THE TRANSITION FROM WINTER TO EARLY SPRING IN THE EASTERN WEDDELL SEA, ANTARCTICA - PLANKTON BIOMASS AND COMPOSITION IN RELATION TO HYDROGRAPHY AND NUTRIENTS

Hydrography and nutrient distribution in relation to plankton biomass and composition were studied during two transects (October and Decembe r) that crossed the ice-covered eastern Weddell Sea (approximately alo ng the Greenwich Meridian) from the ice edge at 58 degrees S to the co ntinental margin at 70 degrees 30'S in 1986. Whereas the winter situat ion still prevailed under the intact ice cover during the October tran sect, extensive melting was underway by December. Despite the very low levels of plankton biomass encountered under sea ice in late winter ( as low at 0.02 mu g chlorophyll al(-1)), distinct differences, particu larly in diatom abundance and species composition, were present betwee n the northern, eastward-flowing and southern, westward-flowing limbs of the Weddell Gyre. On the basis of species composition and physiolog ical state of diatom assemblages, the higher biomass of the northern l imb is attributed to entrainment of plankton-rich water from the ice-f ree Circumpolar Current rather than to in situ growth. The pelagic com munity characteristic of the region under the pack ice throughout the study was dominated by nanoflagellates, ciliates and heterotrophic din oflagellates. Biomass of the latter groups ranged between 12 and 119% of that of autotrophs, and microscopic observations suggested that gra zing pressure was heavy. This winter and early spring community resemb led the regenerating communities of nutrient-limited waters. Break-up and melt of the ice cover in early December occurred simultaneously ov er an extensive area yet did not elicit biomass build-up, not even at the northern ice edge where favorable growth conditions appeared to pr evail. Apparently most of the diatoms sinking into the water from the rich stocks developing in melting ice are grazed by protozoa and krill , hence do not contribute to water column blooms in this region. This situation contrasts with those reported from the western Weddell and R oss Sea ice edges where blooms of ice diatoms were observed in sharply defined melt-water zones adjoining closed ice pack. The role of melti ng sea ice in initiating blooms will hence differ in accordance with r egional hydrography.