Differences of the protozoan biomass and grazing during Spring and Summer in the Indian sector of the Southern Ocean

The dynamics of protozoa were investigated during two cruises in the Indian sector of the Southern Ocean: the early spring ANTARES 3 cruise (28 Septem ber to 8 November 1995) and the late summer ANTARES 2 cruise (6 February to 8 March 1994). Biomass and feeding activity of protozoa were measured as w ell as the biomass of their potential prey - bacteria and phototrophic flag ellates - along the 62 degrees E meridian. The sampling grid extended from the Polar Frontal region to the Coastal and Continental Shelf Zone in late summer and to the ice edge in spring, crossing the Antarctic Divergence. Pr otozoan biomass, although low in absolute terms, contributed 30% and 20% to the total microbial biomass (bacteria, phytoplankton and protozoa) in earl y spring and late summer, respectively. Nanoprotozoa dominated the total pr otozoan biomass. The geographical and seasonal distribution of protozoan bi omass was correlated with that of phototrophic flagellates. However, bacter ial and phototrophic flagellate biomass were inversely correlated. Phototro phic flagellates dominated in the Sea Ice Zone whereas bacteria were predom inant at the end of summer in the Polar Frontal region and Coastal and Cont inental Shelf Zone. Furthermore, bacteria were the most important component of the microbial community (57% of the total microbial biomass) in late su mmer. Phototrophic flagellates were ingested by both nano-and microprotozoa . In contrast, bacteria were only ingested by nanoprotozoa. Protozoa contro lled up to 90% of the daily bacterial production over the period examined. The spring daily protozoan ingestion controlled more than 100% of daily pho totrophic flagellate production. This control was less strong at the end of summer when protozoan grazing controlled 42% of the daily phototrophic fla gellate production.