Autotrophic picoplankton in southern Lake Baikal: abundance, growth and grazing mortality during summer

Abstract
Autotrophic picoplankton were highly abundant during the thermal stratification period in late July in the pelagic area (water depth 500–1300 m) of southern Lake Baikal; maximum numbers were 2 × 10 6 cells ml −1 in the euphotic zone (˜15 m). Unicellular cyanobacteria generally dominated the picoplankton community, although unidentified picoplankton that fluoresced red under blue excitation were also abundant (maximum numbers 4 × 10 5 cells ml −1 ) and contributed up to ˜40% of the total autotrophic picoplankton on occasions. Carbon and nitrogen biomasses of autotrophic picoplankton estimated by conversion from biovolumes were 14–84 μg C l −1 and 3.6–21 μg N l −1 . These were comparable to or exceeded the biomass of heterotrophic bacteria. Autotropic picoplankton and bacteria accounted for as much as 33% of paniculate organic carbon and 81% of nitrogen in the euphotic zone. Measurements of the photosynthetic uptake of [ l4 C]bicarbonate and the growth of picoplankton in diluted or size-fractionated waters revealed that 80% of total primary production was due to picoplankton, and that much of this production was consumed by grazers in the <20 μ.m cell-size category. These results suggest that picoplankton-protozoan trophic coupling is important in the pelagic food web and biogeochemical cycling of Lake Baikal during summer.

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