PHYTOPLANKTON COMPOSITION IN THE WEDDELL-SCOTIA CONFLUENCE AREA DURING AUSTRAL SPRING IN RELATION TO HYDROGRAPHY

During the EPOS leg 2 cruise of the RV ''Polarstern'', carried out in late austral spring of 1988-1989, the composition of phytoplankton in relation to the distribution of hydrographic parameters was studied in four successive transects carried out along 49-degrees-W and 47-degre es-W, across the Weddell-Scotia Confluence (WSC) and the marginal ice zone (which overlapped in part). In all transects, a maximum of phytop lankton biomass was found in the WSC, in surface waters stabilized by ice melting. Different phytoplankton assemblages could be distinguishe d. North of the Scotia Front (the northern limit of the WSC) diatoms w ith Chaetoceros neglectus, Nitzschia spp. and Thalassiosira gravida) d ominated the phytoplankton community. This assemblage appeared to have seeded a biomass maximum which occupied, during the first transect, a n area of the WSC, south of the Scotia Front. The southernmost station s of the first transect and all the stations to the south of the Scoti a Front in the other transects were populated by a flagellate assembla ge (with a cryptomonad, Pyramimonas spp. and Phaeocystis sp.) and an a ssemblage of diatoms (Corethron criophilum and Tropidoneis vanheurkii among others) associated to the presence of ice. During the last three transects, the flagellate assemblage formed a bloom in the low salini ty surface layers of the WSC zone. The bulk of the biomass maximum was formed by the cryptomonad which reached concentrations up to 4 x 10(6 ) cells l-1 towards the end of the cruise. Multivariate analysis is us ed to summarize phytoplankton composition variation. The relationships between the distribution of the different assemblages and the hydrogr aphic conditions indicate that the change of dominance from diatoms to flagellates in the WSC zone was related to the presence of water mass es from different origin.