During active vegetation season of 2000 (spring–autumn), phytoplankton primary production, extracellular release of organic compounds, and heterotrophic utilisation were investigated in the euphotic zone of a mesotrophic Lake Balsys in order to estimate organic carbon flow in the area of algal–bacterial interaction. The maximum values of total produc-tion (cellular and extracellular) were observed in the middle of summer when Chlorophyceae dominated by their abundance and Chryptophyceae with Bacillariophyceae – by their biomass. Relative significance of extracellular production was in inverse ratio to cellular algae production. Bacterial population utilised 2–4 times more excreted organic carbon in the beginning of summer than bacteria in late summer; whereas, the values of assimilation not always depended on the amount of exudates. Most part of assimilated organic carbon (75 %) was used by heterotrophic bacteria as a source of energy.
Keywords: planktonic algae, bacteria, primary production, organic carbon excretion, bacterial utilisation, freshwater ecosystem.