Marine Biology (1995) 124:387-397 �9 Springer-Verlag 1995

R. S. K a u f m a n n �9 K. L. Smith, Jr �9 R. J. Baldwin R. C. Glatts �9 B. H. Robison �9 K. R. Reisenbiehler

Effects of seasonal pack ice on the distribution of macrozooplankton and micronekton in the northwestern Weddell Sea

Received: 14 June 1995/Accepted: 14 July 1995

Abstract The presence of mesopelagic organisms in the guts of surface-foraging seabirds feeding in open areas within seasonal pack ice in the Antarctic has given rise to questions regarding the effects of pack ice on the underlying mesopelagic community. Bottom-moored free-vehicle acoustic instruments were used in concert with midwater trawls and baited traps to examine the abundance, size distribution and vertical distribution of pelagic organisms in the uppermost 100 m of the water column during the austral spring of 1992 in two areas of the northwestern Weddell Sea, one covered by sea- sonal pack ice and the other free of ice cover. Acoustic targets were more abundant and significantly larger at the open-water station than beneath pack ice. How- ever, targets at the ice-covered site exhibited a pro- nounced diel pattern, with the largest targets detected only at night. Samples from night trawls at the ice- covered site contained several species of large, vertically-migrating mesopelagic fishes, whereas these species were absent from trawls taken during the day. In addition, baited traps deployed in pack ice just beneath the ice-water interface collected large numbers of scavenging lysianassoid amphipods, while deeper traps beneath the ice and traps at the open-water sta- tion were empty, indicating the presence of a scaveng- ing community associated with the undersurface of the ice. These results support the idea that mesopelagic organisms migrate closer to the surface beneath pack ice than in open water, exposing them to possible predation by surface-foraging seabirds.

Communicated by M.F. Strathmann, Friday Harbor

R.S. Kaufmann ( ~ ) . K.L Smith, Jr �9 RJ. Baldwin. R.C. Glans Marine Biology Research Division, Scripps Institution of Oceanography, La Jolla, California 92093-0202, USA

B.H. Robison - K.R. Reisenbichler Monterey Bay Aquarium Research Institute, 160 Central Avenue, Pacific Grove, California 93950, USA

Introduction

The waters around the Antarctic continent are some of the most productive on earth, supporting an abundant and diverse pelagic community (Lancraft et al. 1991; Hopkins et al. 1993; Voronina et al. 1994). Perhaps the most prominent feature in this area of the ocean is the Antarctic ice sheet, which covers roughly 4million square kilometers during the austral summer, but may expand during the winter to encompass 20 million square kilometers (Zwally et al. 1983). The presence of an ice layer at the sea surface may have profound effects on the biota of the underlying water column. For example, the biomass of primary producers in the water column beneath ice cover is typically lower than the biomass of primary producers in nearby open water (Kottmeier and Sullivan 1990; Bianchi et al. 1992; Gar- rison et al. 1993); however, the ice itself may support high abundances of algae that can influence primary productivity in the surrounding waters while providing a source of nutrition for organisms living beneath the ice (Stretch et al. 1988; Garrison and Buck 1989; Spindler 1994).

Many species in this area of the ocean exhibit life cycles that are linked to the seasonal pack ice to vary- ing degrees (Marschall 1988; Miller and Hampton 1990; Smetacek et al. 1990; Siegel et al. 1992; God- lewska 1993). However, until recently, little was known about the community living beneath the Antarctic sea ice. Increasing understanding of this unique environ- ment has given rise to a number of questions about the under-ice community and its interactions with other portions of the Antarctic fauna, especially organisms inhabiting the underlying water column and apex pred- ators such as seabirds and marine mammals.

Within the past decade, the discovery of mesopelagic species in the guts of surface-feeding seabirds foraging in areas of open water amidst heavy pack ice (Ainley et al. 1986, 1988) has presented the intriguing possibility

388

of trophic coupling between two apparently disjunct communities. Although many of the mesopelagic spe- cies found in bird guts are known to migrate vertically on a daily basis (Torres et al. 1985; Torres and Somero 1988; Lancraft et al. 1991), most have not been caught at depths of <100 m in open water (Lancraft et al. 1989; Hopkins et al. 1993), while the seabird species sampled are not known to forage deeper than 10 m (Ainley et al. 1986, 1988).

The current interpretation of this phenomenon is that the presence of sea ice and the suite of biological, chemical and physical conditions associated with total ice cover may induce mesopelagic organisms to ascend to much shallower depths than in open water (Ainley et al. 1986). Seabird predation on mesopelagic species occurs when leads or polynyas form in the pack ice, allowing these predators access to organisms near the sea surface. Gut-content analyses of mesopelagic fishes caught under the ice (Hopkins and Torres 1989) and taken from seabird guts (Ainley et al. 1986,1988; Hopkins et al. 1993) have revealed that a number of these vertically-migrating fishes are preying on shallow- water zooplankton (e.g. Euphausia superba, Thysanoessa macrura), many of which live in association with the undersurface of the pack ice (O'Brien 1987; Marschall 1988; Stretch et al. 1988; Siegel et al. 1992). Thus, at least some mesopelagic species may be attracted to surface waters by prey associated with the sea ice com- munity.

To gain a greater understanding of the relationship between pack ice and the underlying pelagic commu- nity, a sampling program was undertaken to examine the effect of pack ice on macrozooplankton and micronekton in the epipelagic zone (defined here as the upper 100 m of the water column). The goals of this program were directed toward addressing three ques- tions: (1) How does the presence or absence of seasonal pack ice affect the abundance of macrozooplankton and micronekton in the epipelagic zone? (2) How does the presence or absence of seasonal pack ice affect the size distribution of macrozooplankton and micronek- ton in the epipelagic zone? (3) How does the presence or absence of seasonal pack ice affect the vertical distri- bution of macrozooplankton and micronekton in the epipelagic zone?

We conducted a short-term feasibility study to evalu- ate the possible use of bottom-moored free-vehicle acoustic instruments to monitor the abundance, size distribution and vertical distribution of animals in the upper 100m of the water column beneath seasonal pack ice and in nearby ice-free waters. Bottom-moored free vehicles are independent of any ship or surface mooring (Smith et al. 1979), while acoustic techniques are non-invasive, minimizing or eliminating mechan- ical disturbance associated with sampling instrumenta- tion. To provide a reference frame for the acoustic data, two additional collecting methods were employed: a large (10 m a mouth), opening-closing Tucker trawl to

collect pelagic animals within discrete depth intervals, and baited traps to sample the scavenging community, including animals that might not commonly be caught in towed nets.

Materials and methods

Two locations in the northwestern Weddell Sea were occupied during the austral spring of 1992, using the research icebreaker "Nathaniel B. Palmer". One of these locations was roughly 40 km south of the ice edge in an area covered by first-year seasonal pack ice (up to 1 m thick), while the other was located in open water to the west of the South Orkney Islands (Fig. 1A). Hydrographic informa- tion for both locations was obtained from conductivity temperature depth (CTD) casts to > 900 m depth (bottom depth in both areas was ~ 1000 m). Temperature and salinity profiles from the ice-covered area (Fig. 1B) revealed a surface layer of relatively cold ( ~ 1.8 ~ water with a salinity between 34.2 and 34.3 %0, becoming substantially warmer ( ~ 0 ~ and more saline ( > 34.6 %0) with increasing depth. These profiles are consistent with published hydrographic data for the northwestern Weddell Sea (Gordon 1988; Hopkins and Torres 1988). Temperature and salinity profiles from the open-water station (Fig. 1B) were noticeably different, with high- er surface temperatures ( ~ - 1.3 ~ and salinities ( ~ 34.4 %0) and a less pronounced gradient with increasing depth. These character- istics are typical of hydrographic conditions generally associated with the Weddeli-Scotia Confluence, an area of intense mesoscale mixing at least partially associated with the relatively shallow topography of the South Scotia Ridge (Deacon and Foster 1977; Gordon 1988).

Two upward-facing, split-beam acoustic instruments were de- ployed at each of these two stations during late September and early October 1992 (Fig. 1A). These instruments were moored on the bot tom and positioned ~ 100 m beneath the sea surface in areas with bot tom depths of 1050 to 1100 m. The operational frequency of each instrument was 72 kHz, resulting in an acoustic wavelength of 2.0 cm. Each instrument was programmed to ensonify an 8600 m 3 conical section of the water column (102.4 m vertically upward from the transducer with a beam angle of _+5 ~ every 5 s for 1 rain (12 acoustic "pings" per group). Twelve-ping groups were separated by 6 rain (10 groupsh 1) and individual deployments lasted 1 to 2 d. Individual acoustic pings were 40 cycles in length, yielding an effec- tive vertical resolution of 41.33 cm over the 102.4 m vertical range of the instrument; this range was divided into 512, 20 cm range bins. Each transducer was divided into four quadrants, and two records from each quadrant (one in phase and one 90 ~ out of phase) were stored for each range bin, resulting in 2048 data points per ping. Unfortunately, on all four deployments, the acoustic transducers were positioned slightly ( ~ 5 to 15 m) too deep to permit ensonifica- tion of the ice-water or air-water interface at the ice-covered and open-water stations, respectively.

During the cruise, calibration tests were run on both arrays in a shallow area in the lee of Coronation Island (South Orkney Islands). The arrays were moored with the transducers at a depth of 96 m, and two copper calibration spheres (23 and 60 mm diam) were placed 5 m apart on a monofilament line and suspended in the field ensonified by one of the arrays at distances of 75 to 85 m above the transducer (Foote 1982,1983); attempts to suspend the same calibration spheres in the field ensonified by the second array were unsuccessful. Measured target strengths (TS) for the two copper spheres differed from theoretical predictions by <1 dB (23 mm sphere: mean measured TS _+ SD = - 44.44 _+ 0.29 dB, predicted TS = - 4 4 . 7 d B ; 6 0 m m sphere: measured TS = - 33.38_+ 0.36 dB, predicted TS = - 33.6 dB; cf. Foote 1983, 1990). A correc- tion factor of 0.24 dB was subtracted from all biological targets, corresponding to the difference between the predicted and measured target strengths for the calibration targets. In the absence of direct

389

Fig. 1 A Vertically-profiling A acoustic array stations in northwestern Weddell Sea, and boundaries of EPOS (European Polarstern Study) and AMERIEZ (Antarctic Ice Edge 5! Zone) studies. Ice-covered stations (O) were 115S (26-28 September 1992; 61~ 41~ 1070 m depth) and 120S (27-29 September 1992; 61~ 41~ 1059 m depth); open-water stations (at) were 140N (4 6 October 1992; 60~ 49~ 1062 m depth) and 141N (4-6 October 1992; 60~ 49~ 1088 m depth). B Conductivity-temperature depth profiles from study sites in ice-covered and open-water portions of northwestern Weddell Sea. Each trace is average of two complete water- column profiles to < 50 m above bottom [cross-hatched traces temperature profiles; smooth traces salinity profiles; Ice cover Station 109S (25 September 1992; 61~ 41~ and Station 135S (1 October 1992; 61~ 41~ Open B water Station 136N (3 October 1992; 60~ 49~ and Station 148N (6 October 1992; 60~ 49~

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measurements for the second acoustic array, the same correction factor was applied to data collected with both arrays.

After recovery of the instruments, the acoustic data were down- loaded to a personal computer and separated into individual pings. The return magnitude (the intensity of the echo from a target at the receiver, measured in arbitrary units), target strength and phase angle (defined as the angle from which the return was received relative to the central axis of the transducer's main lobe and deter- mined by combining phase information for adjacent pairs of trans- ducer quadrants) were calculated for each return, with corrections for transmission loss and non-zero angular elevation of the return (cf. Smith et al. 1989). Corrections for non-zero return angles were made by adding to each target strength a factor equal to the difference (in dB) between an on-axis return and one deviating from the central axis of the main lobe by an angle equal to the phase angle. Correction factors were obtained from beam patterns mea-

sured for each transducer at distances of 3 to 10 m in a large test tank at the Naval Research and Development (NRaD) facility in San Diego, California.

Return magnitudes were plotted as a function of time and distance from the transducer, and individual targets were identified from these plots. Targets were distinguished from background noise by their non-random spatial and temporal distribution, as well as by their phase angle, Targets used in this analysis appeared in a minimum of three adjacent 20 cm range bins over three or more consecutive pings. In addition, potential targets were rejected if the phase angles from all quadrant pairs did not provide a single unique location for the target. This criterion also effectively minimized the possibility of unintentionally considering multiple targets to be a single target, since multiple targets would be expected to generate inconsistent phase angles as their orientation relative to the transducer changed with time. However, this criterion may

390

also have resulted in underestimation of the actual number of targets present.

Target strengths for individual targets were used to generate estimates of animal body length, using the relationships

TS = 10 log

and

)~ = 0.021

where TS is the target strength in decibels (dB), cr is the acoustic cross-section in square meters, 2 is the wavelength of the acoustic signal (in this case 2.00 cm), and E T L is the estimated target length in meters (Love 1977). These equations were derived empirically using measurements for a number of aquatic species over a range of frequencies and have been used effectively for estimating animal lengths from acoustic target strengths measured in the field (Smith et al. 1989,1992).

In both the ice-covered and open-water areas, the upper 300 m of the water column was also sampled during the day and at night using an opening-closing Tucker trawl (9 m 2 mouth opening with a 5 mm mesh liner and 0.333 mm mesh in the cod end). This net was typically towed at a speed of 2.0 knots for periods of 37 to 66 min. A small flowmeter mounted inside the mouth of the net was intended to facilitate estimation of the volume of water sampled during each tow, however occasional mechanical problems with this instrument prompted us to estimate the sampling volume in the following manner. The volume of water sampled was calculated by multiplying the effective cross-sectional area of the net (calculated using the actual mouth area and measured fishing angle) by the average of flow estimates from: (1) the in-mouth flowmeter, (2) the total dis- tance traveled during the tow as determined from Global Position- ing Satellite (GPS) fixes, (3) the average ship speed through water during the tow multiplied by the duration of the tow. In cases where one of these three estimates differed from the others by more than a factor of two, the two most consistent values were averaged to generate an estimate of the volume of water sampled. Trawls were carried out during the day and at night, with daytime trawls taking place between 09.00 and 15.00 hrs local time and nighttime trawls between 21.00 and 03.00 hrs local time, respectively (sunrise oc- curred between 04.30 and 05.30 hrs, with sunset between 17.30 and 18.30 hrs). Because of equipment failure, no useful trawl samples were collected at the open-water station. Hence, all discussions of and references to Tucker trawl collections as components of this study are restricted to samples taken at the ice-covered site.

After recovery of the net, all fishes were identified to species and measured to the nearest 0.1 mm standard length, while invertebrates were preserved in 10% seawater-buffered formalin for later identi- fication. Unfortunately, a number of jars containing invertebrate specimens were broken before their contents could be counted and definitively identified. Thus, quantitative abundance data are avail- able for all fishes but for invertebrates from only some trawls.

In the ice-covered area, two moorings with baited minnow traps at depths of 0, 10,50, 100 and 200 m were deployed concurrently through holes drilled in the ice. A weight was attached to the bot tom of each mooring to maintain the line in a vertical orientation. Each trap was baited with ~1 lb (454 g) of Antarctic cod, with the bait wrapped inside two mesh bags to prevent animals from accessing the bait and feeding to satiation then leaving, and also to prevent destruction of the bait. A similar trap array was deployed from a floating buoy at the open-water site. In addition, a bottom- moored, free-vehicle trap array was deployed at a depth of 2300 m at the open-water site, with traps at altitudes of 2,5,10,25, 50,100,200, 300, 400, 500, 600, 700, 800, 900 and 1000 m. Soak times in both areas ranged from 14 to 24 h. In both areas, the surface- moored traps drifted during the course of their deployments. At the open-water station this drift was due to surface currents, while in the

pack ice the drift resulted from the drift of the ice sheet in which the traps were anchored.

Results

Abundance of macrozooplankton and micronekton

The acoustic instruments were deployed for a total of 66.63 h at the ice-covered station and 98.73 h at the open-water station (Table 1). Acoustic targets were more abundant at the open-water location than at the ice-covered site, with 10.0 and 7.6 targets per hour of sampling, respectively (Table 1). A portion of the open- water acoustic record spanning 1.75 h contained an unusually large surface return, most probably an ice- berg. Acoustic targets were nearly twice as abundant beneath this feature as in the surrounding water col- umn (21.4 vs 10.0 targets per hour of sampling).

The most abundant fishes captured by the Tucker trawl in the ice-covered area were the Antarctic silver- fish Pleuragramma antarcticum and the myctophid Electrona antarctica, with a number of juvenile NotoIepis cf. coatsi collected as well. By far the most abundant invertebrates were the euphausiids Eu- phausia superba and Thysanoessa macrura, with lesser numbers of copepods, hyperiid amphipods, ca- lycophoran siphonophores (Diphyes cf. antarctica), pteropods (Clio sp.) and chaetognaths (Table 2).

At both sites, more targets were detected at night than during the day, although this pattern was less pronounced at the open-water station than beneath the

Table 1 Data collected by free-vehicle bot tom-moored acoustic in- struments deployed in ice-covered and open-water areas of Weddell Sea. Ice-covered stations were 115S (26-28 September 1992; 61 ~ 32.54'S, 41 ~ 54.28'W; 1070 m depth) and 120S (27-29 September 1992; 61 ~ 30.62'S, 41 ~ 39.44'W; 1059 m depth). Open-water stations were 140N (4-6 October 1992; 60 ~ 14.96'S, 49 ~ 47.74'W; 1062 m depth) and 141N (4-6 October 1992; 60 ~ 13.13'S, 49 ~ 50.62'W; 1088 m depth). Mean target strengths were generated by calculating a mean value for each set of return magnitudes and converting the result to target strength

Ice Open "Iceberg" cover water

Deployment duration (h) 66.63 96.98 1.75

Pinging time (min) 322 378 14

Targets detected 41 63 5

Target density (nos h -1) 7.6 10.0 21.4

Target strength (dB) mean - 51.5 - 47.1 - 47.8 min - 59.0 - 62.4 - 50.0 max - 40.1 - 41.3 - 44.1

Estimated target length (cm) mean 9.2 13.3 11.6 min 3.0 1.8 8.5 max 28.7 24.9 17.6

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Table 2 Densities (nos 1000 m- 3) of species collected in day and night Tucker trawls in seasonal pack ice [ + present in trawl sample but quantitative estimate precluded by breakage of jars prior to enumeration (see "Materials and methods")]

Taxon Day depth (m) Night depth (m)

10 35 50-85 91-160 185-290 10 35 50-85 13(>180 180-240

Cnidaria Calycophoran siphonophore Diphyes cf. antarctica + 5.13 1.53 ?Euphysaora sp. 0.04

Ctenophora Aulococtena sp. + + Beroe ?cucumis

Mollusca: Pteropoda Clio sp. + + 0.03 0.41 Unidentified sp. + +

Mollusca: Cephalopoda Unidentified Taoninae + Galiteuthis 91aciaIis 0.27 0.I 1 Psychroteuthis 91acialis

Annelida: Polychaeta Unidentified Alciopidae + + Unidentified sp. + 0.03 0.04

Arthropoda: Crustacea Euphausia superba O. 15 Thysanoessa macrura + + 14.51 21.37 Cyllopus Iucasii 1.56 0.07 Hyperiella dilatata + + Primno macropa 0.06 0.11 Abyssorchomene rossi Unidentified copepod 2.38 3.88

Thaliacea Salpa ?thompsoni Unidentified gymnosome 0.46 0.45

Chaetognatha Unidentified sp. + 2.96 2.68

Larval/juvenile fishes Pleuragramma antarcticum 0.04 Notolepis cf. annulata Notolepis cf. coatsi 0.12 0.41

Adult fishes Lampanyctus cf. achirus Gymnoscopetus braueri Electrona antarctica Bathylaqus antarcticus

249.94 0.20 2.15 0.03 0.44 0.94

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ice (Fig. 2). A t empora l pa t t e rn in species a b u n d a n c e was also a p p a r e n t in the t rawl samples f rom the ice- covered station, due pr imar i ly to the presence of large n u m b e r s of Euphausia superba in the n ight trawls (Table 2). Adul t fishes, krill (E. superba) and si- p h o n o p h o r e s (Diphyes cf. antarctica) were substant ia l ly m o r e a b u n d a n t in n ight trawls t han dur ing the day, with adul t fishes cap tu red only at night. O f the four

Fig. 2 Hourly total numbers of targets (animals) detected acousti- cally with two upward-facing, vertically-profiling acoustic arrays during 2d deployments in ice-covered (continuous line) and open- water (dotted line) areas of northwestern Weddell Sea. Station data as in legend for Fig. 1

392

species of adult fishes collected with the trawl in the upper 300 m of the water column beneath the ice, three (Gymnoscopelus braueri, Electrona antarctica and Bathylagus antarcticus) are known to migrate vertically on a diel basis (Lancraft et al. 1989, 1991). However, the possibility of net avoidance by these species during the day can not be discounted.

Size distribution of macrozooplankton and micronekton

Targets detected in the open-water area were signifi- cantly larger than those under the ice (mean estimated length 13.3 vs 9.2 cm; Mann-Whitney U-test, p < 0.001), although the largest target detected (target strength = - 40.1 dB; estimated length =28.7 cm;

Table 1) was observed beneath the ice (Fig. 3A). Targets beneath the "iceberg" were not significantly larger than those in the ice-covered area (mean estimated length 11.6 vs 9.2 cm; Mann-Whitney U-test, 0.05 < p < 0.10) or in open water.

The size distribution of targets detected acoustically beneath the ice was similar to that of animals > 2.0 cm in length (the detection limits of the acoustic instru- ments) captured in the same general area by the Tucker trawl (Fig. 3B). However, the trawl collected a number of juvenile Pleuragramrna antarcticum and Notolepis spp. between 4 and 6 cm in length, resulting in an abundance peak of a magnitude not observed in the acoustic data. Perhaps more important, however, was the relative paucity of animals larger than ~ 12 cm in the trawl samples compared to the acoustic records (Fig. 3B). This pattern may reflect the greater tendency

Fig. 3 A Frequency histogram A 0.3 of 2 cm size classes of targets (animals) detected acoustically during 2 d deployments in ice- covered and open-water areas of northwestern Weddell Sea; ~ 0.2 B frequency histogram of 2 cm ~- size classes of targets (animals) ~= detected acoustically and captured with 10 m 2 Tucker u_ 0.1 trawl beneath seasonal ice cover in northwestern Weddell Sea; C estimated lengths and times of occurrence for acoustic targets (animals) in ice-covered ([3) and open-water (O) areas of northwestern Weddell Sea ( + targets detected beneath "iceberg") >,0.2

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393

or ability of larger animals to avoid mechanical distur- bances such as ships and trawls.

A temporal pattern in acoustic target size was appar- ent at the ice-covered site, with targets > 12 cm ob- served only at night (Fig. 3C). In contrast, no such diel trend in target size was recorded at the open-water station. In open water, targets > 12 cm were detected at a l l times of the day and night, a fact which is reflected in the significantly larger size of the targets detected in open water, relative to the ice-covered site (Fig. 3A).

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Vertical distribution of react�9 and micronekton

The vertical distribution of acoustic targets differed between the ice-covered and open-water stations, with targets typically farther above the array in open water (Fig. 4A). In contrast to the open-water area, a greater proportion of the targets detected at the ice-covered site occurred > 50 m above the transducer. It is impor- tant to note that in neither instance was the inter- face between the water column and either air or ice ensonified.

A diel pattern in the vertical distribution of acoustic targets was apparent at the ice-covered site but not in open water. Targets were detected at distances > 30 m above the transducer only at night at the ice-covered station (Fig. 4B). This trend was absent from the open- water data, with the shallowest targets occurring near midday and numerous targets > 30 m above the trans- ducer, reflecting the generally shallower distribution of targets at this location (Fig. 4).

Information about the vertical distribution of ani- mals in the upper 100 m of the water column was also provided by the baited traps. All baited traps from the open-water station were empty upon recovery, as were all the subsurface traps deployed beneath the ice. How- ever, each of the surface traps from the two baited trap arrays within the pack ice contained several hundred (280 and 337) scavenging amphipods belonging to the relatively large (up to 3 cm in length) species Abyssor- chomene (formerly Orchomene) rossi. We believe this to be the first record of scavenging amphipods occupying this microhabitat.

Discussion

Abundance of macrozooplankton and micronekton

During the period encompassed by this study (austral spring), the presence of pack ice appeared to have an acoustically-detectable influence on the epipelagic community in the northwestern Weddell Sea. For example, the abundance of acoustic targets was ele- vated in open water relative to estimates from beneath

Open water

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Fig. 4 A Frequency histogram of target distances from acoustic array beneath ice cover and in open water; B distances of targets from acoustic transducer and times of occurrence for acoustic tar- gets (animals) in ice-covered (O) and open-water (O) areas of north- western Weddell Sea ( + targets detected beneath "iceberg")

the pack ice (Table 1). These results are consistent with published trawl data from studies in the same general area (e.g. EPOS, AMERIEZ; Fig. 1A), indicating greater zooplankton abundances in open water than beneath nearby pack ice (Hopkins and Torres 1988; Siegel et al. 1992). A similar pattern has been noted for acoustically-monitored krill swarms, with larger swarms detected more frequently in open water com- pared to ice-covered areas (Sprong and Schalk 1992). Interestingly, however, this trend in abundance, while mirrored in the phytoplankton (e.g. Garrison et al. 1987; Bianchi et al. 1992) and microzooplankton (e.g. Marin 1987) does not appear to be the result of a change in the species composition of the pelagic community, although individual species may not be

394

represented equally in all areas (e.g. Lancraft et al. 1991; Siegel et al. 1992).

The difference in animal abundance between ice- covered and ice-free areas may be attributable to re- duced primary production in the water column beneath pack ice relative to open water (Garrison and Buck 1989; Bianchi et al. 1992; Garrison et al. 1993). During the spring, chlorophyll a concentrations beneath the pack ice in the northwestern Weddell Sea may be more than an order of magnitude lower than in open water (Bianchi et al. 1992). This appears to induce certain life stages of species that winter beneath the ice (e.g. Eu- phausia superba) to migrate in spring away from the pack ice into open waters with higher primary produc- tion (Sprong and Schalk 1992). Such behavior would result in reduced animal abundances beneath pack ice compared to open water, while having a minimal effect on species composition in the two areas.

Another possible explanation for the abundance patterns reported here is a difference in the vertical distribution of animals between our two study areas. Vertical distributions of a number of species are known to be affected by the presence of pack ice in the Weddell Sea (e.g. Atkinson and Peck 1988; Lancraft et al. 1991; Siegel et al. 1992). These differences seem to result from the avoidance by animals of the colder, less saline surface waters found beneath seasonal pack ice (Torres and Somero 1988). Because the acoustic instruments used in this study were located within or above the thermocline in all cases, the effect of vertical displace- ment exceeding 100 m on epipelagic abundances in the two areas can not be evaluated. However, at least within the uppermost 100 m of the water column, ex- cluding the uppermost 5 to 15 m which were not en- sonified, minimal differences were observed between the vertical distribution of acoustic targets at our ice- covered and open-water stations (Fig. 4A).

The observation of greatly elevated abundances of acoustic targets beneath the "iceberg", relative to the surrounding water column, must at this point be re- garded as anecdotal. However, krill have been noted in close association with the undersides of ice floes (e.g. Marschall 1988; BergstrSm et al. 1990; Daly and Macaulay 1991), and it seems reasonable to suggest that the undersides of icebergs may also be populated by these ecologically-important crustaceans. If so, the presence of such potential food items (cf. Ainley et al. 1988) might serve to attract predatory fishes, resulting in a local faunal enrichment relative to the surrounding water column. While such a phenomenon could explain the observed differences in target abundance between the under-"iceberg" environment and the nearby water column (Table 1), it should be stressed that, at present, this proposed mechanism is based on anecdotes and conjecture.

The diel patterns in abundance at both study sites (Fig. 2) suggest the vertical migration of organisms from their daytime depths below 100 m into surface

waters at night, with this behavior particularly well- developed beneath pack ice. A number of species in this part of the Weddell Sea have been observed in surface waters only at night (Lancraft et al. 1989, 1991). Three fish species (Gymnoscopelus braueri, Electrona antarc- tica, Bathylagus antarcticus) were identified in our night trawl samples but were absent from the day trawls, although the influence of daytime net avoidance can not be ruled out as a contributing factor. Nevertheless, the acoustic data corroborate this pattern, as evidenced by the observed nighttime increase in target abundance beneath the pack ice. Although we have no comparable trawl data from the open-water station, published re- ports indicate the presence of the same species in open water (e.g. Lancraft et al. 1989; Siegel et al. 1992), suggesting that vertical migration of certain species may account for diel abundance patterns in open water as well.

Size distribution of macrozooplankton and micronekton

Acoustic targets detected in open water were signifi- cantly larger than those detected beneath the ice (13.3 vs 9.2 cm, Mann-Whitney U-test, p < 0.001), with tar- gets beneath the "iceberg" intermediate in size, al- though the largest individual target was detected beneath the ice (Table 1). The greater size of acoustic targets observed in open water, relative to those re- corded under the ice, is consistent with reports of small- er prey being taken by seabirds foraging in or near pack ice, compared to those feeding in open water (Ainley et al. 1991). Similar trends have also been reported for populations of krill (Euphausia superba), with larger individuals more common in open water and smaller individuals prevalent beneath pack ice at shallow depths (Siegel et al. 1990). This pattern may simply reflect differences in the vertical distribution of certain size classes between ice-covered and open-water areas, with larger individuals occurring at shallower depths in open water, making them more readily available to surface-foraging avian predators.

It is conceivable, however, that the observation of smaller individuals of certain species beneath the ice has an ontogenetic explanation. Because of the stable food supply provided by ice-associated algal communi- ties, the under-ice environment may function as a "nursery" for the larvae and juveniles of some species (cf. Daly and Macaulay 1991). For example, larval and juvenile krill are known to be intimately associated with the undersurface of pack ice, while adults gener- ally avoid this environment (BergstrSm et al. 1990; Daly 1990; Sprong and Schalk 1992). Larvae and ju- veniles of other species may also make use of this environment, but further research is needed. A number of larval and juvenile fishes ( Pleuragramma antarcticum and Notolepis spp.) were identified from our trawl

395

samples beneath the pack ice (Table 2), but trawl data from the open-water area were unavailable for com- parison.

The presence of a diel pattern in acoustic target size at the ice-covered station but not in open water (Fig. 3C) is intriguing, particularly in light of the diel trend in target abundance at both stations (Fig. 3A). Beneath the ice, targets larger than ~ 12 cm were detect- ed only at night, perhaps reflecting an upward migra- tion of relatively large animals from deeper water. As support for this scenario, night trawls from the ice- covered areas contained three species of mesopelagic fishes known to migrate vertically on a diel basis: Gym- noscopelus braueri, Electrona antarctica and Bathylagus antarcticus (see "Results - Abundance of macrozoo- plankton and micronekton"). In addition, individuals of these species constituted virtually all the largest animals (potential acoustic targets) collected with the trawl.

The reasons for this pattern may include predation on ice-associated organisms, particularly larval and juvenile krill (cf. Ainley et al. 1991). Several species of midwater fishes are known to prey extensively on under-ice populations of Euphausia superba (Hopkins and Torres 1989), with krill comprising an increasingly large dietary component as a function of increasing fish size. Two of these, Gyrnnoscopelus braueri, and Elec- trona antarctica, have been identified in this study as contributors to the elevated abundance of large acous- tic targets observed at night in surface waters beneath the pack ice where E. superba occur in abundance, but not in open water where krill are relatively scarce near the surface.

Vertical distribution of macrozooplankton and micronekton

Acoustic targets detected beneath ice cover were typi- cally deeper than targets in open water, although more targets were detected at a distance of > 50 m from the arrays beneath the pack ice (Fig. 4A). Interpretation of these patterns is hampered by the absence of data for the uppermost several meters of the water column, where previous reports indicate that a substantial pro- portion of the ice-associated fauna is located (O'Brien 1987; Stretch et al. 1988; Smetacek et al. 1990). Since no habitat comparable to the under-ice surface is present in the open-water region, we would expect that, had the acoustic records in both locations included the inter- face between the water column and either the ice or air, the under-ice target distributions would have exhibited a pronounced shift toward the uppermost portion of the water column.

One possible prey species associated with the under- surface of the ice is the scavenging amphipod Abyssor- chomene (Orchomene) rossi. The capture of several hun- dred individuals in each under-ice surface trap but not

in any of the deeper traps suggests that these am- phipods may be present in large numbers just beneath the ice surface, as has been noted for krill (e.g. O'Brien 1987; Marschall 1988; Stretch et al. 1988), but not in the underlying water column. Scavenging amphipods in this genus are common inhabitants of the deep-sea (e.g. Ingram and Hessler 1983; Kaufmann 1992) and cold- water (Stockton !982; Sainte-Marie 1986) benthos, where they are ecologically and energetically significant elements of the community (see Sainte-Marie 1992 for review). Related species have been found in association with landfast ice in the Arctic (Boudrias and Carey 1988); however, the Arctic specimens were collected beneath ice covering water 7.5 m deep, compared to 1000-1200 m water depth in the areas where our traps were deployed. A few specimens of A. rossi were identi- fied from our trawl samples, but the number of animals collected in each surface trap was an order of magni- tude higher than in any single trawl sample. These data provide further evidence for the significance of the under-ice environment as an important portion of the Antarctic ecosystem, particularly as a potential overlap zone for surface-feeding predators and their pelagic and ice-associated prey.

Use of bottom-moored free-vehicle acoustic instruments

Much of the existing data on the abundance and distri- bution of under-ice fauna have been collected using ship-based sampling techniques, most commonly towed nets (e.g. Hopkins and Torres 1988; Lancraft et al. 1991) and hull-mounted or towed acoustic instru- ments (Godlewska 1993; Zhou et al. 1994). Although these methods have provided a great deal of very useful information about the pelagic community beneath the Antarctic ice sheet, there are two serious problems associated with the use of such ship-based techniques: the inability to effectively sample close to the under-ice surface (e.g. O'Brien 1987; Daly and Macaulay 1988; Siegel et al. 1990; Lancraft et al. 1991; Hopkins et al. 1993) and the perturbation of animals resulting from local physical disturbances. For example, kritl exhibit strong escape responses when disturbed (O'Brien 1987; Marschall 1988), suggesting that the presence of a ship will at least impact the distribution of this important member of the epipelagic community.

The inability to sample close to the under-ice surface may also lead to biased conclusions. Observations by divers have revealed that a large percentage of krill occur within 1 m of the under-ice surface (O'Brien 1987; Daly and Macaulay 1991), and other species may share this environment. In addition, published reports of the species composition and vertical distribution of pelagic animals identified from the guts of surface-foraging seabirds feeding in open areas within the pack ice differ from those determined from trawl samples (Ainley et al.

396

1988; Lancraft et al. 1989; Ainley et al. 1991), suggesting that samples obtained in the pack ice with conventional trawls do not accurately reflect the composition of the under-ice community.

There is good evidence to suggest that free-vehicle acoustic instrumentation does not suffer from one im- portant bias of ship-based techniques: the physical dis- turbance of animals by the ship and/or the sampling gear. This point is illustrated by the comparison be- tween size-frequency distributions for the animals detected acoustically beneath the pack ice and those collected with a large trawl in the same area (Fig. 3B). The trawl that was used in this study is relatively large for its genre, and therefore should be capable of collect- ing a larger size spectrum of animals than smaller trawls of similar design (Pearcy 1980; Stein 1985). Nonetheless, it is apparent that even this large trawl undersampled animals larger than ~ 12 cm (Fig. 3B), relative to the non-invasive acoustic technique.

It should be noted that there is another possible explanation for the observed discrepancy between the size distributions observed with the acoustic and trawl techniques: the volume of water sampled. Because of the large volume of water ensonified by each acoustic array (8600 m 3 per acoustic ping over a deployment containing 4000 to 5000 pings), compared to the vol- ume sampled by the trawl (15 to 30000 m 3 per trawl), larger and presumably rarer animals might be absent from trawl samples simply by random chance. How- ever, only a greatly increased trawling effort would permit a satisfactory resolution of this question.

Bottom-moored free-vehicle acoustic instruments hold promise for the monitoring of the abundance, size distribution and vertical distribution of macrozooplank- ton and microneckton in the upper 100 m of the water column beneath seasonal pack ice and in open water. Similar instrumentation might also be useful for re- cording the movements of individual animals as well as animal schools. With additional refinements and a somewhat different sampling program, this monitor- ing could be conducted continuously over extended periods of time (e.g. weeks to months). Long-term acoustic monitoring of the epipelagic zone at a single location that experiences seasonal variation in ice cover could provide valuable insights into the influence of ice cover on the dynamics of the under-ice and ice- associated communities. Of course, all acoustic tech- niques suffer from a common shortcoming: the lack of direct observations of the targets being detected. The existence of this problem argues strongly for the use of bottom-moored free-vehicle acoustic systems in combi- nation with one or more complementary, albeit intru- sive, sampling techniques, such as conventional trawls, ship-based acoustic instruments and baited traps.

Aeknowledgements We are grateful to J. Edelman, R. Wilson, K. Wood and J. Scott as well as the captain and crew of the R.V. "Nathaniel B. Palmer" for their invaluable assistance at sea. We

would also like to thank J. Hunt, V. Loeb, L. Madin and G. Matsumoto for identifying species from the trawls. Critical com- ments on this manuscript were provided by two anonymous reviewers. This research was supported by National Science Foundation Grants DPP 91-18997 and OPP 93-15029 and Office of Naval Research Grant N00014-89-J-1540 to K.L. Smith.

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