AUTOTROPHIC PICOPLANKTON IN SOUTHERN LAKE BAIKAL - ABUNDANCE, GROWTH AND GRAZING MORTALITY DURING SUMMER

Autotrophic picoplankton were highly abundant during the thermal strat ification period in late July in the pelagic area (water depth 500-130 0 m) of southern Lake Baikal; maximum numbers were 2 x 10(6) cells ml- 1 in the euphotic zone (approximately 15 m). Unicellular cyanobacteria generally dominated the picoplankton community, although unidentified picoplankton that fluoresced red under blue excitation were also abun dant (maximum numbers 4 x 10(5) cells ml-1) and contributed up to appr oximately 40% of the total autotrophic picoplankton on occasions. Carb on and nitrogen biomasses of autotrophic picoplankton estimated by con version from biovolumes were 14-84 mug C l-1 and 3.6-21 mug N l-1. The se were comparable to or exceeded the biomass of heterotrophic bacteri a. Autotropic picoplankton and bacteria accounted for as much as 33% o f particulate organic carbon and 81% of nitrogen in the euphotic zone. Measurements of the photosynthetic uptake of [C-14]bicarbonate and th e growth of picoplankton in diluted or size-fractionated waters reveal ed that 80% of total primary production was due to picoplankton, and t hat much of this production was consumed by grazers in the <20 mum cel l-size category. These results suggest that picoplankton-protozoan tro phic coupling is important in the pelagic food web and biogeochemical cycling of Lake Baikal during summer.