Dynamics of the plankton communities of the Lazarev Sea (Southern Ocean) during seasonal ice melt

Abstract

Size-fractioned primary production and zooplankton grazing impact were estimated along a repeat grid dunng seasonal ice melt in the Lazarev Sea aboard the MV 'SA Agulhas' (voyage 77) in austral summer (December/January) 1994-1995. During the survey, the phytoplankton size composition shifted from a community dominated by nano-and picophytoplankton ( c 2 0 pm) during the first grid to one dominated by microphytoplankton (>20 pm) during the second grid. Total areal production during the first grid was generally dominated by nanophytoplankton and ranged between 133 and 356 mg C m-2 d-'. During the second grid survey, total areal production was higher, ranging between 263 and 400 mg C m-2 d ' Protozoan grazing removed between 0.5 and 31 'Yo of the initial phytoplankton stock or between 33 and 94':c1 of the potential phytoplankton product~on per day during the first grid, and between 0.5 and 8 % of the initial phytoplankton stock or between 9 and 25% of the potential phytoplankton production per day during the second grid The grazing impact of meso-and macrozooplankton during the flrst grid corresponded to < O 5% of chlorophyll stock or < 2 3 % of the daily primary production Dunng the second grid, the grazing impact of meso-and macrozooplankton was higher, removlng on average 1.12"/0 of integrated chlorophyll or 28.7% of daily production These results suggest that the partitioning of carbon between the various size classes of zooplankton dunng seasonal ice retreat is largely determined by the size structure of the phytoplankton.