Polish Polar Research

Four water masses were distinguished in the upper water layer between Elephant Island and the South Orkneys. Measurements of temperature, salinity, concentrations of dissolved oxygen and silicates were used for the analysis of the hydrological situation and to recognise the origin of water masses. For additional information, nitrates and chlorophyll concentrations were used. Drake Passage and Bransfield Strait waters occupied the western part of the investigated area, from surface to 150 m depth. Below, the Circumpolar Warm Deep Waters (CWDW) were found. The region east of 53.5°W was occupied by winter Weddell Sea water. Above this, a 45 m thin layer of summer modification of Weddell Sea Surface Water was found between 49°W and the South Orkneys. The highestchlorophyll α concentrations were found in this modified water.

Results of an oceanographic survey along the edge of drifting pack ice in the area between Elephant Island and the South Orkney Islands are reported. The influence of sea ice on hydrological factors was very weak. It was not possible to develop oceanographic features characteristic for marginal sea-ice zones in the areas with well marked surface currents and dynamic hydrological conditions. The spatial distribution of chlorophyll was governed by water stability, although during our survey, areas with enhanced vertical stability could not be described in terms of a sea-ice edge influence.

The highest concentrations of algal cells (1.1 x l0 6 litre- 1 ) and of algal carbon (20 μg litre -1 ) were associated with a lens of ice melt water in the northeast of the study area. Phytoflagellates were dominant at all stations with greater numbers always in the 0 - 20 m surface layer and with the peaks of Cryptophyceae in the open waters and also near the ice edge east of 50° W. Picoplankton flagellates and monads (1.5-5.0 μ) were generally next in abundance and most important numerically in the near ice stations in the western part of the study area. Parasinophyceae were usually more abundant than Nitzschia cylindrus (Grunow) Hasle, the only common diatom species found mainly in the western near ice edge stations. The presence olN.cylindrus, dominant in the pack ice and in phytoplankton near the ice edge, shows that algae released from ice may act as an inoculum for the phytoplankton.

Altogether 105 algal taxa were identified including 101 diatom species. Chaetoceros criophilus was dominant in the western part of the study area influenced by waters from the Bellingshausen Sea. Corethron criophilum was abundant in the Weddcll Sea water mass found to the east of 53.5°W meridian. Nitzschia cylindrus common in the ice-melt samples was dominant in only two net phytoplankton collections obtained at the ice-edge zone. Additional samples from Admiralty Bay, at King George Island revealed the dominance of Chaetoceros socialis and the presence of many tychoplankton species. Very few diatom cells were found in the open waters of the Bransfield Strait which combined with the presence of krill, suggested intensive grazing by herbivores. The unstable waters of the Weddell-Scotia Confluence area contained little phytoplankton except for a station dominated by Phaeocystis pouchetii. Greater cell densities were related to warm, lower salinity Weddell Sea water of summer modification found in the surface layer east from 49°W.

At the northern border of pack ice the study on chlorophyll a content, density of cells, species composition and domination in samples from the drifting ice floes and from brash ice was carried out. 102 taxa of algae were found in the pack ice. In the study area algal taxa were rather uniformly distributed. In different ice layers the qualitative composition of diatom assemblages was similar and usually the diatom Nitzschia cylindrus was dominant and most frequent. Chlorophyll a content (from 0.12 to 334.5 mg m-3) and the density of cells (from 0.3 to 362 x l0 6 cm) varied strongly in various habitats. Ice floes near the northern pack ice border contained low values of chlorophyll a (mean value 0.50 ±0.28 mg m-3) . However, brash sea ice originating from ice floes, contained 142.4 ±117.5 mg m-3 of chlorophyll α in visibly discoloured and 30.1 ±24.3 mg m~3 of chlorophyll α in not visibly discoloured parts on average. The range of chlorophyll α content and the presence of characteristic species allow to distinguish brash sea ice infiltration assemblage of diatoms.

Macrozooplankton was caught at 17 stations with a Bongo net from the 0-200 m layer. The stations were located near the pack ice edge, between Elephant Islands and the South Orkney Islands. The cluster analysis of 58 recognized taxa allowed to distinguish three regions: the western — near Elephant Island, the middle and the western one — at the South Orkney Islands. No clear difference in macrozooplankton species composition at the open sea stations and those near pack ice was found. The average biomass of macrozooplankton in the investigated area amounted to 82.8 g/1000 m3 (95% CL: 47.2-94.2 g/1000m3). Macrozooplankton was dominated by salps and krill. The biomass and 95% confidence limits were 52.0 g/1000 m3 (15.6-59.2 g/1000 m3) and 26.1 g/1000 m3 (8.4-30.4 g/1000 m3), respectively. Differences in the biomass distribution of some taxa in three distinguished regions were observed. Except of salps the biomass of particular taxa caught near the pack ice edge and the same taxa caught in stations distant from this edge were similar. The biomass of salps was evidently higher in most northern stations.

At the ice edge krill undergoes diurnal migrations with the period of 12 hours and amplitude of about 6 meters. The mean depth of krill occurrence is 41 m, shallower then for open waters. In our opinion these migration parameters are characteristic of juvenile adolescent krill dominating at the ice edge.

In the investigated area the overall abundance of krill was small and was increasing with the distance from ice. However, with the data available, it was not possible to decide whether this increase was related to the ice border or was a part of a larger scale phenomenon. The depth distributions as well as the mean values of krill depth were similar to those of open water both in this study and reported in literature.

In the region between King George Island and the South Orkney Islands 7 fish species from 6 families were found. The concentration of larvae at the edge of drifting ice was higher (2.55 ind. x 1000 m-3) than in the stations situated at a distance from the ice edge (0.93 ind. x 1000 m-3).