Uncoupled Responses of Bacterial and Algal Production to Storm-induced Mixing in Lake Biwa

Abstract

Production of bacteria and phytoplankton was examined over a three week period during the storm season (August-September) in the South Basin (water depth, 4 m) of Lake Biwa, Japan. Passage of two typhoons caused drastic changes in physical and nutrient conditions, including pulsed increases in the concentration of soluble reactive phosphorus (SRP) associated with sediment resuspension. In response to these meteorological events, phytoplankton increased substantially; up to 6-fold and 3-fold higher values of chlorophyll a and primary production relative to average prestorm levels were observed during the storm period. In contrast, bacterial abundance and production increased little after the storm events. The abundance, production rate, and specific growth rate of bacteria were not significantly correlated with primary production (p>0.4). Furthermore, the ratios of bacterial : primary production were high (maximum, 1.3), suggesting that bacterial carbon demand occasionally exceeded primary production. These results are consistent with the hypothesis that organic carbon from non-phytoplanktonic origins (i.e. allochthonous and littoral organic carbon) are important for bacteria in freshwater lakes. Notably, there was a highly significant correlation (p=0.003; r=0.627; n=20) between specific growth rates of bacteria and SRP concentrations, suggesting that bacterial growth was limited by P. We conclude that the coupling of bacterial and algal responses to the typhoon disturbance was weak in Lake Biwa. The results support a recent model of the lacustrine microbial loop, which suggests that bacteria are largely driven by external carbon supplies and that bacteria may compete with phytoplankton for P.