbound passage of vessels. We suspectthis is due to the vessel plowing through the river anddeflecting large volumes of oppositely flowing water slightlyupstream and laterally, whereby it rushes downstream at a greatervelocity than the ambient current velocity of the river. Thisprocess is a very dynamic one that eventually evens out thedisturbance to reestablish the "normal" flow of the river at someequilibrium level. Downstream vessel passage apparently does notpresent such an extreme displacement of water at least in partbecause the ship and the water it is displacing are traversingthe same direction. In fact, we observed a decrease in currentvelocity with downstream passage but an increase during upboundpassage. This same trend was also observed by Eckblad (1981) inthe upper Mississippi River. In either case, but especially forupbound ships, passage is expected to have a greater effect onincreasing benthic drift depending on the speed and the size ofthe ship (Bhowmik et al. 1981). Under the windy conditions whichwe measured the effect of ship passage, no increase in driftdensity was observed. In a trial run of this experiment in calmweather during May 1985 at Frechette Point, we observedconsiderable disturbance of the bottom with increased turbidityduring passage of an upbound vessel. Based on theseobservations, we conclude that there remains yet an unproven buthighly probable increase in drift components with passage of anupbound vessel during calm weather conditions. However, duringwindy weather, the added effect of ship passage was undetectableand did not substantially alter an already disturbed system.

Drift induced by windy weather conditions and by shippassage during ice-free, calm weather conditions representsexamples of quasi-catastrophic drift. It is difficult toascertain the ultimate effect on the benthos of either and thateffect very likely depends upon factors unique to each portion ofthe affected river bottom. However, we feel that windy-weatherconditions have a greater overall, river-wide effect on driftthan individual, though frequent, ice-free ship passages. In themost clear case of an effect due to weather, benthic driftdensity at Frechette Point, which should have been greatestnocturnally, was highest during the day. We feel this differencewas due to windy conditions during the day and calm conditionsduring the night. This is a remarkable difference, because itsuggests that wind intensity and direction is capable of exertinga greater controlling influence on drift than is light intensity.In addition to drift density differences, current velocities weregreater during windy conditions than during calm conditions.This concurred with the conclusions drawn by the Great Lakes

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Hydraulics and Hydrology Branch of the U.S. Army Corps ofEngineers which concluded that wind speed and directioninfluenced surface and probably subsurface flow of the St. MarysRiver (U.S. Army Corps of Engineers 1984). Similar results wereobserved during the larval fish surveys.

Wind-induced drift is expected to be river-wide, aside fromsheltered areas. By contrast, ship-induced drift is pulsed byfrequency of ship passage. In either case, induced driftingorganisms will resettle, but we suspect they will do so morerapidly when induced by ship passage than by wind. More rapidresettlement of ship-induced drift is expected because thedisturbance generated by a ship passage is short-lived and thatcaused by weather events may last for days or possibly weeks. Inour 2-wk sample period in early June it was windy during daylighthours for all but 2 days.

If during calm conditions there are pulses of ship-induceddrift, there remains the possibility that these "clouds" of driftmay provide an unexpected food source for fish predators. Inthis fashion, ship-induced drift may add to the mortality ofbenthos populations in the river. However, we hypothesize thatthe effect of weather during ice-free conditions far exceedsnormal ship passage effects on drift. Given the observed abilityof river benthic and larval fish populations to maintain adequateproductivity, we expect no adverse effect on these populationsdue to normal ship passage operations during ice-free conditions.Consequently, any loss of benthos attributable to normal shippingactivity during the base shipping season of 1 April to 15December is in all likelihood easily sustainable by the benthicpopulation indigenous to the river.

Addressing the potential effect of ship passage on benthicpopulations in the river during winter, ice-cover conditions ismuch more difficult to answer. The primary reason is the lack ofdata. Poe et al. (1980) and Poe and Edsall (1982) demonstratedwith their ship passage, under-ice drift data (the only dataavailable) that there was a considerable increase in benthicdrift density. This conclusion can not be overlooked and shouldbe impetus for further investigation and analysis. However, re-analysis of the Poe et al. (1980) benthic drift data suggestsvessel passage under conditions of ice-cover may result in apulsed increase in benthic drift density (15-16 February 1979),while at other times (13-14, 17-18 March 1979) displaying littleapparent effect. These variations leave open the solution to theeffect of under-ice shipping effects and lead us to furtherspeculation.

Based upon our knowledge of drift, we would not expect ship-induced drifting benthos to drift greater distances beforeresettling than during summer, thereby not causing any increased

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detrimental effect on existing populations. Most fish predatorsduring winter are expected to be larger than during suunmer,probably located in deeper water, more lethargic, seldom feeding,and not relying heavily on drifting benthos as a food source.This being the case, predation may not be severe on a suddenpulse of drift moving downstream. However, we are concerned aswere Poe et al. (1980) and Poe and Edsall (1982) about anyadditional loss to benthic populations at a time when numbers andproductivity are minimal and about alteration or loss ofpreferred habitat due to physical disturbance of the riverbottom. Benthic invertebrates which may be negatively affectedinclude those capturing food by constructing nets or having afiltering apparatus, e.g., Hydropsychidae, some Chironomidae,Simuliidae, and Pelecypoda. Ephemeroptera would be negativelyaffected by increased siltation or sediment disturbances throughfouling of gills and possibly loss of preferred habitat. Taxawhich graze on algal growth on rocks or collect detritalmaterial, e.g., many Trichoptera, Gastropoda, and manyChironomidae, may find those resources altered by physicaldisturbance of the river bottom. Alteration of habitat andparticularly food resources during conditions of ice cover areimportant, because in winter the river is a much more static,fixed system than after ice break-up. When ice breaks up,benthic food resources increase, e.g., there is increasedproduction of aquatic macrophytes and algae, and increased inputof allochthonous material due to spring melt and rains, makingany alterations less restrictive.

Whether the benthos can survive continued and frequentunder-ice disturbances has yet to be demonstrated. Given thatthere was shipping under conditions of ice cover from 1977through 1979 (personal communication; Don Williams, U.S. ArmyCorps of Engineers, Detroit District) and the benthos of theriver apparently has been capable of maintaining reasonabledensities and productivity rates in the areas and seasonsstudied, this suggests the effect of shipping has been minimal.However, the variable results observed by Poe et al. (1980), andthe concerns expressed herein remain to be addressed empirically.The data of Poe et al. (1980) do not seem adequate to answerwhether the apparent ship-induced pulse of drift evident in theirdata would continue to be evident with additional ship passagesor would level off at some equilibrium level. Regardless ofwhich occurs, the impact of either on the benthos in high impactareas and possibly in low impact areas remains to be determined.Our speculation that previous shipping did not appear todetrimentally affect the benthos during ensuing years is notsufficient to forecast the future. The frequency of ship passagemay vary from year to year and the fate and importance of induceddrift losses remain unknown. This is particularly important tothe cohort surviving from the previous reproductive period.Typically, in mid-winter and early spring this cohort will be at

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a numerical minimum. Finally, reproductive success of thebenthos varies from year to year based on many factors butpossibly foremost may be weather for taxa which have flyingadults, e.g., Ephemeroptera, Trichoptera, and Diptera. Poorweather conditions at critical times, coupled with any potentialadditional loss attributable to ship passage, may be important tosurvivability of some taxa.

ZOOPLANKTON

Zooplankton Abundance and Community Structure

The St. Marys River was characterized by low zooplanktondensities, both in February (winter) and early June (spring),averaging 313. 2/m^ and 93.7/m^ respectively for No. 2-mesh netcollections. For No. 10-mesh net collections, these averageswere 636. 2/m^ and 421.3/m% respectively. In contrast, Selgeby(1975) reported that, in the outflow from Lake Superior, Februaryzooplankton densities averaged approximately 2,000/m' while earlyJune densities averaged approximately l,000/m%- his collectionswere made using a No. 10-mesh net.

Low zooplankton densities in our study appear to be due toseveral factors. First, most collections were made with arelatively coarse (355-Mm) No. 2-mesh net which undersampled allbut the largest zooplankton. Evans and Sell (1985) determinedthat a No. 2-mesh net probably provides representative abundanceestimates only for the largest zooplankton, i.e., Limnocalanusmacrurus copepodites and possibly Daphnia. Immature Cyclops andDiaptomus copepodite abundances are severely underestimated by aNo. 2-mesh net by at least one order of magnitude. Sincezooplankton community structure in the St. Marys River variedseasonally and spatially, such underestimates were not constantover the study. For example, the St. Marys River zooplanktoncommunity was dominated by larger zooplankton in winter thanspring (this study; Selgeby 1975). Furthermore, during June,zooplankton community structure varied along the length of theriver with smaller-bodied zooplankton dominating at Point auxFrenes while larger-bodied zooplankton were dominants at upperriver transects at Frechette Point and Lake Nicolet.

A second factor which may have affected zooplanktonabundance was the sampling method. Plankton nets were suspendedin the river and zooplankton were collected as water flowedthrough the net. Current velocities were low, averaging 18.1 cm/s in February and 20.8 cm/s in June. In contrast, towing speedscommonly used in zooplankton studies average 50 to 100 cm/s.These high towing speeds are necessary to minimize net avoidance,especially by the larger zooplankton. In this study, it ishighly probable that a substantial fraction of the zooplankton

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community vas able to avoid capture by the suspended nets,especially in areas where flow velocities were low. This mayaccount for the fact that zooplankton generally tended to occurin highest densities in the channels, especially in surfacewaters, where current velocities were highest. This may alsoaccount for the greater densities of zooplankton in day versusnight collections made in June at Frechette Point and LakeNicolet.

There are two possible implications to sampling biases inzooplankton population estimates. First, seston (zooplankton anddetritus) concentrations may have been underestimated; similarly,the relative contribution of detritus to total seston may havebeen overestimated. These errors probably varied with temporaland spatial changes in zooplankton community structure andcurrent velocity. Second, if net avoidance was a significantfactor affecting zooplankton drift abundances, it may also havebeen a significant factor affecting drift abundance estimates ofother organisms. For example, fish larvae abundances probablywere underestimated. The mean size of larvae collected in driftsamples averaged only 11.9 ng. Benthic abundances may also havebeen underestimated, especially the highly motile Mysis relicta.

One intriguing aspect of the zooplankton study was thedecline in zooplankton abundances at Point aux Frenes duringwinter and spring, and the decrease in mean animal size in June.The decline in abundance was most pronounced in June. Sincecurrent velocities at Point aux Frenes were higher than at LakeNicolet, flow velocity through the plankton net does not appearto be an important factor. Possible important factors affectingthe loss of zooplankton (especially larger-bodied animals) aremechanical damage in the turbulent river waters and predation bysize-selective planktivorous fish.

Although most of the zooplankton community probablyoriginated in Lake Superior (especially in winter), there weretwo other sources of animals. Small rivers and streams probablycontributed to the St. Marys River zooplankton community. Thiswas most evident at Frechette Point where a Daphnla morph, D.mlnnehaba was collected in low numbers in June at stations 2 and3. A second probable source of zooplankton was the littoral andepibenthic regions of the St. Marys River, especially in summer.The epibenthic and littoral community appeared to be especiallywell developed at Point aux Frenes in sxommer where there werelarge day-night, depth strata, and location (channel versusshallow) differences in mean animal size.

It is not clear whether or not an extension of the winternavigation season would have any effect on the zooplanktoncommunity. Although zooplankton abundances varied significantlyalong the course of the river, across the river, with depth, and

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with current velocity, the study was not designed to provideinformation on the causal factors. Thus it would be premature tospeculate on what effects a potential increase in currentvelocity and resuspension (associated with an extended winternavigation season) may have on the zooplankton community alongthe course of the river.

Biomass

The biomass studies showed the variable nature of the riverload. During winter, when the river was ice-covered, zooplanktonwas the major component of the river load (an average of 63.2% ona dry-weight basis). The second-most abundant component wasdetritus. The mean %AFW(ash-f ree dry weight) :seston was low(4.8%), approximating the mean measured value for benthos,mysids, and fish. This suggests that the detrital component ofseston was comprised largely of organic matter, e.g., plantfragments. In contrast, during the June study, zooplankton was asmaller component of river load (9.4%) with detrituspredominating (86.9%). This occurred primarily due to a 5-foldincrease in detritus biomass. The mean %AFW: seston alsoincreased to 25.6% suggesting that inorganic matter (variousmineral particles) and refractile organic matter increased inconcentration.

The June increase in detritus concentration and %AFW: sestoncould have been due to two factors. First, as flow velocities ofsmall rivers and streams increased with spring snowmelt,increased amounts of terrigenous material may have entered theSt. Marys River. Second, as the river lost its protective icecover, heavier sedimentary particles (with a higher %AFW:sedimentvalue) were more readily resuspended from the river floor aswind-driven waves activity intensified. Such wave activity couldexplain why detritus accounted for a larger percentage of theJune river load at Point aux Frenes (98.7%) than Lake Nicolet(40.6%), although there were only small differences in currentspeed (17.1 cm/s versus 13.9 cm/s) between the two locations.Conditions generally were windy at the Point aux Frenes site butwere calm at Lake Nicolet. Similarly, current velocities werehigher at Lake Nicolet during winter (18.9 cm/s) than spring(13.9 cm/s), and detritus accounted for a smaller percentage ofthe total river load (27.0% versus 40.6%). The mean %AFW:sestonwas lower in winter (3.8%) at this site under ice cover than inspring (24.6%) when ice cover had been lost. Current velocitiesapparently increased under windy conditions. For example, atPoint aux Frenes, June current velocities averaged 17.1 cm/sversus 4.0 cm/s in winter. The June study was conducted duringwindy conditions. In contrast, differences in winter-springcurrent velocities were less at Frechette Point and Lake Nicolet;

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at these sites, June studies were conducted during calm or calmto windy conditions.

The results of this study suggest that if an extended wintershipping season resulted in a substantial loss of protective icecover, one possible consequence would be a change in the natureof river load. It is possible that river load would increase asheavier sedimentary matter became more readily resuspended fromthe river floor. Furthermore, current velocities could increaseunder windy conditions. This potentially could affect thebenthic and fish community, including overwintering eggs.

Benthos was generally a minor component of total river load.Unlike zooplankton, benthic drift (in terms of biomass) was notgreater in the more rapidly flowing channel areas than in shallowregions. Benthic drift apparently was not higher in surface andmid-depth strata where current velocities were high whencontrasted with bottom waters where current velocities were less.In many instances, there were significant day-night differencesin benthos biomass (all winter comparisons; June Lake Nicolet andPoint aux Frenes comparisons) suggesting that benthic organismswere able to retain their desired position in the water columnover a 24-hour period. A possible exception was Frechette Pointin June when current velocities may have been sufficiently high(38.4 cm/s) to prevent the benthos from migrating down to thesediments during daylight hours. At all other locations andtimes, current velocities apparently were not sufficiently highto prevent benthic organisms from returning to the sedimentsfollowing sunrise.

Plants were minor components of river load, especially inwinter. The distribution of plant fragments within the river wasfragments. In contrast, during the June study, zooplankton was asamples containing relatively large amounts of vegetation.Plants were most abundant at Frechette Point although the reasonfor this was not determined. Possibly there were greaterstanding stocks of plants upriver of Frechette Point. Theeffects of an extended winter shipping season on plants isunknown. Assuming that there was greater wave activity in theabsence of ice cover, greater amounts of plant material couldappear in the river load. This may or may not be beneficial tothe river ecosystem. An increase in plant fragments potentiallycould benefit the benthic community (by supplying increasedorganic matter) although, if fragmentation was severe, theoverwintering plant community could be harmed.

Fish larvae were minor components of river load. It isprobable that fish larvae biomass may have been underestimated bythe sampling method used. Since fish larvae were not collectedduring winter, increased winter shipping should not have aneffect on larval fish distributions at that time. However, as

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stated earlier, overwintering demersal eggs and pelagic juvenilesand adults may be adversely affected by increased resuspension ofdetrital and mineral matter.

FISH LARVAE

Lake Herring

Historical and current evidence shows that lake herringspawn in the St. Marys River (Behmer et al. 1980; Goodyear etal. 1982; Listen et al. 1983, 1986). Newly hatched lake herringlarvae that we collected established that spawning occurred in1985. Certainly, suitable depths and bottom types are availablein the river system. The scarcity of larvae in the EdisonHydropower Canal suggests that Lake Superior was not a majorsource.

Lake herring hatching usually begins from 1 to 6 days andpeaks from 5 to 20 days after ice break-up (Colby and Brooke1973; Cucin and Faber 1985). We estimate that ice break-upoccurred around 20-22 April 1985 on the St. Marys River. Sincewe began sampling on 26 April, our collections should havecovered the peak hatching period. Our results and those ofListen et al. (1983, 1986) generally show peak larval fishdensities during the first 3 sampling wk after ice break-up.However, no distinct hatching peak, that is, a consistentlymaximum catch at all or most stations during only one samplingweek, occurred. These findings suggest that either the hatchingpeak was missed or the sampling sites were not located nearproductive spawning sites. We believe the latter reason.However, the former cannot be dismissed totally. John and Hasler(1956) found that increased light and agitation stimulate themovement of lake herring embryos which accelerates larvalemergence. Hatchery-reared eggs subjected to continuousillumination hatched 7 to 8 days earlier than eggs held indarkness. When kept in hatchery trays in a calm water bath, eggsthat were ready to hatch were induced to hatch almost instantlyby agitating the tray for a moment; when not disturbed, the eggsremained unhatched for several additional days (exact number notgiven) (John and Hasler 1956). Since ship traffic in theSt. Marys River begins before the river is completely ice free,we can hypothesize that the agitation caused by ship passages andthe increased illumination from the break-up of channel ice couldcause early emergence of lake herring from eggs laid near theshipping channel. In addition, there is one reported incidence,over 2 yr, of lake herring emergence 4 to 5 wk before ice break-up in an Ontario lake (see Cucin and Faber 1985: page 24).

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Compared with peak densities of some spring-hatched species,the production of lake herring larvae in the St. Marys Riverappears to be moderate (Table 43). Densities of rainbow smeltand yellow perch were substantially greater than lake herring.However, densities of lake herring in the St. Marys River aresimilar to larval fish densities in productive areas of LakeSuperior (Selgeby et al. 1978) and Lake Huron (Loftus 1979a,1979b, 1982). Maximum densities from these three water bodieswere in the same order of magnitude. Listen et al. (1986)concluded that lake herring is an abundant species which supportsa major sport fishery in the St. Marys River. Goodyear etal. (1982) reported that lake herring spawn in the river, but noinformation was given on the magnitude of this reproduction.

Since lake herring are pelagic spawners, and the slightlyadhesive eggs are deposited in shallow depths over no particularbottom type (Smith 1956; Colby and Brooke 1973; Scott andCrossman 1973; Cucin and Faber 1985), the newly hatched larvaetend to be widely distributed (Cucin and Faber 1985).Consequently there may be no major spawning area (that is, anarea where intensive spawning occurs) in the St. Marys River.

Regarding larval fish production among the six riverstations. East Neebish Island (station 6) appeared to be the mostproductive. Moderate spawning must have occurred in or upstreamof this area. Some spawning apparently occurred at north LakeNicolet and north Lake Munuscong (stations 2 and 7). Listen etal. (1986) collected moderate numbers of lake herring larvae atthese two stations. Stations 2, 6, and 7 are all near smallislands which may play a role in lake herring spawning or larvalfish behavior. The remaining three stations in Lake Nicoletappear unimportant regarding lake herring reproduction. Larvaemay have emigrated into these three stations after emergingelsewhere. Lake herring larvae are positively phototactic duringthe first few days after hatching and swim toward the surface.They then move to shallow inshore areas (Cucin and Faber 1985).

Lake Whitefish

Historically, lake whitefish spawned in the St. Marys River(Goodyear et al. 1982). Lake whitefish spawn in shallow depths,usually over sand, gravel, and rocks (Scott and Crossman 1973).Certainly this type of habitat is available in the St. MarysRiver. Although currently some spawning occurs in the river(Goodyear et al. 1982), the importance of this spawning appearsto have diminished in recent years (Listen et al. 1986). Listenet al. (1986) did not consider lake whitefish to be an abundantspecies in the river. However, densities of larvae that we andListen et al. (1986) found in the river are similar in magnitude

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to densities reported from productive areas of Lake Huron (Loftus1979a, 1979b, 1982).

The data on the number of lake whitefish and lake herringcaught per week and the percentages of larvae that contained yolkshowed two regular patterns that would be expected for larvalfish that are residents of the St. Marys River system. First,the number collected declined in a regular fashion over the

theySecond, as larval fish grow older they develop more fins andgreater agility which helps them to avoid plankton nets towed intheir environment. Since no lake whitefish and very few lakeherring larvae passed through the Edison Hydropower Canal(station 1) during any given week's sampling, we concluded thatthe contribution from Izaak Walton Bay and Lake Superior to theSt. Marys River population was very small. However, in 1983,Listen et al. (1986) found the greatest densities of lakewhitefish larvae were at their Lake Superior station (near IzaakWalton Bay) compared with six other stations below the St. MarysRapids. Densities in 1982 were much lower at this station thanin 1983. Thus, in some years, lake whitefish production can besubstantial in Lake Superior near the St. Marys River. We wouldexpect lake herring and lake whitefish to hatch some days laterin the Lake Superior area than in the St. Marys River, wherewater should heat faster and ice break up (cues for hatching)sooner than in Lake Superior. However, lake herring larvaecaught in the Hydropower Canal entering the river from LakeSuperior were low in abundance over the entire 6-wk period, andwe collected no lake whitefish larvae in the canal.

Our data establish the St. Marys River as a spawning andnursery area for lake herring and lake whitefish. Larval fishdensities of both species were similar in magnitude to densitiesreported from Lake Huron and Lake Superior. However, theimportance of this reproduction to the river populations remainsunanswered. One aspect of this question might be answered bysampling in an area away from the channelized portion of theriver. The area around the north side of St. Joseph Islandappears, at least from physical maps, to have suitable areas forcoregonine spawning and, presumably, has not been disturbed byeither dredging or large ship traffic. Densities of larval lakeherring and lake whitefish in this relatively undisturbed areacould be compared with those in the river where peak densitiesoccurred in the past.

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Impacts of Winter Navigation on Fish

The impact on fish of the proposed extension of winternavigation past the normal closing date of 15 December must beexamined in light of the life history of each of the importantspecies that inhabits and reproduces in the area. Lake herringand lake whitefish spawn in the fall during November to earlyDecember; lake whitefish usually spawn over rocky, shallow areaswhile lake herring are pelagic spawners in shallow water with noparticular preference for substrate type. One of the criticalperiods in the reproductive phase occurs just after fertilizationand water hardening of the eggs. If eggs are subjected toincreased agitation during this critical period, increasedmortality can result. Since ship passages already occur over thespawning period, this potential impact has existed for manyyears. Another critical period for the St. Marys Rivercoregonids is hatching in the spring. Lake herring eggs beginhatching from 1 to several days after ice break-up (Colby andBrooke 1973; Cucin and Faber 1985). There is also evidence thatincreased light and agitation can speed up larval emergence (Johnand Hasler 1956). If ice break-up in the St. Marys River occursprematurely because of ship traffic in the spring, timing of lakeherring emergence and seasonal production of its zooplankton androtifer food may be mis-matched. Increased mortality ofcoregonid larvae could be the end result. So in the spring thepotential for damage already exists as the navigation seasonusually opens on 1 April. Ice broken up early by ship trafficmay cause early emergence of lake herring larvae and presumablylake whitefish. There is evidence (see Cucin and Faber 1985) oflake herring hatching under the ice on two occasions in anOntario lake. However, there was only this one reportedincident; all other studies show hatching just after ice break-up(John and Hasler 1956; Colby and Brooke 1973; Scott and Crossman1973; Cucin and Faber 1985).

Another species which has the potential to be impacted bywinter navigation is the burbot. The burbot spawns from Decemberto April (Auer 1982), mostly during the winter and sometimesunder ice in shallow water over sand or gravel shoals. Eggs aresemibuoyant and scattered randomly over the substrate (Auer1982). Our larval burbot data indicate that adults spawned overa prolonged period or that eggs were deposited in habitatsvarying greatly in water temperature. We collected larval burbotin moderate abundances and of the same newly hatched size in eachof the 6 wk of the study. Since eggs of the burbot aresemibuoyant, there is considerable potential for their movementfrom optimal spawning substrate because of the currents and wavesgenerated by ship passage in the St. Marys River. Apparentlythis impact was not extremely detrimental as moderate numbers ofburbot eggs hatched over the entire 6 wk of the study.

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Resuspension of sediments may be an important impact onoverwintering fish eggs. The surge caused by ship passage inwinter may resuspend fine sediments which could resettle on theeggs. Suffocation of the embryos would result in increasedmortality for all three species.

If extension of the shipping season results in substantialice breaking activity, another impact on overwintering eggs couldoccur. Channel ice pushed into shallow areas could scour thebottom. Overwintering coregonid and burbot eggs may be crushed,dislodged, or moved to unsuitable habitats.

Another possible effect on lake herring, lake whitefish, andburbot reproduction is the possible dislodgement of spawned eggsfrom the spawning substrate by the surge from ship passages.However if the fish are spawning on optimal substrates, some eggsshould be deposited deep in the interstices of rocks, waterharden and expand there, and be subject to very littledislodgement due to current and waves caused naturally or by shippassage.

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drift of zooplankton, benthos, and fish...appendixes242-261.macrophyte,benthos,fishlarvae,and...

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