A total of sixty five taxa of marine phytoplankton (diatoms, dinoflagellates, silicoflagellates and cyanoprokaryotes) were recorded in the transect from the cold region of the Antarctic (Weddell Sea) up to La Plata Bay, Argentine in the late austral summer (March 1989). Diatoms were the dominant group in a south-north transect from the Seal-Bay (Princess Martha Land, the Antarctic). Most of the phytoplankton species of the cold Antarctic region disappeared around 50°S where there is a steep water temperature gradient. The diatom flora declined in the regions of increasing temperature, i.e. between 60° and 50° S and was replaced by dinoflagellates of the genus Ceratium. Large centric diatom genera Corethron, Rhizosolenia, Chaetoceros and Dactyliosolen represented the most apparent phytoplankton part. The most common of the small centric diatom genera were Thalassiosira, Asteromphalus, Actinocyclus and Coscinodiscus, while several species of Navicula and Nitzschia were the most abundant pennate forms. The presence of a considerable number of freshwater pennate diatoms, characterized as indifferent in the halobion spectrum and mostly periphytic, might be attributed to survival strategies during their development on the floating coastal ice.
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.