Process of 14CO2 assimilation and organic matter transformation in the community of planctonic organisms (chain algae–bacteria) in the littoral zone of mezotrophic Lake Gulbinas (group of lakes Žalieji ežerai in Vilnius district) were investigated in summer of 1998. It was estimated that in the beginning of summer, when the representatives of Bacillariophyceae and Chrysophyceae classes dominated by their abundance and biomass, the excretion of organic carbon from algae was more intensive comparing with late summer when Chrysophyceae and Chlorophyceae algae dominated by their abundance, and Bacillariophyceae class algae – by their biomass. In the beginning of summer the cenoses of bacteria utilised ten times more than organic carbon for their biomass synthesis than bacteria in late summer. Major amount of assimilated organic carbon (90 %) was used by heterotrophic bacteria as a source of energy irrespective of the investigation period.
Keywords: planktonic algae, bacteria, 14CO2 assimilation, organic carbon excretion, trophic relation, freshwater ecosystem.