We investigated the changes in bacterioplankton abundance, composition, and fractions of cells incorporating leucine associated with a Microcystis aeruginosa bloom and enhanced protozoan grazing in a eutrophic reservoir. Bacterial community composition and leucine incorporation were studied by means of fluorescence in situ hybridization and microautoradiography. To alter the levels of protozoan grazing and the interactions of bacteria with the M. aeruginosa bloom, a manipulation experiment using size-fractionation and dialysis bag techniques was conducted. Samples collected from the Microcystis-poor dam area and the Microcystis-rich middle part of the reservoir were incubated as