Bacteria ingestion by Daphnia galeata in a biomanipulated reservoir: a mechanism stabilizing biomanipulation?

We determined clearance rates and ingestion rates of Daphnia galeata on bac teria and phytoplankton in order to test if bacteria are an important alter native food resource for daphnids during periods of low phytoplankton bioma ss in the biomanipulated Bautzen reservoir (Germany). D. galeata was able t o feed on bacteria with the same efficiency as on algae during most of the time. In spite of similar clearance rates, bacteria ingestion was usually l ower than phytoplankton ingestion due to lower bacterial biomass. Only at l ow biomass of algae in late fall and during the clear water phase, bacteria contributed up to 87% and 42%, respectively, to total carbon ingestion of D. galeata. However, even a short period of relatively high bacteria grazin g by daphnids may be important for biomanipulation. Bacteria might bridge o ver periods of food limitation of daphnids thus promoting the maintenance o f high Daphnia biomass. Therefore, ingestion of bacteria by daphnids is tho ught to stabilize biomanipulation and may hold a key position in the food w eb of biomanipulated lakes.