Early summer distribution of Antarctic krill sexual development in the Scotia-Weddell region: a multivariate approach

Summary The sexual Development of antarctic krill was studied during the EPOS leg 2 cruise (November 1988–January 1989) in the seasonally ice covered Scotia-Weddell Confluence area. Multiple Correspondence Analysis (MCA) was used to elucidate the general trends of variation of biological (body-size, molt stage) and environmental data (geographical position, sea-ice extension, sampling time) associated with female development. In November, female krill from the ice covered area (Weddell Sea) had a juvenile ovary, while pelagic female krill from Scotia Sea were in advanced previtellogenesis, and one third of them had already spawned. The successive samples from the Confluence illustrated a rapid advance of sexual development during the whole period. Both observation of live krill maintained on board and the MCA confirmed the general trend of distribution of sexual development in relation with size structure and environmental factors. Previtellogenesis occurs in relation with the ice-edge, while vitellogenesis is performed in short cycles in the summer pelagic habitat. The degree of sexual development attained by krill samples (measured by the sexual development index, SDI) is then a good indicator of the biological activity of the krill population and of its impact on the pelagic ecosystem.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Acoustic observations on krill spring-summer migration and patchiness in the northern Weddell Sea

Summary Continuous acoustic observations with a 30 kHz and a 150 kHz echo-sounder were made from November 1988 to January 1989 on one repeatedly sampled transect, running along 49°W from the open waters of the Scotia Sea into the Weddell Sea pack-ice. Swarm signals occurring on the echorecords were mainly found in the upper 100 m of the watercolumn, in size varying vertically from 1 to 70 m, and horizontally from less than 5 to over 3,000 m. Catches with a RMT 1+8 indicated that the observed swarm signals were most probably caused by krill, Euphausia superba. From late November to early January the swarms migrated northwards away from the ice-edge and towards greater depths, while simultaneously growing in size. The average number of swarms observed per 10 nautical miles along the transects remained fairly constant throughout the cruise, but the average swarmsize and total aggregation size increased during the period studied. The echo-data give evidence of the spring-summer migration of major parts of the krill stock from under the ice-cover towards open waters.Data presented here were collected during the European Polarstern Study (EPOS), funded by the European Science Foundation     

Top predators as indicators for ecosystem events in the confluence zone and marginal ice zone of the Weddell and Scotia Seas,Antarctica, November 1988 to January 1989 (EPOS Leg 2)

Summary The pelagic summer distribution of Antarctic seabirds, seals and whales was studied in the marginal ice zone of the northwestern Weddell Sea from November 1988 to January 1989. In order to relate top predators to other components of the ecosystem studied simultaneously, their distribution is mainly described in terms of energy flow. Bird, seal, and probably also whale requirements were highest in ice-covered areas. There was no evidence of higher numbers of top predators along the ice edge: densities generally increased further into the ice. In the pack ice, combined energy requirements of top predators often amounted to about 200.000 kJ/day/km², or about 45 kg fresh food, indicating high abundance and availability of prey under the ice. There was a lack of conformity between top predator abundance on the ice and abundance of other life in the water column below. In open water, bird requirements were generally less than 25.000 kJ/day/km², seals were virtually absent and whales were distributed unevenly. Tubenosed birds concentrated along the outer ice edge in early summer but they moved north to open water during December, leaving the area of maximum phytoplankton biomass associated with the retreating ice edge. This pattern matched northward movements of krill swarms that may be related to changes in quality rather than quantity of phytoplankton stocks.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Aggregation of algae released from melting sea ice: implications for seeding and sedimentation

Summary Factors influencing the fate of ice algae released from melting sea ice were studied during a R V Polarstern cruise (EPOS Leg 2) to the northwestern Weddell Sea. The large-scale phytoplankton distribution patterns across the receding ice edge and small-scale profiling of the water column adjacent to melting ice floes indicated marked patchiness on both scales. The contribution of typical ice algae to the phytoplankton was not significant. In experiments simulating the conditions during sea ice melting, ice algae revealed a strong propensity to form aggregates. Differences in the aggregation potential were found for algal assemblages collected from the ice interior and the infiltration layer. Although all algal species collected from the ice were also found in aggregates, the species composition of dispersed and aggregated algae differed significantly. Aggregates were of a characteristic structure consisting of monospecific microaggregates which are likely to have formed in the minute brine pockets and channels within the ice. Sinking rates of aggregates were three orders of magnitude higher than those of dispersed ice algae. These observations, combined with the negligible seeding effect of ice algae found during this study, suggest that ice algae released from the melting sea ice are subject to rapid sedimentation. High grazing pressure at the ice edge of the investigation area is another factor eliminating ice algae released during melting.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Nitrogen uptake in the Weddell Sea during late winter and spring

Summary Uptake rates of ammonium, nitrate and urea were measured during the EPOS leg 1 cruise to the Weddell Sea in October–November 1988 using the isotope ¹⁵N. Nitrate was the most important nitrogen source both for ice algae (f-ratio 0.88) and for phytoplankton in the water column (f-ratio 0.85). Indications of a gradual decrease in % new production with time were found in the outer marginal ice zone. Nitrogen uptake rates in ice algae from the sub-ice assemblage were light-limited at in situ irradiances. Significant regeneration of ammonium was found in ice algal samples only.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Community structure of the epipelagic zooplankton community under the sea-ice of the northern Weddell Sea

Summary The present paper describes the composition, abundance, biomass and diversity of the meso- and macrozooplankton in the epipelagic zone of the open water and under the ice of the northern Weddell Sea. Samples were collected in October/November 1988 with a multiple RMT1+8 net during the European Polarstern Study (EPOS). Multivariate analysis resulted in two distinct site clusters, a northern one mainly located in the open water/marginal ice zone and a southern one extending from the marginal ice zone into the consolidated pack-ice. Clusters were, however, faunistically coherent with a high degree in positive covariation of species. There was no basis for the separation into communities, but differences occurred on the population level in numerical abundances, biomass (wet weight) and in a shift in species dominance. Different ice zones and vertical layers were tested among each other with regard to their relative species abundance. Significant differences were found between the upper 60 m layer of the open sea, the upper 60 m layer of the closed pack-ice and the so called transitional zone. Species richness and diversity was lowest directly under the closed pack-ice. Abundance and biomass was highest in the surface layer of the open water, while both variablès decreased dramatically under the ice. Copepods dominated numerically in open water, while salps dominated in biomass. Euphausia superba and Thysanoessa macrura were the dominant species in the upper water column of the closed pack-ice zone. Krill was the only species with increasing abundance in the sub-ice area and a dominance in biomass of more than 91% demonstrated its unique importance for the sub-ice habitat.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Effects of Antarctic sea ice biota on seeding as studied in aquarium experiments

Summary The potential seeding impact of sea ice microbial communities was studied during late austral winter early spring 1988 in the Weddell Sea, Antarctica. Experiments were performed in seawater aquariums with natural seawater and seawater enriched with crushed ice. Algal, protozoan and bacterial cell numbers were followed, as well as nutrients and DOC levels. The results showed a potential seeding effect of sea ice communities to the water column. However, the type of ice communities differed greatly from each other and the effect of such seeding will be patchy. In our experiments seeding of seawater by ice rich in algae, flagellates and/or particulate organic carbon lead to the development of communities dominated either by diatoms or bacteria.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Phytoplankton distribution in relation to sea ice,hydrography and nutrients in the northwestern Weddell Sea in early spring 1988 during EPOS

Summary Phytoplankton biomass and distribution of major phytoplankton groups were investigated in relation to sea ice conditions, hydrography and nutrients along three north-south transects in the north western Weddell Sea in early spring 1988 during the EPOS Study (European Polarstern Study), Leg 1. Three different zones along the transects could be distinguished: 1) the Open Water Zone (OWZ) from 58° to 60°S with high chlorophyll a concentrations up to 3.5 g l^–1; 2) the Marginal Ice Zone (MIZ) from 60° to about 62.5° with chlorophyll a concentrations between 0.1 and 0.3 g l^–1, and 3) the closed pack-ice zone (CPI) from 62.5° to 63.2°S with chlorophyll a concentrations below 0.1 gl^–1. Nutrient concentrations increased towards the south showing winter values under the closed pack-ice. Centric diatoms such as Thalassiosira gravida and Chaetoceros neglectum forming large colonies dominated the phytoplankton assemblage in terms of biomass in open water together with large, long chain forming, pennate diatoms, whereas small pennate diatoms such as Nitzschia spp., and nanoflagellates prevailed in ice covered areas. Fairly low concentrations of phytoplankton cells were encountered at the southernmost stations and many empty diatom frustules were found in the samples. The enhanced phytoplankton biomass in the Weddell-Scotia-Confluence area is achieved through sea ice melting in the frontal zone of two different water masses, the Weddell and the Scotia Sea surface waters.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     

The overwintering strategy of Antarctic krill under the pack-ice of the Weddell Sea

Summary Antarctic krill (Euphausia superba Dana) occurs in enormous swarms in Antarctic waters during the ice-free summer months. The winter whereabouts of this stock were hitherto unknown. Evidence collected during the Winter Weddell Sea Project 1986 (WWSP'86, G. Hempel 1988) covering a large area of the eastern and southern Weddell Sea indicates that the seasonal sea ice cover sustains the bulk of the krill population. Results presented here, show that known aspects of krill morphology and behavior are actually adaptations to the ice habitat, suggesting that the dominance of krill in the Antarctic marine ecosystem is a result of its capacity to grow and reproduce in the water column in summer, and find both food and shelter in the ice cover during the rest of the year. This conclusion has far-reaching implications for our understanding of Southern Ocean biology and ecology.     

New results of the “EPOS” leg 3 cruise to antarctica: Horizontal and vertical distribution of isopods (Crustacea) in the eastern Weddell Sea

Summary A comparison of the EPOS leg 3 material of Weddell Sea Isopoda with the known literature data revealed some new results for the horizontal and vertical distribution of isopods in the eastern Weddell Sea. The number of isopod species known for the Weddell Sea almost doubled to 118 species. New results on the vertical distribution of 11 isopod genera are presented.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Egg size and composition in Ceratoserolis (Crustacea: Isopoda) from the Weddell Sea

Summary Samples of ovigerous female Ceratoserolis trilobitoides carrying newly spawned (stage A1) eggs in the marsupium were obtained from Borge Bay; Signy Island (South Orkney Islands), the continental shelf near Elephant Island (South Shetland Islands), and two sites in the inner Weddell Sea. The dry mass of a newly spawned egg was greater at the inner Weddell Sea sites even when the larger size of the females from the Weddell Sea and the positive relationship between egg size and female size were taken into account. Larger eggs contained more yolk, and there was a slight but significant tendency for larger eggs to have a lower percentage nitrogen content. Eggs from Ceratoserolis meridionalis were smaller in size than sympatric Ceratoserolis trilobitoides, but of similar composition. The reason for a greater investment per egg by females at higher latitudes is not clear, but it may be related to a slightly longer development period, itself associated with the lower water temperatures in the inner Weddell Sea.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Krill diet affects faecal string settling

Summary Free-floating sediment traps used on a transect from Scotia Sea to Weddell Sea collected larger, more degraded, krill faecal strings in the deeper (150 m) than in the 50 or 75 m traps. The smallest faecal strings were only present in the shallower traps. Sinking velocity of smaller faecal strings was — as expected — much lower than for larger ones, with a total range of 50 to 800 m · day ^–1 for faecal string volumes of 0.007 to 0.53 mm³. Krill feeding largely on diatoms produced faeces with higher settling velocity than those feeding on non-diatom phytoplankton. Smaller krill faecal strings do not leave the upper mixed layer. Potential settling velocities as measured in settling tubes (without turbulence), may in this respect be misleading. Small oval faecal pellets of unknown origin showed relatively high settling velocities (80 to 250 m·day^–1 for 0.002 to 0.013 mm³) due to higher compaction and lower form resistance to sinking.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Winter aggregations of marine mammals and birds in the north-eastern Weddell Sea pack ice

Summary A seabird and mammal census was carried out in the north-eastern Weddell Sea during the austral winter of 1986. The German research icebreaker Polarstern operated in heavy pack ice along the Greenwich Meridian between the northern sea ice boundary and the Antarctic coast. Crabeater seals (Lobodon carcinophagus), minke whales (Balaenoptera acutorostrata), Adélie penguins (Pygoscelis adeliae), Antarctic petrels (Thalassoica antarctica) and snow petrels (Pagodroma nivea) were found to be more abundant in the vicinity of the submarine Maud Rise, about 700 km north of the continental margin, than in other areas of substantial ice cover traversed during that cruise. The aggregations of birds and mammals are expected to reflect aggregations of their principal food, krill (Euphausia superba) wintering underneath the ice cover. The distribution pattern of krill predators coincides with the course of a warm water belt upwelling near Maud Rise. This upwelling could induce local ice melting which in turn may result in an increased release of sea ice algae.     

Biological activity in the antarctic zooplankton community

Summary Zooplankton biomass and respiratory ETS activity were studied along a transect at 49°W, running from open Scotia Sea water (57°S) into the Weddell Sea pack ice (62°S) in November–December 1988. Zooplankton biomass and respiratory activity were relatively high in the ice edge region and in the frontal zone separating the Scotia Sea from waters in the south; low biomasses and activities were encountered under the pack ice. The higher activities in the frontal zone were mainly attributed to locally higher water temperatures, while in the ice edge region they were probably related to changes of the Zooplankton population into more active developmental stages. This development in life stages is possibly a response to ice edge generated phytoplankton blooms. A comparison of the most abundant Zooplankton taxa indicated that amphipods and euphausiids had relatively the highest weight specific ETS activities. The developmental stages of Euphausia superba, from juvenile to sexually mature adults, showed a large variability in ETS activity; only the most inactive stages were found under the pack ice. The difference in respiration between the most active and inactive stages was at least a factor of 6 and is of importance to the overwintering of the species in the under ice habitat.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

On processes determining the vertical stability of surface waters in the marginal ice zone of the north-western Weddell Sea and their relationship with phytoplankton bloom development

Summary Ice-edge-related phytoplankton blooms following the retreating sea-ice in the marginal ice zone are frequently observed phenomena. Such blooms are generally short-lived and are followed by a strong decrease in the chlorophyll concentration towards the open ocean, generally explained by the degradation of the vertical stability. Solar heating and ice melting, which control the stability of the surface water of the north-western Weddell Sea during sea-ice retreat in spring were analysed in order to relate the spatial features of the phytoplankton ice-edge bloom in 1988 to the density field. Solar heating has little effect on the density of seawater in ice-covered areas because the thermal expansion coefficient is very low close to the freezing temperature. Outside the marginal ice zone, the temperature effect on stratification increases and the combined contribution of melting and heating on buoyancy input to the surface layer is roughly constant. As a consequence, the low phytoplankton stocks in the open ocean adjacent to the marginal ice zone, after an initial bloom peak following the retreating ice in spring, are not necessarily caused by deep turbulent mixing, in contrast to what is often assumed.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Ice algae during EPOS,leg 1: assemblages,biomass, origin and nutrients

Summary Ice algae in infiltration assemblages were the dominating primary producers in the northwestern Weddell Sea during the austral spring 1988. Band and sub-ice assemblages were encountered at a few stations only. Maximum ice algal biomass measured was 424 g Chl. a I^–1 compared to less than 0.4, g Chl. a I^–1 in the water column. Biomass and nutrient concentrations in the infiltration layer decreased inward from the edge of ice floes. The composition of algal groups indicated that the concentric distribution was due to migration by mobile taxa. Various procedures for melting of ice-containing samples of algae were tested. Melting in dialysis tubing seemed to have advantages over other methods, especially for cells to be used in physiological experiments.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Late winter microbial communities in the western Weddell Sea (Antarctica)

Microbial communities in the water column and sea ice were studied during the EPOS-cruise on R/V Polarstern in the western Weddell Sea in late winter (October–November 1988). Samples were taken from four transects from heavy pack-ice to open water. The results indicated the important role of protozoans especially in the ice-edge area. Heterotrophic nanofiagellates, dinoflagellates, ciliates and sarcodines showed significant positive correlations with chlorophyll a. Autotrophic picoplankton and autotrophic flagellates, which were probably motile zooids of Phaeocystis pouchetii (up to 3×10⁶ cells 1^–1), were most abundant in the areas of low or medium chlorophyll a concentration. Sea ice contained high numbers of heterotrophic organisms, and the distribution of the different groups showed distinct vertical zonation. At two sites, the microbial assembly beneath the ice was clearly influenced by communities from the melting ice.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Short-term variations in particulate matter sedimentation off Kapp Norvegia,Weddell Sea,Antarctica: relation to water mass advection,ice cover,plankton biomass and feeding activity

Summary A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The ¹³C-composition of the sedimented organic carbon was about-24. The isotope ratio decreased sharply by about 5.5 at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/C_org ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m^–2 day^–1).Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Photosynthesis-irradiance relationships and carbon metabolism of different ice algal assemblages collected from Weddell Sea pack ice during austral spring (EPOS 1)

Summary Photosynthesis-irradiance relationships and the carbon metabolism of different ice algal assemblages collected from Weddell Sea pack ice were investigated during the EPOS 1 cruise. Infiltration- and interstitial assemblages exhibited the photosynthetic characteristics of high-light adapted ice algae with a mean assimilation number of 1.81±0.93 mg C (mg Chl a)^–1 h^–1. A higher light harvesting efficiency under light limited conditions (alpha^B-value), as well as a lower light intensity for light saturation (I_K-value) was determined for the interstitial assemblage. An increase in light intensity from 3.5 to 106 mol m^–2s^–1 resulted in increased synthesis of polymeric carbohydrates (presumably reserve material) in a band assemblage. However, the absolute incorporation of radiolabel into lipid- and amino acid fractions remained essentially constant over this range of photon flux densities. Light-saturated rates of photosynthesis of three infiltration assemblages under hypersaline conditions (approx. 50 and 110%) decreased by 13–55% (controls: approx. 32–34%). The adverse effect of salinity treatment was much less pronounced under hyposaline conditions (approx. 20), where maximal photosynthetic rates were only slightly decreased (-9%) or even stimulated (14–22%). These observations suggest that sea ice microalgae in the ice edge region of the Weddell Sea during spring, being in a metabolically active stage, may have the potential to initiate or contribute to phytoplankton blooms upon release into the water column.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation