Role of Daphnia in the decomposition of organic matter in the surface layer of Lake Biwa

To examine the role of Daphnia in the decomposition and elimination of organic carbon at the surface layer, organic matter from Lake Biwa was incubated with and without Daphnia at 20°C in the dark for 15 days in May, July and October. In all of the experiments, total organic carbon decreased during the incubation, regardless of treatment; however, the net effect of Daphnia on this decrease differed among experiments. Daphnia stimulated the decrease in particulate organic carbon in two out of three experiments. Bacterial abundance was higher in treatments with Daphnia than in treatments without Daphnia in all experiments. Higher bacterial abundance in treatments with Daphnia could not be explained fully by zooplankton grazing effects on phagotrophic protozoans, rather Daphnia seemed to stimulate bacterial growth by supplying substrates for that growth. The results of the present study suggest that Daphnia reduces sinking flux, not only by direct grazing, but also by the channelling of particulate organic carbon into dissolved forms that are then available for bacterial growth.