Loss processes influencing growth of planktonic bacterial populations in Lake Constance

Abstract

The fate of bacterial production in a deep mesotrophic lake has been investigated on the basis of a multiple approach combining field and experimental studies. Bacterial production gains as estimated by measurements of methyl-[ ³ H]thymidine incorporation into macromolecules appeared to be more or less balanced by corresponding grazing losses. This was suggested by considering abundance and grazing potential of presumed grazers as well as by following in time the binding of labelled bacteria by larger sized organisms. Moreover, strong increases in bacterial net growth were always observed after removal of grazers from the water samples. As was indicated by independent approaches, phagotrophic microflagellates were mainly responsible for the observed grazing losses. Support for the importance of alternative loss processes such as sinking or autolysis was given neither by field observations nor by long-term expenments with exclusion of grazers. It is concluded that phagotrophic microflagellates may be regarded as important functional components of planktonic communities in freshwater lakes.