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.     

Bacterial activity in sea ice and open water of the Weddell Sea,Antarctica: A microautoradiographic study

Metabolic activity of bacteria was investigated in open water, newly forming sea ice, and successive stages of pack ice in the Weddell Sea. Microautoradiography, using [³H]leucine as substrate, was compared with incorporation rates of [³H]leucine into proteins. Relation of [³H]leucine incorporation to the biomass of active bacteria provides information about changes of specific metabolic activity of cells. During a phytoplankton bloom in an ice-free, stratified water column, total numbers of bacteria in the euphotic zone averaged 2.3 × 10⁵ ml^–1, but only about 13% showed activity via leucine uptake. Growth rate of the active bacteria was estimated as 0.3–0.4 days^–1. Total cell concentration of bacteria in 400 m depth was 6.6 × 10⁴ ml^–1. Nearly 50% of these cells were active, although biomass production and specific growth rate were only about one-tenth that of the surface populations. When sea ice was forming in high concentrations of phytoplankton, bacterial biomass in the newly formed ice was 49.1 ng C ml^–1, exceeding that in open water by about one order of magnitude. Attachment of large bacteria to algal cells seems to cause their enrichment in the new ice, since specific bacterial activity was reduced during ice formation, and enrichment of bacteria was not observed when ice formed at low algal concentration. During growth of pack ice, biomass of bacteria increased within the brine channel system. Specific activity was still reduced at these later stages of ice development, and percentages of active cells were as low as 3–5%. In old, thick pack ice, bacterial activity was high and about 30% of cells were active. However, biomass-specific activity of bacteria remained significantly lower than that in open water. It is concluded that bacterial assemblages different to those of open water developed within the ice and were dominated by bacteria with lower average metabolic activity than those of ice-free water.     

Diving and haulout behavior of crabeater seals in the Weddell Sea,Antarctica, during March 1986

Summary Time-depth recorders were used to study the diving and haulout behavior of six crabeater seals in the marginal. ice edge zone of the Weddell Sea during March 1986. Haulout patterns revealed the seals' clear preference for diving during darkness and hauling out onto sea ice during daylight. Seals did not necessarily haul out every day; individual seals hauled out on 80–100% of days during the study period. Four general dive types were identified: 1) traveling dives, 2) foraging dives, 3) crepuscular foraging dives, and 4) exploratory dives. Nearly continual diving occurred for extended periods (about 16 h) nightly, with one individual diving up to 44 h without interruption. Foraging dives occurring during crepuscular periods were deeper than those made during the darkest hours. The authors suggest that the distinct diel pattern of dive timing and depth may be related to possible predator avoidance behavior by the seals' principal prey, Antarctic Krill.     

A new bryozoan genus from the Weddell Sea,Antarctica

Summary Brettiopsis gen.nov. is erected within the family Scrupariidae. Its affinities with the genera Brettia, Leiosalpinx, Brettiella, Bugulella and Scruparia are briefly discussed. Its type species, Brettia triplex Hastings, is redescribed on the basis of new material collected in the southern part of the Weddell Sea by the RV Polarsirkel. Bivalved brood chambers, previously unknown in this species, are described and illustrated.     

Copepods in summer platelet ice in the eastern Weddell Sea,Antarctica

Copepods in platelet-ice layers underlying fast ice and in the water column below were studied at Drescher Inlet, eastern Weddell Sea in February 1998. Three copepod species were found: Drescheriella glacialis and Paralabidocera antarctica occurred in platelet-ice layers, while Stephos longipes was only present in the water column. The distribution of all species varied considerably between station and depth. D. glacialis dominated the platelet-ice community and occurred at all five platelet-ice sampling sites, except one, with numbers of up to 26 ind. l^–1. In contrast, P. antarctica was only found in low numbers (up to 2 ind. l^–1) at one site. The total copepod abundance in the platelet ice was not associated with algal biomass, although it was strongly correlated with high ammonium concentrations (up to 9 M) in the interstitial water between the platelets. This is the first indirect evidence to support the hypothesis that zooplankton excretion can partly account for the high ammonium values often found in platelet-ice layers.     

Phytoplankton biomass,P-I relationships and primary production in the Weddell Sea,Antarctica, during the austral autumn

An investigation into the changing phytoplankton biomass and total water column production during autumn sea ice formation in the eastern Weddell Sea, Antarctica showed reduced biomass concentrations and extremely low daily primary production. Mean chlorophyll-a concentration for the entire study period was extremely low, 0.15±0.01 mg.m⁻³ with a maximum of 0.35 mg.m⁻³ found along the first transect to the east of the grid. Areas of low biomass were identified as those either associated with heavy grazing or with deep mixing and corresponding low light levels. In most cases phytoplankton in the <20-μm size classes dominated. Integrated biomass to 100 m ranged from 7.1 to 28.0 mg.m⁻² and correlated positively with surface chlorophyll-a concentrations. Mean P^Bmax (photosynthetic capacity) and α^B (initial slope of the photosynthesis-irradiance curve) were 1.25±0.19 mgC. mgChla ⁻¹.h⁻¹ and 0.042±0.009 mgC.mgChla ⁻¹.h⁻¹.(μmol.m⁻².s⁻¹)⁻¹ respectively. The mean index of photoadaptation,I _k, was 32.2±4.0 μmol.m⁻².s⁻¹ and photoinhibition was found in all cases. Primary production was integrated to the critical depth (Z _cr) at each production station and ranged from 15.6 to 41.5 mgC.m⁻².d⁻¹. It appears that, other than grazing intensity, the relationship between the critical depth and the mixing depth (Z _mix) is an important factor as, ultimately, light availability due both to the late season and growing sea ice cover severely limits production during the austral autumn.     

Summer diet of Weddell Seals (Leptonychotes weddelli) in the eastern and southern Weddell Sea,Antarctica

Summary Stomach and intestine samples from 21 adult Weddell seals were used to study the diet of these seals from the eastern and southern Weddell Sea coast from January to February 1983 and 1985. Fish occurred in all seals, squid in five, octopods in three and Euphausia crystallorophias in one seal. Pleuragramma antarcticum was the predominant fish in the diet, constituting 61.1% of otoliths in 1983 samples and 93.8% in 1985. Aethotaxis mitopteryx, Dissostichus mawsoni, unidentified Trematomus spp. and channichthyids were also recorded. Size and wet weight of P. antarcticum were calculated from uneroded otoliths, found in 6 seal stomachs with liquid food pulp, collected during early morning hours in 1985. Size distribution of P. antarcticum from individual seals was reasonably constant, ranging between 5.0 and 22.0 cm SL; adult fish from about 14.0 to 19.0 cm SL predominated. P. antarcticum in seals from the southern area had a larger median size (16.5 cm SL), than those from further east (15.5 cm SL). Calculated wet weights of all P. antarcticum from individual seal stomachs ranged between 4.7 and 16.9 kg the mean was 12.8 kg. Comparisons with net-hauls from the southern Gould Bay suggest that Weddell seals feed mainly in deeper water layers (>400 m) where adult P. antarcticum occur at higher densities.     

A study on octopodids from the eastern Weddell Sea, Antarctica

A comprehensive study of octopodids in the Weddell Sea was conducted during cruise ANT XIII/3 of RV "Polarstern". The study yielded eight species of incirrate octopodids, three of which were undescribed. There appeared to be no differences in octopodid species assemblage in the two sampling areas (Kapp Norvegia and south of Drescher Inlet), supporting the theory that there is a single macrobenthic assemblage on the eastern shelf of the Weddell Sea. Six of the octopodid species belonged to the genus Pareledone, illustrating the extensive radiation of this endemic genus within the Southern Ocean. The fragmented nature of suitable habitats and disturbance caused by glaciation cycles are proposed as mechanisms for this radiation. Comparative data illustrate that both the number of octopodid species present and their abundance are greater at higher latitudes than close to the Antarctic Polar Frontal Zone.     

Sipuncula of the southeastern Weddell Sea (Antarctica)

Sipunculans collected during the EPOS (European Polarstern Study) leg 3 cruise (13.1.89–10.3.89) to the Weddell Sea shelf area from depths ranging from 186 to 2037 m are recorded. Seven species in three genera are recognized; Nephasoma capilleforme and Phascolion convestitum have not been recorded hitherto in Antarctica. Significant morphological characters as shown by light (LM) and electron (SEM) photographs are described and illustrated. Antarctic distribution maps of the species collected and a checklist of the sipunculan species south of the Antarctic Convergence with depths and bibliographic references are included.     

Build-up and decline of summer phytoplankton biomass in the eastern Weddell Sea,Antarctica

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the first half of the study (15 Jan–13 Feb) in an area off Vestkapp, favourable irradiance/mixing regimes initiated net phytoplankton growth in ice-free waters on the shelf and in stretches of open water over the partially ice-covered deep ocean. Chi a concentrations in the upper water column were moderate (0.2–0.8 g l^–1), but significantly above winter values. Later in the season (16 Feb–11 March), a phytoplankton bloom with surface Chl a concentrations ranging from 1.6–2.3 g l^–1 was encountered in an area further to the east. We suggest that the upper water column must have been stratified in this region for time scales of weeks to faciliate bloom development. Bacterial biomass and productivity generally paralleled the seasonal development of the phytoplankton. Nitrate concentrations in the upper mixed layer were substantially lower than would be expected from the existing phytoplankton standing stock, suggesting that heterotrophic consumption of organic matter by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by the passage of a storm, resulting in a homogeneous distribution of phytoplankton biomass over the entire water column, followed by sedimentation and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high, amounting to more than half of the primary production (range: 10%–120%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values <1 g Chl a l^–1. The combination of low incident irradiances and incessant deep mixing prevented the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A major fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankton in the eastern Weddell Sea.     

Nanoplanktonic coccolithophorids (Prymnesiophyceae, Haptophyceae) from the Weddell Sea, Antarctica

The examination of whole mounts prepared for transmission electron microscopy has resulted in the finding of thirteen taxa of nanoplanktonic coccolithophorids from the Weddell Sea, Antarctica. The material was collected as part of the AMERIEZ programme, March 1986. Cold-water adapted nanoplanktonic coccolithophorids have previously been shown to constitute a recurrent plankton element at subarctic and arctic localities. Three of the Weddell Sea species, Wigwamma annulifera, W. arctica , and Papposphaera sagittifera , are conspecific with northern hemisphere material, while two species, Calciarcus alaskensis and Turrisphaera arctica , are possibly identical with previously described arctidsubarctic material. Six taxa new to science have been described from the Weddell Sea, Wigwamma antarctica, W. triradiata, Trigonaspis melvillea, Pappomonas weddellensis, Papposphaera obpyramidalis , and P. simplicissima . The cooccurrence of identical forms at the two poles, and the fact that the species described are allocated to "arctic" genera, indicate a geologically relatively recent exchange of biological material between the poles.     

Peltidiidae (Copepoda, Harpacticoida) from the Weddell Sea (Antarctica)

Alteutha polarsternae sp.n. is described and Eupelte villosa (Brady, 1910) is redescribed in detail. The distribution of both these species in the Weddell Sea is recorded.     

Shrimp (Decapoda,Natantia) occurrence and distribution in the eastern Weddell Sea,Antarctica

Summary 108 successful ground and Agassiz trawl catches were taken between 155 and 2031 m depth in the eastern Weddell Sea on board RV Polarstern in spring and summer (October–February) 1985–1989. In addition, 7 hauls were taken with a semipelagic trawl. Only 19 hauls (16.5%) contained no shrimps. The others yielded large numbers of Notocrangon antarcticus, Chorismus antarcticus, and Nematocarcinus lanceopes as well as 20 Lebbeus antarcticus and 11 specimens of an Eualus species new to science. 8 Pasiphaea scotiae were caught in a pelagic krill trawl. No reptant decapod crustaceans were detected in the study area. Shrimp densities determined from trawl catches were lower than estimates derived from underwater photography but in the same order of magnitude. Although yields of the three common shrimp species in some cases exceeded 20 kg per 0.5 h haul, shrimp stocks in the area cannot be considered to be of commercial significance. A wider geographical distribution and greater frequency of shrimps in high Antarctic waters was found than described hitherto. There was considerable variation in numbers, sex composition, occurrence at different depths, and size-frequency distributions. C. antarcticus and N. antarcticus grow to a larger size compared with individuals from the Antarctic Peninsula area. Within the area of investigation, length frequency distributions are skewed towards larger sizes at higher latitudes. In the eastern Weddell Sea larger specimens of the three common species live at greater depths than smaller individuals. Potential reasons for these differences are discussed.AWI Publication No. 124     

Habitat Use by Weddell Seals and Emperor Penguins Foraging in the Ross Sea, Antarctica

The only apex predators that live year-round at high latitudesof the Ross Sea are the Weddell seal and emperor penguin. Theseasonal distribution, foraging depths, and diet of these twospecies appear to overlap. What makes it possible for emperorpenguins and Weddell seals to co-exist at high latitude throughoutthe winter when other marine tetrapods apparently cannot? Bothspecies have similar adaptations for exploitation of the deep-waterhabitat, forage on the same species, and routinely make longand deep dives. Yet, despite these similarities, there is probablylittle trophic overlap between the adults of both species dueto geographical and seasonal differences in habitat use. Forexample, during the winter months while female emperor penguinsare ranging widely in the pack ice, adult seals are foragingand fattening for the upcoming summer fast, literally beneaththe feet of the male penguins. However, there is more extensiveoverlap between juvenile seals and adult penguins, and shiftsin prey abundance and/or distribution would likely affect thesetwo groups similarly. In contrast, juvenile penguins appearto avoid inter- and intra- specific competition by leaving theRoss Sea once they molt.     

Vertical distribution and abundance of mesoplanktonic medusae and siphonophores from the Weddell Sea,Antarctica

The composition, abundance and vertical distribution of mesoplanktonic cnidarians collected along a transect across the Weddell Sea have been analysed. The transect was characterized by a thermocline, approximately between 200 and 100 m, which deepened significantly towards the shelf edges. In total, 10 species of medusae and 18 species of siphonophores were identified. The most abundant medusae were Pantachogon scotti (up to 11,671 specimens/1,000 m³) and Arctapodema ampla (up to 960 specimens/1000 m³). The most abundant siphonophores were Muggiaea bargmannae (up to 1,172 nectophores/1,000 m³) and Dimophyes arctica (up to 230 nectophores/1,000 m³). Five assemblages of planktonic cnidarians were distinguished: (a) epipelagic species located in and above the thermocline; (b) epi- and upper mesopelagic species located in, above and just below the thermocline; (c) epi- and mesopelagic species located in and below the thermocline; (d) mesopelagic species; (e) lower mesopelagic species. Differences in the depth distribution of the various species gave rise to a clear partitioning of the mesoplanktonic cnidarian population throughout the water column. This vertical partitioning was related to the existence of a thermocline, the structure of the water column and the vertical distribution of prey.     

Macrofaunal communities on the continental shelf and slope of the southeastern Weddell Sea,Antarctica

Summary During the third leg of the European Polarstern Study (EPOS leg 3) in the austral summer season 1989, benthic macrofaunal communities were sampled from the Elephant Island area (61° southern latitude) and from Kapp Norvegia (71° southern latitude) to Halley Bay (75°30 southern latitude) using a commercial bottom trawl and an Agassiz trawl. Thirty-six trawl samples from a depth range of about 200–2,000 m were considered, with most of the samples being from the shelf and upper slope. Multivariate analysis techniques (clustering and TWIN-SPAN) discriminated between an eastern and a southern community in which parallel subgroups can be distinguished at increasing distance from the ice shelf.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Spring distribution and ecological role of seabirds and marine mammals in the Weddell Sea,Antarctica

Summary The early spring distribution of seabirds, pinnipeds and cetaceans was quantitatively determined in the northern Weddell Sea during the EPOS 1 cruise of the icebreaking RV Polarstern. Two hundred and ninety-one half hour counts were made in the Antarctic region between October 18 and November 16, 1988 (+94 counts in sub-antarctic and sub-tropical water). The bird populations were dominated by the Adélie Penguin Pygoscelis adeliae, which represented 90% of the birds counted in the closed pack ice (CPI), with a mean density of 31 penguins per km². Crabeater seal Lobodon carcinophagus, the most common pinniped, had a mean density of 1.2/km² in the CPI with local concentrations reaching 14/km². Expressed as biomass, mean values of about 90 kg/km² were found in the whole Antarctic zone for seabirds, 50 for seals, and 35 for baleen cetaceans. Densities were higher in the CPI: 140, 180 and 115 kg/km² respectively, were found for birds, pinnipeds and baleen whales, and lowest in open water (8 kg/km² for the birds, 0.2 for the pinnipeds and no whales). A minimum value of food intake by seabirds and marine mammals was estimated to be 2.7 mg C/m²/day for the Antarctic zone (4.1 in the CPI). Taking into account that pinnipeds density and cetacean food uptake are underestimated, and that Antarctic seabirds consume much more fish than generally suspected, a minimum krill production of 16 mg C/m²/day is proposed for the Antarctic zone: 22.5 in the CPI, 3 in open water, and intermediate values in the marginal ice zones.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation