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Ecological Characterization vi'the Benthic Community of

Lake Pontchartrain, Louisiana

CQby Q IWalter B. Sikora

Jean Pantell Sikora pI

Aprl 1982

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U.S. Army En~ie District, New OrleansLAJ Contract No. DACW2-7O-C-MO

Coastal Eoolg Laboratory, Center for Wet4041 ReOW0000Lowlelona $tat* Vrdvefty. Soon Reuge, Louisiana T000

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Ecological Characterization ofthe Benthic Community of

Lake Pontchartrain, Louisiana

by

Walter B. SikoraJean Panteil Sikora D I

April 1982

Apr,.sitribution Ujnlin2Ited

Prepared forU.S. Army Engineer District, New Orleans

Contract No. DACW2-79-C-0099

Coastal Ecology Laboratory, Center for Wetland Resources,Louisiana State University, Baton Rouge, Louisiana 70803

Publication No. LSU-CEL-82-05

PREFACE

This report represents the results of an investigation of the benthiccommunity of Lake Pontchartrain, Louisiana and the subsequent ecologicalcharacterization. This study was sponsored by the New Orleans Districtof the U.S. Army Corps of Engineers (COE) under Contract No. DACW29-79-C-0099 to the Coastal Ecology Laboratory, Center for Wetland Resources,

Louisiana State University, Baton Rouge, Louisiana. This report hasbeen designated by the Coastal Ecology Laboratory as contribution No.LSU-CEL-82-05.

Contracting Officer's Representative was Larry Hartzog. Addi-tional assistance in the technical review was provided by Monica

Farris, Dr. Eugene Cronin, and Dr. Wiley M. Kitchens. This review wasco-ordinated by John C. Weber and Suzanne R. Hawes.

The authors would like to acknowledge significant contributions inthe field and in the laboratory by J. Wilkins, laboratory manager, andK. Westphal, N. Walker, E. Parton, M. Lindsay, R. Robertson, and C.Cardiff during the course of the study. We wish also to acknowledge K.Westphal, figures; J. Bagur, editorial suggestions; C. McLean, typing;

E. Parton, assistance with data management; R. Wilson, boat captain;and Dr. Richard W. Heard, taxonomic assistance.

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OVERVIEW AND CONCLUSIONS

The overall objective of this study was the ecological characteriza-tion of the benthic community of Lake Pontchartrain. This study alsoincluded an investigation of the physical or geological factors andanthropogenic or cultural factors which, while affecting the benthiccommunity measurably, also affected the entire ecosystem. Changeswhich have been identified in the benthic community will be discussed,particularly when they are strong indicators of ecosystem-wide trends.

Lake Pontchartrain is a modified bar-built estuary formed between2600 to 2800 years ago. It has the narrow connections with the

ocean, the reduced tidal action, shallow depths, and wind-inducedmixing typical of estuaries so classified.

Lake Pontchartrain is oligohaline, with mean salinities of about5 ppt, and a horizontal stratification. Salinities are higher in theeast end and lower in the west end. Periodic flooding by waters fromthe Mississippi River can lower the salinities to essentially fresh-water conditions.

The high levels and sources of pollutants in Lake Pontchartrainare described in a recent study (U.S. Army Engineer District, NewOrleans 1980). The specific effects of hydraulic clam shell dredgingare described in a recent study (Sikora et al. 1981). This reportdescribes the general effects of both of these anthropogenic impacts onthe benthic community.

The most striking change which has taken place in the 25 yearssince the last major study occurred is the loss of the larger sizeclasses of the clom Rangia cuneata. Darnell (1979) reports meandensities of i. %uneata longer than 20 mm of 135 ± 16/m 2 found duringthe survey studies in Lake Pontchartrain in 1953 and 1954. During thepresent study densities of 0.85/m2 of this clam were found. Dominancein numbers and biomass has shifted from R. cuneata to two smallhydrobiid gastropods, Texadina sphinctostoma, and Probythinellalouisianae, which now make up 70 to 80% of the numbers of animals foundin Lake Pontchartrain.

Although both of these gastropods are found in all samples fromLake Pontchartrain, their proportion varies from station to station.Overall, T. sphinctostoma was more numerous the first year (1978-1979)of this study, and P. louisianae was dominant, with greater biomass,the second year of the study (1979-1980). Both of these gastropods,however, are much smaller than the clans which they have replaced.They have a length of 2-3 - and a weight of 0.2-0.3 mg. The usual15,000-20.000 gastropods per square meter total only 3.75 - 5.0 gum2

AJDW. The 135 clams longer than 20 = per square meter, which thegastropods have replaced, would have had a biomass of 20 to 50 g/u 2

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iii

This change in average size, and in total biomass has resulted ina benthic infauna which can be characterized as being much smaller thanusual (Table 15). The low biomass would have an effect on the benthic-dependent predators, providing one-tenth of the food, for instance, forblue crabs that was available 25 years ago.

One of the reasons for this decline in benthic biomass, examinedin this report, appears to be related to the lower levels of carbon inthe sediments. Steinmayer (1939) found 6 to 8% organic matter in thesediments, where we now find about 11. Primary production in the watercolumn, which would have been a source of carbon for the sediments, islower than expected for the level of nutrients in the lake. Loweredprimary production may be related to the higher levels of toxic sub-stances entering the lake, or to the turbidity caused by the re-suspension of sediments which have been destabilized by shell dredging.

In addition to a decline in the benthic comaunity, we have foundand presented evidence for a decline in the numbers of zooplankton, anda shift in dominance in the nekton. Diversity in the lake is low.Lowered diversity in the benthic community appears to be related to thelow number of species able to live In the lake. The lack of seasonality inspecies composition is unusual in estuarine benthic communities. Totalnumbers of species present is lower than average for brackish systems,which are usually characterized by fewer species.

The direct effect of the destabilization of the sediments by shelldredging has been the loss of the larger size classes of the clam, R.cuneata. The replacement of the clam-dominated community by a muchlower diversity gastropod-dominated community, instead of the usualpolychaete-amphipod community common in estuaries of this region, isprobably attributable to the high levels of toxic substances found inthe sediments. All measures of benthic community structure, such as Idiversity and species -mposition, and community function, such as

niche breadth, examinou during this study indicate that the benthiccom=nity of Lake Pontchartrain is showing unmistakable signs ofstress.

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TABLE OF CONTENTS

PAGE

PREFACE ........... .......................... i

OVERVIEW ............. ................. iii

LIST OF TABLES ....... ...................... . viii

LIST OF FIGURES ........ ......... .... . x

INTRODUCTION

Purpose and Scope ...... ...... ... 1Geomorphic History of Lake Pontchartrain ... 2Width-Depth Equilibrium ................ . 3General Description of Lake Pontchartrain ....... 3Anthropogenic Impacts on Lake Pontchartrain .. ..... 7

Hydrologic Impacts ......... ..... 8Bonnet Carre Floodway Opening ... ......... . 8Pollution ....................... 15Hydraulic Shell Dredging in Lake Pontchartrain . . 17

Sediments and Mineralogy ...... ......... 18Salinity Regime of Lake Pontchartrain ........... 24

METHODS . . . . ........................ 28

Location of Sampling Stations ............ .. 28Determination of Precision of Sampling Regime . . . . 29Sampling Methods ...... . ........ 30Statistical Methods ................ . 31

RSLSto 2 . . . . . . . . .. .. .. .. . . 38Station 3. .. ... . . . . . . . . . . . . . 33Station 4 . . . . . . . .. . . . . . . .3 . 4Station 5 ............. . . . . . . . . . 40Station 6 . . 0 0 . . . . . . . . 0 . 46

Stat..in Treads. . . . . . . . . . . o . . . . . . 62

Station 7 48

Station 9 . 0.00..05station 10 . .. .. .0. .. .. .. ... .. 6 54

Station 11 . .. .o. .. .... . .. .. 56I

Station 12 . .. .. .. .W... . .. .. 54Station 132 . .. ... .58.6

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CONTENTS PAGE

Changes in the Macrofauna ........... ... 62Changes in the Meiofauna . ............ ... 69Changes in Community Structure . ........ 69Changes in Community Function . .......... ... 76

DISCUSSION . ............... . . . . . . . . 80

Loss of Large Rangia cuneata from theBenthic Community . . ......... ..... . 80Primary Production and the Benthic Community . . . .. 83Zooplankton and the Benthic Community ........ . . 85Feeding Modes in the Benthic Community ....... . 86Niche Breadt h of the Benthic Community ........ . 87Species Diversity of the Benthic Community . ... 87Abundance and Biomass of the Benthic Community . 89Changes in Benthic Comunity Structureand Function as a Response to theOpening of the Bonnet Carre Floodway 89

Predation by Nekton on the Benthic Community . . . . . 93Changes in the Benthic Community resultingfrom Sbell Dredging ................ 95

Changes in the Benthic Community resultingfrom Toxic Substances ................ 95

SUMARY . . . ....... .................. 98

LITERATURE CITED . . ................... 99

APPENDIX A

MACROFAUMA DATA . . . . . . . . . . ......... 111MEIOFAUNA DATA ................... 160

APPENDIX B

SYSTEATIC LIST OF BENTHIC INVERTEBRATESIN LAKE PONTCHARTRAIN . . . . . . . . . . . . . . . 185

DISCUSSION . . . . . . . . . .. ... . . .. . 191

APPENDIX C

DENDROGRAMS, CLUSTER ANALYSIS OF SAMPLESBY STATION . . . . . . . . . . . . . .. 193

APPENDIX D

DIUTEIC SAMPLING CRUIS DATES . . . . . . . . . .. 200TRAISRCT S ... .. . . . ..... 202

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CONTENTS PAGE

APPENDIX E

SEDIMENT METHODOLOGY .. ...... . ......... 204

APPENDIX F

DESCRIPTION OF THE "DEAD ZONE"

INCIDENT, AUGUST 1980. .. ..... ............ 206

vii

LIST OF TABLES

TABLE PAGE

1 Carbon loading, 1979 opening of BonnetCarre Floodway. .... ........ ........ 11

2 Salinities in ppt, for all stations,all sampling dates .. ........... ...... 12

3 Water temperatures, f or all stations,all sampling dates o. .. .. .. .. .. ...... 13

4 Organic carbon content of sediments,all stations. .. ........................ 21

5 Lake Pontchartrain sediment bulk densities . . . 23

6 Location of Lake Pontchartrainsampling stations. .. ....... ........... 34

7 Macrofauna. diversity measures andniche breadth by station. ....... ........ 36

8 Macrofauna. ranked by abundance and biomass,1979-1980 .... ...... ... ........... 64

9 Hacrofauna, ranked by abundance and biomass,1979-1980 ..... .... ... ............ 65

10 Ash-free dry weights of Rangia cuneata,Probythinella louisianae, and Texadina,sphinctostoma.. .. ........ .......... 68

11 Meiofauna. ranked by abundance and biomass,1978-1980 . . . o.o. .. .. ...... .... 74

12 Hacrofauna. diversity measures and nichebreadth, by month . . . . . . . ..... o . 75

13 Regression analysis; nematodes,dependent variable . o. . o. ... ooo. . .. 77

14a Regression analysis; P. louisianaea,dependent variable .oo. . o.. o.... .. 79

14b Regression analysis, To sphinctostomadependent variable . .. o......0. . .. . 79

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TABLE PAGE

15 Macrofauna abundance and biomass;a comparison ................. 90

Al Macrofauna abundance, biomass, diversitymeasures; for all months, at all stations . . . .

A2 Meiofauna abundance, biomass; forall months, at all stations ........... 160

Dl Benthic sampling cruise dates .......... 200

D2 Transect from north shore, Rangiacuneata data .......................... 203

F1 Oxygen, temperature, conductivityand salinity by depth at "deadzone" stations ....... ................... 208

F2 Macrofauna densities, August 1978,August 1979, and August 980..... ........... 209

ix

i4|-

LIST OF FIGURES

FIGURE PAGE

1 Map of Lake Pontchartrain, Louisiana,the major rivers and passes . .............. 4

2 Map of Lake Pontchartrain showing samplingstation locations and isohalines . ........ 6

3 Flow during 1979 opening of BonnetCarre Floodway ....... .. ................. 10

4a Distribution of organic carbon in thesurface sediments, Initial Survey ....... 19

4b Distribution of organic carbon in thesurface sediments of sampling stations ..... 19

5 Distribution of sediment types inLake Pontchartrain ...... .. ............... 22

6 Mean monthly salinities, LakePontchartrain ...... ... ................. 25

7 Map of Lake Pontchartrain showingsalinity regions .... ................ ... 27

8 Response of Lake Pontchartrain toS seasonal salinity fluctuations ... ... ......... 27

9 Macrofauna abundance, Station 1 ........ .33

10 Meiofauna abundance, Station 1 ......... ... 37

11 Macrofauna abundance, Station 2 ........ 38

12 Meiofauna abundance, Station 2 ......... .39





17 Macrofauna abundance, Station 5 . . . . . . .. 44


x

FIGURE PACE

19 Macrofauna abundance, Station 6 ......... 46

20 Meiofauna abundance, Station 6 ........... . 47

21 Macrofauna abundance, Station 7 ............. 48

22 Meiofauna abundance, Station 7 ....... ... 49

23 Macrofauna abundance, Station 8 ............. 50

24 Meiofauna abundance, Station 8 ........... 51

25 Macrofauna abundance, Station 9 ......... .52

26 Heiofauna abundance, Station 9 .. ......... . 53

27 Macrofauna abundance, Station 10 ............ 54



30 Meiofauna abundance, Station 11 . . . . . . . .. 57

31 Macrofauna abundance, Station 12 . . . . . 58

32 Heiofauna abundance, Station 12 ......... .59

33 Macrofauna abundance, Station 13 . . ......... 60

34 Heiofauna abundance, Station 13 o.......... .62

35 Annual mean gastropod abundance,Lake Pontchartrain ............... 63

36 Annual mean langia cuneataabundance, Lake Pontchartrain . ......... . 66

37 Annual mean Mulinia pontchartrainensisabundance, Lake Pontchartrain . o . o ... ... 67

38 Annual mean chironomid abundance, LakePontchartrain . o. .. . .. . . . . . . . .. . 70

39 Annual mean polychaete abundance, LakePontchartrain .................. 71

40 Annual mean amphipod abundance, LakePontchartrain . . . . . . . . . .. . . .... 72

xi

FIGURE PAGE

41 Annual mean nematode abundance, LakePontchartrain .. ...... ......... .... 73

42 Mean biomass and abundance of macrofauna, inLake Pontchartrain, by month .... ....... 94

C1 Dendrogram; cluster analysis ofsamples for Station I. .... ........ .... 193

C2 Dendrogram; cluster analysis ofsaples f rtton 2.tati.o2. .. ........ 193

C3 Dendrogran; cluster anlaysis ofsamples from Station 3 ..... ....... . . 194

C4 Dendrogram; cluster analysis ofsamples from Station 4.... ............. 194

C5 Dendrogram; cluster analysis ofsamples from Station 5 .. ... ............. 195

C6 Dendrogram; cluster analysis ofsamples from Station 6 . . . . . . . . . . . . . . 195

C7 Dendrogram; cluster analysis of

samples from Station 7. .. .. .... ..... 196C8 Dendrogram; cluster analysis of

samples fromStation8 .. . .. ... ... .... 196

C9 Dendrogram; cluster anlaysis ofsamples from Station 9 ... .... ........ 197

C10 Dendrogram; cluster analysis ofsamples from Station 10. .. ..... . . . . . . 197

C11 Dendrogram; cluster analysis ofsamples from Station 11. .... ......... 198

C12 Dendrogras; cluster analysis ofsamples from Stationl12 . ... .. . ... . .. 198

C13 Dendrogram; cluster analysis ofsamples from Stationl13 . . . . . . . . . . . .. 199

DI Map of Lake Pontchartrain shovingnorth tranuect stations . . . * o . * . . o . . . 202

F1 Map of Lake Pontchartrain showingstation locations in "dead zone" area . . . . . . 207

xii

INTRODUCTION

Purpose and Scop e

The obJecti'9.s f or this study were the result of recommendationsdeveloped during a series of meetings held with participants from theU.S. Army Corps of Engineers, the U.S. Fish and Wildlife Service,National Marine Fisheries Service, and the Louisiana Department ofWildlife and Fisheries. These objectives included:

1. The-determination of the structure and function ofthe benthic community of Lake Pontchartrain,

2. An examination of the effects of the Bonnet CarreSpillway opening on the benthic community of LakePontchartrain, and

3. An analysis of the altered bulk density due todredging on the benthic community of LakePontchartrain.

In order to fulfill the first objective, a comprehensive samplingprogram was designed and implemented. After the organisms collectedhad been enumerated and identified, certain measures of communitystructure such as species diversity were examined. Factors whichaffect community structure such as biological interactions (predationand/or competition) or physical disturbances (alteration in sedimentstability, or presence of toxic substances) were investigated andassessed. Measures of community function included a study of communityrespiration, which has been discussed in a previous report (Sikoraet al. 1981). In addition, community functions such as resourcepartitioning by the dominant populations or feeding groups present andniche breadth were examined and quantified.

The second objective, the evaluation of the effects of theBonnet Carre Spillway opening on the benthic community, wasaddressed by examining the evidence of changes in the communitystructure attributable to the opening. The interaction of otherimpacts on the lake with the opening of the spillway was alsoexamined and will be discussed.

An extensive field and laboratory investigation was necessaryto fulfill the third objective. Measurements of bulk densities atall stations were made in the field, and changes of bulk densitythrough time were made in the laboratory. Analyses of thesedeterminations were compared and will be presented.

Our overall objective, the ecological characterization of thebenthic community of Lake Pontchartrain. Includes an investigationof the physical or geological factors and anthropogenic or culturalfactors which affect the entire ecosystem. Any pollutant or

natural stress on an estuarine ecosystem will be reflected in analteration in the benthic community. These changes which have beenidentified in the benthic community will be discussed, particularlywhen they are strong indicators of ecosystem-wide trends.

Geomorphic History of Lake Pontchartrain

Worldwide lowering of sea level associated with the build-up ofmassive continental ice sheets occurred during periods of glaciationin the late Cenozoic (Flint 1971). After the last, or Wisconsin,glacial stage, the worldwide sea level rose in two stages. During thefirst stage, sea level rose to about -75 m at about 35,000 B.P.(Before Present). The second rise began about 18,500 B.P. and continueduntil about 3000 B.P., when it reached approximately the present stillstand (Morgan 1967). Saucier (1963) gives a detailed account of theformation of Lake Pontchartrain from geological, paleontological, andarcheological evidence. From this and from later studies, it appearsthat the lake has evolved through two distinct types of estuaries (asclassified by Pritchard 1967) into its present form. When sea level wasabout -12 m, a small, drowned river valley estuary was formedwhen the Gulf waters flooded the Amite trench, a river system that hadbeen formed by the confluence of the Amite, Tangipahoa, and TchefuncteRivers and some smaller streams. About 5000 B.P., Pontchartrain Bay, ashallow bay of the Gulf of Mexico, was formed. This large, open baycovered most of present day Lakes Pontchartrain and Maurepas. As sealevel rose, two barrier spits were formed. The first, called Milton'sIsland beach trend, was a typical recurved spit that extended from GoosePoint out into the bay and curved back north. A second, larger spit,the Pine Island beach trend, formed later and extended from the PearlRiver down into the New Orleans area, over which the city now stands.Saucier places the formation of Lake Pontchartrain at about 3500-4000B.P. when the prograding Cocodrie Delta of the Mississippi River curvedeastward and to the north, burying the Pine Island beach trend. Otvos(1978), however, based on foraminifera assemblages, disputes this earlydate for the formation of the lake. According to his scenario, theCocodrie Delta extended only to the western tip of the Pine Island spit,which later broke up into a series of barrier islands. ThusPontchartrain Bay became a bar-built estuary. This condition allowedopen channels to exist between the islands, which would have allowedconsiderable water exchange between the Gulf and the bay, and probablywould have given rise to mesohaline salinity conditions in the estuary.

About 2600 to 2800 B.P. Lake Pontchartrain was formed when the St.Bernard Delta of the Mississippi River buried the Pine Island beachtrend completely, covering most of the area and extending out to theChandeleur Islands. A constricted opening was left approximatelywhere The Rigolets now exists, thus beginning the oligohaline salinityregime that persists to the present. As the St. Bernard Delta wasabandoned and began to deteriorate through subsidence and erosion; LakeBorgne and the western part of Mississippi Sound were formed, afterwhich a second opening, Chef Menteur Pass, cut through. Thus, from ageomorphic perspective, Lake Pontchartrain is a modified bar-builtestuary. It is modified in the sense that a wide area of subaerial

2 |

deltaic land covers the original barrier islands and separates them fromthe sea. It meets Pritchard's definition of an estuary (Pritchard 1967)by having a "free connection with the open sea," which he furtherdefines as allowing "an essentially continuous exchange of water betweenthe estuary and the ocean." He further states of bar-built estuariesthat because the inlets connecting this type of estuary with the oceanare usually small compared to the dimensions of the sound within thebarrier, tidal action is considerably reduced, and these systems areusually shallow, with the wind providing the important mixing mechanism.Lake Pontchartrain meets all these criteria.

Width-Depth Equilibrium

Tidal basins of the Texas and Louisiana coast were found to be ina dynamic equilibrium with relation to geological processes and physicalforces by Price (1947). Price found all 31 tidal basins studied to havea predictable relationship between width and depth regardless of basinorigin, although most were bar-built estuaries. Two relationships werederived, one for the "non-humid coast" of south Texas and another forthe "humid coast" of eastern Texas and Louisiana. Lake Pontchartrainfits well on a regression line for the 16 basins of the latter group,with 48 km as an average width and 5 m maximum depth. Pricediscusses several possible reasons for the existence of the relationship.These include fetch of the wind, depth of wave action, character andabundance of incoming sediments, and subsidence. Of these, it wouldseem that the abundance of incoming sediments is the most important asevidenced by Atchafalaya Bay. In 1947, this bay also fit the regressionrelationship of width and depth, along with West C8te Blanche Bay andVermillion Bay. However, since the flood year of 1973 the AtchafalayaRiver has overcome equilibrium conditions and has built a subaerialdelta in the bay. Lake Pontchartrain has not experienced any significantshallowing in historic times despite numerous crevasses and six openings iof the Bonnet Carre Spillway, which allowed flood waters of theMississippi River to enter the lake. The Mississippi River brokethrough the levees in 1874 in forming the Bonnet Carre Crevasse. Floodwaters flowed into the lake for eight years (Steinmeyer 1939, Gunter1953), yet the lake apparently maintained the equilibrium. Exactly howthe mechanism for maintaining this equilibrium works is not preciselyknown. If wind induced vaves do function in this capacity byresuspending sediment, which is then flushee nit of the basin, then LakePontchartrain must have been a turbid system throughout its history.

General Description of Lake Pontchartrain

Lake Pontchartrain is a shallow (mean depth 3.7 m, maximum 5 m)body of water of about 1630 km2 lying In the middle of a large,southeastern Louisiana estuarine complex (Figure 1) with a diurnal tidalregime and mean tidal range of 11 cm (Outlaw 1979). To the west is LakeMaurepas, connected to Lake Pontchartrain by Pass Manchac; to the east,Lake Pontchartrain Is connected to Lake Borgne by The Rigolets Pass andby the Chef Menteur Pass. In the southeast, the man-made Inner HarborNavigation Canal (INNC)-issisuippi River Gulf Outlet (MMG) complex

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connects the lake to the Gulf of Mexico. The tidal passes located onthe east end of the lake have cross sections of 6850 m2 (Rigolets), 3200m2 (Chef Menteur) and 800 m2 (IHNC). The mean tidal current velocities

within these passes during nonflood riverine flows are 70 cm/sec, 100cm/sec, and 50 cm/sec, respectively (Chuang et al. 1980) with volumescorresponding to 60%, 30%, and 10%, respectively. The total tidal prismof all three passes was calculated to be 1.56 x 108m%, with nosignificant flood or ebb tide dominance (Swenson, personal communica-tion). Higher salinity water enters the lake through these passes. Thewestern end of the lake is characterized by riverine input offreshwater.

Circulation within the lake has been shown to be primarily winddriven (Gulf South Research Institute 1972; Gael 1980) with currentspeeds reaching velocities of about 15-20 cm/s (U.S. Army Corps ofEngineers 1962), with speed and direction dependent on the wind. Themean monthly lake level shows a seasonal pattern with a spring and afall peak. These peaks correspond to similar peaks in the easterlywinds, indicating that the long-term lake level trends appear to beresponding to tha wind regime. A similar bimodal curve for the levelin the Gulf of Mexico also has been documented (Marmer 1954). Chuangand Swenson (1981) have shown that the transport of water in and out ofLake Pontchartrain via the tidal passes at time scales shorter than 15days is directly related to the east-west wind stress. This patternindicates a coupled coastal ocean-lake response. Long-term water levelvariations may be due to the seasonal heating cycle, rainfall, and riverrunoff into the lake.

Analysis of winter current meter records from the tidal passes(Chuang et al. 1980) has shown that tidal variations account for 50% ofthe volume transport through the passes (tidal prism is about 1.5 x 108m3)and subtidal or fiontidal effects (meteorological events) account for theother 502. During calmer months of the year (summer) the tidal effectsare probably more significant.

The salinity of the lake is quite low, with the mean being about 5ppt. The lake is horizontally stratified (Figure 2), with salinitiesbeing highest in the east end (tidal passes) and lowest in the west end(fresh water input) (Swenson 1980a). This gradient rarely may be as muchas 12 ppt but is often less than 3 ppt. Analysis of 10 years ofsalinity data collected by the U.S. Army Corps of Engineers (1962)indicate that the salinity of the lake has a seasonal pattern with aaininum in the summer (June- July) and a maximum in the fall (October--November).

Lake temperatures show a general pattern in which the lake isessentially isothermal, the maximum spatial gradient being about 4* C(Swenson 1980a). In addition, the lake temperature closely follows theair temperature through the year (Thompson and Verret 1980) with amaximum water colum mean temperature of about 30* C in August-Septemberand a minimm of about 6' C in January-February.

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Vertical salinity (and temperature) changes are usually smallenough to be inconsequential, indicating that the lake is vertically awell-mixed system. Occasionally some salinity stratification doesoccur. The gradient can be as high as 6 ppt; however, it is usuallfless. Oxygen stratification may also occur occasionally, with lowoxygen conditions resulting at the bottom.

The majority of the fresh water input to the lake is from threeriver sources: the Tickfaw, the Amite-Comite, and the Tangipahoa.These rivers supply about 85% of the river input to the lake. TheAmite-Comite system alone supplies 52%. The remaining input is fromnumerous small rivers and bayous (8%), marsh drainage (3%) and runofffrom the city of New Orleans (4%) (data from Swenson 1980b).

During flood years, Lake Pontchartrain can receive MississippiRiver water via the Bonnet Carre Floodway and Pearl River water via TheRigolets. In 1979 the floodway was open for 38 days tAeasing a volumeof water equal to 1.5 x 1010 m', a volume that is more than twice (240%)the volume of the lake (Swenson 1980a).

The flushing time of an estuary is defined as "the time required toreplace the existing fresh water in an estuary at a rate equal to theriver discharge" (Dyer 1973). Using the mean streamflow of250 m3/s, it is estimated that the flushing time for the lake is about60 days.

Owing to the lake's large area, and hence large fetch, wind inducedwaves play an important role in the lake system. Wind and wave datacollected by the Corps of Engineers in the 1950's and 1960's (un-published) indicate that there is little lag time between an increasein wind speed and the corresponding increase in wave height. Analysisof this data indicates that resuspension of silt-clay sediments, the Imajor type in Lake Pontchartrain, (Bahr et al. 1980) would occur withwave heights of about 1 m. Waves of this height occur with wind speedsof 9 m/s (20 MPH) or greater. Wind data for the lake (Gael 1980)indicate that winds of this magnitude occur about 15% of the time.Thus, one can conclude that the bottom sediments of the lake are inmotion at least 15% of the time due to natural causes. This phenomenonhas been recognized as having occurred in Lake Pontchartrain for sometime. Steinmayer (1939) states "at times even the sediments in thedeepest part of the lake are agitated and moved by waves."

Anthropogenic Impacts on Lake Pontchartrain

In order to understand the dynamics of biological processesoccurring in Lake Pontchartrain it is necessary to consider the culturalinfluences which have modified the physical and chemical environment ofthe lake. It is not possible to view the faunal parameters in LakePontchartrain in the context of a pristine, natural system. Any de-scription of Lake Pontchartrain which neglected these cultural impactswould be woefully incomplete.

7

Hydrologic Impacts

Lake Pontchartrain has probably been subjected to periodic floodingby waters from the Mississippi River throughout most of its history.This is true in recent times, because of the proximity of the presentmain channel of the Mississippi to the western shore of the lake.Numerous crevasses or breaks in the natural, and later man-made leveeshave occurred. The "crevass period' in the Lake Pontchartrain basin isconsidered as having extended from about 1750 to 1927 (Gunter 1953). Sincethe floodway was built it has been opened six times: in 1937, 1945, 1950,1973, 1975, and 1979. The impact of lengthening the interflood periodsis not fully known. The impact of the opening of the floodway and thusflooding of Lake Pontchartrain will be addressed later in this report.

The IHNC was built in 1921 (Schweitzer 1979), and the connectionto Lake Pontchartrain was made at this time. The MRGO Canal wascompleted in 1963 to put the Port of New Orleans 65 km closer to the seaby way of a channel 160 m wide and 12 m deep (Schweitzer 1979). TheMRGO is connected to the Intracoastal Waterway which connects to theIHNC which in turn is connected to Lake Pontchartrain. High salinityGulf water enters Lake Pontchartrain via this route causing salinity andoxygen stratification in the southeastern region of the lake (Poirrier1978). Although stratification does occur, the extent to which overallsalinities on a lake-wide basis have been increased has not been fullyquantified and will be addressed later in this report.

Many sections of the city of New Orleans are below sea level andleveed, necessitating that storm runoff water be pumped up and out ofthe city. Much of this water is pumped into Lake Pontchartrain,amounting to 4% of the annual fresh water input budget of the lake

U (Swenson 1980b). The southern shore of the lake is impacted by this waterand its chemical constituents (U.S. Army Engineer District, New Orleans,1980).

Changing land use patterns in the upper drainage basin of the lakehave increased floodwater discharges primarily in the Amite-Comite Riversystem (Turner and Bond 1980a). Urbanization tends to increase stormwater runoff as well as adding to the pollution load.

Bonnet Carre Floodway Opening

The Mississippi is considered to have occupied its present channelfor the past 700 years. For much of the time, before the building ofartificial levees, the river flooded over a wide area. Lake Pontchar-train still received floodwater by overland flow and via Bayou Manchac,which was one of the floodwater distributaries. Thus it is safe toconclude that Lake Pontchartrain has received floodwaters from theMississippi for the last fourth of its existence, and certainly duringIts formation by two Mississippi Delta systems. The Mississippi Riverhas played a major role in the evolution of the Lake Pontchartainecosystem. Levee building along the Mississippi River is considered

Sd

I -. --

to have begun about 1727, and extended to Baton Rouge by 1812. Thebuilding of levees increased the height of the flood crest on theriver, and crevasses became more violent because of the increasedgradient (Saucier 1963). From 1849 to 1927 Gunter (1953) statesthat there were 38 crevasses in the levee system allowing water toflow into Lake Pontchartrain or Lake Borgne, or an average of onecrevasse opening every 2.1 years. Saucier (1963), on the otherhand, shows the locations of only 11 crevasses, through whichwater flowed into Lake Pontchartrain, during the same 78 yearperiod, for an average of one every 7.1 years. Gunter's figure of38 crevasses is, thus, too high, as not all of these crevassesaffected Lake Pontchartrain. In order to prevent the floodingof New Orleans and other commnunities, the Bonnet Carre Floodwaywas built in 1931 to divert floodwaters from the MississippiRiver via Lake Pontchartrain to the Gulf of Mexico thus loweringthe river stage downstream. Since 1931, the floodway has beenopened six times: in 1937, 1945, 1950, 1973, 1975, and 1979. Thisis an average of one opening every 8.3 years over the past 50 years.

During the course of this study, the Bonnet Carre Fl1bodwaywas opened on April 17, 1979. It was considered fully closed byMay 26, 1979, with only four of the 350 bays remaining open untilMay 30, 1979. The opening and closing dates are relative becauseriver water began leaking through the wooden "needles," or slatsthat close the bays, a considerable period of time before they wereopened. Officially, however, the opening is considered to havebegun April 19, 1979, with discharge data beginning on that date,and to have remained open for 32 days until May 20, 1979, when dis-charge data ceased to be taken. The total discharge is given inFigure 3 (Swenson 1980a) and Table 1. The total volume of waterwas roughly 1.5 x 101 0m3,about 240% of the volume of the lake.As seen from Table 2, the whole lake essentially turned fiesh

in May 1979. Despite this large influx of colder river water,the effect on the temperature of the lake water was not as greatas might have been expected (Table 3). Water temperatures inMay 1979 remained close to the temperatures recorded in April1979, about 2-3* below the May temperatures recorded in 1980. Inall, the 1979 opening (32 days, mean discharge 4785 m3/8 wasmost similar to the 1950 opening (38 days, mean discharge4049 m'/s Saucier 1963).

Of the effects of the 1950 opening, Gunter (1953) states thatthe general effects were beneficial to oysters, speckled trout weredriven eastward, and "the plant growths were greatly stimulated,and associated animal life, such as scuds and grass shrimp, wasfound in great concentration. Plankton feeders, such as mullet,anchovies, menhaden, and shad were seen in great abundance every-where." Gunter does say, however, that the 1945 opening wasgenerally considered to have caused considerable damage to oysterbeds in Mississippi Sound. The 1945 opening was of much greaterduration and volume (open 57 days, mean discharge 6088 m3 /sSaucier 1963), and was opended later in the year, March to Hay.The 1950 opening was open from February to March.

*00

* ~400

0 o0

CA 0 4IL) 0% 90

0) 0

6 4 4U 0

4r410 I" 00. 14

44 AJ 4

I'sI 0 Ira.0cp).u A .0

4J 44.4

@10.0 %0 "W

a n4 44IL v

I . 4 4

10_____ ____ ___ o

.~ _____ -.

- -P-

Table 1. Carbon loading into Lake Pontchartrain from the 1979 openingof the Bonnet Carre Floodway.

chlorophyll chlorophyll carbon

volume flux* carbonT carbon flow chlorophyll carbon flow

Date 1u03 cfgh 36/ sm/1 8/204 USA gil 8/sc

April

19 98 2.775.4 6.9 19.150.3 3.94 13.2 x 10- 5

360.8

20 130 3.681.6 12.0 44,179.2 2.98 9.9 x 10- 5

367.4

21 149 4,219.7 7.3 30,803.8 2.45 8.2 x 10"5

346.4

22 179 5.069.3 -- - --

23 - - 6.9 - 1.85 6.2 x 10- 5 --

24 198 5,607.4 7.5 42.055.5 0.17 0.6 x 10"5

31.9

25 190 5,380.8 9.0 48.427.2 1.39 4.6 x 10- 5

250.7

26 235 6.655.2 6.6 43.924.3 1.18 3.9 x 10-5

262.8

27 205 5,805.6 7.4 42.961.4 0.00 0.0 0.0

2P8- - 8.3 - 1.30 - --

29 216 6,117.2 7.2 44.0413.8 0.52 1.7 x 10-5

106.4

30 226 6,400.3 6.7 42,862.0 - --

May

1 244 6.910.1 7.3 50.443.8 1.56 5.2 x 10"5

360.7

2 238 6.740.2 6.5 43,811.3 0.00 0.0 0.0

3 224 6,343.7 - - - -

4 243 6.881.8 6.1 41,978.9 .18 0.6 x 10"5

41.5

5 21.7 6,995.0 8.0 55,960.0 .89 2.9 x 105

208.4

& 249 7.051.7 - - 1.49 5.0 x 10"5

356.8

7 218 6,173.7 6.5 40.129.0 1.47 6.9 x 10-5

303.7

8 216 6,117.1 5.1 33.032.3 1.95 6.5 x 10-5 399.4

9 204 5,777.3 6.5 37,552.4 2.94 9.8 x 105

569.1

10 177 5,012.6 - -

11 166 4,701.1 5r.5 28,586.1 3.50 11.7 i 10- 5

550.0

12 161 4,559.5 6.4 29,180.8 2.14 7.2 x 10"5 326.9

13 141 3,993.1 - -- - --

14 123 3,483,4 - -- - --

15 108 3,058.5 9.2 28,138.2 1.76 5.9 % 10"5

180.1

16 97 2,747.0 6.0 16,.82.0 1.47 6.9 . 10. 145.1

17 83 2,350.6 6.0 14,103.6 2.73 6.6 % 10"3

214.8

18 58 1,642.6 5.8 9,527.1 1.97 6.6 % 105

108.2

19 55 1,357.6 5.5 8,566.8 2.07 6.9 x 10"

107.9

20 42 1,189.4 4.3 5,114.4 2.27 7.6 a 10" 90.4

mesa 169 4.732.0 6.9 33,376. 1.15" 6.2 x 10's 258.6

agaca ftrm Outlac, (1979). Table 31.

tuts frm U.S. Geolo"IcaL Survey (1980).

_;. U

4

V W cc c cc - 0- cc

J~44.

0J m 04m mmI

54 00A Ci. C* *44 * MW 4 *4 4 Zo44 4 .

Aj 4)

*4 5 * 0 -. 44 ** 4* 4 -

C *

*4* 44 cM 00 64* c; 44 c4* 60 c;c

S44 1.4

41~ .r.- 6 6 6 6 6 6 C,0 IV

4-

0 1=.44 ** 44 .. 4 ** .. 4 .4* 44 ** 4 ** 44 **

0., 'J

c- m 4 @m .ftt c

M 0

.4 to >

W-f 4C 9

wCO

5.4 -% 4.cc AU

COW

CI

01

Am sts AN s- N x R4 . r, A I. st 44 UA UM MA n GxR

'v 0

0 41 41 2

1-4 AJ 44 44 41 V 4 4 14 4 O ..

41 4) 0 mo Ch~~N e A 0iP MA NN Nl N- Nm NN No co Ch

02 41 4141 .. N c; 0; inA NA. 0;. NN 14 00 *InI 4

Ia4.M 44. I

Ca 41 l 0D A. 40 01 @4 *4 m0 at 0 5t 014 @ A A4 @14

0 ~1 . 11 r. a. mm a. *a *a aa *0a.

001 eW 0% 0 - Ia.9OfI 41 54 0% P 41 z .4 1.4 A C a 44 44 A . NA

.0 > r.0

cc $ 14 C: "Ia. u C 0

0 4 41 5 0. NN NN NC!N A N N N. N

o 1041a a

t20' v 44 f 0c41 44 0" 0 0 5 - 0V.)-

0) Qja.

to0 1: 4 0

0 14I0 0 NO540.050 Pa4 A.9

W1 IW410 U

I 0 NN 4.4 NW 0W 44 Wa . 44 5 @3t cc -H 41 .y NA A 14 - 4- .. A

.0' V0o410

-02 41 r. AJ oc v------------- a- Ch 00 10 O 0 -to 0 v 0 41 -C; 0; NN NA. NA. NA NN NN NA NN NN N. m.

to 41 a41 2 .l0 0I I

41 a41 0 2 $ 0 Wa a 0 aa a41 41 Ia4. 15 N 4 U 41 44 aN0. 4U 4-

r 42 IA 0 Ia 00o 41 u-I M2 U 0 4-

0a141 54 9 Av'4A 41 41 N ON .S 44 - N N 4 S N. 11

41 t W4 0.0 4 6 0 UU. 44 44 44 a4 44 44 W41 4 W41 41

U4 ".4 W0 go 01 4 0 a a

*A x 4 N- NA. r4A4 in4N OftN14N .

41

14

1

Other floodway studies include Porrier and Mulino (1975) whichlooked at the effects of the 1973 opening on the epifaunal foulingcommunity, and Porrier and Mulino (1977) which looked at the effects ofthe 1975 opening on the same community. The former study of the 1973opening concluded that although five out of 28 species were eliminatedfrom the community, the 21 remaining species appeared to be present inthe same relative abundance before and after the spillway opening. Inthe study of the 1975 opening, a similar conclusion was ceached; thatthere was a lack of a significant change in the epifaunal communityafter the opening. However, the same five species which were eliminatedin the 1973 opening were absent before the 1975 opening. In a study ofthe effects of the 1973 opening on plankton populations at two stationsin eastern Lake Pontchartrain, Hawes and Perry (1978) concluded that theeffects on the plankton had been dramatic but short-termed. The endemicassemblages had been replaced temporarily by a freshwater assemblage.However, the original assemblage returned after the closing of thefloodway.

In general, it appears that the brackish-oligohaline system of LakePontchartrain can tolerate periodic lowering of salinities to 0.0 pptcaused by an opening of the Bonnet Carre Floodway. However, the impactsof the floodway opening can not be evaluated in terms of loweredsalinities and freshwater impacts alone. Waterborne constituents needto be considered, particularly in a historic sense. Walsh et al. (1981)point out that, on a global scale, anthropogenic nitrogen loading of theworld's rivers is increasing. They state that the nitrate content ofthe Mississippi has at least doubled to 9.3 mg/L during spring floodover the past 25 years. Gunter (1953) reports a nitrate value of 0.4mg/L for water coming through the Bonnet Carre in 1950. Samples takenby the U.S. Corps of Engineers and analyzed by the U.S. GeologicalSurvey during the 32 days of the 1979 opening have a mean value of 1.76

U mg/i nitrate nitrogen or about 7.66 mg/i nitrate, a twentyfold increase.Gunter's value appears too low. It was probably either an error inanalysis, or was taken on a particularly low day. In either event, itappears that certain waterborne constituents are increasing in con-centration in Mississippi River water.

Organic carbon is another constituent of concern. Table 1 givesthe total organic carbon measured daily by the U.S. Geological Surveyduring the 1979 opening. To calculate total loading the concentrationis multiplied by the flow to give grams C per second (mean 2.76 x 1066)which equals 9.21 x 1010 g C brought into Lake Pontchartrain. If wecompare this to the total carbon fixed in the lake by taking a mean ofthe midlake stations, 157.5 g Cm2yr (Dow and Turner 1980) times thetotal area of the lake 1.66 x 109m2 (Swenson 1980b), we find that thetotal amount of organic carbon brought into the lake by the 1979 openingis 35.3Z of the total carbon fixed in the lake in a year. The BonnetCarre Floodway was only open for 32 days. When we compare the carbonpulse to the amount fixed in the lake during an average 32 day period,we find that more than four times as much carbon entered the lake

14

k • • _ -I

as was fixed in a 32 day period. This represents a significant pulse,even if some of this carbon passed through without being deposited inthe lake. Combined, the allocthonous carbon, and whatever amount ofautochthonous production that may have been stimulated by the nutrientinflux brought into the lake by the 1979 Bonnet Carre opening, amountsto a significant portion of the lake annual carbon budget.

Pollution

Lake Pontchartrals receives surface water drainage from two largedrainage basins; thp Lake Pontchartrain watershed, which includes thedrainage basin of the Amite-Comite, Tangipahoa, Tchefuncte, and numerouslocal rivers, as well as the New Orleans area (Turner and Bond 1980b),and the Pearl River Drainage Basin. Both of these drainage basins carryurban, industrial, and agricultural runoff, which finds its way into thelake. Three regions of the lake are primarily impacted by toxicmaterials (U.S. Army Engineer District, New Orleans 1980). However,because of the shallowness of the lake and the nature of the wind-drivencirculation patterns, Lake Pontchartrain is considered a well-mixedsystem, thus an impact in any region of the lake will impact the wholesystem.

The following four paragraphs are excerpted from the New Orleans-Baton Rouge Metropolitan Area Water Resources Study (U.S. Army EngineerDistrict, New Orleans 1980).

North and West Shores of Lake Pontchartrain

Maximum recorded concentrations of mercury exceeded the EPAcriterion at stations located at the mouth of the Tangipahoa andTchefuncte Rivers, Pass Manchac, North Shore, and Greater NewOrleans Expressway. The maximum concentration of mercury was 1.3ug/l, recorded at the month of the Tangipahoa River. The EPAcriterion for mercury is 0.1 ug/t.

PCB, chlordane, parathion, dieldrin, and aldrin violated theEPA criteria of 0.001 ug/l, 0.004 ug/t, 0.04 ug/t, 0.003 ug/tand 0.003 ug/L respectively, for these parameters. PCB frequentlyexceeded the criterion at stations located at the mouth of theTangipahoa River and Pass Manchac, and it occasionally exceeded thecriterion at the mouth of the Tchefuncte River. Maximum recordedconcentrations occurred at Pass Manchac and were 0.2 ug/l.Chlordane and parathion frequently exceeded the criteria at thestation at Pass Manchac. Maximum recorded concentrations of thesetwo parameters were 0.7 ug/t and 6 ug/L respectively. Dieldrinand aldrin concentrations exceeded the EPA criteria at the mouth ofBayou Lacombe, with maximum recorded concentrations of 0.025 ug/Land 0.025 ug/l. Fecal coliform counts frequently violated the EPAcriterion of 200 colonies/100 ml at all of the stations sampled.The maximum recorded value was 2,400 colonies/100 ml.

15

-iI~I7. -

South Shore of Lake Pontchartrain

All samples analyzed for aldrin and dieldrin exceeded the EPAcriteria at all stations. The maximum and minimum concentrationsrecorded for both parameters were 0.025 Ug/i. The EPA criteria forboth parameters are 0.003 ug/i.

The Corps of Engineers no-discharge criteria are exceeded byDO, pH, fecal coliform, aldrin, and dieldrin. The maximum andminimum recorded values were given in previous paragraphs. Theno-discharge criteria for DO, pH, fecal coliform are 5.0 ug/i6.0-8.5, and 200 colonies/100 ml, respectively. The no-dischargecriteria for aldrin and dieldrin are zero.

It should be noted that several of the pesticides and PCB's ex-ceeded the EPA criteria by several orders of magnitude at maximumrecorded concentrations. In addition, high levels of PCB's (mean of0.32 ± 0.04 ug/i) were found in Lake Pontchartrain sediments at stationsin the center of the lake (Sikora et al. 1981). Unfortunately, nosediment criteria exist at this time. However, to put it in perspectiveon a concentration basis, the EPA water criterion is 0.001 ug/i or 0.001ppb while the sediment concentrations are 320.0 ppb, five orders ofmagnitude higher.

Pass Manchac, which is estimated to supply over 60% of the fresh-water inflow into Lake Pontchartrain (Swenson 1980b), is the source ofsignificant input of the herbicides 2,4-D and 2,4,5-T. From waterresources data (U.S. Geological Survey 1980), Pass Manchac was sampledon 44 days between October 1978 and September 1979 with most of thesample in April, May, and June 1979. Out of 44 sampling days, 2,4-D waspresent 42 days.in concentrations which ranged from 0.01 and 0.12 ug/iand 2,4,5-T was present 37 days in concentrations from 0.01 to 0.03ug/i. This represents an almost continuous input of these two biocidesfor which the no-discharge criteria is zero. Unfortunately, no flowdata is available for Pass Manchac. A conservative estimate of flowsinto Lake Maurepas by Swenson (1980b) for 1978 is a mean flow of 203.6mS/s. Assuming at least that the same amount of water flows out throughPass Manchac, and multiplying the flow by the mean concentration of 0.05pg/L 2,4-D an estimated 315 kg or 693 lb enters the lake in a year fromthis source alone.

The Bonnet Carre Floodway also brings in a load of toxic substancesfrom the Mississippi River. During the 1979 opening significant concen-trations of lead (380 ug/i max), zinc (60 Ug/i max) and copper (32 ug/imax) were measured (U.S. Geological Survey 1980). Also detected werecalcium (1 pg/i), mercury (0.1-0.2 g/i), selenium and vanadium. Thebiocides present in detectable amounts were dieldrin, diazinon, DDT,2,4-D and 2,4,5-T. Although endrin, heptachlor, epoxide, and chlordaneare reported to be present in the Mississippi River (Brodtmann 1976),they were below detectable limits during the 1979 flooding. PCB's werealso not detected in the 1979 floodwater samples but are reported from

16

the Mississippi River (Giam et al. 1977, Giam et al. 1978). Anunfiltered water sample containing a considerable amount of sediment wastaken during the present study off the spillway structure and analyzedby the EPA laboratory. It contained 210 ppb arochlor 1260, probablyadsorbed to sediment particles, and thus not detectable in filteredwater samples.

Hydraulic Shell Dredging in Lake Pontchartrain

Hydraulic dredging for Rangia shell in Lake Pontchartrain beganaround 1933 and has steadily intensified to the present time. Thistrend can readily be seen from clam shell production estimates by theLouisiana Wild Life and Fisheries Commission, (1968), which range from230,000 ms statewide in the mid-thirties to 3,820,000 m, mainlyfrom Lakes Pontchartrain and Maurepas, in 1968. The shellsof the brackish water clam Rangia cuneata are deposited in shallowlayers, 0.5 to 1 m deep, blanketing the entire bottom of an estuary.The strategy for harvesting this resource is for the dredges to movecontinuously at 5 to 8 km/hr, constantly dredging a shallow trench 1 to2 m wide. Nearly 0.25 km2/day will pass through the processing plant ofa single dredge, with a larger area being affected by spoil. In thisrespect clam shell dredging differs from oyster shell dredging, becauseoyster shells are concentrated in reefs buried 5 to 15 m in depth.Harvesting these shells results in a potholed effect, with severedisturbance in concentrated areas, yet with large areas leftundisturbed.

The magnitude of clam shell dredging in Lake Pontchartrain isdiscussed by Sikora et al. (1981). There are between five and sevendredges operating in the lake continuously. Allowing for breakdowns andrepairs, it is estimated that in a year's time between 3.17 to 5.08 x10 m2 will be dredged.

GSRI (1974) describes a series of zoning restrictions imposed bythe Louisiana Wild Life and Fisheries Commission, which include, 1) aone-mile band around the perimeter of the lake, 2) a one-mile strip oneach side of the Lake Pontchartrain Causeway, 3) a one-half-mile stripcrossing the lake diagonally to protect high pressure gas pipelines,and 4) a four-mile wide area encompassing the eastern end of the lakefrom Goose Point to New Orleans. Dredging operations are thusprohibited in 56Z of the lake. The total area of the lake is estimatedby Swenson (1980b) to be 1.66 x 109m2 of which 7.17 x 108m2 is open todredging. Dividing this figure by each of the two estimates of the areacovered by dredging annually, we find that an area equal to that whichis open to dredging will be covered in from 1.4 years (with 7 dredgesworking 360 days/yr) to 2.3 years (with 5 dredges working 270 days/yr).At this rate, one can see that not only has much of the lake beendredged, but because the entire 44Z of lake bottom open to dredging isnot covered in any one year, some of the lake bottom is dredged andredredged, possibly several times each year.

17

Sikora et al. (1981) conclude that shell dredging produces fluidmud and low bulk density sediments which persist for long periods inthe lake. Over the two year study period, benthic biomass production wasreeuced by an average of 32% in a dredged area as compared to a nondredgedcontrol area.

Sediments and Mineralogy

There are three major species of clay minerals transported by riversto the marine environment; kaolinite, illite, and montmorillonite(Parham 1966). The surface charges or cation-exchange capacities ofthese three minerals are quite different, with montmorillonite havingroughly three times the capacity of illite and twelve times that ofkaolinite (Strahler and Strahler 1971). The amount of salt in thewater necessary to cause flocculation of each clay mineral is thereforedifferent. Kaoli.te, with the lowest capacity, will flocculate in thelower salinity waters. Hontmorillonite, on the other hand, will remainsuspended in the lower salinity waters and be sedimented in areaswith water of higher salinity. Generally, in a gradient from fresh tomarine water, the sequence of sedimentation is kaolinite, illite, andmontmorillonite. The distribution of clay minerals in Lake Pontchar-train was studied by Brooks and Ferrell (1970) based on 1967 fieldsampling. They found all three species of clay minerals in LakePontchartrain, with kaolinite content decreasing from west to east,illite increasing from north to south-southeast and montmorilloniteincreasing from west to east-southeast. Montmorillonite is the mostimportant in the central lake region, amounting to 60% of the clay;kaolinite, about 30%; and illite, 20% or less. The authors concludethat, on a large scale, salinity gradients and correspondingmineralogical gradients do exist. This study illustrates the factthat Lake Pontchartrain does have considerable periods of time witha stable salinity gradient, or it would not have retained anidentifiable gradient of clays in the sediment.

The total organic carbon content of the sediments has beenmeasured in the past (Steinmayer 1939), during the lake-wide initialsurvey (Bahr et al. 1980), and at the sampling stations during thepresent study. On comparing the present levels to those measured in1939, a striking fact emerges: the organic content of the sedimentsin Lake Pontchartrain has drastically decreased. Steinmayer (1939)states that the organic content of Lake Pontchartrain sediments is ofvegetable origin; what he terms as comminuted vegetable matter andhumus matter. He gives an average value for organic content of claysediments as 6.72% by dry weight. He also gives an "isorganic chart"on which are plotted isopleths of organic matter in the lake, andstates that "organic content is high in the deeps and gentle slopesand low on the beaches and ridges." Much of the center of the lakeIs shown to have an organic content of 6% and a considerable area inthe very center of the lake 8Z organic content. During the lake-wideinitial survey in 1978-1979 (Bahr et al. 1980) 86 stations wereanalyzed for organic carbon. Only 7 stations had organic contentabove 1.8% (Figure 4a). Only two of these were over 5%; one in a peat

18

Organic Carbon inSedimnt

- 03 00 @.o .

3 o.• .cO.3

a Q 0 a

0 o C 0 C3

0 0 0 0 0

a0 o 0 0 0 ] 03

Figure 4a. Distribution of organic carbon in sediments of Lake PontchartrainLa. determined in the initial survey, from Bahr ei al. (1980).

otqauia Cwa at 8aSob Sai

1.40 @(.20

Figure Ab. Sampling stations of present study showing organic carbon insediments as percent dry weight. Numbers in parenthesesi/ndicae depth of sample i.n centimeters.

____121_ 1 4

(9(3.3) M )o

• .0

bed near the shore, and one near a sewage outfall in Jefferson Parish.The mean value for the 86 stations in the lake was 1.06% t 0.49%. Inthe present study the organic carbon content of the sediments wasmeasured by depth at each station. Values for the top layer of sediment(1.7 cm to 5.8 cm except Station 3, in which the top 2 cm were lost)range from a high of 2.05% at Station 8 to 0.69% at Station 13 (Figure4b, Table 4) with a mean value of 1.33 t 0.11%. It is evident that asignificant drop in the organic content of the sediments has takenplace. The methodology used in the two studies is not entirelydissimilar. Steinmayer used loss on ignition of dried sediment (driedat 1100 C). In the intial survey, and in the present study, carbon wasdetermined by burning dried sediment in an induction furnace andmeasuring carbon with a gasiometric carbon analyzer (Appendix E), thusthe results should be comparable.

Grain size analysis (methods appear in Appendix E) from Bahr et al.(1980) was used in the present study and is illustrated in Figure 5.The major portion of the open lake region is composed of silty-claysediments. Ten of the 13 stations had this sediment type, except thefollowing: Station 4, a fine clayey sand; Station 9, a doarse clayeysand; Station 13, a fine sandy silt with some clay. Bahr et al. (1980)show that the distribution of benthic organisms in Lake Pontchartrainwas not related to sediment grain size distrib..tion. Therefore, nofurther effort to refine the grain size analyses at the samplingstations was made in the present study.

Bulk density is the actual weight per unit volume of intact sed-iment, with differences between sediments of the same grain size dis-tribution, and similar organic content, being the result of sedimentconsolidation. Consolidation of sediments, corresponding most closely tocompaction in geological terminology, is defined as "every processinvolving a decrease of the water content of a saturated sedimentwithout replacement of the water by air" (Terzaghi 1943) or "the gradualprocess which involves, simultaneously, a slow escape of water and agradual compression, and which also involves a gradual pressure adjust-ment" (Taylor 1948). Bulk density can be used as a measure of theconsolidation state provided that the grain size distribution is takeninto account. As sediment grain size increases toward larger grainsizes, i.e., toward sand, the bulk density also increases. This increasein bulk desnity of coarser sediments is not due to consolidation.Richards and Parks (1976) report bulk density of 1.42 g/cm' for siltyclay in North Pacific Continental Shelf and slope sediments.

The methods used to collect samples and determine bulk density aregiven In Appendix E. Bulk densities were determined in 5 cm depthintervals; however, because of the method employed, the uppermost samplevaried in depth. Thus it is difficult to compare the bulk densities ofthe upper-mot samples in an absolute stratigraphic sense. Even on arelative basis, comparison of all the stations is difficult because ofdifferences in sediment type and grain size. Stations 4, 9, 10, 11, and13 (Table 5) have large components of silt and sand, and which give higherbulk densities. Stations 1, 2, 3, 5, 7, 8, and 12 all have basicallysilty-clay sediments. Station 8 has anomalously low bulk density in the

20

co. . .l N .7 .

00.4 c4 C;

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U 44-

(0

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I.

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-Ir

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* - a - 0 - N 0N N N N N N - .*

- -N a- a

19 N a N * .4-1 .9 .9 - ** '9

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upper-most sample, perhaps because it also has the highest organiccarbon content (Table 1). The other stations in this group all havecomparable bulk densities ranging from 1.28-1.33. The next tier ofsamples by depth (5 cm deeper) shows some differences. Stations 1, 3,and 12 show the same bulk density at this depth while Stations 2, 5, 7,and to some degree, 8 show higher bulk densities at this depth. AlsoStations 2 and 7 show the same or higher bulk densities at the nextdeeper depth, while Stations 1, 5, 8, and 12 all decrease in bulkdensity at the next deeper depth. Perhaps one factor that may haveinfluenced the uniformity of the bulk density in the uppermost sampleswas the opening of the Bonnet Carre Floodway in April 1979, since samplesfor the measurement of bulk densities were all taken in July 1979.Another possible factor is the redistribution of sediments altered byshell dredging activity by wind-induced waves and water circulation(Sikora et al. 1981).

Salinity Regime of Lake Pontchartrain

By definition, salinity is an important physic.l parameter ofestuaries. Unless sea water is measurably diluted with fresh waterthere is no estuary (Pritchard 1967). Of importance to the distributionof the biota is not only the degree of sea water dilution but how thedilution process varies spatially and temporally. Estuaries aregenerally characterized by changing salinities which are governed byfresh water inputs and mixing processes. In estuaries with large freshwater inputs and large tidal heights, significant changes in salinitiescan occur in a matter of hours, particularly with a semidiurnal tide.Estuaries with large tidal excursions may have large salinity variationsover large areas. Usually estuaries are thought of as stressed environ-ments not only because of the physical variability but because of the

* temporal variability associated with wide ranges of physical parameters.

The more stressful an environment, the fewer the number of specieswhich can adapt and prosper in it. Slobodkin and Sanders (1969) listthree types of low diversity (i.e., species poor) environments: (1)ftnewl environments, (2) "severe" environments, and (3) "unpredictable"environments. The category, "unpredictable" is defined as thoseenvironments in which the variances of environmental properties aroundtheir mean values are relatively high and unpredictable both spatiallyand temporally. The authors go on to predict that given two regions ofidentical geometric and geologic properties, the less severe and morepredictable will probably have greater species diversity.

An examination of the mean monthly salinities taken daily duringthe 12 year period from 1946 to 1958 at Little Woods, Louisiana on thesoutheastern shore of Lake Pontchartrain (U.S. Army Corps of Engineers1962) reveals that Lake Pontchartrain does have a predictable salinitypattern (Figure 6). Salinities are low in late spring and early summerand high In the fall period in October-November. Note that thevariability in any one month does not exceed 3 ppt and the range overthe 12 year period is about 5.5 ppt.

24

00 4)u.P4

44"

w~ 0

= 4.114 N j41 0

o 4) 04.(10

0;r4 00

0 A0 r4w~ Ua

r-4 M-

434

4~

r-4 0

4.1 4.l. 4.4

4.04

04

004.1

LL 0

'-a 43

Idd~~ N S.~l

25l..

The other aspect of predictability in an estuary is the stabilityof the salinity gradient. Data gathered during a hydrographic study ofLake Pontchartrain by Swenson (1980a) indicate that the lake responds asa unit to salinity fluctuation (Figures 7 and 8). The lake was dividedinto four regions: A. northwestern riverine region, B. southwesternregion, C. eastern region, and D. a far eastern, east bay region.These regions were determined by their salinity response, and they appearto follow each other closely in their salinity fluctuations. It is alsoevident from Figure 8 that the salinity gradient runs from lowersalinities in the west to higher salinities in the east. This was foundby previous workers Brooks and Ferrel (1970), Tarver and Savoie (1976),and others. An examination of salinities recorded during the course ofthe present study reveals that the range in salinities from west to eastis usually less than 3 ppt with the largest range recorded in August1980 of 9.5 ppt. These data also reveal that, although there is littleor no salinity stratification most of the time, stratification doetoccur, particularly at Station 1, and occasionally in the eastern sectionof Stations 9, 10, and 13. Of interest also is the fact that virtuallythe whole lake had fresh water conditions in May 1979 after the openingof the Bonnet Carre Floodway. Salinities had not fully recovered twomonths after the closing of the floodway but had returned to normalconditions by August 1979. Although the opening of theBonnet Carre Flopdway may at first seem drastic, the lowering ofsalinities in an oligohaline system, even to 0 ppt, is not in realitythat great a variation from the mean.

On hree occasions the salinity was reversed, albeit slightly,so that lower salinities occurred in the eastern region in March, May,and July 1979. That this should occur at all indicates that the PearlRiver may influence salinities in Lake Pontchartrain to a greater extentthan previously thought. In order to actually lower salinities in theeast, Pearl River water would have to actually enter the lake in abundance.However, the Pearl River could also prevent salinities from becominghigh in the eastern region by pushing out and/or diluting Gulf waterbefore it enters Pontchartrain. This appears to have occurred on atleast two occasionc; in September 1972 (Tarver and Savoie 1976) (highsalinities used by Bahr et al. (1980]) and during the present study inAugust 1980. Both times the Pearl River had flows below normal incombination with low river flows in the Pontchartrain Basin. A closerexamination needs to be made of river flow data since it appears thatboth drainage basins must have low flow times in unison in order tocause high salinities in Pontchartrain.

Thus it appears that Lake Pontchartrain has quite stable andpredictable salinity patterns. Other estuaries on the Gulf coastexhibit extreme fluctuations. Parker (1955) reports that several baysin Texas that normally experience fluctuations of 20 ppt experiencedsalinity fluctuations of 40 ppt in the early 1950's. Apalachicola Bayin northern Florida, another bar-built estuary, experiences yearlysalinity fluctuations between 0 and 25 ppt (Livingston et al. 1978).Lake Pontchartrain then would be predicted to have normal to higher thannormal diversities for an oligohaline system.

26

Figure 7. Four regions of Lake Pontchartrain determined by t--linitycharacteristics: A northwestern, riverine region; B southwesternregion; C eastern region; D far eastern, east bay region.

10

1 03- C

AU2-2

Q

0'M A MJJA8ONDJF A

1978 1979

Figure 8. Lake Pontchartrain responds as a whole to seasonal salinity

fluctuations despite salinity differences between the four

IIregions. (data from Swenson 1980a)

27

- ~ - - . . - - - - -~ ~ - - - - - - - - - - - - - - - - . - - - - - - - - - -

METHODS

Location of Sampling.Stations

In designing a sampling regime to yield the greatest amount ofinformation from a fixed number of samples, the allocation of samples ina stratified random sampling design will produce large gains inprecision if stratification is proportional to some variable (Cochran1977, Green 1979) or to some identifiable deviation in the habitats orbiotopes (Elliott 1971). Two environmental variables which were con-sidered as important in the optimal allocation of samples for thecharacterization of benthic community structure and function weresalinity and sediment grain size.

The selection process for the locations of the monthly stationsbegan with an extensive initial survey of the lake from February to May1978. The lake was divided into a grid of 86 quadrats, each in 23.2 km2in area. The results of this initial survey are given in Bahr et al.(1980). The benthic characterization study was conceived as an openlake study rather than a littoral investigation because of relative sizeof the open lake compared to its shoreline. The lake was furtherdivided into five geographical regions: an eastern, western, northern,southern, and central regions. The intent was to place stations in eachof these regions in combination with other factors such as riverineinput, salinity regime, sediment grain size, and urban influence.Maximum salinities from Tarver and Savoie (1976) were used to divide thelake into a low salinity portion, 60% of the lake and a higher salinityportion, 40% of the lake. Since salinity was considered one of the mostimportant physical variables governing animal distribution, six stationswere allocated to the low salinity portion and four stations to the high

* salinity portion. A similar type of apportionment was followed forgrain size and total organic carbon. Figure 2 shows the locations ofthe monthly sampling stations (1-10), and the seasonal stations (11-13).

The exact placement of the stations was done with the followingrationale: two stations (1 and 2) in the area of urban influencenear New Orelans, a transect of stations in a northwesterly direction(4, 5, and 6), a midlake station (3), a riverine station (7), a stationfurther out in the lake which might be impacted by the rivers and PassManchac (8), a station in sandy area in the north region (9), and astation which would be in the eastern region subject to water movementfrom the east (10). Later, after the logistics of sampling these 10stations and analyzing the samples in the laboratory had been worked out,the 3 seasonal stations were added in areas where additional informationwould be useful. Dates of benthic sampling cruises are given inAppendix D.

28

Determination of Precision of Sampling Regime

Before the initiation of the regular sampling cruises a preliminarysampling cruise was made on August 1, 1978 in order to determine thenumber of replicate samples required to yield statistically validpopulation estimates. The macroinfauna sampling was accomplished with10 replicate cores from a modified J&O boxcorer (Jonassen and Olausson1966). The meiofauna was sampled from three of the boxcores withacrylic core tubes (4.9 cm2) for a total of 12 replicate cores.Additionally, 8 meiofauna samples were carefully taken by divers todetermine the extent to which the sampling gear being used might biasthe population estimates. The smaller meiofauna are subject to re-suspension by the bow wave from certain types of sampling gear and makegood indicators of bias. Meiofauna samples from the boxcores had a meanof 48,000 ± 8900/m. Meiofauna samples from the diver-taken cores had amean of 47,100 ± 6200/m2 , which was not significantly different(p > 0.05) from the mean of the boxcore samples. Analysis of varianceestablished that the variability in the meiofauna was no greater betweenboxcores than within the boxcores. From these preliminary analyses weconcluded that the box corer was sampling infauna without bias, and thatthe boxcore samples taken at one station were sampling one habitat,since differences between boxcores were no greater than differenceswithin box cores.

After the macrofauna from the 10 boxcore samples had been countedand identified the mean was found to be 6477 ± 844/m2 . To establish thenumber of samples needed for an adequate sampling regime, this infor-mation and a formula from Elliott (1971) was used,

n 8 2

E2 x

2

where n represents the number of samples required; a , the variance; E,the error that can be tolerated in the estimate of the population mean;and x, the arithmetic mean of the samples. At least 17.1 samples wouldbe required to achieve a 10% error (or 90% confidence limits to themean). Another method of establishing an adequate number of replicateswas also used (Green 1979), /n - t (s/x), which gave an estimatedreplicate number of 27. Finally, n&mBs were assigned to these 10preliminary samples, and random numbers were gener-a-d to form unbiasedgroups of samples. The mean and variance of groups of 3 randomly chosenboxcore samples were tested against the mean and variance of the groupof 10 boxcores and were found not to be significantly different

(p > 0.05). In other words, 3 box cores would yield as much informationabout the density of the population, with as much precision, as 10boxcores would.

29

The meiofauna cores were tested in a similar fashion, and itwas found that 4 samples would be necessary to yield as muchinformation as 10 cores. The variability of meiofauna is somewhathigher than that usually found in macrofauna, and therefore requiressomewhat more sampling effort to attain the same degree of precisionof estimates of mean densities.

Sampling Methods

To obtain quantitative information on the density and distributionof the benthic community of Lake Pontchartrain, a series of 3 replicateundisturbed bottom sediment samples were taken at the 13 stations with amodified J&O box corer (Jonassen and Olausson 1966), 0.09 m2 formacrofauna, and subsampled with acrylic core tubes (4.9 cm2) for the4 meiofauna samples. Under the following conditions the contents of thebox corer were discarded and another sample was taken.

1) If rough water caused the boat to move sufficiently to causethe corer to enter the sediments at an angle so that thesurface was disturbed

2) If the corer was found on retrieval to have entered thesediments too deeply so that organisms could have been lostthrough the top

3) If, on retrieval, the corer appeared to be leakingbecause a piece of shell was preventing full closure of thejaws of the device.

The contents of the box corer were sieved first through a 1.27 cmmesh; then through 0.32 cm mesh to remove large shells and organisms;then through a 0.5 mm screen to retain the remainder of the macrofauna(animals > 500 Us). All fractions were preserved onboard ship in abuffered formalin solution with Rose Bengal stain. Meiofauna samples(animals between 500 Ur and 44 Us) were preserved unsieved. All sampleswere returned to the laboratory for further sieving, enumeration,identification, and archiving. Organisms were identified to the lowestpractical taxon, with certain forms being sent to specialists forconfirmation or for further identification.

Meiofauna samples were sieved in the laboratory through a series ofsieves. Animals retained on the 500 mm sieve were returned to the macro-fauna sample from the box core from which the meiofauna core was removed.Animals retained on the 63 mm and 44 mm sieves were counted as meiofauna.As animals were identified, they were placed in appropriately labelledvials. The contents of all vials were recounted, and the residue fromwhich the animals had been removed was searched for animals that mighthave been overlooked. Sample vials have been archived for further useby taxonomic specialists if requested.

All macrofauna were first sorted to major taxa and later identifiedto species. Some specimens were sent to consulting taxonomic

30

specialists for confirmation of identification. Some undescribedspecies are listed as such. Since macrofauna are rechecked as speciesidentifications are made, the additional recounting to maintainlaboratory efficiency in meiofauna sorting is not necessary for macro-fauna sorting.

Stations 1 through 10 were sampled monthly from August 1978 throughAugust 1979, then quarterly through August 1980.

Stations 11, 12, and 13 were sampled quarterly from November 1978through August 1980.

Statistical Methods

Statistical analyses were performed using computer programs accordingto Barr et al. (1979). programs. Certain special programs for classifica-tion and ordination of benthic data were provided by Dr. Stephen A.Bloom of the University of Florida (Bloom et al. 1977).

With 582 collections of macrofauna, the use of multivartatetechniques of analysis became mandatory. Numerical analyses,particularly clustering methods, are a conservative method of treatingvery large data sets. No subjective elimination or combining of data isnecessary, and interpretation is straightforward, eliminating the needfor intuitive approaches (Clifford and Stephenson 1975, Green 1979).Clustering analysis is a grouping procedure to separate ecologicalentities - in this case, monthly collections at each station. Itcan show semilarities or differences, depending on the form ofcoefficient used. An objective decision on the degree to which theopening of the Bonnet Carre caused changes in the benthic comsmunitystructure of each station was based on cluster analysis of the macro-fauna for each station over time.

The Canberra metric coefficient in its dissimilarity form is

Djk - - I - Ikm I (Xj + xlkjk i

where X is the value of the i-th species in the J-th collection andm is theJnumber of attributes (species). The Canberra metriccoefficient is often used in benthic studies (Clifford and Stephenson1975) because it suppresses the importance of the wide ranges of numbers

I of individuals of certain species. With particularly "spikey" datasets, the use of a simultaneous double standardization of the data isrecotomended to alleviate scale problems (Boesch 1973). This yields astandardized element:

31

.. ...... ... . . .... .. .- I I ILI I

Y i x £ 1/L xij L

x ij)I/

for entry into the matrix. No transformations of the data wereperformed.

The flexible sorting strategy with 8 - -0.25 that was used isconsidered an intensely clustering, moderately space dilating strategy.It is considered better to use an intensely clustering strategy when thedata set is very large (Boesch 1977).

Diversity of all collections was measured using the Shannon-Wienerindex, H' - Epilog p , where p is the proportion of the populationthat is of the i-tfi Ipecies. Evenness was measured using J' -H'/H1 where H'max - log eS, or J' - H'/log eS, and S is the number ofspecies.

Niche breadth (B ) for a given species (i) over all environments(h) was measured usini a formula derived from a function, A - Epi2proposed by Simpson (1949) as a measure of concentration. Levins (1968)proposed as a measure of niche breadth B - 1/X. The formula

h 2

SZPih

was also used to obtain niche breadth of the total community (9) bysumming all of the species abundances in each environment and thencalculating community percentages in the formula given above (Lane etal. 1975). Niche breadth of an individual species Bi was comparedwith the niche breadth of the total community i in order to establishthe degree to which that species utilized all resources available (thegeneralist) or utilized only a part of the total resource (thespecialist).

To obtain biomass values for all taxa, intact individuals wereplaced in preweighed miniature aluminum foil weighing pans and dried at100 ° C for 24 hr (Cumnins and Wuycheck 1971). All weighings were done ona Cahn Automatic Electrobalance, model 21. Empty pans were also weighed,dried, and muffled, so that corrections for oxidation of the aluminumcould be made in the calculations of the ash free dry weights (AFDW).

At least 10 replicates for each taxa were weighed. Some speciesof macrofauna that had been sorted into size classes, such as Rangiacuneata, had over 40 replicates. The smaller organisms, such asnematodes, required as many as 50 organisms per replicate. Somespecies that exhibited seasonal changes in biomass required additionalreplicates for seasonal values. These data were then incorporated intothe statistical analysis programs so that estimates of ash-free dryweights could be made for all collections (Appendix A).

32

• i i l iSow,%"

RESULTS

The results for various measures of comunity structure will bedescribed separately for each of the stations. In the section for eachstation there will be a figure showing macrofauna abundance. Theaverage for the lake is shown as a dashed line with small filledcircles. Values for the station being described are represented bylarger filled triangles and a solid line.

In each section there will be a similar figure showing nematodeabundance. Total meiofauna abundance dynamics are often masked byerratic settlements of temporary meiofauna. The presence or absence ofsuch taxa as benthic ostracods and rotifers, which are sometimes in thesediments, and sometimes just above the sediments, can also contributeto the high variability of meiofhuna values. Nematode abundance, onthe other hand, varies less and probably represents a more accuratemeasure of the patterns of true meiofaunal abundance than the morevariable total meiofauna.

Tables in Appendix A give average abundance and biomass of mio-fauna and macrofauna for each station and each sampling period. Thesetables also include the diversity, number of species, and evenness ofthe macrofauna. Table 6 gives the station locations, depths, andsediment types.

Station 1

Iso" STATION 1. MACROFAUNA ABUNDANCE hr-'&

MEAN FO",R ALL STATIONS 0---*

70 s

go-.

50-

30e -- ------ %% ---A. .O .o.... -e . . .. . .. .4 . . . .

W . . 6 A I 1 , L

A 0 N O J P M A M J J A 0 P M A1075IOT IWO

Figure 9. Kacrofauna Abundance, Station 1.

33

i"PoaiBI-

Table 6. Location, depth, and sediment type of sampling stationsin Lake Pontchartrain, Louisiana

Station Depthnumber Location in Sediments

meters

I 30' 03' 06" N, 90- 04' 55" W 4.6 Soft silty-clay

2 30' 03' 04" N, 90' 07' 25" W 4.6 Firm silty-clay

3 30" 10' 19" N, 90' 10' 28" W 4.6 Soft silty-clay

4 30- 05' 35" N, 90' 12' 57" W 4.5 Hard silty-sand

5 30' 10' 54" N, 90' 19' 42" W 3.5 Soft silty-clay

6 30' 12' 36" N, 90° 21' 17" W 3.0 Soft silty-clay

7 30' 20' 51" N, 90* 13' 16" W 3.0 Soft silty-clay


9 30' 13' 27" N, 89' 59' 12" W 3.8 Mixed; coarse sand,silty-clay


11 300 07' 45" N, 90° 16' 20" W 4.1 Hard silty-sand

12 300 07' 44" N, 900 02' 09" W 5.0 Soft silty-clay

13 30* 09' 19" N, 89° 49' 00" W 2.7 Hard silty-sand

34

Station 1, located in the southern portion of Lake Pontchartrain(Figure 2, Table 6) was 2.8 km north of the mouth of Bayou St. John,about 2 km northwest of the IHNC, and 6 km east of the Causeway. Thisstation was under urban and industrial influence. Higher salinitywater from the IHNC at times caused salinity and oxygen stratification.Porrier (1978) has also recorded this stratification in the same area.

Macrofauna abundance is relatively low, but not significantlydifferent from the overall abundance at Station 12, which had thelowest densities of all stations. Average abundance for the first yearwas 9,759/M2; the second year it was 12,698/m2 ; and for the entirestudy period, 11,228/ 2 . No seasonal peaks were observed (Figure 9).Densities at Station 1 did not appear to be coupled with the samefactors that regulate densities in the lake as a whole. The ususalpattern of low numbers in the warmer months, due to heavier predationpressure (Levine 1980), and higher numbers in the colder months with apeak in February (due to the movement of many predators out into theGulf) was not followed. No response to the opening of the Bonnet CarreFloodway was seen.

Fewer species were found at Station 1 than the average for thelake (Table 7). A small hydrobiid gastropod, Texadina sphinctostoma,was the dominant species found, making up 80% of the numbers. Thehighest numbers of a capitellid polychaete, Mediomastus californiensis,that occur in the lake were found at Station 1. Two clams, Rangiacuneata and Mulinia pontchartrainensis, occurred in low numbers. Thesefour species plus a few chironomids comprised more than 96% of thenumbers of animals found at Station 1.

Species diversity at Station 1 was the lowest of any station inthe lake. The diversity of 0.832 1 0.59 for the first year wassignificantly lower than the lake mean of 1.117 ± 0.015. Diversity at !Station I fell even lower the second year, to 0.582 ± 0.085. Averagenumber of species the first year was 9.85 ± 0.44; the second year,8.00 ± 1.24. Evenness, calculated at 0.365 ± 0.025 the first year,dropped to 0.291 ± 0.021 during the second year. Dominance of T.

4 sphinctostoma had increased to 90% with a concomitant drop in percentof other species.

Station 1 also ranked lowest in biomass of any of the stations.The average for the two year study of 2.7677 ± 0.4823 g/m2 AFDW issignificantly lower than the mean of 8.8238 g/m2 AFDW found for thelake.

Cluster analysis of the macrofauna for Station 1 over the 17sampling periods (Figure C1, Appendix C) showed three major clusters.The first group, characterized by intermediate abundance and diversity,included December 1978 and January, February, and March 1979. Thesecond cluster, with higher abundance and lower diversities than thefirst cluster, included all sampling periods from April 1979 throughMay of 1980. The third cluster was the extremely low abundancesamples from August through November of 1978 and August of 1980.

35

-- 0

LA r4 mA -e 1- mh m %D-4 0 w O w h m O wOh~~ ~ e4 m0 ' ) ~ ~ L 0~J O -4 ")~

NO0 M) M' W) 0 Q~N O ) 0 ' +1

4 J

4J K

wUe v 0 m m m w 0 %

044 > t 0 Go a) (0 No m 0 M K- Oh M O '00el

0 -e 4 0 .4 0 4 r-4 %D4 r--4 0 - ) ('4 4

-4 U)fI L % o q L 0 r

4) u 00 0 0 000a00 0co 4)

-4 4- 0 NO 4 .- I -ON CO N

4)

* ) O 00 Oh N4 -4 U N- 0 -4 N0 M

U)-4 m v lnU ('4 - CO C '0 -4 M) IX

.0c4 K )K) K ) ) K)nN L) '

4.) (36

lw STATION i. NEMATODE ABUNDANCE A -"-AMEAN FOR ALL STATIONS e--- e

1400ao

12W000

21000

0

aoo .-A..J/ . I '4a00

200 -

0A S 0 N 0 J F M A M J J A 0 F M A

1978 1979 1980

Figure 10. Nematode Abundance, Station 1.

As with the very low abundance, biomass, and diversity of themacrofauna of Station 1, the total meiofauna abundance at Station I(Figure 10) was very near the lake mean, but very low numbers of othertaxa (Table A2, Appendix A) keep the total abundance down. Biomass wasquite low; the mean value of 0.7133 g/m2 AFDW was significantly (p <0.05) lower than the mean of 0.9424 g/m2 AFDW found for the lake. !Biomass was lower than any other station except Station 12, and was notsignificantly different (p < 0.05) from Station 12.

Copepod densities at Station 1 were below average for the lake,which probably contributes to the low biomass, since copepod ash-freedry weights were approximately three times that of the relatively smallnematodes.

In addition to the group of five dominant copepods, Halicyclopscoulli, Halicyclops fosteri, Pseudobradya sp., Scottolana canadensis,and Acartia tonsa that were found at all stations, two rarer specieswith marine affinities occurred at Station 1. These were Enhydrosomasp. and Pseudostenhelia wellsi, which did not occur at many otherstations.

Other maiofauna taxa found to be below the lake average wereostracods and rotifers. Certain taxa found in very low numbers in thelake as a whole, such as kinorhynchs, gastrotrichs, and tardigrades,did not occur at all at Station 1.

37

_________________________

Station 2

so STATION 2. MACROFAUNA ABUNDANCE A--MEAN FOR ALL STATIONS 6---O

70

0-

10 e _ -

A S O N D J F M A M J J A 0 F M A1978 1979 1980

Figure 11. Macrofauna Abundance, Station 2.

Station 2 was west of Station 1, (Figure 2) in the southernportion of Lake Pontchartrain (Table 6). Located 2.7 km north of theSouthern Yacht Club and 2.4 km east of the Causeway, this station wasunder urban influence with occasional salinity and oxygenstratification. Station 2 probably had never been dredged; largeamounts of shell were always present in the boxcores.

I| Macrofauna abundance (Figure 11) was slightly above the averagefor the lake the first year and below the average for the second yearof the study. Average abundance the first year was 29,962/m2 ; for thesecond year, 23, 338/m 2; for the entire study period, 26,650/m2 .Seasonal peaks in abundance (Figure 11) followed the general trends forthe whole lake, unlike Station 1 that was only 4 km to the east.

A response to the opening of the Bonnet Carre Floodway was seen inthe increase in macrofauna abundance in April 1979, possiblyattributable to the effect of the colder, fresher flood waters on theusual iacrofauna predators.

The number of species found both years of the study was averagefor the lake. Texadina sphinctostoma, the small gastropod, was thedominant species, accounting for 772 of the abundance. Rangia cuneata,Mulinia pontchartrainensis, Mediomastus californiensis, andchironomids, together with the gastropod, made up 99.5Z of the totalabundance.

Although macrofauna abundance was average, measures of speciesdiversity for the first year were not significantly different fromStation 1, with the lowest measured diversity. During the second year,

38

diversity at Station 2 dropped from 0.832 ± 0.059 to 0.791 ± 0.106,which was not a significant decrease. Three other stations had greaterdecreases in diversity. Average number of species decreasedsignificantly, from 11.72 ± 0.32 the first year to 9.67 ± 1.39 thesecond year. Evenness remained quite similar: 0.344 ± 0.016 the firstyear, and 0.352 ± 0.029 the second year. The significant change inspecies numbers was not reflected in a significant change in diversity,but rather was a consequence of the lowered densities during the secondyear.

Biomass of the macrofauna at Station 2 was 7.9712 ± 0.927 g/m2

AFDW, near the mean for the lake.

Cluster analysis (Figure C2, Appendix C) of Station 2 macrofaunaover time yielded three clusters. The first cluster included allsampling periods from January 1979 through August 1979, and wascharacterized by higher abundances and diversity. The next clusterinclude November 1978, December 1979, and February and May 1980, withlower diversity, and intermediate abundance. The last cluster includedAugust, September, and October 1978, and August 1980, and wascharacterized by low abundance.

1wo STATION 2. NEMATODE ABUNDANCE A --- AMEAN FOR AU. STATIONS *---.

1400

I/"1200

10 19918

0

0" - ~ ~ -. . .. .- .

A SO0 N0.J F M A M J1. A 0 F M A1975 1079 1980


Unlike macrofauna abundance at Station 2, which was higher than at

the neighboring Station 1, the abundance of nematodes was lower atStation 2 than at Station 1 the first year of the study (Figure 12).Although numbers of nematodes increased the second year (Table A2,

Appendix A) all other muiofauna taxa decreased, with a significantdecrease in copepod numbers. Copepod species at Station 2 were the

39

_______________- --~ 2

same as at Station 1. Meiofauna biomass was low, 0.7803 ± 0.0596 g/m2

AFDW, not significantly different (p < 0.05) from Station 1, andsignificantly lower than the mean of 0.9424 ± 0.0901 g/m2 AFDW for thelake as a whole.

Station 3

so STATION 3. MACROFAUNA ABUNDANCE h-&

MEAN FOR ALL STATIONS 0---@

o 50

20 /0 -o-"'

0

0................... . .. , , ,A A, n I n , ,- , , , , ,

A S 0 N D J F M A M J J A 0 F M A1978 1979 1980

Fiiure 13. Macrofauna Abundance, Station 3.

Station 3 was 4 km west of the Causeway and about 16 km north ofthe south shore near the central portion of the lake (Figure 2, Table6). Macrofauna abundance was relatively high with only two otherstations ranking higher. Average abundance of the first year was35,185/mz; the second year, it was 30,303/m2 ; and for the entire studyperiod, it was 32,744/m2 .

A strong seasonal peak (Figure 13) occurred in February 1979. Thehigh numbers consisted of a settlement of the smaller of the twogastropods, Texadina sphinctostoa. A second peak, which was probablyrelated to the opening of the Bonnet Carre Floodway, occurred in May1979. Lower abundance in the summer, and higher abundance in thewinter and early spring followed the general pattern for macrofaunaabundance in Lake Pontchartrain.

The dominant species, Texadina sphinctostoma accounted for almost70% of the total abundance the first year. This species with threeother molluscs, the clams, Rangia cuneata and Muliniapontchartrainensis, and the other gastropod, Frobythinella louisianae,made up 96% of the abundances.

During the second year, a change in dominance occurred with thenumbers of P. louisianae increasing, and T. sphinctostoma decreasing.

40

Species diversity the first year (1.043 ± 0.043) was notsignificantly different from the lake mean (1.117 ± 0.015), or from thediversity for the second year (0.990 ± 0.052). Average number ofspecies the first year was 12.13 ± 0.34; the second year, 11.58 ± 0.80.Evenness, like diversity and species numbers, remained constant, with0.42 ± 0.02 the first year and 0.41 ± 0.02 the second year.

Biomass of 11.7858 ± 1.6497 g/m2 AFDW was slightly higher than theaverage for the lake and was typical of the four midlake stations.

Cluster analysis for Station 3 (Figure C3, Appendix C) shows aweak seasonality. The first cluster incuded all the months fromNovember 1978 through June 1979 and February 1980, characterized byhigher abundance and somewhat lower diversity. The other group,August, September, and October 1978, July, August, and November 1979,and May and August 1980, included all three Augusts and wascharacterized by lower abundance and somewhat higher diversity.

law0 STATION 3, NEMATODE ABUNDANCE -

MEAN FOR ALL STATIONS e---e

lo -- . . ..1400

E1200

Z 1000 1

0oZ- S0W -

00aw %


In contrast to the relatively high macrofauna abundance, the totalmiofauna abundance at Station 3 was low, although not the lowest inthe lake. Nematode abundance (Figure 14) was significantly below theaverage for the lake from December 1978 through April 1979, during thetime of peak macrofauna abundance. eiofauna biomass (0.7895 0.0866

g/m2 AFDW) was only slightly lower than the lake mean, reflecting theslightly lower total meiofauna abundance.

Copepod species numbers were lover at Station 3 than at many ofthe stations. The five comon species Halicyclop. coulli, Halicyclops

.. .41

fosteri, Pseudobradya sp., Scottolana canadensis, and Acartia tonsa arepresent, but the two species with marine affinities, Enhydrosoma sp.and Pseudostenhelia wellsi, do not occur. Distribution of the othermeiofaunal taxa are not significantly different from the mean for thelake.

Station 4

80 STATION- 4. MACROFAUNA ASUNOANCE b&MEAN FOR AU. STATIONS ---

70

E0-o50.

40

0



Station 4 was 7.2 km west of the Causeway and 6.4 km north of thesouthern shores of Lake Pontchartrain (Figure 2, Table 6). Macrofaunaabundance was slightly above the mean for the lake the first year andbelow the mean for the lake the second year of the study. Averageabundance was 26,195/m the first year; 22,426/mn for the second year;and 24,311/m2 for the entire study period. There was, however, aresponse to the Bonnet Carre Floodway opening, seen in the increasedabundance in April 1979.

The dominant species, Texadina sphinctostoma, made up only 53Z ofthe total macrofauna. The highest numbers in the lake of the mussel,Mytilopsis leucophaeta, were found at Station 4. These two molluscsand Rangia cuneata, Mulinia pontchartrainensis, and Probythinellalouisianae together made up 96% of the total macrofauna abundance.

Measures of species diversity showed Station 4 to be among thethree highest in diversity in the lake. The diversity of 1.288 1 0.037for the first year was significantly higher than the mean for the lakeof 1.117 t 0.015. During the second year, the diversity of 1.124 ±0.056, although significantly lower than the first year, was still

42

significantly higher than the mean for the lake, 0.987 ± 0.027. Theaverage number of species the first year was 12.56 ± 0.45; the secondyear, 11.92 ± 0.43, not significantly different. Evenness, calculatedat 0.51 ± 0.01 the first year, was both significantly higher than thelake mean and that of the second year, 0.46 ± 0.02.

Station 4 was also significantly high in biomass, with 12.8457 ±1.7788 g/m2 AFDW. Despite an average total abundance of macrofauna,the high numbers of Mytilopsis leucophaeta, which have a higher AFDWthan most species present, increased the biomass significantly.

Cluster analysis of the macrofauna for Station 4 over the 17sampling periods (Figure C4) showed three major clusters. The firstincluded the samples from April 1979 and May 1979, which are the twomonths most affected by the opening of the Bonnet Carre Floodway. Thenext cluster included the sampling periods before the Floodway wasopened, and third cluster those after. Stations 4, 5, and 11 are thoseclosest to the Bonnet Carre Floodway, and all showed a distinctdifference in the pattern of clustering from the other stations.

law0 STATION 4, NEMATODE ABUNDANCE A"--MEAN FOR ALL STATIONS 0---e

1600

E12000

am0 400 -

0

0O -. 6_ & A I . I. . . . .

A S 0 N D J F M A M j J A 0 F M A1978 1979 1980


Abundance of total meiofauna at Station 4 was somewhat low.Nematode abundance was low, except for a peak in response to the BonnetCarre Floodway opening in April 1979. Meiofauna biomass was low,0.8071 ± 0.750 g/m2 AFDW, but not significantly different from the meanfor the lake, 0.9424 ± 0.0901 n2 AFDW. The same copepod speciesoccurred at Station 4 as described for Station 1.

43

Station 5

80 STATION 5. MACROFAUNA ABUNOANCE h-mMEAN FOR ALL STATIONS --- e

To-

0

ao

05

0

A SO 0 t)j M A M J1.J A 0 F M A1978 1979 1980


Station 5 was in the western part of the lake, 9.7 km east of thewest shore, 2.8 km east of the power lines and 12.9 km south of PassManchac (Figure 2, Table 6). Macrofauna abundance was higher the firstyear than the second year, and slightly above the average for the lake.Average abundance the first year was 30,673/m2 the second year,27,283/m2 ; and for the entire study period, 28,978/m 2 . Seasonalpeaks in abundance (Figure 17) followed the general trends for thewhole lake, with higher population densities in the colder months andlower densities in the warmest months.

A distinct response to the opening of the Bonnet Carre Floodway isevidenced by the increased abundances in March, April, and May 1979.

Station 5 was one of the few stations in the lake characterized bydominance of the small gastropod Probythinella louisianae, rather thanthe more common Texadina sphinctostoma. The two gastropods, the clamRanaia cuneata, and the polychaete Hypaniola florida make up 94% of thetotal numbers.

Species diversity for the first year of the study was higher atStation 5 than the mean value for the lake (Table 6). Diversity fellfrom 1.185 t 0.025 to 1.033 1 0.046 the second year, which was notsignificantly different from the mean for the lake. Average number ofspecies the first year was 11.77 t 0.38; the second year, 10.000 t0.54. Evenness, calculated at 0.484 ± 0.008 the first year, dropped to0.453 ± 0.020, which was not significant.

44

Biomass of 11.4344 ± 1.2875 g/m2 AFDW was not significantlydifferent from the mean of 8.8238 g/m2 AFDW found for the lake.

Cluster analysis of the macrofauna for Station 5 over the 17sampling periods (Figure C5, Appendix C) showed four major clusters.The first group includes all the months from December 1978 to theopening of the Bonnet Carre Flooduay in April 1979. The second groupincludes August, September, October, and November 1978. The thirdgroup includes all sampling dates following the opening of the Floodwayfrom May 1979, except for the last month, August 1980, which stoodalone. Station 5 shows the effect of the opening of the Floodway onmacrofauna as a division between clusters just as the cluster analysisfor Station 4 did. The division, however, comes one month later.

lowm STATION 5, NEMATODE ABUNDANCE r'---"MEAN FOR ALL STATIONS *---e

1400

So

4O

1200

10

A S O N 0 J F M A M J J A 0 F M A1978 1979 1980


Meiofauna abundance (Table A2, Appendix A) is high at Station 5 onthe average. This abundance was the result of high numbers ofnematodes (Figure 18) since numbers of other taxa are quite moderate(Table A2, Appendix A). Biomass of 1.1215 ± 0.1031 g/ma AFDW was notsignificantly different from the mean for the lake of 0.9424 g/m2 AFDW.The five comon copepods, which occurred at all stations, were presentat Station 5. In addition, three rarer copepods, Nitrocra lacustris,Eurytemora affinis, and Mesocyclops edax, were found with somefrequency. The two copepods with marine affinities found at Stations1 and 2 did not occur at Station 5.

45

_III n- .j ... .

Station 6

so STATION G, MACROFAUNA ASUNDANCE A"---&MEAN FOR AU. STATIONS -o

A AS O NDO J F M A M J J A D F M A970 1979 1980


Station 6 was 3 1cm to the north and vest of Station 5 (Figure 2,Table 6) and the most westerly of all the stations. Macrofauna

~abundance was near the mean for the lake the first year and second fromq highest the second year. Average abundance the first year was

25,488/in2; the second year, 37,101/in2; and for the entire study period,28,230/me. Seasonal peaks in abundance (Figure 19) were masked by theeffects of an apparent disturbance the first year of the study, butquite distinct during the second year. Response to the opening of the

t Bonnet Carre Floodway was also masked.

t Station 6 was characterized by the dominance of ProbythinellaI louisianae, and above average numbers of the polychaete Hypaniola! florida and chironomitd larvae. These three plus the other gastropod,I Texadina 8phinctostoma, made up 87Z of the total macrofauna the first

yaand 95Z of the total the second year. The density ofSP. louisianae was more than twice as high the second yerthan it was

yea

the first.

Species diversity was significantly higher than the mean for thelake the first year, at 1.200 ± 0.030. A significant decrease,attributable to the increased dominance of P louisianae. to 0.9650.053 occurred the second year. Average number of species did notchange significantly writh 10.13 t 0.33 the first year, and 9.83 ± 0.77the second year. Evenness dropped from 0.527 ± 0.014 to 0.439 ± 0.031,which was a significant decrease, and a reflection of the change indominance.

46

30--

Biomass of the macrofauna of Station 6 was :0.4714 - 1.2809g/m2 AFDW, near the mean for the lake.

Cluster analysis (Figure C6, Appendix C) of Station 6 macrofaunaover time yielded two clusters. The first cluster included Novemberand December 1978, January and February 1979, November 1979, andFebruary 1980, the fall and winter months. The second cluster includedall the spring and summer months; August, September, and October 1978,May, June, July, and August 1979, and May and August of 1980. Station6 appeared to be clustering strongly on a seasonal basis, however, thiswas probably a reflection of the seasonal changes in abundance.

lso STATION 6. NEMATODE ABUNDANCE Ar---AMEAN FOR ALL STATIONS 0---e

1400

=SW

00aw

00-

0

A S O N O J F M A M J J A 0 F M Ais7B 1979 180 .1Figure 20. Nematode Abundance, Station 6.

Meiofauna abundance was very near the mean for the lake. Nematodeabundance (Figure 20) exhibited a strong seasonal pattern, and wassomewhat higher than the mean the first year. Copepods at Station 6occurred in numbers close to the mean. The species present were thesame as those occurring at Station 5. Meiofauual biomass 0.9129 -0.0745 g/n2 AFM was not significantly different from the mean for thelake.

47

Station 7

so I STATION 7. MACROFAUNA ABUNDANCE Apo--&MEAN FOR ALL STATIONS 0---@

70

050-

~40 -.o

x 30o ------ -

20---

10

A S O N D J F M A M J J A D F M A1978 1979 1980


Station 7 is the northernmost station, 2.5 km from shore, 6.4 kmsouthwest of themouth of the Tchefuncte River and 5.2 km northeastof the mouth of the Tangipahoa River (Figure 2, Table 6). Macrofaunaabundance at Station 7 was quite low the first year, and near the meanfor the lake the second year. Average abundance the first year was10,757/m2; the second year, 22,089/ma; and for the entire study period,14,103/22. Abundance patterns did not follow the seasonal trends forthe lake the first year, but appear to follow closely the second year.

The number of species found both years was low. The dominantspecies both years was the gastropod Probythinella louisianae. Itaccounted for 56% of the total abundance the first year and 85% thesecond year. Station 7 was unique in having the smallest percentage ofTexadina aphinctostoma of any of the stations. T. sphinctostoma

accounted for only 10Z of the abundance the first year and 4Z thesecond year. The highest numbers of oligochaetes were found at Station7. This is the only station at which the two amphipods Grandidierellabonnieroides and Gamarus mucronatus were found.

Species diversity at Station 7 was significantly higher at 1.287 ±0.039 than the mean for the lake the first year. The second year,diversity at Station 7 dropped to 0.699 ± 0.094 which was a significantdecrease, and significantly lower than the mean. The average number ofspecies (9.718 1 0.346 the first year and 8.563 ± 0.690 the secondyear) were not significantly different from each other, but weresignificantly lower than the mean for the lake. Evenness, which wassignificantly higher than the mean for the lake at 0.578 ± 0.021 the

48

-

first year, decreased significantly to 0.345 ± 0.050 the second year,which was significantly lover than the mean for the lake. Thissignificant change in evenness was a reflection of the increase in the'dominant species, and concomitant decrease in relative abundance of theothers.

Biomass of the macrofauna at Station 7 was 5.8154 ± 0.9101g/m2 AFDW, significantly lower than the mean for the lake.

Cluster analysis of the macrofauna of Station 7 over time yielded3 clusters. One cluster included August, September, and October 1978,and August 1980 and was characterized by low abundance. Another smallcluster included March 1979, and February and May 1980, and wascharacterized by high abundance. A large central cluster included allremaining collections. Seasonality was weak, and reflected control bysome factor other than time of year.

1600 STATION 7. NEMATODE ABUNDANCE --- iL

MEAN FOR ALL STATIONS o---e, 1600

am

1400Z 1000)

200

0 1 ( I] . .I,. , , .. ,,4 .1 . . .

A 3-O NO J F M A M d J A 0 F M A1978 1979 1980


Melofauna abundance at Station 7 was near the mean for the lake(Table A2, Appendix A), although nematode abundance (Figure 22) wasgenerally low. Components with freshwater affinities, the ostracodsand rotifers, wre present in significantly higher numbers than atother stations. Copepod species wre th same as those found atStations 4, 5, and 6. Keiofauna biomass of 0.8856 t 0.0811 S/21 AFN

was not significantly different from the mean for the lake.

49

-- - II

Station 8

STATION a. MACROFAUNA ABUNOANCE h-mMEAN FOR ALL STATIONS *--

I-

go

AS ON 0J F M A M J J A 0 F M A1978 1979 1980


Station 8 is 9.3 km south of the mouth of the Tchefuncte River and12.5 km east of Pass Hanchac (Figure 2, Table 6). This station islocated over what used to be a fossil oyster reef. It has been dredgedat some time in the past, since only small, flat fragments of oyster

$shell remain, often scattered over the sediment surface. This stationqhad the highest sediment organic carbon values (2.05%) of any station.

Macrofauna abundance was the second highest for the lake,significantly above the mean both years. Average abundance the firstyear was 35,314/m2; the second year, 42,234/m2; and for the entireperiod, 36,945/m2 . Seasonal peaks in abundance were well defined(Figure 23). No distinct, immediate response to the opening of theBonnet Carre Floodway was observable in abundance patterns. Station 8was the only station sampled during this study where a change in thedominant species occurred during the study. The dominant speciesduring the first year of this study at Station 8 was Texadinasphinctostoma. During the second year of the study, numbers ofProbythinella louisianae more than doubled, establishing it as thedominant species. One of the two small hydrobiid gastropods wasdominant at all stations in the lake. Both occurred at all stations atsome time during the year in varying proportions. Distributional mapsof the abundance of these two species resulting from quantitativesamples collected during a survey in June, July, and August 1954(Darnall 1979) show quite similar distributions to those present duringthis study. Darnall (1979) shows roughly equal numbers of the twospecies in the lake. T. ephinctostoma numbers remain relatively

5o

stable, with changing dominance resulting from the variations in thenumbers of P. louisianae. Station 8 is also characterized by higherthan average abundance of the polychaete Hypaniola florida.

Species diversity of 1.303 ± 0.025 for the first year was signi-ficantly higher than the-mean for the lake. Species diversity droppedsignificantly the second year to 1.096 ± 0.038, a value not signi-ficantly different for the mean for the lake. The change in averagenumber of species from 12.05 ± 0.29 the first year to 10.83 t 0.55 thesecond year was not significant. The change in evenness from 0.527 ±0.010 the first year to 0.467 ± 0.023 the second year was significantand is related to the increase in the number of P. louisianae.

Biomass at Station 8 was the highest of all stations, 15.2317 ±1.4906 g/m2 AFDW.

Cluster analysis of the macrofauna at Station 8 (Figure C8,Appendix C), yielded two clusters with a strong seasonal component.

The first, a winter and spring grouping included all collections fromDecember 1978 through May 1979, and December 1979 through May 1980.The second cluster, the summer and fall grouping, included August,

September, October, and November 1978; June, July, and August of 1979;and August 1980.

1wo STATION &. NEMATODE ABUNDANCE h,---AMEAN FOR ALL STATIONS 0---@

"I12W -1oo

11000 -0 =-- ~-*

' . ..... . ............ .... ....

A S 0 N 0 J F M A M J J A 0 F M At97 1076 100


51

OPP-

Unlike macrofauna abundance and biomass at Station 8, which wasrelatively high for the lake, meiofauna abundance and biomass were veryclose to the mean. Nematode abundance (Figure 24) followed the generaltrend for the lake, as do other taxa (Table A2, Appendix A). Copepodspecies were essentially the same as those found at the neighboringStation 7. Meiofauna biomass of 0.8677 ± 0.0644 g/m2 AFDW was notsignificantly different from the mean for the lake.

Station 9

STATION 9. MACROFAUNA ABUNDANCE h-MEAN FOR ALL STATIONS --- e

t~~~~T A

60.

"40-

X 30-- ----. -- S

1978 1979 1980


Station 9 was in the northeastern portion of Lake Pontchartrainand lies 4.5 km south of Goose Point and 12.5 km east of the Causeway(Figure 2, Table 6). The sand at this station is probably derived fromthe ancient Milton's Island Beach Trend. This station was dredgedduring the study sometime between November 1979 and February 1980.Macrofauna abundance was relatively low, with only two other stationsaveraging lower. Average abundance for the first year was 12,241/m2;for the second year it was 12,046/a2; and for the entire study period,12,143/32. The peak in abundance in April 1979 (Figure 25) was neithera seasonal peak, nor a response to the opening of the Bonnet CarreFloodway. A settlement of over 70,000/mz tiny bivalves (so small thatit was not possible to identify them as either Rangia cuneata orMulinia ontchartrainensis) occurred, which did not survive until thenext sampling cruise.

52

IWO-

Slightly fewer macrofauna species were found at Station 9 than theaverage for the lake. Texadina sphinctostoma was the dominant speciesfound, and it, with the two clams, made up 96% of the total macrofauna.

Species diversity at Station 9 of 1.056 ± 0.070 for the first yearwas not significantly different from the mean for the lake. The slightdecrease to 0.948 ± 0.067 for the second year was not significant.Average number of species of 9.205 ± 0.427 for the first year, and of10.250 ± 0.687 for the second year, were not significantly differentfrom each other. Evenness, calculated at 0.484 ± 0.030 the first year,dropped to 0.409 ± 0.023 the second year, which was not a significantchange.

Biomass at Station 9 of 5.0359 ± 1.7948 g/m2 AFDW was relativelylow with only two other stations ranking lower.

Cluster analysis of the macrofauna for Station 9 over the 17sampling periods (Figure C9, Appendix C) yielded 4 clusters separatedat high levels of dissimilarity. The first cluster included Octoberand November 1978, and July 1979, and was characterized by extremelylow abundance. April 1979 stood alone, separated by the extremely highabundance of tiny clams. The third cluster included December 1978,January 1979, March 1979, and December 1979, all characterized byintermediate abundance. The last cluster included all other samplingtimes and was also characterized by low abundance, and lower evennessthan the first cluster.

100 STATION 9. NEMATODE ABUNDANCE A"--*MEAN FOR AL STATIONS 6---@

1400

am

am

. . . . . . °_. . . . .. ." . . .--- -"- ""

A 8 0 N 0 J F M A M J J A 0 F M A1076 1070 1980


53

Meiofauna abundance, unlike macrofauna abundance at Station 9, wasnot significantly lower than the average for the lake the first year.Nematode abundance (Figure 26) was not significantly different atStation 9 for the two years. The mean for nematodes over the wholelake increased substantially the second year, so that the abundance forStation 9 was significantly lower the second year than the mean for thelake. More species of copepods occurred at Station 9 than at any ofthe stations previously discussed, probably because of the coarser,sandy sediments. Melofauna biomass of 0.8254 ± 0.0816 g/m2 AFDW wasnot significantly different frov the mean for the lake.

Station 10

s STATION 10. MACROFAUNA ABUNDANCE *-i

MEAN FOR ALL STATIONS 0---o

0

E 60a soz

040-

0

A & 0 N D 1 F M A M J J A D F M A1978 .1979 1980


Station 10 lies 7 km to the south and east of Station 9, 6.0 kmnorthwest of South Point, in the eastern portion of Lake Pontchartrain(Figure 2, Table 6). Macrofauna abundance was below the mean for thelake the first year and slightly above the mean the second year.Average abundance the first year was 13,711/m; the second year,25,296/m2; and'for the entire study period, 17,209/m2. The peak inFebruary 1979 (Figure 27) was not a seasonal response. Settlement ofthe small gastropod Texadina sphinctostoma is often quite patchy. Onesample at Station 10 at that time contained 6 times as many(a 50,000/ma2) as the other samples (Table Al, Appendix A) of the newlysettled gastropods, which did not survive until the next samplingperiod.

54

Station 10 had relatively large numbers of Rangia cuneata andMulinia pontchartrainensis. Texadina sphinctostoma was dominant,contributing 43Z to the total abundance. The three species togethermade up 94Z of the total the first year. The second year of the study,T. sphinctostoma made up 53%, and Probythinella louisianae hadincreased from 2% to 25Z of the total.

Species diversity was 1.089 t 0.039 the first year, notsignificantly different from the mean for the lake. The second yeardiversity increased to 1.236 ± 0.078, significantly higher than themean for the lake. Average number of species increased from 10.79 ±0.39 to 12.25 ± 0.71; not significantly different from the mean for thelake or from each other. Evenness increased from 0.464 ± 0.014 to0.498 ± 0.029; again not significantly different from each other orfrom the lake mean.

Biomass of the macrofauna at Station 10 was 6.5716 ± 0.9522 g/m 2

AFDW, not significantly different from the mean for the lake.

Clust~r analysis (Figure C10, Appendix C), of the macrofauna ofStation 10 over the 17 sampling periods yielded three clusters. Thefirst included August, September, and October 1978, and was charac-terized by extremely low abundance and low species numbers. Thesecond cluster included only November and December of 1978, and January1979, and was characterized by low abundance and intermediate speciesnumbers. The last very strong, very large cluster included all theremaining sampling dates, with remarkably even abundance and higherspecies numbers.

IWO8 STATION IM NEMATODE ABUNDANCE A.--MEAN FOR ALL STATIONS 0---

1400W

1200

Z 1000

4W

A SON 0 J F MA M J J A 0 F M A

1076 190 1 gao


55

Unlike macrofauna abundance at Station 10, which was lower thanthe mean for the lake the first year, meiofauna abundance was notsignificantly different either year. Nematode abundance (Figure 28)follows the general pattern for the lake. Although numbers ofnematodes were greater the second year (Table A2, Appendix A), theincrease was not significant. Copepod species were high, including notonly the common species found at all stations, but the less frequentones. Biomass of the meiofauna was 0.9340 ± 0.0731 g/m2 AFDW, notsignificantly different from the mean of the lake.

Station 11

s STATION 11, MACROFAUNA ABUNDANCE A

MEAN FOR ALL STATIONS 0---O

0 so

@~40-

0j~ 30-

10 *

0A S O N DJ F M A M J JAFM A1978 1979 1980


Station 11, one of the three seasonal stations sampled on aquarterly basis during both years of the study, lies halfway along aline between Station 4 and Station 5, 12.9 km northeast of the BonnetCarre Floodway (Figure 2, Table 6), in the western portion of LakePontchartrain. Macrofauna abundance was close to the mean for the lakethe first year, but below the mean the second year, among the fourlowest in the lake. Average abundance for the first year was24,209/m2; the second year it vas 18,268/n 2 ; and for the entire studyperiod, 21,239/m 2 . No seasonal peaks were observed (Figure 29). Astrong response to the opening of the Bonnet Carre Floodway was shownin the increase in abundance in May of 1979. Since Station 11 was theclosest station to the floodway, it would be expected to show thisresponse.

56

- -°---

The number of species was average for this station. It wasdominated by the small gastropod Texadina sphinctostoma which made up65Z of the total abundance the first year, and 61Z of the total thesecond year (Table Al, Appendix A).

Species diversity at Station 11 the first year was 1.105 ± 0.063,not significantly different from the mean for the lake. Speciesdiversity increased the second year to 1.273 ± 0.063, which wassignificantly higher than the mean for the lake. Average number ofspecies increased from 11.50 ± 0.65 the first year to 12.50 ± 0.62 thesecond year, which was not a significant increase. Evenness,calculated at 0.458 ± 0.028 for the first year, increased to 0.508 ±0.025 for the second year, which was not significant.

Biomass of the macrofauna was 8.2022 ± 1.4489 g/m2 AFDW, notsignificantly different from the mean for the lake.

Cluster analysis of the macrofauna yielded two clusters. Thefirst included only the May 1979 collections, when the Bonnet CarreFloodway was opened. The second cluster included all other samplingperiods (Figure C1I, Appendix C).

low STATION 11, NEMATODE ABUNDANCE Ah---&law MEAN FOR ALL STATIONS o---e

*)1400

12000

a,

o - 0-0- Ile

400

200

0.............. . .A......................A 850 ND0 J F M A M J J A D F M A

1078 1970 1080


57

-•|

In contrast to the very "average" macrofauna at Station 11, whichshowed no significant changes, the meiofauna made a dramatic change inthe second year. The abundance of nematodes more than doubled (Figure30). This increase in abundance was significantly different from theprevious year, and from the mean for the lake. Other meiofaunal taxaincreased in abundance also.

Copepod species at Station 11 included the five common species;Hylicyclops coulli, Halicyclops fosteri, Pseudobradya sp., Scottolanacanadensis, and Acartia tonsa. Only two of the rarer species, Nitocralacustris and Cyclops bicolor occurred there.

Meiofauna biomass was at 1.1599 t 0.1555 g/m2 AFDW, slightlyhigher than the mean for the lake.

Station 12

s 80 STATION 12. MACROFAUNA ASUNDANCE h-MEAN FOR ALL STATIONS o---e

70

M0 J0

0.

10

0A S ON 0 J F M A M IJ J A D F M A1978 1979 1980


Station.12 was in the eastern portion of Lake Pontchartrain, 9 kmdue north of the Lakefront Airport (Figure 2, Table 6). Macrofaunaabundance at this station was the lowest of all stations.

Average abundance for the first year was 8,651/m2; the second yearit was 11,748/32; and for the entire study period, 10,200/ma.Densities at Station 12 did not follow the general pattern formacrofauna in the lake as a whole (Figure 31). Neither evidence of theusual seasonal trends nor any response to the opening of the BonnetCarre Floodway can be seen.

58

Species numbers were low. Average number of species the firstyear was 7.75 ± 0.88; the second year, 8.67 ± 1.27. The dominantspecies, Texadina aphinctostoma, makes up 68% of the total macrofauna.This spcies together with the two clams, Rangia cuneata and Muliniapontchartrainensis, makes up 93% of the total abundance (Table Al,Appendix A).

Species diversity was low. Diversity dropped from 1.033 ± 0.059the first year, to 0.687 ± 0.116 the second year. Species diversityfor the second year of the study was significantly lower than the firstyear at Station 12, and significantly lower than the mean for the lakeas a whole. Evenness, calculated at 0.538 ± 0.032 for the first year,dropped to 0.411 ± 0.067 during the second year. This change inevenness is related to the change in abundance of the dominant species,and its concomitant change from 68% to 77% of total abundance.

Biomass of the macrofauna at Station 12 was also very low. Theaverage for the two year study of 3.2738 ± 1.0997 was not significantlydifferent from the biomass at Station 1, which was the lowest for anystation. It was significantly lower than the mean for the lake.

Cluster analysis of the macrofauna for Station 12 over allsampling periods (Figure C12, Appendix C) yielded three major clusters.The first included the first three quarterly samples; November 1978,February 1979, and May 1979. The second included the next fourquarterly samples; August 1979, December 1979, February 1980, and May1980. The last sampling period, August 1980, was separated from theothers at an absolute level of dissimilarity. The collections at thisdate from Station 12 were almost completely devoid of the usualspecies.

lo STATION 12. NEMATODE ABUNDANCE A"-MEAN FOR ALL STATIONS 6---e

wo* 140 .

612W 0

00aw

A A -


59

, , |I Sl

Total meiofauna abundance was consistently below the mean for thelake (Table A2, Appendix A). Nematode abundance (Figure 32) was thelowest of all the stations. Copepod abundances were less severelydepressed than nematode abundances. The copepods were dominated byScottolana canadensis, one of the harpacticoid copepods with pelagicnauplii. Heiofauna biomass at Station 12 was the lowest of all thestations in the lake (Table A2, Appendix A). The mean value for thetwo year study period was 0.5561 ± 0.0863 g/ma2 AFDW. This wassignificantly lower than the mean for the lake of 0.9424 g/m2 AFDW.

Station 13

s- STATION 13. MACROFAUNA ABUNDANCE AP--

0-MEAN FOR ALL STATIONS *.--

1U

UO

00-

10-.S

0 A 0N 0. J F MA M J J A 0 F M A

1978 1979 1980 IFigure 33. Macrofauna Abundance, Station 13.

Station 13 was in the east bay of Lake Pontchartrain, 6.4 km northof the mouth of Chef Henteur Pass (Figure 2, Table 6). Macrofaunaabundance at Station 13 (Figure 33) was often the highest in the lake.Average abundance for the first year was 32,504/m2 ; for the secondyear, 45,789/m2 ; and for the entire study period, 39,141/m a.

The dynamics of the macrofauna populations at Station 13 areclearly being influenced by different environmental factors duringdifferent portions of the study. The very low densities at the firstsampling period (November 1978) were made up of the highest number ofspecies found at all the stations that month. This pattern would bemore typical of an area under heavy predation pressure than one underoxygen or salinity stress, which would have also lowered speciesnumbers. The rise in abundance between May 1979 and August 1979 wasalso atypical. Late sumer, for most of the lake stations, was a timeof heavy predation and decreasing benthic populations. Abundance

60

• m m -|4

patterns during the second year of the study seem to follow the samegeneral patterns as the rest of the lake stations.

The dominant species Probythinella louisianae made up 33% of totalabundance the first year, and 52% of the total the second year. Thisspecies, plus the clam Macsma mitchelli, the other common gastropodTexadina sphinctostoma, and the tube-dwelling amphipod Cerapusbenthophilus, together made up 77% of the total abundance the firstyear and 94% of the second year.

Species diversity was the highest of all the stations in the lakeat Station 13. Diversities of 1.363 ± 0.085 the first year, and of1.385 ± 0.085 the second year, were both significantly higher than themean for the lake. Average number of species increased significantlyfrom 15.75 ± 0.41 the first year to 20.33 ± 0.50 the second year. Thetotal number of species found at Station 13 was 33 (Table Al, AppendixA). Many species were found at this station that occurred at no otherstation sampled. Others occurred in much higher numbers at Station 13than at the other stations. The mean of the total numbers of speciesfound at the other stations was 23.2 ± 2.0.

Evenness was measured at 0.495 ± 0.031 the first year, and 0.4590.026 the second year.

Biomass at Station 13 was 13.3026 ± 2.2242 g/m2 AFDW, which, wassignificantly higher than the mean for the lake.

Cluster analysis (Figure C13, Appendix C) of Station 13 macrofaunaover time yielded three clusters. The first included only the November1978 collection, with extremely low abundance and higher diversity, thesecond included February, May, and August 1979, and February 1980, andwas characterized by high dominance of Probythinella louisianae. Thethird cluster included December 1979, and May and August 1980, and wascharacterized by more nearly even numbers of Probythinella louisianaeand Texadina sphinctostoma.

In addition to having the highest overall abundance of macrofauna,Station 13 also had the highest abundance of meiofauna (Table A2,Appendix A). Average abundance of nematodes (Figure 34) was very high.The collections at Station 13 in February 1979 had the highest numbersof nematodes collected in any month at any station.

Copepod species at Station 13 included both the more commonspecies, Ealicyclops fosteri, Pseudobradya op., Scottolana canadensis,and Acartia tonsa; and the rarer ones, Nitocra lacustris, Eurytemoraaffinis, NesioM-- 0ps edax, yrooa up.,Onychocamptus mohammed, andHicroarthridion littoral. The last three have strong marineaffinities.

61

STATION 13 NEMATODE ABUNDANCE h-ia - /72\ MEAN FOR ALL STATIONS 0---*

140-000

12M -

Z iW -

A - A.



Lake-wide Trends

Changes-in the Macrofauna

The results of this study show many differences between thestations, and some similarities. The dominant macrofaunal species atall stations examined was a small hydrobiid gastropod (Figure 35). Atnine stations (Station 1, 2, 3, 4, 8, 9, 10, 11, and 12) the first yearthe dominant species was Texadina sphinctostoma, and at four stations(Stations 5, 6, 7, and 13) the dominant species was Probythinellalouisianae. Table 8 shows the ranks by both number and biomass ofthose species accounting for more than 0.05% of the abundance. Onlyseven species occur with more than 1.0% abundance. Molluscs make up 95%of all animals found.

The second year of the study the dominance of the gastropodsshifted (Table 9). Numbers of Texadina sphinctostoma remained quitesimilar; the numbers of Probythinella louisianae, however, more thandoubled. An examination of the biomass columns on Tables 8 and 9 willshow that although numbers of the former gastropod still exceeded thostof the latter the biomass of the latter gastropod was greater. Thiscaused a shift in rank by biomass from fourth to first position.

Other changes included the decrease in numbers in three species ofbivalves; Ranzi cuneata (Figure 36), Mulinia pontchartrainensis(Figure 37 and Mytilopsis leucophaeta. Rgia cuneata showed thegreatest change in numbers of any species other than Probythinellalouisianae. The second year of the study Probythinella louisianae wasthe dominant species at five stations (Stations 5, 6, 7, 8, and 13),Texadina sphinctostoma was dominant at seven elations (Stations 1, 2,4, 9, 10, 11, and 12), and at Station 3, biomass of the tvo specieswas equal (Table 10). Changes in abundance patterns in other taxa are

62

Total Gastropods, 1978-1979

Diamte r z 1O. O/m2

7 C] Texadina sphinctostoma

Probythinell louisiana

0

Total Gastropods, 1979-1980~~Diameter: '- = 1O0O0/M2 !

7= Texadina sphinctostoma

= Probythinella louislanae

_ _ 63

10

Filgure 35. Annual mean gastropod abundance for each sampling station inLake Pontchartrain. 'Upper map, 1978-1979; lower map, 1979-1980.

ml m 63

Table 8. Macrofauna ranked by abundance and biomass, 1978-1979

Rank Biomass Z ZN SBiomass Species -2 (AFDW) N Biomass

g/ms

1 1 Texadina sphinctostoma 11,869 2.3453 51.72 25.73

2 4 Probythinella louisianae 4,625 1.5179 20.16 16.65

3 3 Rangiacuneata 3,794 1.7968 16.53 19.71

4 6 Mulinia pontchartrainensis 1,072 0.5076 4.67 5.57

5 9 Hypaniola florida 435 0.0154 1.90 0.17

6 2 Mytilopsis leucophaeta 399 2.0645 1.74 22.65

7 5 CHIRONOMIDS 365 0.7924 1.59 8.69

8 10 Mediomastus californiensis 119 0.0051 0.52 0.06

9 13 Monoculodes edwardsi 48 0.0018 0.21 0.02

10 12 Edotea montosa 41 0.0029 0.18 0.03

11 11 Corophium lacustre 34 0.0038 0.15 0.04

12 7 Mysidopsis almyra 27 0.0309 0.11 0.34

13 15 Streblospio sp. 18 0.0003 0.08 >0.01

14 8 NEDERTEANS 18 0.0165 0.08 0.18

15 16 OLIGOCHAETES 15 0.0001 0.07 >0.01

16 14 Cerapus benthophilus 15 0.0011 0.07 0.01

ALL OTHERS 53 0.0131 0.22 0.14

TOTAL 22,947 9.1155

First 8 species, cumulative percent98.31 Numbers

99.23 Biomass

64

Table 9. Macrofauna ranked by abundance and biomass, 1979-1980

Rank Biomass Z z(Ar W) N Biomss

-Biomass Species -/m2 (N ia

1 2 Texadina sphinctostoma 10,070 1.9898 42.01 24.94

2 1 Probythinella louisianae 9,753 3.2009 40.69 40.12

3 4 Rangia cuneata 1,508 0.7063 6.29 8.85

4 6 Mulinia poutchartrainenais 837 0.3964 3.49 4.97

5 8 Hypaniola florida 645 0.0228 2.69 0.29

6 3 CHIRONOMIDS 410 0.8910 1.71 11.16

7 7 Cerapus benthophilus 322 0.0232 1.34 0.29

8 5 Mytilopsis leucophaeta 121 0.6261 0.50 7.85

9 9 Monoculodes edwardsi 75 0.0029 0.31 0.04

10 15 OSTRACODS 44 0.0003 0.18 >0.01

11 13 Streblospio sp. 29 0.0004 0.12 >0.01

12 10 Edotea montosa 27 0.0020 0.11 0.03

13 16 OLIGOHAZTES 24 0.0001 0.10 >0.01

14 14 Capitella capitata 19 0.0003 0.08 >0.01

15 12 Mediomastus californienSis 15 0.0006 0.06 0.01

16 11 Corophium lacustre 12 0.0013 0.05 0.02

ALL OTHERS 58 0.0143 0.27 0.18

TOTAL 23,969 7.8778

First 8 species, curulative percent

98.41 Numbers

98.24 Bionass

0

65

... .. . . -- - . ... .II , :-! ]I

Ranala cuneata, 1978-1979Diameter: -i=3000/rn2

00

Rangia cuneata, 1979-1980Diameter: -43000/mn

2

I'7

4o VI0

Figure 36. Annual mean lRsnvia, cuneata abundance for each sampling stationin Lake Poncartrain. Upper map, 1978-1979; lover map, 1979-1980.

Mulinla Pontchartrainenals,

197-1970Diameter: -4 3000/M2'

'10

.13

4

figure 37. Annual mean H~~ulinia ontchartrainni bnac o ahsmlning

1979-191980

Dimtr:P-7 OOM

01I0

Table 10. Ash-free dry weights, g/m2: Rangia cuneata, Probythinellalouisianae, and Texadina sphinctostoma

SpeciesRangia cuneata Probythinella louisianae Texadina sphinctostoma

Stations 1st yr 2nd yr 1st yr 2nd yr Ist yr 2nd yr

1 0.31 0.84 0.41 0.04 1.55 2.55

2 1.93 0.83 0.03 0.18 4.57 2.40

3 1.34 0.41 1.69 2.80 4.79 2.82

4 3.03 0.88 0.54 1.22 2.73 2.14

5 1.02 0.69 4.29 4.30 2.51 1.52

6 1.10 0.69 3.59 8.24 1.97 1.44

7 0.82 0.59 1.98 6.18 0.21 0.18

8 2.83 0.99 2.97 7.33 3.40 3.03

9 3.52 0.43 0.02 0.03 0.74 1.73

10 1.81 1.08 0.10 2.04 1.36 2.67

11 1.92 0.89 0.70 0.90 3.11 2.20

12 0.41 0.49 0.04 0.29 1.09 1.79

13 3.22 0.37 3.55 7.86 1.58 1.70

X 1.78 0.71 1.52 3.20 2.35 1.99

SE 0.15 0.05 0.11 0.35 0.11 0.11

68

not as well defined. Chironomids (Figure 38), for instance, decreasedin abundance the second year of the study at Station 1, and increasedat Station 6. Similar trends are seen in the polychaetes (Figure 39)and the Amphipods (Figure 40).

Changes in the Meiofauna

Changes between the first year of the study and the second year inthe meiofauna were confined to the nematodes (Figure 41). A significantchange from 454.18 ± 15.19/10cm2 to 594.00 ± 30.01/10cm 2 occurred. Rankby numbers and biomass for the meiofauna is given in Table 11. Changesin the nematode populations have been discussed for each station. Avariety of patterns was seen. At some stations an increase in nematodesoccurred, while at others there was no increase. Overall, no stationexhibited a significant decrease in nematode abundance.

Changes in Community Structure

The change in species diversity of the macrofauna from 1.1170.015 the first year to 0.985 ± 0.028 the second year was significant.The decrease in eveness from 0.473 ± 0.006 the first year to 0.423 ±0.010 the second year was significant, and was the component thatcaused the change in diversity, since there was no significant changein average number of spezies (Tables 7 and 12).

There is no discernihle pattern in the variation of speciesdiversity from month to month over the lake (Table 12). No seasonalincrease in species diversity caused by migratory species or seasonalincreases in some species through reproduction occurs. Significantlygreater variation occurs in species diversity from station to stationover all months (Table 7). Briefly, this indicates that spatialdifferences are more important than temporal; where a station is willaffect species diversity more than when it was sampled.

In addition to examining the abundance and biomass of the benthicpopulations, we have looked at some measures of community structure,such as species dominance, species diversity, evenness and speciesrichness. One addi'ional measure of comunity structure was made.The percent occurrence through time or constancy of each species wasmeasured. Ranking for constancy was similar to the ranking forabundance; the same 6 species were ranked 1 through 6, although indifferent order.

Texadina sphinctostoma ranked first with 100% constancy. Nocollection was made that did not contain at least one of thesegastropods. Chironomids are second with 98.92 constancy, yet theymake up less than 2% of the total abundance. Third, with 98.45%constancy, is Rangia cuneata, which varied from 6 to 16% of totalabundance. Probythinella louisianae, second in dominance by numbers,ranked fourth with 96.39% constancy. The polychaete Hypaniola florida

69

- -- - .. ZTL......--

-, II m II I . U I --

Chironomids, 1978-1979Diameter: i : 600/M2

0'

09

05 010U

112 .13

oo2

Chironomids, 1979-1980Diameter: = 001m 2

0 7

6 /I

0

, i 02

Figure 38. Annual mean chironomid abundance for each sampling station inLake Pontchartrain. Upper map, 1978-1979; lower map, 1979-1980.

70

Polychaetes, 1978-1979Diameter: s-. 3000/mg

r110 312

12 013

o4

o 2 01

Polychaetes, 1979-1980

Diameter: =-.3000/n 2

0

.10

0 2 91

Figure 39. Annual mean polychaete abundance for each sapling station inLake Pontchartrain. Upper map, 1978-1979; lower map, 1979-1980.

71

- -__ _ _ . . . .. . . . I I I

I I I I I I I II II i i ,

Amphipods, 1978-1979

Diameter: i = 6001m2

07

00

~Diameter: p-= 600Irn 2

6 05 03 001

4

e 2 •

Fiue 40. Annual mean mphipod abundance for each sampling station in

Lake Pontchartrain. Upper map, 1978-1979; loer map, 1979-1980.

,72

8- ~ ~ ~ ~ ~ ~ ~

cmIu

1- M

cc 0'n cYwq c

4c1 41 CI

OC 9 v N 11%

LU 00.

0 0I

WI.- omq

(A 44A' o

0 0

41' -N

coo -

733 -EA)

Table 11. Meiofauna, ranked by abundance and biomass, 1978-1980

Rank Abundance BiomassBiomass Taxa NI0o c2 (AFDW) 2 % z

9g/10 cm N Biomass

1 1 Nematodes 491.66 419.14 61.57 46.41

2 2 Copepods 69.23 142.96 8.67 15.83

3 6 Copepod nauplii 62.20 39.37 7.79 4.36

4 7 Ostracods 50.95 26.70 6.38 2.96

5 9 Rotifers 37.52 16.28 4.70 1.80

6 4 Turbellarians 26.42 94.97 3.31 9.41

7 3 Gastropods 24.88 95.86 3.12 10.618 8 Bivalves 15.30 24.65 1.92 2.73

9 5 Polychaetes 14.20 40.28 1.78 4.46

ALL OTHERS 6.09 13.00 0.76 1.43

TOTAL 798.56 903.21 Pg/10 cm2

i7

II

74

rr

0000 P, 0 a v0 M~ in 00-4 0- m 0mm- 0 m' N4 IV eq 0 0 vn %A -40 LA %D Nq V

-W r 00 v 004 0 co C4 N r 0 m 0 mn 0 v

ce

Ln N- %0 1- Ch le4 0C ON 0)0 ) 0 U) coC q0 ~ ~ C v4 Ln V IV le -e v'I it t n V - - - - v

mn ena t) G O L L o L

ca 00 0 0 0 0 00M00

m. m) Go40 0 0) -e On 00 %0 0 r le N0 4 O C 4 L n in rn r, en G 00Go00so 0

.0 . in le Ln q n v v in v m fen

4) 4

in >n m i (4 0o in 00 'A 0 in n 00 v - N00%V4 in4 '0 .a* v n qn r-n * l C4. in . C. in in e* n In 040c

M 0 0 a in N V-n N- NO a 000

0 c* ~ ~ 4 50. 'o in 0 0 n ' n '0 '0 N1$4 ~ t '0 c .4 n i 0 00 0 ) i

.0 *. . . . . . . . . . . . . . . . . . .

as0-4 N 91 0 ) n 0)0

o - -4 -4 4 - -4 -4 4 - -4 -4 475

0WMMF

i . . - - - -_ -

which makes up about 2% of the abundance is fifth, with 95.36%constancy. Mulinia pontchartrainensis with 4% of the total abundanceis sixth, with 90.21% constancy. This high constancy is anotherindicator of the lack of seasonality, which will be discussed atgreater length in another section.

Changes in Community Function

In addition to community structure, some measures of communityfunction were made. Niche breadth (B) as measured for each stationover all months (Table 7) and for each month over all stations (Table12). Similar to diversity, there was slightly more difference betweenstations than there was between months. This indicates that althoughall stations studied are functioning in a similar manner, partitioningresources similarly, resource availability changes less seasonally thanit does spatially.

Niche breadth for individual species (Bi) was measured for thedominant macrofauna species.

In order to gain some insight into the possible functional relation-ships between major groups within the benthic community, a series ofstepwise multiple regression analyses were performed using lakewidedata. The variables in these exploratory analyses included totalmolluscs, total bivalves, total gastropods, individual species ofmolluscs, total nonmolluscs, total polychaetes, individual polychaetespecies, and many other combinations of macrofauna groups. Inaddition to the macrofauna, variables were formed from the meiofaunadata set similarly (total meiofauna, nematodes, copepods, total non-nematodes, etc.) and merged with the macrofauna variables.

Physical variables such as temperature (using the values from justabove the bottom), conductivity, and presence or absence of sedimentdisruption (indicative of recent dredging) were included.

The results of these exploratory analyses were not highly signifi-cant, but did indicate some strong patterns. By performing the analyseson data from single stations, instead of including all stations, agreat improvement in significance of the results was possible. Usingonly those variables which had shown some association in previoustrials, and using 4a option in the program which tests each variablefor maximum improvement of r , the correlation coefficient, it wasfinally possible to demonstrate highly significant associations- betweencertain groups, at certain stations.

Table 13 shows the results of these regression analyses for eachstation with nematodes as the dependent variable. In all but twocases, a significant model with one of the two gastropods as anindependent variable emerged after the addition of one, two, or threevariables. In one case (Station 4) a one variable model, with tempera-ture as the independent variable, emerged. At Station 9 no combinationof variables led to a significant result vith nematodes.

76

dl _ _ . .... _-_ _ _

Table 13. Stepwise multiple regression analysis; nematodes, dependent variableall stations

Station Independent Variable r2 F Prob., F

1 Probythinella louisianae, Rangia cuneata 0.98 67.89 0.0032

2 Texadina sphinctostoma 0.88 28.22 0.0060

3 T. sphinctostoma, temperature 0.96 40.14 0.0068

4 Temperature 0.95 84.41 0.0008

5 T. sphinctostoma, conductivity 0.94 23.56 0.0146

6 P. louisianae, temperature 0.99 75.55 0.0131

7 P. louisianae, R. cuneata, temperature 0.90 11.72 0.0189

8 P. louisianae 0.93 40.05 0.0080

9 T. sphinctostoma, conductivity 0.55 1.86 0.2983 (NS)


11 T. sphinctostoma, temperature 0.88 10.99 0.0416

12 P. louisianae, conductivity 0.96 32.21 0.0094


77

Using one of the gastropods as the dependent variable at onlythose stations where that gastropod was the dominant species yieldedhighly significant results. At the Probythinella louisianae dominatedstations, the regression analyses showed a consistent associationbetween that species and nematodes. At all but one of these stationstemperature was also associated with P. louisianae (Table 14a). Wheretemperature appeared it always had a negative slope, showing an inverseassociation.

At those stations where Texadina sphinctostoma was the dominantspecies a different pattern emerged. Results were not as consistentas at the other stations, but, generaly, Probythinella louisianae wasstrongly associated with changes in Texadina sphinctostoma numbers.Other associations were with nematodes, temperature, conductivity,dredging, and chironomids (Table 14b). An occasional weak associationwith the coumon polychaete Hypaniola florida was present-, but nothighly significant.

i7

78I l

Table 14a. Stepwise multiple regression analysis Probythinella louisianaeas dependent variable for stations where P. louisianae isdominant species

Station Independent Variables r2 F. Prob., F

6 Nematodes, temperature 0.99 2252.3 0.0004

7 lematodes, temperature 0.96 36.29 0.0023

V 8 Nematodes 0.93 40.05 0.0080


Table 14b. Stepwise multiple regression analysis; Texadina sphinctostoma as

dependent variable for stations where T. sphinctostoma is dominant

2Station Independent Variables r F Prob., F i


2 Nematodes, conductivity 0.97 47.09 0.0054

3 Nematodes, chironomids 0.99 350.90 0.0003


9 P. louisianae, dredging 0.97 53.91 0.0045

10 P. louisianae, conductivity, dredging 0.99 55.61 0.0177

11 P. louisianae, temperature, conductivity 0.94 10.77 0.0862

12 P. louisianae, dredging, chironomide 0.99 2752.32 0.0004

79

_ _ _ -

DISCUSSION

Loss of Large Rangia cuneata from the Benthic Community

Benthic infauna offer several advantages as ecological indicators.Not the least of these is that both the habitat and the community arestationary. Benthic organisms will occupy the same location in space aslong as conditions are conducive to the community through time. Thetime frame for which this is true can be greatly extended into the pastif the organisms in the community leave evidence of their habitation,either as trace fossils, or as actual fossils of the organisms themselves.This is particularly true of the mollusca, especially the bivalves. Ifwe are able to compare fossil information with that gleaned from extantcommunities, much can be inferred about the ecosystem which supportsthese communities now and in past times.

One striking pattern that has emerged from the present study is theloss of larger Rangia cuneata (over 20 mm long) from the open-lakebottom community. This is one of the most significant faunal changes inLake Pontchartrain because it represents a complete change in thedominance, biomass, and energy flow patterns of the benthic community.This change is apparently widespread over the entire lake, and reflectsa profound change in the lake ecosystem as a whole. That large R.cuneata were ever present in the open lake is beyond question, sincetheir fossil shells are there in such abundance that a sizeable industryis supported by harvesting these shells. The shell dredging industryremoves almost 5 million cubic yards (3.8 x 106m3) of shell annually,with 62,200 clams (with 2 valves each) represented in each cubic yard.That large living R. cuneata were present in the near past is evidenced fromDarnell (1979) who, during a survey study of Lake Pontchartrain between1953 and 1954, found a mean density of 135 ± 16/m 2 of R. cuneata longerthan 20 mm at 23 open lake stations. At 12 mid-central stations hefound a mean of 112.75 ± 16/m a of R. cuneata over 20 mm. A few yearslater, in 1157 and 1958, working in the nearshore, in waters depths of2.4 m, and slightly deeper, Fairbanks (1963) found R. cuneata largerthan 21 um in densities of 31/m2 .

Rangia cuneata does not become sexually mature until lengths> 20 m. Fairbanks (1963) determined that R. cuneata developed re-cognizable gonads at a mean length of 23 mm. Older adult clams areconsiderably larger, and it is not unusual for these larg(er clams tobe found in high densities in favorable habitats. In Texas, Hopkinsand Andrews (1970) report 45 m clams occurring at a density of250/M2. In Georgia, Godwin (1968) reports 52 m size clams occurringin densities of 132/32 in the Altamaha River Delta. Pfitzenmeyer andDrobeck (1964) report 35 - 45 u clams occurring in densities of225/u2 in Maryland.

In the present study of 13 permanent open-lake stations in LakePontchartrain, 582 box core samples were taken over a two-year period.In those samples only 10 R. cuneata over 30 mm and 33 R. cuneata

so

between 20-30 mm were found, for overall mean densities of 0.19/m2 and0.62/M2. Expressed another way, there is an average of 1 clam > 30 mm,per 5.2 u2, and I clam 20-30 mm per 1.6 m2. R. cuneata larger than 20mm equaled less than 0.03% of the overall mean density over the twoyear period of 3256.5/ma for all sizes of R. cuneata. R. cuneatabetween 10-20 m averaged only 21.68/m. for 0.67%, so that all R.cuneata over 10 mm combined are less than one percent (0.692%) of theoverall mean population density.

In July 1980 a transect was made from inshore on the northshore to the open lake (Table D2, Appendix D). When the stations withwater depths comparable to the depths Fairbanks (1963) sampled (2.1 m to2.7 m) are examined, we find R. cuneata over 30 m in size at densitiesof 4.54/m2, or about 24 times as many per m2 as found in the open lakestations in the present study, but still only 0.12% of overall mean ofall the R. cuneata found at these transect stations. R. cuneata between20-30 mm were found at the same densities, 4.54/m2. The density ofall R. cuneata > 20 mm was 9.08/m2 on this transect during the presentstudy, which is only 29% of the density of 31/M2 found by Fairbanks(1963). Only a single individual was found between 10-20 mm in size.The density of all clams over 10 m found on this transect, from0.4 km to 1.2 km offshore was 0.26%. The density of largeR. cuneata has declined precipitously in the last 23 years both inshoreand offshore. In fact, the decline may have taken place before theearly 1970's, because Tarver and Dugas (1973) and Dugas et al. (1974)both report R. cuneata larger than 16 mm were conspicuously absent fromthe open-lake region of Lake Pontchartrain, and that "none was recordedfrom samples taken in areas that were continually dredged" (Tarver andDugas 1973). Both these studies covered the entire area of the lake andsampled from 83 qut~rts.

Fossil R. cuneata shell larger than 18 mm from two midlake stations(3 and 5) were measured. From Station 3, 131 shells had a mean lengthof 24.5 t 0.48 mm (range 14-40 mm) and from Station 5, 154 shells had amean length of 27.0 ± 0.03 m (range 18-31 mm) which suggests that R.cuneata commonly grew larger than 20 mm. We do not know how old theclams were that provided the shells, however, it can be said that largeRangia cuneata have survived in the midlake region of Lake Pontchar-train in the distant past and in the recent past.

What change has occurred in the ecosystem that would prevent largeR. cuneata from surviving in the open lake, and cause the decline in theperipheral regions? There are a number of possible reasons why largeclams do not survive: (1) the sediments have become softer, lessstable, and large R. cuneata sink beneath the sediment surface; (2)resuspended sediments silt up and choke the larger clams; (3) primaryproduction has declined so that the rate of carbon input is too low tosupport the larger biomass of large clams; (4) toxic substances killlarge clams because larger clams accumulate a larger body burden over alonger time than do smaller clams; (5) R. cuneata only spawn success-fully in certain years so that only in those years do sufficient numbersof larvae reach the open lake to insure survival of the year class to

81

the larger sizes; or (6) predation pressure has increased to the extentthat virtually no clams survive to the larger sizes. There is also thepossibility, of course, that a combination of the above factors isresponsible.

Although Rangia cuneata are important in the diet of many demersalfishes that inhabit Lake Pontchartrain, increased predation pressure tothe extent of near complete elimination of clams larger than 10 mm seemshighly unlikely. In a year-long study of the nekton of the lake,Thompson and Verrett (1980) found the lowest overall mean biomass ofdemersal fish at their one midlake station, lower than 11 other stationsin the lake. Unfortunately, the authors gave no statistics on thenumbers of the principal invertebrate predator, the blue crab,Callinectes sapidus, because trawls are considered inefficient gear forblue crabs.

Rangia cuneata in Lake Pontchartrain have two spawning periods,March though May and late summer through November (Fairbanks 1963). Ourdata shows that the smallest size clams were always present in thebenthos. Even assuming a one percent per year survival rate, thereshould be greater numbers of large clams in the open lake. In a studyof the growth of Rangia cuneata, Wolfe and Petteway (1968) calculatethat one year old clams should attain a size of 16 mm. Fairbanks (1963)estimated that one year old clams in Lake Pontchartrain attained a sizeof 15-20 mm. Thus, it appears that the greatest majority of clams nowin the open lake are less than 10 mm, and survive less than one year.

Toxic substances are present in Lake Pontchartrain as previouslydescribed in another section of this report. One of the commontoxicants entering the lake is the pesticide dieldrin. Petrocelli etal. (1973) have shown in short-term laboratory experiments that Rangiacuneata is capable of concentrating dieldrin from water concentrationsof 0.55 ug/L by a factor of 800 times (maximum 2000 times) over ambient.-In a later study (Petrocelli et al. 1975a) it was shown that R. cuneatafed algae exposed tv dieldrin, exhibited a magnification of dieldrinresidues of up to 54 times greater than the concentration resulting fromresuspending contaminated algae in clean seawater. It can be seen thatR. cuneata can concentrate toxic substance- directly from water andthrough the food chain. Unfortunately we do'not yet know what thelethal body burden is, and more research is necessary.

The possibility of an ecosystem-wide decline in primary productionwill be discussed later in this section.

Resuspension of sediment does take place in Lake Pontchartrain andis primarily caused by wind induced waves as pointed out in a previoussection (description of study area). Lake Pontchartrain has probablyalways been turbid. Darnell (1958) points out "In Lake Pontchartrainthe common Rangia is most abundant on the muddiest bottoms in waters ofmaximm turbidity." In a later paper, Darnell (1961) states that ofparticular significance is the fact that the waters are highly turbidand "the great turbidity was found to be directly related to wind actionwhich disturbs. and raises the bottom sediments." The critical question

82

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is: has turbidity increased since the 1950's? Good, qu& titative datais lacking, although perhaps some increase can be inferrLe from theliterature. Alishahi and Krone (1964) in a series of experiments haveshown resuspension of sediments by waves results from the bed shearstress causing scour. They also showed that greater sediment bulkdensity also has greater resistance to scour. Sikora et al. (1981) haveshown that hydraulic dredging for clam shells in Lake Pontchartrainresults in lowered sediment bulk density; and as pointed out in aprevious section (Cultural Impacts), shell dredging activity covers asignificant area of the open lake. To determine whether sedimentresuspension has increased or not will take further research; however,even given the same amount of sediment resuspension as occurred in the1950's, the consequences would be far greater if the sediments werecontaminated with toxic materials (Peddicord 1980).

The question of whether the sediments are too soft to support largeR. cuneata is complex. Rhoads (1974) reviews the problem and states thatthe density factor may be important for mud-dwelling organisms withmineralized tissue, and that the evolution of some morphological featureof soft-mud bottom benthos is necessary, such as thin shells or someother adaptation. With regard to R. cuneata, Stanley (1970) shows thatit is adapted for shallow burrowing in stable substrate with thickvalves and short siphons, and is a slow burrower. Stanley goes on topoint out that another way to deal with the sinking problem is to remainsmall, thereby keeping the surface/volume ratio as large as possible tomaximize support from the substratum per unit animal weight in "soupymud substrate." It has been shown (Sikora et al. 1981) that shelldredging produces fluid mud of lower bulk density, fitting the categoryof a "soupy mud substrate". The size distribution data of Rangiacuneata in the present study (99.3% smaller than 10 mm) could beinterpreted as corroborating the hypothesis that small size is anecessity in'soft mud.

Primary Production and the Benthic Community

Primary production in Lake Pontchartrain is considerably lower thanmight be expected. The mean levels of nutrients in the water columnfrom Lake Okeechobee, Florida were compared with those from LakePontchartrain over a 12 month period. Total phosphorus levels of0.75 US-at/L, and NO -N of 1.08 Ug-at/l in Okeechobee, (Davis andMarshall 1975) are piobably not significantly different from the levelsof 1.60 ug-at/L total phosphorus, and 1.34 iig-at/1 NO -N reported forPontchartrain (Stoessel 1980), since the coefficient if variation forfield samples for these nutrients is reported as 100Z, and laboratorysamples as 801 (Stoessel 1980). Both lakes are large, shallow, turbidsystems, with no significant differences in secchi disk depths, ornutrient regime. Both have been classified as mesotrophic (Witzig andDay 1980, Joyner 1974). PrimarZ production in Lake Okeechobee wassignificantly higher at 1.86 go 'day -, than in Lake Pontchartrain,0.5 m-day- ', averaged over an annual cycle (Dow and Turner 1980,Davis and Marshall 1975).

83

Organic matter in the sediments is thought to be the carbon sub-strate which is at the base of the deposit-feeder food chain. Muchof the literature on detritus-based systems is in turn based on thispremise. It is well known that organic content of sediments variesinversely with sediment grain size so that clay and silty-claysediments usually have the highest organic matter content. The organiccontent of Lake Pontchartrain sediments was measured by Steinmayer(1939) and at that time the clay sediments in the central region of thelake had between 6 and 8% organic matter. The mean value for organiccarbon in lake sediments from Bahr et al. (1980) was 1.06% and from the13 stations occupied during the present study a mean of 1.33% organiccarbon. There appears to have been a decline in sediment carbon since1939. However, both sets of sAmples from which the latter two valueswere determined were collected in the spring and it is not known whetherthere is a seasonal component to the organic matter in the lake.

Warwick and Price (1975) report the organic content of sediment from amud flat in the Lynher estuary in England as 12.2 - 13.6% while Rhoadsand Young (1970) report values for silty-clays in Buzzard's Bay,Massachusetts of 2.0 - 2.2%. From the present data, it would appear ineither case that the organic content of Lake Pontchartrain sediments waslow, and lower than it once had been.

Steinmayer (1939), in describing the organic matter, refers to itas being of "vegetable origin", what he terms as comminuted vegetablematter and humus matter. Darnell (1961) refers to the "offshore de-position of much humus from the marsh and swamp area" and says "thevegetable detritus of Lake Pontchartrain seems to be from the decay ofcertain marsh grasses and phytoplankters". Darnell repeatedly refers tothe "allochthonous" origins of the basic organic matter supporting theconsumers in the lake community. He defines the primary components ofallochthonous material as marginal marsh vegetation, sedges, cordgrasses etc., phytoplankto outside the lake, and Mississippi Riveroverflow material.

The question of organic carbon in detritus-based systems is complexbecause, although macrodetritus is the most obvious component to theunaided eye, it may represent the most refractory elements in thedetritus system. There is evidence that Spartina detritus is notdirectly available to macroheterotrophs (Wetzel 1975), and the microbialcomponents of the systems are actually what are utilized (Odum and de laCruz 1967, Pomeroy 1979). It follows then, that the more labilematerial in detritus will be utilized first, at a faster rate. Presentday sediment carbon techniques do not distinguish between labile andrefractory components of detritus. Refractory material, though presentin seemingly sufficient quantity, may represent a very slow rate ofcarbon transfer to haterotrophs.

At the other end of the spectrum, dissolved organics are readilyavailable to microorganisms (Pomeroy 1979). Autochthonous dissolvedorSanic production by phytoplankton may be very important in open

84

water systems. Phytoplankton grazing by zooplankton is another pathwayby which labile material reaches the bottom in open water systems.Elmgren (1978) has demonstrated in an area of the Baltic Sea thatbenthos increases roughly in proportion to the primary production in thewater column above, and that the benthic system is intimately coupled tothe pelagic system and may respond to events in the plankton withinweeks. In the open water region of Lake Pontchartrain this phenomenoncan be demonstrated at Station 8 and Station 13.

An investigation of primary production by Dow and Turner (1980)in Lake Pontchartrain showed higher production at two of their surveystations. Survey Station 5 is at the point at which the influencesof the Tchefuncte and Tangipahoa Rivers and Pass Manchac combine, andis quite close to our benthic Station 8. Survey Station 13 is near theChef Menteur and our benthic Station 13. Flooding of the Pearl Rivercauses pulses at Station 13 similar to the influence of the rivers onStation 8. These two stations, which had the highest primary pro-duction, also had the highest benthic biomass.

Caution should be exercized ip comparing Lake Pontchartrain, whichis actually an estuary, with Lake Okeechobee, which is not. It ispossible that the exchange of water with the gulf through The Rigoletsand Chef Menteur Passes removes some portion of the phytoplankton. Acomparison should also be made with another estuary. Sellner et al.(1976) report primary production of 234 gCm 2yr 1 in North Inlet estuarynear Georgetown, South Carolina. Summer peaks in phytoplankton pro-duction are correlated with similar peaks in zooplankton production(Lonsdale and Coull 1977). Pontchartrain primary production of 157gCm-2yr- 1 (Dow and Turner 1980) is 67Z of that of North Inlet.

Zooplankton and the Benthic Community (Annual mean zooplankton biomass in North Inlet was measured at

16.18 mg m dry weight (Lonsdale and Coull 1977). Dry weight ofzooplankton from midlake stations in Lake Pontchartrain was measured at8.35 mg m (Stone et al. 1980). This value includes both theirseparately listed macrozooplankton and microzooplankton, in order todecrease differences caused by different net mesh sizes. If we compareonly the macrozooplankton with that of a previous study in Lakepontchartrain (Tarver and Savoie 1976) where the same mesh nets wereused we find little difference in the abundances.

Haves and Perry (1978) reported their findings on zooplankton inLake Pontchartrain in settled volume for 10 minute tows. Convertingthis to a volume basis in order to compare with Darnell's earlierstudies of the lake (Darnell 1961, 1962) we find that their samples,were, on the average, slightly less than half of what Darnell's settledvolume was reported to'be.

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If we assume that the three recent studies of Lake Pontchartrainzooplankton (Stone et al. 1980, Haves and Perry 1978, and Tarver andSavoie 1976) were measuring roughly equivalent populations, then we canassume that Darnell's zooplankton biomass would have been more nearlyequivalent to that found by Lonsdale and Coull (1977) in the unpolluted,pristine North Inlet Estuary.

Copepods make up less than nine percent of the total meiofauna. Ofthese, Acartia tonsa makes up ll.6Z of the copepods found. This is theequivalent of 80301e found on or near the bottom sediments. Noexchange with the water column is possible when the closed box corer isretrieved during sampling. When meiofauna samples are taken from thebox corer they include the overlying 20 to 30 cm of water. It issignificant that almost 300 times more Acartia tonsa are found feedingepibenthically than exist in the water column.

Through alterations over time Lake Pontchartrain is becoming a oneresource system. The distinction between the zooplankton and thebenthos is disappearing. Not only are so-called planktonic forms foundliving epibenthically, many benthic forms are found as "tychoplhnkton."The tube-dwelling amphipod Cerapus benthophilus and tube-dwellingpolychaetes were not infrequently found in the plankton tows. Adults ofTexadina sphinctostoma, the small gastropod, were frequently found inthe plankton. The same rotifer species were found in both series ofsamples. The resuspension of low bulk-density sediments by wind-inducedwater movement would account for the appearance of benthic forms in thewater column. The high incidence of planktonic forms feeding epi-benthically is not so easily accounted for.

Feeding Modes in the Benthic Community

In addition to the disappearance of the distinction betweenplankton and benthos in our turbid, easily resuspended, one-resourcesystem, the distinction between deposit feeders and suspension feedersis blurred. This situation is not uncommon in fine-sediment systems.Eagle and Hardiman (1977) in discussing the feeding modes of the speciespresent at their study site comment that "at the interface layer theanimals probably make no clear distinction between material insuspension and material recently deposited, and further more there isprobably a continual flux of food particles between the two states.Thus, for the most part, the polychaetes present use a pair of palpa ortentacles to collect food from the sediment surface and from suspension;the molluscs filter out suspended material from just over the sea bed,or suck up the sediment surface layer; while the crustaceans stir up thesurface layer and filter the resulting suspension." Boesch (1973) indiscussing the same phenomenon states that "generalizations based onbroad feeding-type categories suffer because they are imprecisedescriptions of feeding behavior and because of the considerablefeeding-plasticity of many benthic animals." Holland et al. (1977), indescribing the community structure of another low salinity mud-bottomestuary, suggest that in physically stressed coumunities biotic inter-actions, such as interactions between feeding types,-may be masked by

86

physical controls. They note also that many of the opportunisticspecies, such as Macoma balthica and Nereis succinea, important inrecolonization processes, have the capability of obtaining food by morethan one mechanism.

Niche Breadth of the Benthic Community

In commenting on the loss of distinction between planktonic filterfeeders, benthic deposit feeders, and benthic suspension feeders we are,in essence, discussing the loss of the specialist with a narrow nicheand the increase in the number of species that are functioning asgeneralists with broad niches.

If we define the environment of an organism as where it lives, thenwe can define the niche of an organism as how it lives, or how it usesits environment. An organism's niche can be discussed in terms ofpatterns of resource utilization. A growing body of theory predictsthat niche breadth should generally increase as resource availabilitydecreases (Schoener 1971, Charnov 1976, Pianka 1978). Levins (1968) andHespenheide (1975) relate increased niche breadth to loweredresource productivity. If we calculate , the niche breadth for thetotal community, for each station over all months, we find that they arenot significantly different from each other, evaluating the differenceby an F test (Petraitis 1979), and have a mean of 34.25. This is notsignificantly different from the 2 values determined for the perturbedGull Lake System (Lane et al. 1975). The Gull Lake System experiencedan order of magnitude increase in filter-feeding plankton aftereutrophication, which lowered the diversity of the system, and loweredthe resourcep available to each organism. Niche breadth expansion isone mechanirm by which low diversity in a community may be compensated(Cody 1975). .1Species Diversity of the Benthic Community

Species diversity in Lake Pontchartrain is uniformly low throughoutthe year. Some shallow, low salinity estuaries such as the CalvertCliffs region of Chesapeake Bay, experience similar low (9' - 1.00)diversities during the summer when low oxygen conditions occur duringsalinity stratification, but return to more normal levels (H' - 2.25)during winter months (Holland et al. 1977). Values for speciesdiversity have been shown by various investigators to have a negativecorrelation with environmental stress, particularly with pollution(Wilha and Dorris 1968, Woodwell 1970, Copeland and Bechtel 1971,Goodman 1975, and luggiero and Merchant 1979).

An empirical categorimation, based on the results of extensiveenvironmental sampling has been proposed by Wilhm and Dorris (1968).These widely accepted standards (converted to log ) for stressed orpolluted systems are H' < 1.443, severe pollutiont 1.43 < Ho < 4.328,

87

moderate pollution; H' > 4.328, clean water. These standards are forfreshwater systems. We can not expect a brackish water system to haveas high a diversity as either a freshwater system or a full marinesystem. It is probable, however, that in the past Lake Pontchartrainwould have had a species diversity of 2.2 to 2.8 similar to that ofother healthy brackish systems (Rosenberg and Moller 1979).

Gray (1978) has pointed out that the use of Caswell's neutral modelof diversity (Caswell 1976) can place diversity measures related topollution on a more theoretical basis. Caswell (1976) enjoins us toconsider the factors which may be operating to increase dominance of oneor a few species at the expense of all the rest, leading to a very.uneven distribution of abundance and a lowered value of H'. In thetheory of community structure, the undisturbed operation of biologicalinteractions acts to eliminate extreme dominance, maintaining a largernumber of species at reasonable levels of abundance, thus increasingthe diversity. Deviations in the directions of increased dominance areattributed to the action of disturbance, upsetting the internal balanceattained by the community.

In testing the neutral model predictions of diversity, a serie" ofcurvilinear regressions was constructed using Caswell's table in orderto extend the range to cover greater abundance. The diversity predittedby the neutral model for Lake Pontchartrain was 1.164. This issignificantly higher than the actual H' of 1.086 ± 0.023, the mean forall stations, over all months. A diversity less than or equal to theneutral model is predicted for highly polluted or disturbed systemstheoretically, where species equilibrium is altered, and higherdominance and lower species diversity ensues, or wherever the influenceof abiotic factors swamp the influence of biological interactions.Other low salinity, polluted estuaries have similarly low diversities;the Baltic, 1.05 (Ankar and Elmgren 1976) and 1.3 (Rosenberg and Moller1979),.Hampe'-. Aoads Area "mud" stations, 1.59 t 0.26 (Boesch 1973).

Huston (1979), using computer simulations of periodic disturbance,showed lowering of diversity as disturbances became more frequent. Hismodel also predicts low diversities under extreme conditions such as lownutrient availability or presence of toxic substances. In short, anyfactor which could cause density independent mortality will alterdiversity.

We have discussed one mechanism of compensation for low diversity;the incTease in niche breadth. An alternate mechanism frequentlyencountered (Cody 1975) is density compensation. If for any reason thespecies total at a particular site is relatively impoverished, theexisting, or remaining, species can use at least a part of the resourceswhich would have gone to the missing species. The densities of suchopportunistic species would thereby increase because they have access toadditional resources, assuming that density is limited by resourcesavailability. The loss of the large Rangia cuneata, found in con-siderable numbers by DarneIl (1979) in Lake Pontchartrain in his studies25 years ago, have made resources available for the great numbers oftiny gastropods.

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Abundance and Biomass of the Benthic Commnity

Information on mean abundance and mean biomass of several otherbenthic communities has been summarized in Table 15. The averageorganism in other benthic communities is 21 times larger than theaverage for Lake Pontchartrain macrofauna. Not only are the specieswhich occur in the lake smaller representatives of their genera orfamilies, but the size of the individuals are smaller than the averagefor collections of some of the same species from other areas inLouisiana and from other states. This condition has been noted bytaxonomic experts who have confirmed species identifications for us.Whether this is a "stunting," similar to the small bluegills in anoverstocked farmpond that have decreased growth rates because ofinadequate resource availability (Lagler et al. 1962) or a response tofactors other than competition for resources, remains undetermined.

If intense competition for limited resources is one of the factorsaffecting community structure, then changes in resources availabilityshould be reflected in measures of community structure such as"abundance. When the Bonnet Carre Floodway was opened in 1979 such aresponse was seen. The increased carbon input into the sediments causedquite noticeable responses at several stations, which were described inthe earlier section detailing results at each station. Figure 42 showsthe mean biomass and abundance for all stations over the two year studyperiod. Values for February 1979 and February 1980 were notsignificantly different. Values for May 1980 and August 1980 arerepeated as open circles on a broken line beneath the values for 1979 toemphasize the difference in the two years.

Changes in Benthic Community Structure and Functionin Response to the Opening of the Bonnet Carre Floodway

A response to the opening of the floodway is seen in the increasedabundance during March 1979, when it was leaking, through July 1979.Whether this was an immediate response to the carbon input or whetherthe i mmediate response was to lowered predation and the sustainedresponse was to the carbon input is a debatable point. Different taxaat different stations appeared to respond. Overall differences in thefirst and second years of the study, indicative of increased carboninput were these:

1) There was an overall increase in nematodes, which, with theirshort turnover time, respond quickly to additional organicsubstrate. The average increase over the lake as a whole was32Z. Not all stations experienced an increase; no stationexperienced a significant decrease.

2) There was an overall increase in biomass of Probythinellalouisianae in the lake. The average for all 13 stations morethan doubled; no station experienced a significant decrease.

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Table 15. Comparison of macrofauna abundance and biomass distribution

Sieve - Biomass BiomassStudy Site Size, N/m g/M2 mg/animal

Long Island Sound (N.Y) 1.0 16,466 54.627 3.32(Sanders 1956)

Martha's Vineyard (Mass.) 1.0 2,477 10.362 4.18(Wigley and McIntyre 1964)

Goose Greek (N.Y.) 1.4 1,201 29.460 24.53(Kaplan et al. 1974)

Lynher Estuary (V. K.) 0.5 1,436 13.240 9.22(Warwick and Price 1975)

Baltic (Sweden) 1.0 3,547 10.480 2.95(Ankar and Elmgren 1976)

Tampa Bay (Florida) 0.5 18,550 27.505 1.48(Conner and Simon 1979)

Lake Pontchartrain -- - - --

(This study 1978-79) 0.5 22,947 9.1155 0.40(This study 1979-80) 0.5 23,969 7.8778 0.33

t0

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3) Biomass of Rangia cuneata declined overall. Average biomassfor all 13 stations the second year was only 40% of the firstyear. First year biomass was not significantly different fromthat of Probythinella louisianae, but fell to 22% of secondyear P. louisianae. Rangia cuneata showed an increase inbiomass at two stations where P. louisianae accounted for lessthan one percent of the gastropod population. There were twostations where nematodes did not increase significantly (1and 12).

4) Texadina sphinctostoma biomass showed a small but significantdecrease of 18% from the first year of the study to the secondyear, averaged over all 13 stations. An increase was seenonly at those stations (1, 9, 10, and 12) where Probythinellalouisianae numbers and biomass were low.

5) No station where P. louisianae was dominant and showed anincrease, or where Rangia cuneata increased, showed asignificant increase in nematodes (Stations 1, 6, 7, 8, 12,and 13).

These changes in community structure from the first to the secondyear lead toward certain conclusions. The increase in organic carbonbrought in during the opening the Bonnet Carre Floodway permitted anincrease in microbial production. This would have resulted in animmediate increase in nematode production. At those stations whereProbythinella louisianae increased, nematodes did not. We speculatethat the increased nematode production was rapidly transferred to P.louisianae production.

The results of the series of stepwise multiple regressions tend tosupport this hypothesis. Table 13, which shows the results withnematodes as the dependent variable demonstrates the influence of thetwo gastropods on nematode abundance. This does not indicate, however,whether the gastropods are acting as predators or as competitors.

Hydrobiid gastropods are important components of some estuaries;not only in this country, but along the coasts of Europe and Africaalso. They occur in very high numbers. An average of 90,000/m, forthe period 1969-1975 was reported for the Lower Medway estuary (Waltersand Wharfe 1980). This study also reports 663,000/m2 Hydrobiids in theDanish Waddensee, and 420,000/m2in the Clyde estuary. The abundancemost frequently reported was 50,000/m2 (Fenchel 1975a, Kofoed 1975a).Many studies on these numerically important organisms have been done.Briefy, they are classified as deposit feeders (Newell 1965) that ingest"particles" with attached micro-organisms; bacteria (Kofoed 1975b),diatoms (Fenchel 1975b, Fenchel et al. 1975), and meiofauna (Lopez andLevinton 1978, Levinton 1980). Hydrobiids very often occur in largenumbers, with two to four closely related species competing for limitedresources. Various mechanisms for partitioning limited resources aredescribed: different "particle" size selection (Fenchel 1975b), anddifferent feeding strategies (Levinton 1979).

91

Both mechanisms appear to be acting to limit competition in LakePontchartrain. Texadina sphinctostoma feeds on the surface;Probythinella louisianae feeds 2 to 4 mm below the surface (Heard 1979).This strategy, coexistence at different depths by hydrobiid gastropods,is a frequently described phenomenon (Levinton 1977, 1979).

An examination of Table 14 shows P. louisianae abundance changes tobe strongly associated with nematode abundance, and temperature.Temperature, as a variable, always had a negative slope; if thetemperature is low, P. louisianae numbers are high, and vice-versa.This is an artifact of the analysis; the actual control on P. louisianaenumbers is predation pressure, which is much higher in the warm monthswhen many fish, shrimp, and crabs are in the lake feeding on them(Darnell 1958, Levine 1980), and lower during colder months whenpredators have gone out to the gulf. Nematodes were a positiveassociation; numbers of P. louisianae increased when nematodesincreased.

Texadina sphinctostoma, which suffered a decrease in abundance the-second year, showed a negative association with nematodes, a negativeassociation with temperature, and a negative association withProbythinella louisianae. In addition a weak negative association withHypaniola florida, the most common polychaete, appeared in later models.

Niche breadth (Bi) was calculated for major species. Texadina

sphinctosoma, with Bi - 29.91 ± 2.94, had a niche breadth that was notsignificantly different from the overall community niche breadth,

34.25 ± 2.90. This indicates that T. sphinctostoma is anonselective deposit feeder. Niche breadth for Probythinella

I| louisianae, B - 18.48 t 2.90 is significantly lower. If twopopulations olcur in the same habitat (have access to the same resourcebase), then the population whose members as a group tend to useresources in proportion to their availability has a broad niche relativeto a population whose members as a group tend to concentrate on someitems and bypass others (Levine 1968, Colwell and Futuyma 1971, Cody1975, Feinsinger et al. 1981). It would appear that P. louisianae withits narrower niche is concentrating on, or actively selecting,nematodes.

Niche breadth for Hypaniola florida, a polychaete, B - 16.33 ±2.18, was not significantly different from that of Probyt inellalouisianae. It also increased the second year.

The first year of the study, before the opening of the BonnetCarre Floodway, Texadina sphinctostoma, the nonselective deposit feeder,was dominant. After the opening of the floodway the increase in carboninput to the sediments caused an increase in microbial substrate whichpermitted enough increase in nematodes that the selective deposit feederProbythinella loulsianae could increase and become the dominant species.We can only speculate that the increased benthic productivity was passedon to the usual predators (fish, shrimp, and crabs) since the nektonportion of the Lake Pontchartrain studies was completed before theopening of the floodway (Thompson and Verret 1980).

92

- - -IN

I

Predation by Nekton on the Benthic Community

Abundance of macrofauna and meiofauna is strongly affected bypredation by the nekton. Levine (1980), in a recent study of feedinghabits of fish in Lake Pontchartrain, described small spot (Leiostomusxanthurus) as feeding on meiofauna until they reached 51 -, when theincidence of hydrobjid gastropods in the diet increased rapidly withgrowth. As they grow older, mollusca make up to 94% of the diet ofspot, 81% of this hydrobiids. Another important predator is the bayanchovy (Anchoa mitchelli), which is described as being an opportunisticor nonselective feeder, preying on whatever invertebrates are mostabundant.

Darnell (1958) describes the blue crab (Callinectes sapid asfeeding on molluscs. Small bivalves and gastropods constitutet c-thirds of the food volume of adult crabs. Small shrimp are mel unalfeeders (Sikora 1977); larger shrimp are efficient benthic pre, -q,ingesting, according to Darnell (1958) "many minute clams," ga. ds,ostracods, and harpacticoid copepods.

The "peaks and valleys" in the macrofauna abundance (Figure 42) aredirectly related to the migrations of these predatory nekton speciesinto and out of the. lake. Most of these species start leaving the lakefor the deeper gulf waters in November, and we see macrofauna abundancestart to rise. Many species are entering the lake again by February andthe abundance starts to drop. The first year of the study the entranceof the major predatory species was delayed by the somewhat colder riverwaters, and the decline did not begin until April.

Benthic feeders dominate the food web in Lake Pontchartrain, and

even fish that are considered planktivores in other systems are feedingnear the bottom, or on resuspended bottom animals. Levine (1980) tcoments that many of the copepods ingested were harpacticoids, probablythe dominant Scottolana canadensis.

We can see the trends we have been discussing in the benthicmacrofauna, the loss of the specialist, or the broadening of nichebreadth, in the fish fauna also. We have described the lake as a oneresource system with the distinctions between the plankton and benthosbecoming less, and the loss of distinction between filter feeders anddeposit feeders. The great increase in dominance of the opportunisticAnchoa mitchelli is an example of the changes in the fish community.

In the first comprehensive study of Lake Pontchartrain, Anchoaaitchelli, the bay anchovy, was second in abundance at 282 toMicroposonius undulatus, the croaker, at 38% (Suttkus et al. 1954). Inthe recent comprehensive survey the bay anchovy had increased to 35% andthe croaker had decreased to 18.2Z. The bay anchovy is described as anindicator of toxic stress (Livingston et al. 1978). Its increaseddominance is a generally accepted indication of toxic effects (Bechteland Copeland 1970).

93

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94.

Changes in the Benthic Community Resulting from Shell Dredging

A major and continuous perturbation which affects the benthiccom mity is shell dredging. On a lake-wide basis between 19 and 31% ofthe total lake bottom is dredged annually. In a study of the effects ofshell dredging on the benthic community it was found that a mean loss of38Z of the estimated benthic biomass production occurred (Sikora et al.1981). The reported loss was calculated from a comparison of a nearbycontrol station which was not dredged during the study. It isdifficult, if not impossible, to estimate what the loss would have beencompared to an undredged station in the 1950's when large Rangiacuneata were present in abundance in the midlake region. The loss couldonly have been greater.

Other studies have reported similar observations. Tarver and Dugas(1973) reported that R. cuneata of 2-5 am were noticeably absent fromgrid number 58 and 73 in eastern Lake Pontchartrain which wascontinually dredged by industry. Their grid number 73 corresponds tothe location of our Station 12. In a map of overall R. cuneatadistribution they show that species to be totally absent from grid 58which is adjacent to grid 73. Dugas et al. (1978) also state that R.cuneata larger than 16 mm were not recorded from any samples taken inareas that were continually dredged.

During the present study, we have observed direct evidence ofdredging at Stations 7, 9, and 10. Although dredging was not observedat Station 12 during the present study, measurements of bulk density ofthe sediments indicated that it had been intensively dredged. Tarverand Dugas (1973) state that this area was continuously dredged previousto their study. Station 12 has the next to the lowest biomass of allthe stations averaged over the entire study period (Station 1 has thelowest). The next three stations, from low to high biomass, are Station I9, Station 7 and Station 10 in that order.

Changes in the Benthic Community Resulting from Toxic Substances

Anthropogenic nitrogen loading of rivers has increased in the past 25years on a global scale (Walsh et al. 1981). The Lake Pontchartrainwatershed has experienced increases in both agricultural land use andurbanization (Turner and Bond 1980a, 1980b). These developments in landuse are the underlying cause of increased nutrient loading. It followsthen, that Lake Pontchartrain should have higher levels of primary pro-duction now than in the past.

There are other factors such as pollution by toxic substances, whichalso affect primary production, and which must be considered. Poly-chorinated biphenyls (PCB's) are currently entering the lake (U.S. ArmyEngineer District, 1.o. 1980) and found in high concentrations in midlakesediments (Sikora et al. 1981). The insolubility of PCB's, their rapidremoval from the water column by adsorption to fine particles, and sub-sequent sedimentation was once thought to be a removal process. Theopposite is actually true as shown by Harding and Phillips (1978) who

95

e| -

demonstrated that particle-bouLd PCB's are readily transferred to phyto-plankton. PCB's have the direct effect of reducing photosynthesis andgrowth, as well as chlorophyll a concentrations. A secondary effect ofPCB's on the phytoplankton community is shown by O'Conners et al. (1978).The larger species are generally more susceptible to this toxicant and areeliminated, while smaller species survive, resulting in an overall sizereduction of the phytoplankton community. Smaller sized phytoplankters maynot be grazed as efficiently by large zooplankton, which could lead tochanges in that community, ultimately altering one of the modes by whichfixed carbon reaches the bottom.

The potential effects of PCB's on the deposit-feeder food chain do notstop with phytoplankton and primary production. Bourquin et al. (1975)have shown that PCB's inhibit the growth and metabolism of many estuarinemicroorganisms. There is, then, the potential for an additive effect,further reducing the available food for deposit-feeding benthic organisms.

PCB's also have direct toxic effects on estuarine organisms such asshrimp (Nimmo et al. 1971), in concentrations of 1 ppm. PCB's can alsoalter comunity structure by selectively eliminating certain groups oforganisms. Hanson (1974) found that crustaceans, particularly amphipodsand crabs, were sensitive and suffered increased mortality; while molluscssuch as the oyster experienced significant reduction in growth withoutmortality. This selective mortality would lead to changes in speciescomposition. Lake Pontchartrain has a particularly poor amphipod fauna,usually less than one percent.

Another biocide found entering Lake Pontchartrain regularly is theherbicide 2,4,-D. Butler (1965) found that concentrations of 1 ppm reducedthe uptake of labelled CO2 by 16% in a natural plankton community composedmainly of dinoflagellates and diatoms. So we must add yet another toxic,photosynthesis-reducing agent to the list.

The organochloride insectides, dieldrin, aldrin, and chlordane,frequently exceeded the EPA criteria. In the south shore region allsamples analyzed for aldrin and dieldrin exceeded the EPA criteria at allstations. The full impact of these persistent toxicants on the LakePontchartrain ecosystem may never be known. In a recent study (Brown 1980)it was found that a British hydrobiid, Hydrobia Jenkinsi possessed anextrordinary resistance to dieldrin. This species failed to show a toxicresponse to doses in excess of 30 pg/1 of dieldrin. To what extentresistance to chlorinated hydrocarbons occurs in other species ofhydrobiids is unknown, however it is possible that as a group these smallgastropods could be more resistant, just as amphipods as a group are moresensitive to these compounds. If this were true, hydrobiids would possessan advantage over other members of the benthic comiunity which were not asresistant. Such an advantage would enable hydrobiids to utilize theresources left by the elimination of susceptible species. The LakePontchartrain benthic commnity is dominated to an overwhelming extent innumbers and biomass by two species of hydrobiids, Texadina sphinctostomaand Probythinella louisianae (Tables 8 and 9). That this was not always socan be inferred from Darnell (1962) who states "The bottom comnitythroughout the lake is now dominated by enormous populations of the clam

96

'dip,

Rangia cuneata . ... and the small gastropods Littoridina sphinctostomaand Profbthinella protera are also widespread and abundant." The exactreverse is true today.

Thus far we have discussed primarily the direct effects of toxicants onestuarine oranisms. There is also the aspect of biomagnification throughthe food chain. The brackish-water clam, Rangia cuneata has been shown toaccumulate the insecticide dieldrin directly from water (Petrocelli et. al.1973), blue crabs (Callinectes sapidus) were shown to have accumulateddieldrin when fed contaminated R. cuneata (Petrocelli et al. 1975b). Inanother experiment the same authors (Petrocelli et al. 1975a) demonstratedthat phytoplankton concentrated sublethal doses of dieldrin 1210 times;that R. cuneata fed these contaminated phytoplankton concentrated dieldrin54 times, and that blue crabs fed the contaminated Rangia cuneata con-centrated dieldrin an average of 5.75 times. In this simple food chain, wehave the theoretical potential to concentrate dieldrin over 375,000 timesthat of ambient concentrations. R. cuneata have the potential of con-centrating dieldrin over 65,000 times ambient concentration. LakePontchartrain receives dieldrin in concentrations which exceed EPA criteriafrom the north shore, the south shore, and from Pass Manchac (U.S. ArmyEngineer District, N.O. 1980) as well as from the Bonnet Carre Floodway(U.S. Geological Survey 1979).

In all the laboratory studies cited above, a single toxicant at a timewas tested. Lake Pontchartrain unfortunately receives many toxicants invarying combinations. Many of these, such as PCB's, dieldrin, otherorganochloride insecticides, and various breakdown products, persist forlong periods of time. It is well know to pest-control specialists thatcertain combinations of insecticides together have synergistic effects,which render the combination many times more lethal than either agentalone. We do not know what the possible synergistic effects are in naturalsystems. In'Lake Pontchartrain there exists a milieu of known toxicants,the individual and combined effects of which have an enormous potential toalter the ecosystem.

97

97

s= ! ! - i

SUMMARY

We have examined and documented several changes in the benthiccomunity structure of Lake Pontchartrain. The change from dominance bylarge Rangia cuneata to dominance by very small hydrobiid gastropods, forinstance, has occurred since the last large-scale study was done 25 yearsago '(St-tkue et al. 1954, Darnell 1958, 1961, 1962, 1979). We have alsodiscussed changes in benthic community function in Lake Pontchartrain. Wehave characterized the lake as functioning as a one-resource system, withthe filter-feeding zooplankton, the deposit-feeding benthos, andsuspension-feeding benthos all feeding together at the sediment-waterinterface. This loss of distinction between feeding types reflects theloss of specialized species and the dominance of the generalist or thebroad-niched species. This trend appears to be a response to lower levelsof primary production. Several factors appear to be involved in thelong-term changes we have documented. Increasing levels of pollution inthe lake and the increasing destabilization of the sediments by dredgingapparently have acted together to reduce primary production and to diminishthe number of species and abundance of the zooplankton and the benthos.Changes in dominance in the nekton appear to be related to the same chuses.The only species remaining in the lake in abundance are those known to betolerant to pollution stress, to the degree that some are consideredpollution indicator species.

The response to the Bonnet Carre Floodway opening would indicate thatthe lake as a whole is carbon limited. A management decision concerningfrequent opening of the floodway for non-flood-related purposes wouldindeed be a dilemma, since the much needed organic enrichment provided bythe river also brings toxic substances into a system that is alreadyshowing unmistakable signs of toxic stress. Changes in community structuresuch as lowered diversity and fewer species, and changes in community.function such as altered food chains and broader niches are some of theseforeboding signs.

98

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Ino

iu

1-1.

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APPENDIX B

Systematic List of Benthic Invertebratesin Lake Pontchartrain

The following list comprises a taxonomic inventory of the softbottom benthic invertebrates, exicuding the epifaunal (fouling)coummunity, found during the present study, the initial survey(Bahr et al., 1980) and a preliminary qualitative survey of 17littoral areas of the lake. The list is not exhaustive but rather

comprises the commonly encountered organisms. Numbers in bracketsafter each organism indicate the habitat in which the organism wasfound:

[1] open lake only[2] open lake and littoral[3] littoral only, intertidal or in marshes[4] passes only or in close proximity to passes.

MACROFAUNA

Phylum Rhynococoela

Unidentified species [2]

Phylum Annelida

Class PolychaetaOrder SpionidaFan. Spionidae

Boccardia op. [1]Polydora cf. socialis (Schmarda, 1861) [1]Stroblosvio benedicti (Webster, 1879) [1]

Order CapitellidaFan. Capitellidae

Capitella cf. capitata (Fabricius, 1780) [1]Mediomostus callorniensis (Sartuan, 1947) [1]

Order NereidiformYem. Pilanildae

Parandalia americana (Eartuau, 1947) [1]Fas. Nereida

Laeopereis culverl (Webster, 1880) [2]Vereis succines (Frey and Leackart, 1847) [2]

185

-- -Sil l li 1

Order TerebellidaeFan. Ampharetida.

Hypaniola florida (Hartman, 1951) now Rob sonia floridaClass Oligochaeta

Fam. NaididaeParanais litoralis (Muller, 1784) [M and [2?]

Fam. TubificidaeAulodrilus pigueti Kowalevski, 1914 [1] and [2?]Limodrilus cervix Brinkhurst, 1963 [1] and (2?]Llanodrilus claparedeianus Ratzel, 1868 [1] and [2?]Limnodrilus hoffneisteri Claparede, 1862 [1] and [2?]Monopylephorus sp. (11 and [2?]Tubificoides. heterochaetus (Michaelsen, 1926)

(aeloscolex) [1] and (2?]

Phylum Mollusca

Class GastropodaFan. Neritidae

Neritina reclivata (Say, 1822) [3]Fan. Littorinida.

Littorina irrorata (Say, 1822) (3]Fam. Ty-drobiidae

Littoridinops palustris Thompson, 1968 [3]Probythinella louisianae (Morrison, 1965)

(w. Vioscalba [1FTexadina sphinctostoma (Abbott and Ladd, 1951)

(- Littoridina) [1Pam. Ellobiidae

Detracia floridina Pfeiffer, 1856 [3]Melampus bidentatus Say, 1822 [3]

Fan. Physida.Physa spp. (31

Class BivalviaOrder Mytiloida

Pan. Mytilida.Ischadium recurvum (Rat inesque, 1820)(-. Brachidontes) [2]

Asygdalum, paM ria (Conrad, 1846) [3] and [4]Geukensia demiss, (Dillvyn, 1817) (-i Modiolus) [3]

Pan. Drelusenida*Mytilopsis loucopheeta (Conrad, 1831) (- Congeria) [2]

Order PteriodaFes. Otraidae

Crasoostres virsinica (Quaelm, 1791) [2] and [4]Order Vemero ida

tam. CorbiculidaoPolymesoda emlaw (Boae, 1802) [31

Fam. MactridaeMulinia Ponchartrainensis Morrison, 1965 (2]Rangia cuneata (Gray, 1831) [2]

Fan. TellinidaeMacoma mitchelli Dali, 1895 [1]

Fam. SolecurcidaeTagelus plebeius (Lightfoot, 1786) (2?]

Phylum Arthropoda

Class Crustaceaorder HyaidaceaFam. Mysidae

Mysidopsis almyra Bowman, 1964 [2]Mysidopsis bahia Molenock, 1969 [3]Taphromysis cf. bovmani Bacescu, 1961 [3]Taphromysis louisianae Banner, 1953 [3]

Order CumaceaFan. Nannastacidae

Almyracuma sp. (undescribed species) [21

Order TanaidaceaFan. Paratanaidae

Hargaria rapax (Rargar, 1879) (- Leptochelia) [3] and [41

Order IsopodaFan. Idoteidae

Edotea montosa (Stimpson, 1853) (2]Pan. Anthuridae

Cyathura 2olita (Stimpson, 1855) [2]'Fan. Sphaerotnatidae

Cassidinidea lunifrons (Richardson, 1900) (2]Sphaeroma terebrans Bate, 1866 (S. destructor

Richardson, 1897) [3]Pan. Asellidae

Asellus op. [3]Lirceus op. (3]

Fan. munnidaMunna cf. reynoldsi Frankenberg and Menzies, 1966 [31

Fan. LigidaeLigia exotica Roux, 1828 (3]

Order hAphipodaFan. Gamaridae

Ga ma rue uncronatue Say, 1818 [2]Gamarus ti ins Sexton, 1939 [2]Goamarus' u.(zcronato form") [31

187

Pam. MelitidaeMelita nitida Smith, 1873 (may be a complex of species) [2]

Pam. AmphilochidaeGitanopsis sp. (underscribed species) [2]

Pam. OedicerotidaeMonoculodes edwardsi Holmes, 1905 [2]

Fain. HaustoriidaeLepidactylus sp. [3]

Pam. HyalellidaeHyalella azteca Saussure, 1857 [2]

Pam. TalitridaeOrchestia grillus (Bosc, 1802) [3]Orchestia, platensis Krqiyer, 1845 [31Orchestia, uhleri Shoemaker, 1936 [3]

Pam. AoridaeGrandidierella bonnieroides Stephenson, 1948

Pan. CorophiidaeCerapus benthophilus Thomas and Heard, 1979 (4]Corophium lacustre Vanhoffen, 1911 [2]Corophium louisianum. Shoemaker, 1934 [3]

Order DecapodaSuborder NatantiaPam. Penaeidae

Penaeus aztecus Ives, 1891 (2]Penaeus setiferus Linnaeus, 1767 [2]

Pam. PalaemonidaeMacrobrachium ohione (Smith, 1874) [3]Palaemonetes kadiokensis Rathbun, 1902 [3]Palaemonetes intermedius Holthius, 1949 [3]Palaemonetes paludosus (Gibbes, 1850) [3]Palaemonetes 2ugio Holthius, 1949 [3]Palaemonetes vulgaris (Say, 1818) [3]

Suborder ReptantiaSection MacruraFam. Callianassidae

Callianassa jamaicense Schmitt, 1935 (4]

Section Brachyura,Fan. Portunidae

Callinectes sapidus Rathburn, 1896 [2]Fan. Zanthide

Panopeus herbstii Milne Edwards, 1834 (4]Rhithropanopeus harrisii (Gould, 1841) [2]

Fan. GrapsidaeSesarma cinereum (Bosc, 1801-02) [3]Semarm reticulatum (Say, 1817) (3]

188

Fan. Ocypodida.Uca loniinis Salmon and Ateides, 1968 [3]Uca minax (LeCoiite, 1855) [3]Uca panacea Novak and Salmon, 1974 [3]Uca spinicarpa Rathbun, 1900 [3]

Class InsectaOrder DipteraFan. Chironouidae (larvae)

Ablabesmyja sp. [2]

Coelotanypus sp, [2]

Cryptotanypus op. [21

MEIOFAUNA

Phyl~um Platybelziuthes

Class Turbellaria.Undentified app.

Phylum Rotif era

Class Monogononta.Brachionus op.Chromoggeter op.Cordylosoma sp.

Voshoa op.Keratella. up.Polyarthra vulgaris Carlin, 1943Proaleu op.Sixiantherina sp.Synchasta up.I

Phylum Kinorhyncha

Unidentified op.

189

I7-

Phylum Neinatoda.

Class Adenophorea (-Aphasinida)

Fam. CoinesoinatidaeSabatiera sp.

Fam. SphaerolaimidaeSphaerolainus sp. 1Sphaerolaimus sp. 2

Fain. MonhysteridaeTheristus sp. 1Theristus sp. 2

Phylum Arthropoda

Class CrustaceaSubclass OstracodaOrder Podocopida

Unidentified spp.

Subclass CopepodaOrder CalanoidaFain. Temoridae

Eurytemora affinis (Pappe, 1880)Fain. Acartiidae

Acartia tonsa Dana, 1849Order HarpacticoidaPam. Canuellide(ily 193

Fam. EctinosomidaePseudobradya sp.

Fam. TachidiidaelMicroarthridion littorale (Pappe, 1881)

Fan. DiosaccidaeSchizopera knabeni (Lang, 1965)

Fan. AmeiridaeNitocra lacustris (Schmankevitsch, 1875)

Fan. CletodidaeEnhydrosoma sp.

Fam. LaophontidaeOnychocamptus mohammed (Blanchard and Richard, 1891)Pseudoatenhelia weilsi (Coull and Fleeger, 1977)

190

OEM._ _

DISCUSSION

The above systematic list is composed primarily of organismsfound in and on soft bottom substrates in Lake Pontchartrain duringthe present study. For coverage of the hard substrate, epifaunalfouling community in Lake Pontchartrain the reader is referred toPorrier and Mulino (1975, 1977). Some species, however, such asMytilopsis leucophaeta are never found singly in soft mud but rathercolonize dead Rangia shells lying on the surface of soft mud. Otherspecies which deserve mention but which were omitted from the list areparasites which parasitize benthic invertebrates. The leech Myzobdellalugubris Leidy, 1851 is found on blue crabs and catfish. The isopodsProbopyrus floridensis Richardson, 1904 and P. pandalicola (Packard1879) parasitize Palaemonetes paludosus and Palaemonetes pugio, re-pectively and Probopyrus bithynes Richardson, 1904 parasitizesMacrobrachium ohione.

Several species are notable because of their absence. Rangiaflexuosa (Conrad 1839) was absent. No dead shells of this specieswere ever found either, although several "flexuosa" shaped shellswere found that were actually Rangia cuneata as diagnosed by thehinge teeth. Another species which was not found during the presentstudy but which has been reported by Tarver and Savoie (1976) wasTellina texans.

A curious situation presents itself in the case of Polymesoda,

carolinians, the carolina marsh clam. Both Dugas et al. (1974) andTarver and Savoie (1976) report numerous individuals from Petersongrab stations in open water. This clam was never encountered at anyof the open lake, box core stations in the present study nor wereany dead shells found at these stations. This clam is usually re-stricted to the intertidal or shallow subtidal littoral zone. Its Ireported occurrence in open water areas of the lake is unexplainedunless there was some error in identification or sample labellingin previous studies.

Tagelus plebeius was reported as occurring in low densities ineastern Lake Pontchartrain by Dugas et al. (1974). Two specimenswere encountered in the present study, a juvenile at Station 10 andan adult at Station 13, both in February 1977. This species mayoccur in the littoral area but because of its deep burrowing habits,it may have been missed.

Thomas and Heard (1979) mentioned in the ecological notesaccompanying the description of Cerapus benthophilus that Ampeliscaabdita usually occurs abundantly in close proximity to C. benthophilusin salinities of 1-15*/.. No Ampelisca abdita were found in thepresent study even though the lake bottom, where there was little orno current influence, should be a suitable habitat.

191

The insect family Chironomidae is represented by three genera.However, the fauna is overwhelmingly dominated (99%) by a singleundetermined species of Ablabesmyia. Mem.e:s of this genus are reportedto be predacious.

The meiofauna is composed of the true meiofauna and the temporarymeiofauna which are the smallest larval and post larval stages ofmacrofaunal polychaetes, gastropods and bivalves. Only the truemeiofauna are included in the list. Determinations were made to thelowest practical taxon.

LITERATURE CITED

Dugas, R. J., J. W. Tarver, and L. S. Nutwell. 1974. The molluskcommunities of Lakes Pontchartrain and Maurepas, Louisiana.Louisiana Wild Life and Fisheries Commission, New Orleans, LA.Tech. Bull. 10. pp. 1-13.

Porrier, M. A., and M. M. Mulino. 1975. The effects of the 1973 openingof the Bonnet Carre Spillway upon epifaunal invertebrates insouthern Lake Pontchartrain. Proc. Louisiana Acad. Sci. 38:36-40.

, and . 1977. The impact of the 1975 BonnetCarre Spillway opening on epifaunal invertebrates in southernLake Pontchartrain. J. Elish Mitchell Sci. Soc. 93:11-18.

Tarver, J. W., and L. B. Savoie. 1976. An inventory and study of theLake Pontchartrain-Lake Maurepas estuarine complex, Phase II-Biology, Section III Mollusks collected in Peterson samples.Louisiana Wildlife and Fisheries Commission, New Orleans, LA.Tech. Bull. 19. pp. 87-99.

Thomas, J. D., and R. W. Heard. 1979. A new species of CerapusSay, 1817 (Crustacea: Amphipoda) from the northern Gulf ofMexico, with notes on its ecology. Proc. Biol. Soc. Wash.92:98-105.

192

-- P-

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APPENiDIX C

Dendrogramit resulting from numerical classification of themacrofauta data from each sampling period for each station.Description of the analysis used is in the Methods section (pp. 31-32)Evaluation of each cluster is found in the Results section for eachstation (pp. 33-62).

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APPENDIX D

Table Dl. Benthic sampling cruise dates.

Cruise Date Stations Sampled

Preliminary Cruise 1~ Aug 78 Near STA 8

August 16 Aug 78 3, 4, 5,6, 817 Aug 78 1, 2, 7, 9, 10

September 14 Sep 78 3, 4, 5, 6, 815 Sep 78 730 Sep 78 1, 2, 9, 10

October 18 Oct 78 1, 2, 4, 9, 1019 Oct 78 3, 4,S, 7, 8

November 13 Nov 78 1, 2, 9, 10, 12, 1314 Nov 78 4,5, 6, 7,8, 1115 Nov 78 3

December 18 Dec 78 3, 4, 5, 6, 719 Dec 78 1, 2, 8, 9, 10

January 15 Jan 79 416 Jan 79 3,5, 6,717 Jan 79 1,2, 8, 9,10

February 13 Feb 79 1, 2, 9, 10, 12, 1314 Feb 79 3,5, 6, 7

15 Feb 79 4, 11

March 20 Mar 79 5, 6, 821 Mar 79 9, 1022 Mar 79 1, 2, 3, 4, 7

April 17 Apr 79 3, 4, 718 Apr 79 1, 2, 8, 9, 10

19 APr79 5, 6

may 1S5May 79 4,5, 6, 8,1116 May 79 3, 9, 10, 12, 1317 May 79 1, 2, 7

June 12 Jun 79 4, 5,6, 713 Jun 79 1, 2,3, 8,9, 10

July 17 Jul 79 919 Jul 79 1, 2, 3, 4, 1020 Jul 79 83 Jul 79 5, 6, 7

200

-w -a-,

Table Dl. (Continued)

Cruise Date Stations Sampled

August 21 Aug 79 922 Aug 79 1, 2, 10, 1323 Aug 79 3, 7, 1224 Aug 79 4, 5, 6,8, 11

November 30 Nov 79 71 Dec 79 3, 83 Dec 79 9, 10, 12, 134 Dec 79 1, 2, 45 Dec 79 5, 6, 11

February 27 Feb 80 1, 9, 10, 12, 1328 Feb 80 2, 4,5, 6,1129 Feb 80 3, 7, 8

May 20 May 80 9, 10, 12, 1321 May 80 1, 227 May 80 3, 4, 5,6, 8,11K28 May 80 7

VAugust 2S5Aug 80 3, 7, 826 Aug 80 2, 4, 5,6, 1127 Aug 80 9, 103 Sep 80 1, 12, 13

201

reen .. :.:Station locations transectfrom north shore

... ... ...

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Figure Dl. Station locations on transect from north shore,Lake Pontchartrain, Louisiana, 9 July 1980.

Table D2. Rangia cuneata data from north shore transect,9 July 1980

Station.Dls tance fromt e a. 0I.5 2 6 2. . c -a s s , " n. c u n a ta , I n s

depth Ln t Core 2-10 1020 20-30 '30

71* A 8 219 0 0 00.4 ke a 3 270 0 0 12.1 . C 0 209 0 L 0

T2* A 16 234 0 2 10.8k. 8 0 273 0 0 02.1. C 0 251 0 0 1

T3* A 0 507 0 0 01.2 k 8 20 309 0 1 12.7 . C 27 348 0 1 0

T4* A 0 550 0 0 01.6 k 8 0 255 0 0 02.7 a C 0 680 1 0 1

6.17 342.08 0.08 0.42 0.42

-D/2 67.17 3726.21 0.91 4.54 4.54

i~ Total /.2 3803.37

2 1.77 97.97 0.02 0.12 0.12

T5 A 0 491 0 0 02.4 km 1 0 656 0 0 03.0. C 0 553 0 0 0

T6 A 0 337 0 0 07.2 k. 8 101 417 0 0 04.2. aC 0 380 0 0 0

*'Thea. $tcatoos correspo d to Falrbasks' (1963) statione In depth and dLsanceftrom $Mrs.•

203

APPENDIX E

Sediment Methodology

Sediment Organic Carbon Analysis

Sediment samples for organic carbon determination were preparedin the following way: the sample was throughly mixed and 5-10 gmof sediment was removed and pretreated with 0.4 HCl for 24 hours toremove carbonates. The sample was then washed to remove the excessHCI, and centrifuged. The supernatant was discarded, and the pelletwas resuspended in distilled water and recentrifuged 5 times, eachtime discarding the supernatant. A pH check was made during the lastwashing to be certain that pH - 7. The sample was then oven-driedat 60* C for 24 hrs and ground with mortar and pestle. A subsampleof 250 mg was then combusted in a LECO model 521 induction furnacecoupled to a LECO semiautomatic gasiometric carbon determinator.Two determinations were made for each sample.

Grain Size Analysis

The hydrometer method of particle size analysis as describedby Day (1956) and modified by Patrick (1958) was used to determinesediment grain size. Briefly, the method consists of floating anA.S.T.M. 152H hydrometer in a sample of sediment suspended in acylinder and taking readings at predetermined intervals at knowntemperatures. Statistical treatment of data followed McBride(1971). Hydrometer methods have been determined to be sufficientlyaccurate by the Committee on Physical Analysis of the Soil ScienceSociety of America to separate soil samples into the three sizefractions of clay, silt, and sand (assuming no gravel is present).

Sediment Bulk Density Determinations

Bulk density samples were obtained using core tubes, which weremade of 5 cm long core segments taped together with waterprooftape to form one core tube. The in situ samples were taken withI core tubes constructed of core segments cut from standard 50 ccplastic syringes, 2.55 cm in diameter, with the leading segmentbev.eled to form a cutting edge. Each core segment was numberedand premeasured for volume. The core tube was inserted slowlyinto the box core sample with a gentle rotation. After removing

the core tube containing the sample, the outside was washed and thetape holding each segment was cut, and a piece of preweighedaluminum foil was inserted as the segment was removed and placedin a preweighed plastic vial with a tight-fitting cap. The samples

204

were refrigerated, brought back to the laboratory, and-weighed. Totalsediment weights for each 5 cm sediment interval were calculated bysubtracting plastic vial weight, core segment weight, and aluminumfoil weight for each sample. Sediment bulk densities in g/cm8 werecalculated by dividing total sediment weight by the core segmentvolume.

LITERATURE CITED

Day, P. R. 1956. Report of the committee on physical analysis, 1954-55, Soil Science Society of America: the hydrometer method ofparticle size anlaysis. Soil Sci. Soc. Amer. Proc. 20:167-169.

McBride, E. F. 1971. Mathematical treatment of size distributiondata. PP. 109-127. In: R. E. Carver (ed.). Procedures insedimentary petrology. Wiley-Interscience, New York.

Patrick, W. H. 1958. Modification of method of particle sizeanalysis. Soil Sci. Soc. Amer. Proc. 22:366-367.

2

205

APPENDIX F

LAKE PONTCHARTRAIN "DEAD ZONE" INCIDENT

During the last scheduled cruise, August 1980, of the present study(Ecolcgical Characterization of the Benthic Community of LakePontchartrain, Louisiana), a large area of bottom in southeastern LakePontchartain was found to be defaunated. This occurrence was labelledthe "Lake Pontchartrain dead zone" by local news media in Baton Rougeand New Orleans. The following account and discussion of the "deadzone" incident is included here because of the large size of theaffected area and possible ecological consequences to the lake as awhole.

The cruise began on 25 August 1980 with stations 7, 3, 8, dredgingexperimental, and dredging control being sampled. The following day, 26August 1980 stations 6, 5, 11, 4, and 2 were sampled; seas increased toover 2 feet and winds of 30 knots were reported during a storm between0900 and 1030. On the third day 27 August 1981 stations 9 and 10 weresampled. After station 9 was sampled the port engine stopped andcouldn't be restarted. Station 10 was sampled after which we returnedto port, with winds increasing to 20 knots. After the engine wasrepaired the sampling cruise was resumed on 3 September 1980 . The re-mainder of the stations (1, 12, and 13) were sampled that day. Seas wererough and at 0920 hrs and later at 1600 hrs white caps were visible.

The "dead zone" was first discovered on 3 September 1980 atstations 1 and 12 (Figure Fl). The sediment surface at both stationswas totally black in color instead of the usual brown color and therewas a slight odor of hydrogen sulfide present. Both these conditionsindicate a depletion of oxygen in the surface sediments at the time ofsampling. What made the discovery alarming was the distance between thetwo stations. Station 1 is located approximately 1 mile off Bayou St.John while station 12 is 6.5 miles out into the open lake from station1. Analysis of the biological data confirmed our initial appraisal thatthe area had been subjected to an acute environmental perturbation.

Results

Biological data (Table Fl) as well as physical parameters (TableF2) measured are given for stations 1, 2, and 12 for August-September1980, as well as, August 1979 and August 1978.

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Figure Fl. Map shoing station locations in the "dead zone" area. Mapcoordinates of the stations are as follovs: Station 1, 30003'06"N,-90004'55"W; Station 2, 30013'04"N, 90;07'25'V; Station 12,30*07'44"N, 90 02109'V; Station P1, 30 05'00"N, 90'03'48'V;Station P2, 30°03'04"N, 90007'25"V. Station TB is the turningbasin at Seabrook in the Inner Harbor Navigation Canal.

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Table Fl. Oxygen, temperature, conductivity and salinity by depth at"dead zone" stations in August 1978, August 1979, andAugust-September 1980.

Au8 . - Sept. Aug. Aug.1980 1979 1978

"/* PP.4 C "/*. PPM *C "I,0 2

t Temp Cond Sal * 02 Temp Cond Sal * 02 Tamp Cond Sal *

Station ISurfa.e 29.4 8.48 4.87 7.2 30.0 6.87 3.90 9.2 30.5 6.53 3.70IN 29.4 8.50 4.88 7.0 30.0 7.02 3.99 9.0 30.4 6.52 3.692. 29.4 8.41 4.89 6.4 30.0 7.49 4.28 6.7 29.5 6.99 3.9734 29.4 8.50 4.88 4.4 30.3 11.50 6.69 5.8 29.4 7.56 4.32S M, 29.4 8.53 4.90 1.1 30.5 L4.20 8.32 2.1 29.3 10.24 5.93

Station Isuri. 30.2 7.68 4.39 7.2 29.9 6.15 3.47 9.0 30.5 7.12 4.05IM 30.2 7.69 4.40 7.1 29.9 6.16 3.47 9.0 30.5 7.12 4.05

0 2M 30.2 7.70 4.40 7.1 30.0 6.27 3.54 8.8 30.0 7.12 4.053X 30.1 7.70 4.40 7.6 30.0 6.38 3.61 6.6 29.6 8.49 4,944M 30.1 7.70 4.40 2.8 30.4 12.10 7.06 2.4 29.1 9.05 5.20

station I2Suriace 29.8 9.09 5.24 8.1 29.9 6.98 3.97-IM 29.8 9.14 5.24 7.5 29.6 6.88 3.85 in

Z 2. 29.7 9.14 5.24 7.2 29.4 6.86 3.83 30?3M 29.7 9.13 5.26 7.1 29.4 6.87 3.84 t oG -

4M 29.6 9.15 5.27 4.6 29.6 7.87 4.75

toxygens not taken in 1980 - oxyten moter brokani

*salinity valuum converted from conductivity (salinicy uonduc vMX -0.4)1.63-

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Discussion

It became immediately apparent from the analysis of the biologicaldata that station 2 was affected also, despite the fact that station 2sediments appeared normal on the day they were sampled. All threestations (1, 2, and 12) exhibited the same pattern of macrobenthicdefaunation in the 1980 samples: all bivalves, all polychaetes andall crustaceans were killed. In fact, all groups were killed at allthree stations except the two species of hydrobiid snails and chironomidlarvae. The chironomids were also completely missing from station 12which was apparently the most seriously affected station. Total numbersof organisms per square meter at Station 12 were reduced to less than 1%of what they were in August 1979, or a 99% reduction. Station 1suffered an 82% reduction and station 2 suffered a 79% reduction.

This great a reduction in numbers per square meter from previousyears during the same season is reason enough for alarm. Even moresignificant, however, is the pattern of reduction. As shown in TableF2, these stations have been subjected to low oxygen concentrationsin August 1979 and 1978. Yet in these years an intact community,characteristic of those stations, was present. In fact, not only aremany benthic organisms capable of surviving low oxygen concentrations,but continue anaerobic metabolism even when exposed to fully oxygenatedconditions (Pamatmat, 1980). Members of many groups including bivalves,gastropods, polychaetes, and crustacea have been shown to be facultativeanaerobes which normally undergo anaerobic metabolism as an energysaving strategy (Pamatmat, 1980). Chen and Awapara (1969) kept Rangiscuneata in deoxygenated water for three weeks without apparent harm tothe animals during a study of glycolysis in R. cuneata. These authorsgo as far as to consider R. cuneata, from biochemical standpoint, asessentially an anaerobic organism. It appears unlikely that low oxygen tconcentrations in the bottom water of the lake would kill the benthic

infaunal community. It is uncertain whether low oxygen conditions inthe bottom waters developed during the period of sampling in 1980. Eventhough we were unable to directly measure oxygen concentrations duringthe August-September 1980 sampling, we were able to measure conductivityand convert to salinity. As Poirrier (1978) points out, low oxygenconditions in the bottom waters of this area of the lake are caused by a

4non-mixing bottom water layer of higher salinity, as he states which "ifweather conditions were such that mixing did not occur for extendedperiods, dissolved-oxygen values would be lowered."

Weather conditions during the time of sampling were such thatstorms caused 30 and 20 knot winds on August 25 and 26, 1980, re-spectively, and white caps developed on September 3, 1980. Waves causedby winds of 15-20 mph affect bottom sediments in Lake Pontchartrain(Swenson 1980). From the salinities measured by depth at station 2 onAugust 26, 1980 and stations 1 and 12 on September 3, 1980 the watercolumn appears to be well mixed with no salinity stratificition. Aslight salinity stratification at all statiotis in August 1979 and August

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1978 was present as were low oxygen concentrations. It appears unlikelythat low oxygen concentrations were present in 1980 when these stationswere sampled because prevailing weather conditions precluded watercolumn stratification for an extended period of time.

The fact that some hydrobiid gastropods survived at all threestations may be highly significant. Brown (1980) working wit1i anotherspecies of hydrobiid, Hydrobia lenkinsi reports that this species isextremely resistant to the toxic effect of the chlorinated hydrocarbonpesticide dieldrin. H. jenkinsi did not show any toxic effects todieldrin in concentrations in excess of 30,000 ppb. It is possible thatresistance to chlorinated hydrocarbons is a characteristic shared byother hydrobiids, and may explain why some of the hydrobiids remainedalive in the "dead zone" if chlorinated hydrocarbon compounds caused thedefaunation. Hydrobia Jenkinsi was, on the other hand quite sensitiveto the toxic effects of heavy metals particularly cooper and chromium.It should also be pointed out that both species of hydrobiids in LakePontchartrain have been collected in the plankton and could be moved inthe water column. Some individuals could have been transported intothe dead zone areas after the initial defaunation occurred.

At first it was thought that pentachoropenol (PCP) might havecaused the "dead zone" because of a ship collision in the MississippiRiver-Gulf Outlet near Shell Beach which resulted in a large PCP spillin late July 1980. One of the ships involved in the collision (the SeaDaniel) was brought back to the Inner Harbon Navigation Canal and dockedat the Seabrook turning basin. Several hundred pounds of PCP reportedlywashed off the deck of this ship and into the canal during a heavy rainstorm.

One week later, on September 11, 1980 another cruise was made tothe area of the "dead zone" in order to collect sediment samples andbiological samples. Sediment samples were collected in specially pre- !pared glass containers supplied by the Center for Bio-Organic Studies ofthe University of New Orleans. Surface sediments were scooped with theglass contiiners from box cores taken at stations 1, 2, and 12, and atstations P1 and P2 half way between station 12 and the southern shore ofthe lake (Figure Dl), and at the IHNC turning basin. The samples wereimmediately frozen with dry ice onboard and deposited in a freezer thenext day at UNO.

The Center for Bio-Organic Studies was at the time, routinelyanalyzing large numbers of sediment samples from the area near the siteof the ship collision and kindly offered to do a preliminary analysis ofthe samples for PCP. The results of the preliminary analysis showedthat no PCP was present in the "dead zone" samples, however, the samples

did contain relatively high concentrations of three unknown halogenatedhydrocarbons.

In light of the forgoing considerations there is sufficientreason to suspect that an event of acute chemical toxicity occurredin southeastern Lake Pontchartrain in the late summer of 1980.

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LITERATURE CITED

Brown, L. 1980. The use of Hydrobia jenkensi to detect intermittenttoxic discharges to a river. Water Res. 14:941-947.

Chen, C., and J. Awapara. 1969. Effect of oxygen on the end-productsof glycolysis in Rangia cuneata Coup. Biochem. Physiol. 31:395-401.

Pamatmat, M. 1980. Facultative anaerobiosis of benthos pp. 69-90. InK. Tenore and B. Coull (eds.) Marine Benthic Dynamics. The BelleW. Baruch Library in Marine Science No. 11. University of SouthCarolina Press, Columbia, S.C.

Poirrier, M. 1978. Studies of salinity stratification in southernLake Pontchartrain near the Inner Harbor Navigation Canal. Proc.Louisiana Acad. Sci. 41:26-35.

Swenson, E. 1980. General hydrography of Lake Pontchartrain, Louisiana.PP. 157-215 In: J. H. Stone (ed.) Environmental analysis of LakePontchrartraln, Louisiana its surrounding wetlands and selectedland uses. CEL, CWR, LSU, Baton Rouge, LA. 70803. Prepared forU.S. Army Engineer District, New Orleans,. Contract No. DACW 29-77-C-0253.

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