THE ONSET OF BIOLOGICAL WINTER IN THE EASTERN WEDDELL GYRE (ANTARCTICA) PLANKTONIC COMMUNITY

Data on hydrography, phyto- and zooplankton, obtained on a transect al ong the 0 degrees meridian during the Winter Weddell Gyre Study, June 1992, revealed peculiarities of the early winter situation in the east ern Weddell Gyre. The vertical distribution and developmental stage co mposition of Rhincalanus gigas, Calanoides acutus, Calanus propinquus and krill, Euphausia superba larvae, were a good index for a general a ssessment of the seasonal condition of the plankton communities. There were five zones differing in seasonal situation: (1) The Polar Front and the southern ACC (not studied in detail), (2) The Weddell Front, ( 3) The Weddell Gyre interior, (4) The Maud Rise area, and (5) The Coas tal Current zone. In the Weddell Front, the planktonic community resem bled an autumnal situation with moderate phytoplankton biomass; the ov erwintering stock of copepods was not completely formed and the occurr ence of calyptopes larvae of E. superba indicated that krill continued to reproduce until May. In the Weddell Gyre interior, a typical winte r plankton community was found even before sea ice had formed. The spe cific hydrographic regime of the Maud Rise (governed by the mesoscale circulation over the seamount) support the late autumn conditions simi lar to the Weddell Front (but without early krill larvae). The plankto n of the Coastal Current was a winter community. We conclude that in t he eastern part of the Weddell Front (compared to the western part) se asonal development of both phytoplankton and herbivorous zooplankton i s delayed in spring but prolonged in late autumn. Furthermore, it appe ars that the Weddell Sea ecosystem exhibits a much higher degree of sp atial and temporal variability than thought before. This may have an i mpact on seasonal pattern of organic carbon transport from the pelagic realm to deeper water layers and to the sediment.