Cycling of organic carbon in the photic zone of a eutrophic lake with special reference to the heterotrophic bacteria

Abstract

Cycling of organic carbon in the photic zone was investigated in a eutrophic Norwegian lake during the ice‐free period. Special attention was paid to heterotrophic bacteria and the factors influencing their production and biomass development.The average proportions of the biomass of phytoplankton, zooplankton, and bacteria were 17 : : 1 on a carbon basis. Phytoplankton primary production averaged 580 mg C m−2 d−1, and 22% of the fixed carbon was exuded. Bacterial production, estimated by frequency of dividing cells, averaged 160 mg C m−2 d−1 and equaled 25% of the gross primary production on a seasonal basis. Increased bacterial activity was either correlated with increased exudation or followed a collapse in the phytoplankton community. Grazing was estimated to amount to 35–180% and 41–84% of the total losses of phytoplankton and bacteria. Cladocerans were the main grazers on phytoplankton, and heterotrophic flagellates were the main grazers on bacteria.Because of high death rates of zooplankton and cycling of carbon within the microbial loop, the available substrate for bacteria equaled the primary production on a seasonal basis. Exudates, egestion products from grazers, and dead zooplankton constituted two‐thirds of the available substrate, and these sources were of equal magnitude.