Within the SIBEX study area greater concentrations of net-phytoplanklon biomass and numbers were found in waters with clearly defined physical characteristics of either Bellingshausen Sea (an area north and west of Anvers Island and northern part of the Bransfield Strait) or Weddell Sea (south eastern entrance to the Bransfield Strait). Low biomass and cell numbers occurred in the southern Drake Passage and in the south central Bransfield Strait, which in the latter case, appears to be a characteristic feature of ihsse waters during the entire phytoplankton growing season. Early summer blooms of Chaetoceros neglectus and C. tortissimum were mainly responsible for high values of phytoplankton biomass. Some diatoms, such as C. socialis were exclusively associated with Weddell Sea water; a replacement of one water type by another in the Bransfield Strait may considerably modify phytoplankton populations which are present there.
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.
Net phytoplankton cell numbers in 50 m water column of Admiralty Bay ranged between 0.2 x 10 5 x m-2 on 24 August 1990 and 2.3 x 10 7 x m-2 on 15 November 1990. Cluster analysis has confirmed the presence of two groups of samples: spring and summer ones (October to April), rich in cells and in species, and, on the other hand, winter samples (June to August) impoverished in algae. Spring and summer fluctuations of diatoms were mainly due to Corethron criophilum, Rhizosolenia alata and its varieties, R. hebetata f. semispina, Thalassiosira spp., Chaetoceros spp., and Nitzschia spp. (Fragilariopsis and Pseudonitzschia groups). The abundance and succession of species in Admiralty Bay reflect seasonal differences in diatom growth; they also reflect mixed populations of the Weddell and Bellingshausen seas entering Admiralty Bay via Bransfield Strait. Striking poverty of algae in some summer samples can most likely be attributed to zooplankton grazing.