The zooplankton community in the vicinity of the ice edge,western Weddell Sea,March 1986

Summary The zooplankton community in the vicinity of the ice edge in the west central Weddell Sea was investigated in the late austral summer (March 1986). Sampling was done with two ships operating concurrently, one in the pack ice and the other in the adjcent open sea. Metazoan microzooplankton (<1 mm) was most abundant in the epipelagic zone. It consisted mostly of copepod nauplii and copepods of the genera Oithona, Oncaea, Ctenocalanus and Microcalanus. While species composition was similar in both areas, vertical patterns differed in that the microzooplankton had sparse populations in the upper 50 m under the ice. This may have been related to water temperature which in the upper 50 m under the ice was more than 1°C cooler than in the open sea. Zooplankton in the 1–20 mm size range was dominated by the calanoid copepods Metridia gerlachei, Calanus propinquus and Calanoides acutus which constituted half the biomass in the upper 1000 m. Their populations had highest densities in the upper 150 m, though much of the C. acutus population resided below 300 m. Metridia gerlachei and C. propinquus underwent diel vertical migrations in both areas whereas C. acutus did not migrate. Species diversity in the epipelagic zone was moderate and the fauna was characterized by species typical of the oceanic east wind drift. Diversity increased with depth and was due primarily to the appearance of circumpolar mesopelagic copepods in Weddell Warm Deep Water. Biomass of 1–20 mm zooplankton in the 0–1000 m zone was low (1.1–1.3 gDWm^-2) compared to other Southern Ocean areas investigated with comparable methods. It is suggested that this is related to Weddell circulation patterns and the resulting low annual primary production in the central Weddell Sea.     

Gut contents and diel feeding rhythm in dominant copepods in the ice-covered Weddell Sea,March 1992

Zooplankton was sampled at a diurnal station during the drift of the Ice Station Weddell-1 in the western Weddell Sea in March 1992. Gut contents of Metridia gerlachei, Calanoides acutus, Calanus propinquus and Rhincalanus gigas were studied. Calanoides acutus was trophically inactive, but 18%, 33% and 45% of Calanus propinquus, R. gigas and M. gerlachei, respectively, contained food in their guts. Diel changes in gut levels and percentage of individuals with full guts with maximum during the dark period were observed in M. gerlachei copepodite stage V and females. The possible reasons for observed differences in foraging patterns of the four copepod species are discussed.     

Seasonal and spatial variations in the zooplankton community of an Eastern Antarctic coastal location

Summary The zooplankton community of a shallow coastal area in Eastern Antarctica was found to be one of low species diversity dominated by Copepoda. It was comprised of the more common Antarctic oceanic copepod species, medusae, molluscs, euphausiids, several copepod species associated with the ice-water interface and, in summer, benthic fauna larvae. Most species of copepods displayed a marked seasonality in abundance with peak numbers between March and May. It is proposed that several factors, including phytoplankton seasonality contribute to the zooplankton species composition, zooplankton seasonality, and to the temporal differences in the period of maximum abundance between copepod species. Annual vertical migratory behaviour in conjunction with the circulation of Prydz Bay are important determining factors for those species which can be considered as oceanic; Calanoides acutus, Calanus propinquus, Ctenocalanus citer, Metridia gerlachei, Oithona similis and Oncaea curvata. However, for copepod species which can be classified as inshore residents, such as Stephos longipes, Paralabidocera antarctica and Drepanopus bispinosus, it is their association with the ice water interface that determines their seasonal appearance and abundance. Some differences were established between the zooplankton community of the Vestfold Hills and that of other Antarctic coastal regions. This may be attributed, in part, to the extensive shallow areas of the Vestfold Hills coastal region. Spatial distribution of the zooplankton with depth and between sites was investigated and found to be essentially homogenous. When differences were established, in the majority of cases all species present, all age classes and both sexes contributed to the differences.     

Effects of a rapidly receding ice edge on the abundance,age structure and feeding of three dominant calanoid copepods in the Weddell Sea,Antarctica

Open-water, marginal-ice and in-ice zones were sampled in the Weddell Sea during November and December, 1993 in an effort to examine the influence of the early spring bloom on the diet and population structure of the three biomass dominant copepods: Metridia gerlachei, Calanus propinquus, and Calanoides acutus. The abundance of all three species in the upper 200 m was highest at stations in the open water, but individually, each species displayed a unique trend. M. gerlachei, which showed the least variability, was significantly more abundant in open water than in the marginal-ice zone. The abundance of Calanus propinquus was higher in open water than in the marginal-ice zone or in the ice. Calanoides acutus displayed the highest variability, with significant differences between all three ice-cover zones. Diet analysis revealed no significant differences in the number of food items within each ice-cover zone and diatoms were the most numerous item identified in the guts of all three species. However, M. gerlachei and Calanus propinquus also contained metazoan material, while Calanoides acutus did not. There were dramatic differences in the age composition of the species between the zones. Early copepodite stages of all three species predominated at the ice edge and in open water. Numbers of M. gerlachei adult females were roughly equivalent in all three zones while Calanoides acutus and Calanus propinquus adult females composed a higher fraction of the total population within the ice. These results compare well with life-history data compiled by other authors and reinforce the importance of the ice edge to bloom-dependent Antarctic zooplankton. Accepted: 5 April 1999     

Meso- and macrozooplankton communities in the Weddell Sea,Antarctica

Summary The present paper describes composition and abundance of meso- and macrozooplankton in the epipelagic zone of the Weddell Sea and gives a systematic review of encountered species regarding results of earlier expeditions. Material was sampled from 6 February to 10 March 1983 from RV Polarstern with a RMT 1+8 m (320 and 4500 m mesh size). In agreement with topography and water mass distribution three distinct communities were defined, clearly separated by cluster analysis: The Southern Shelf Community has lowest abundances (approx. 9000 ind./1000 m³). Euphausia crystallorophias and Metridia gerlachei are predominating. Compared with the low overall abundance the number of regularly occurring species is high (55) due to many neritic forms. Herbivores and omnivores are dominating (58% and 35%). The North-eastern Shelf Community has highest abundances (about 31 000 ind./1000 m³). It is predominated by copepodites I–III of Calanus propinquus and Calanoides acutus (61%). The faunal composition is characterized by both oceanic and neritic species (64). Fine-filter feeders are prevailing (65%). The Oceanic Community has a mean abundance of approximately 23 000 ind./1000 m³, consisting of 61 species. Dominances are not as pronounced as in the shelf communities. Apart from abundant species like Calanus propinquus, Calanoides acutus, Metridia gerlachei, Oithona spp. and Oncaea spp. many typical inhabitants of the Eastwind Drift are encountered. All feeding types have about the same importance in the Oceanic Community.     

Composition and abundance of zooplankton under the spring sea-ice of McMurdo Sound,Antarctica

Summary Zooplankton was sampled through holes in the sea-ice of McMurdo Sound from 8 November to 10 December, 1985. Replicated vertical hauls were made to 100 and 300 m off Pram Point in the inner Sound, near the edge of the permanent McMurdo Ice Shelf. The zooplankton was sparse, averaging 2.5 mg/m³ wet weight. The numbers of individual species varied between catches, depths, and occasions. Generally, small copepods, particularly Oithona similis, Ctenocalanus citer and Oncaea curvata, numerically dominated the catches, and higher densities of these were present in the shallower 100 m layer. Deeper hauls contained higher numbers of larger crustaceans, particularly copepods Metridia gerlachei, Calanoides acutus and Euchaeta spp., ostracod Conchoecia belgicae and euphausiid Euphausia crystallorophias. Pteropods Limacina helicina and Clione limacina were also consistently caught, but in equal densities in 100 m and 300 m hauls. Numerous other plankters were caught in low numbers, including amphipods, chaetognaths, medusae, radiolarians, and larval nemerteans, barnacles, shrimps, polychaetes and echinoderms. Comparative samples from 40 km further north, off Cape Royds and near the sea edge of the fast sea-ice in Wohlschlag Bay, and to 100 m deep, contained a similar species diversity to those near the McMurdo Ice Shelf, but always with higher densities of L. helicina. On the last sampling occasions, when microalgae were conspicuous under the ice off Cape Royds, there were increased densities of microcopepods and Paralabidocera antarcticus, indicating different ecosystem processes from the inner Sound location.     

Effect of the abundance of three predominating copepod species on adequate sample volume and sample size in Bransfield Strait (Antarctic Peninsula) and waters north of the Weddell Sea

Summary The effect on adequate sample size and sample volume of the abundances of three predominant copepod species, Metridia gerlachei, Calanus propinquus and Calanoides acutus, were studied in Bransfield Strait (Antarctic Peninsula) in the austral summer of 1988–1989 and waters north of the Weddell Sea in 1989–1990. Copepod abundances were higher in the area north of the Weddell Sea, with the exception of Metridia gerlachei, which was evenly distributed over both areas. Local (intra-station) patchiness was not found, indicating random distribution over small areas. In the assessment of inter-station variability in Bransfield Strait, with standard error of the mean set arbitrarily at 20% of the average abundance and a sampling volume of 150 m³, the theoretical minimum sample size (number of sampling stations) ranged from 6 to 17 for juvenile copepods and from 11 to 25 for adults. The minimum number of stations in the area north of the Weddell Sea reached from 5 to 7, and from 7 to 10 respectively.     

Feeding patterns of post-larval and juvenile notothenioids in the southern Weddell Sea (Antarctica)

Summary The food of 163 juvenile specimens of 13 species of notothenioid fishes collected in the southern Weddell Sea (Antarctica) was analyzed. Investigated fish size range was 3–13 cm SL. Principal food items were calanoid copepods Metridia gerlachei, Calanoides acutus, and Calanus propinquus; all developmental stages of Euphausia crystallorophias, and post-larval nototheniid fish Pleuragramma antarcticum. Diet of juvenile channichthyids is limited to few species of euphausiids and fish in the size > 10 mm, but does not include significant numbers of copepods. Pelagic stages of nototheniids feed on copepods and/or larval euphausiids smaller than 10 mm. At similar size, nototheniids and bathydraconids take smaller prey items than channichthyids.     

Distribution of six major copepod species around South Georgia during an austral winter

Summary A net sampling survey was conducted around the island of South Georgia during July/August 1983. This study compares the age structure and vertical and horizontal distributions of the dominant copepods Calanoides acutus, Calanus simillimus, C. propinquus, Rhincalanus gigas, Metridia lucens and M. gerlachei. The chief physical and biological factors affecting the distributions of these species are assessed and the results are compared with those from a similar survey around the island carried out in early summer (1981/1982). The survey grid lay within the Polar Front during the winter survey, and horizontal changes in copepod abundance corresponded well to the temperature gradient across the front. This pattern was interrupted by the South Georgia shelf where the seasonal migrants (Calanoides acutus, Rhincalanus gigas and Calanus simillimus) occurred in high abundance. The concentration of these migrants over the shelf relative to the oceanic surface layer was attributed to the shelf having prevented their seasonal migration. Within the oceanic area the copepods occupied differing depths, with Calanoides acutus and Metridia gerlachei living deeper than Calanus simillimus, C. propinquus and M. lucens. The populations also tended to live deeper in the warmer (NW) portion of the oceanic survey area. In contrast to the summer survey the age structure of each species showed little variation throughout the survey area. This was attributed mainly to the decreased rates of copepod growth and metabolism in winter.     

Detection of zooplankton items in the stomach and gut content of larval krill, Euphausia superba, using a molecular approach

The usefulness of a molecular approach based on polymerase chain reaction (PCR) was investigated to identify and quantify the feeding of larval krill on zooplankton organisms in the Lazarev Sea during winter in 2006. Different primers and probes of dominant copepod species (Oithona sp., Ctenocalanus citer, copepodid stages of Metridia gerlachei and Calanoides acutus), co-occurring with larval krill under sea ice during winter, were developed for quantitative PCR (qPCR) and their species specificity was tested on target and non-target species. The qPCR results showed that larval krill were exclusively feeding on Oithona sp. This result was confirmed by microscopic analysis of stomach and gut contents of larvae from the same stations.     

Use of oxygen microsensors to measure the respiration rates of five dominant copepods and Euphausia crystallorophias furcilia from the Amundsen Sea,West Antarctica

The individual respiration rates of five biomass-dominant copepods (Calanoides acutus, Rhincalanus gigas, Metridia gerlachei, Calanus propinquus and Paraeuchaeta antarctica), and Euphausia crystallorophias furcilia, from the Amundsen Sea, West Antarctica, were determined using a Clark-type oxygen microsensor affording high temporal resolution. Measurements were conducted on specimens collected from waters exhibiting a very narrow temperature range (?1.68 to ?1.32 °C), at sites located between 71 and 75°S, during the summer (31 January–20 March 2012). A short incubation time (3 h) was sufficient to reveal significant declines in dissolved oxygen concentrations by 12–45%. The respiration rates of the copepods and E. crystallorophias furcilia were within the ranges of previously reported values. The respiration rates of relatively large-bodied species were rather low, whereas the smaller species generally exhibited higher respiration rates. The data show that this simple microsensor technique is a useful high-resolution non-invasive means of investigating the metabolism of zooplankton in the Southern Ocean. The method could be used in other situations when such information is required.     

Zooplankton communities of the Arctic’s Canada Basin: the contribution by smaller taxa

Zooplankton was sampled at ten stations in the Canada Basin during August 2002 using both 53- and 236-m mesh nets to examine the contribution by smaller and less studied species. Copepod nauplii, the copepods Oithona similis, Oncaea borealis and Microcalanus pygmaeus, and the larvacean Fritillaria borealis typica dominated the upper 100 m of the water column numerically, while biomass was dominated by the copepods Calanus hyperboreus, Calanus glacialis and Paraeuchaeta glacialis, and the chaetognath Eukrohnia hamata. Zooplankton biomass ranged from 3.7 to 14.5 mg AFDW m^–3, with a mean of 9.6 mg AFDW m^–3 . While the three microcopepods contributed less than 5% of the biomass, estimates of their potential growth rates suggest they might contribute upwards of 25% of the metazoan zooplankton production. The true rates of growth and development of these microcopepods in the Arctic need to be determined to conclusively ascertain their importance.     

Summer feeding activities of zooplankton in Prydz Bay, Antarctica

Grazing of dominant zooplankton copepods (Calanoides acutus, and Metridia gerlachei), salps (Salpa thompsoni) and microzooplankton was determined during the austral summer of 1998/1999 at the seasonal ice zone of the Prydz Bay region. The objective was to measure the ingestion rates of zooplankton at the seasonal ice zone, so as to evaluate the importance of different groups of zooplankton in their grazing impact on phytoplankton standing stock and primary production. Grazing by copepods was low, and accounted for <1% of phytoplankton standing stocks and 3.8-12.5% of primary production for both species during this study; even the ingestion rates of individuals were at a high level compared with previous reports. S. thompsoni exhibited a relatively high grazing impact on primary production (72%) in the north of our investigation area. The highest grazing impact on phytoplankton was exerted by microzooplankton during this investigation, and accounted for 10-65% of the standing stock of phytoplankton and 34-100% of potential daily primary production. We concluded that microzooplankton was the dominant phytoplankton consumer in this study area. Salps also played an important role in control of phytoplankton where swarming occurred. The grazing of copepods had a relatively small effect on phytoplankton biomass development.     

Effects of the ice-edge bloom and season on the metabolism of copepods in the Weddell Sea,Antarctica

The metabolic responses of several species of Antarctic copepods to primary productivity and changes between seasons were investigated. To examine the influence of the spring ice-edge bloom on the metabolism of copepods, oxygen consumption rates were determined on specimens from three zones of widely different ice coverage and chlorophyll biomass: pack ice (pre-bloom), ice edge (bloom) and open water (post-bloom). Summer metabolic rates were compared with published winter rates. Field work was done in the Weddell Sea in the region of 60 °S, 36°W in late November and December 1993. Oxygen consumption rates were determined by placing individuals in syringe respirometers and monitoring the oxygen partial pressure for 10–20 hours. Higher metabolic rates were observed in the primarily herbivorous copepods, Calanoides acutus, Rhincalanus gigas and Calanus propinquus in regions of higher primary production: ice edge and open water. The carnivorous Paraeuchaeta antarctica showed a similar pattern. The omnivorous copepods Metridia gerlachei and Gaetanus tenuispinus showed no changes in metabolism between zones. Data on routine rates of copepods from the winter were available for C. propinquus and P. antarctica. In P. antarctica, rates were higher in the summer. Calanus propinquus showed a higher metabolic rate in the summer than in the winter, but the difference was not significant at the 0.05 level. It was concluded that copepods near the ice zone in the ice zone in the Antarctic rely on the spring ice-edge bloom for growth and completion of their life cycle.     

Cnidaria from the Croker passage (Antarctic Peninsula) with a special focus on Siphonophorae

Studies of “gelatinous” zooplankton are rather rare, and little is known about the biology and ecology of Antarctic Cnidaria, especially for siphonophores. More investigations are necessary for complementing the current information on the “gelatinous” zooplankton inhabiting this important but little know biogeographical region, especially because siphonophores very likely play a significant role in the Antarctic food chains. The species composition, abundance and vertical distribution of planktonic cnidarians in the Croker Passage were evaluated using the data obtained from three expeditions. Zooplankton were sampled with a double plankton net between 0 and 1,200 m water depth in both summer and winter seasons. In total, ten species of siphonophores and four species of medusae were identified. Siphonophore assemblages were dominated by Dimophyes arctica, Pyrostephos vanhoeffeni, and Diphyes dispar. D. arctica and D. dispar occurred in greatest numbers in summer, mainly in the upper mesopelagic zone. P. vanhoeffeni, a cryophilic species, occurred most abundantly in winter (in the lower mesopelagic zone), when they probably reproduce. Restricted circulation from the Bellingshausen Sea over the continental shelf into Croker Passage may exclude the deeper-living oceanic fauna from the area of investigation, as indicated by the low abundances of Crystallophyes amygdalina, Muggiaea sp., and Heteropyramis spp. Four species of medusae were identified. The highest abundance was noted for Solmundella bitentaculata and Arctapodema sp. These occurred most abundantly in the middle (January) and end (April) of summer.     

Distribution of antarctic zooplankton around Elephant Island during the austral summers of 1988, 1989, and 1990

Year to year variation and vertical distributions of epipelagic Zooplankton around Elephant Island and King George Island were examined with samples collected with bongo nets and a 1 m² MOCNESS during the austral summers (Jan–Feb.) of 1988, 1989 and 1990. Copepods were the major components of epipelagic Zooplankton (in numbers) with dominance of Metridia gerlachei (1988 and 1989) and small calanoids and cyclopoids (1990). Euphausiids and salps were the next most abundant groups. The percent composition of euphausiids decreased from 1988 to 1990 while that of salps increased. The abundance of salps exceeded euphausiids and major taxa of copepods in 1990. Local patches of polychaetes and amphipods were also found. Statistically significant annual variations with increased numbers in 1990 were found by analyses of variance in total abundance, abundances of copepods, salps, chaetognaths and amphipods, but abundances of euphausiids, polychaetes and fishes showed no significant annual variations. When the study area was divided geographically, horizontal variability in abundance within each year showed no significance in total abundance, abundances of copepods, euphausiids, amphipods and fishes, but significance in salps, polychaetes and chaetognaths. Results of site clustering based on covariances of abundances of eleven major taxa were well matched, though not perfect, with the distribution of surface water temperatures which could be used as a tracer of source water masses suggesting that spatial variation was related to hydrodynamic conditions. Factor analyses showed that annual and spatial variations in abundance were mainly caused by only two taxa, Metridia gerlachei and salps (mostly Salpa thompsoni). These two taxa were responsible for about 60% and 30% of total variance, respectively, and were useful indicators of the interannual variation. That is, 1988 and 1989 were the years of M. gerlachei, and 1990 was the year of salps. From vertically stratified MOCNESS samples, it was shown that the major taxa in this study area were active vertical migrators. While most samples obtained by relatively shallow tows (uppermost 100 m depth) were composed of exclusively one or two taxa, those from relatively deep tows (down to 200 m) showed various patterns of vertical stratification suggesting that the patterns of vertical migration were species specific. Species specific ontogenetic vertical migration associated with elevated habitat temperatures also seemed to be responsible for the annual variation in zooplankton distribution in the upper water column.     

Inter-annual variation in summer zooplankton community structure in Prydz Bay, Antarctica, from 1999 to 2006

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330-μm mesh, 0.5-m² Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill E. crystallorophias, and large copepods (C. acutus, C. propinquus, M. gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.     

Distribution of six major copepod species around South Georgia in early summer

Summary An intensive net sampling survey was conducted around the island of South Georgia during November/December 1981. The distribution and copepodite stage structure of the dominant copepods Calanoides acutus, Calanus simillimus, C. propinquus, Rhincalanus gigas, Metridia lucens and Metridia gerlachei were compared. The herbivorous species had completed their spring vertical migration and their summer generations were developing during the survey. At every station, Calanoides acutus was noticeably more advanced than Rhincalanus gigas in its reproductive cycle. The species were also more advanced in their development in the SE oceanic part of the survey area than in the NW. However, copepod development rates are rapid at this time of year, and this apparent regional difference may be due mainly to temporal variation during the four weeks of the survey. A truly regional variation in timing of reproduction was found when development was compared between shelf and oceanic waters. Spawning of Rhincalanus gigas and particularly of Calanoides acutus was later over the shelf. The age structure of the epi-mesopelagic metridinids also differed between shelf and oceanic waters, but no age differences were found for either Calanus simillimus or C. propinquus, both of which live and spawn higher in the water column than the other species. The presence of the South Georgia shelf also limited the overall abundance of the two deeper living metridinids. However, the length of time between spawning (and production of large numbers of early copepodids) and sampling had the largest influence on observed species abundance within the survey area.     

The effect of the receding ice edge on the condition of copepods in the northwestern Weddell Sea: results from biochemical assays

We compared six biochemical measures of nutritional condition: citrate synthase activity (CS), malate and lactate dehydrogenase activity (MDH and LDH), RNA:DNA ratio, and percent body protein and lipid. Adult females of five species of calanoid copepod (Calanoides acutus, Calanus propinquus, Metridia gerlachei, Rhincalanus gigas and Paraeuchaeta antarctica) were collected in the marginal ice zone of the northwestern Weddell Sea at the time of the annual phytoplankton bloom that occurs in association with the receding ice edge during austral spring. Three zones within the marginal ice zone were sampled: heavy-ice-cover pre-bloom, ice-edge bloom and low-ice-cover post-bloom. Lipid generally increased greatly from ice-covered to open water zones, and its importance in the life of polar copepods cannot be overstated. Increases in protein from ice-covered to open water were also observed, but were of less significance. Each species exhibited significant changes in at least one enzyme activity level. Citrate synthase activity in C. acutus, C. propinquus and R. gigas, all herbivores, increased between pre- and post-bloom stations. C. propinquus and M. gerlachei, which feed during winter, had large increases in LDH activity between pre- and post-bloom stations. Rhincalanus gigas and P. antarctica, the two largest species studied, showed variations in MDH activity, with peak enzyme activity occurring in post-bloom stations. RNA:DNA ratio did not change in any species. The effects of size, shipboard handling and freezer storage were easily corrected statistically, and did not alter any conclusions. The patterns observed in copepod nutrition at the Antarctic ice edge were consistent with existing models of life history for each species. The observations reported here, in conjunction with previously reported data, suggested that measurement of metabolic enzyme activity, especially in concert with lipid, enables estimation of nutritional condition in adult copepods. Additional studies comparing metabolic activity and ecology of common species should yield more information on the ecology of rarer species.     

Trophic structure in open waters of the marginal ice zone in the Scotia-Weddell confluence region during spring (1983)

The structure of the food web was investigated in open waters adjacent to the marginal ice zone in the southern Scotia Sea in spring 1983. Diets were defined for dominant zooplankton, micronekton, and flying seabird species and then aggregated by cluster analysis into feeding groups. Most zooplankton were omnivorous, feeding on phytoplankton, protozoans, and in some cases, small metazoans (copepods). Only two species were found to be exclusively herbivorous:Calanoides acutus andRhincalanus gigas. Micronekton were carnivores with copepods being the dominant prey in all their diets. The midwater fishElectrona antarctica was the dominant food item in seven of the nine seabird species examined. Cephalopods, midwater decapod shrimps and carrion were also important in the diets of a few seabird species. Comparison (cluster analysis) of diets in spring with other seasons (winter, fall) indicated that over half the species examined (18 of 31) had similar diets in all seasons tested. The significant intraspecific shifts in diet that did occur were attributable to regional, seasonal, and interannual effects. A scheme is presented that describes the major energetic pathways through the open water ecosystem from phytoplankton to apex predators. At the base are phytoplankton and protozoans which are the principal food resource for the biomass copepods and krill. Krill and the biomass copepods are the principal forage of the midwater fishElectrona antarctica which, in turn, is the central diet component of flying seabirds as well as important food for the Antarctic fur seal and cephalopods. Krill are a major diet element for the fur seal and cephalopods, and the principal food of the minke whale.