Field sampling and experiments were performed in Lake Biwa to clarify mechanisms regulating temporal and vertical changes in bacterial abundance. The bacterial abundance was vertically homogenous during holomixing period but differed vertically in stagnant period. During the stagnant period, bacterial abundance was high at layer above the thermocline but their abundance was about 5 to 10 times low below the thermocline. Laboratory experiments without grazers showed that bacterial abundance closely related with algal abundance because they rely on DOC supply from algae as carbon and energy sources. However, in Lake Biwa, bacterial abundance was not related with DOC nor phytoplankton abundance. In-situ incubation experiments revealed that bacterial growth rate was severely limited by phosphorus but not by DOC and nitrogen at shallow layers (above thermocline). Further analysis demonstrated that bacterial growth rate in Lake Biwa was potentially determine by temperature, but potentially high growth was not realized in summer at the surface because of phosphorus deficiency. In addition to bioassay, grazing experiments were conducted, which showed that bacteria were grazed by protozoan (flagellates) at almost the same rate of their growth. However, effects of these grazers were not limited in negative aspect alone. By releasing nutrients, protozoan grazers indirectly stimulated bacterial growth rate especially when bacterial growth was P limited. Thus, nutrient return from grazers play a crucial role for bacteria to balance grazing loss with their growth especially at the surface warm layer. Besides these experiments, empirical analyses indicated that although protozoan and Daphnia affected negatively on bacterial abundance, Eodiaptomus stimulated bacterial abundance positively, because they can consume protozoan grazers but not bacteria. This fact suggests that effects of grazers on bacterial abundance are species-specific. In contrast to the surface water, temporal changes in bacterial abundance were less affected by chemical and biological factors at the deep layers, because bacterial growth was severely limited by low temperature. As a result, bacteria in the cold deep layer contributed less than 30% of bacterial production during the stagnant period, although bacterial production correspondent to substantial fraction of primary production on whole water column. These results imply that during the stagnant period in Lake Biwa, dissolved organic matter transfer into bacterial biomass mainly at the surface layer and its magnitude depends highly on phosphorus supply and effects of grazers.
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