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

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 : 2: 1 on a carbon basis. Phytoplankton primary production averaged 580 mg C m-2 d-l, and 22% of the fixed carbon was exuded. Bacterial production, estimated by frequency of dividing cells, averaged 160 mg C m-2 d-l 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. The new methods developed for assessing bacterial production (reviewed by Azam and Fuhrman 1984; Staley and Konopka 1985; Moriarty 1986) allow quantitative estimation of the different bacterial substrates in natural systems. We know that bacteria are directly supplied with organic carbon from the phytoplankton through exudation from healthy cells and by lysis of senescent and dead cells. A more indirect effect of phytoplankton on bacteria is through egestion products from grazers. This pathway has been suggested to represent an important source of reduced carbon for bacteria (JoI Present address: SINTEF, Division of Applied Chemistry, Group of Aquaculture, N-7034 Trondhcim, Norway. Acknowledgments We thank Arnc Haug for identifying zooplankton and estimating biomass, and Ruth Waadeland and Marte Morkved for drawing the figures. Yngve Borshcim, John Korstad, Amfinn Langeland, and Svcrre M. Myklestad provided comments on earlier drafts of the manuscript. This study forms part of the research program on eutrophication of inland waters financed by the Norwegian Council for Scientific and Industrial Research. hannes and Webb 1970; Olsen et al. 1986). To our knowledge no studies have attempted to include all these sources in a quantitat ive evaluation. 13acterial production estimated for various aquatic systems has been in the range of 18-44% of phytoplankton primary production on an annual basis (Jordan and Likens 1980; Larsson and Hagstrom 1982; Peldros-Aho and Brock 1982; Riemann 1983; Love11 and Konopka 1985), and the review by Cole et al. ( 1988) suggests an average of 30%. The fate of this production is poorly understood, but several workers have suggested that grazing by protozoans may be important (e.g. Pomeroy 1974; Williams 198 1). This microbial loop, most precisely formulated by Azam et al. ( 1983), has rarely been quantified, and its overall existence has been questioned by Ducklow et al. (1986). The main intention of the present study was to simultaneously assess inputs and outputs for the bacterioplankton in the photic zone of the eutrophic Lake Nesjovatn to provide a basis for evaluating the factors influencing bacterial production and bio-