Direct effects of zooplankton grazing activities on the natural assemblage of bacterioplankton and algae were evaluated at monthly intervals, from June to October of 2000, in the middle part of the River Spree, Germany. We quantified bacterioplankton, algae, zooplankton abundance and measured carbon ingestion rates (CIRs) by zooplankton according to two zooplankton size classes: (i) micro zooplankton (MICZ), ranging in size from 30 to $150{mu}m$ and including rotifers and nauplii, excluding protozoans and (ii) macrozooplankton (MACZ), larger than $150{mu}m$ and including cladocerans and copepods. CIRs were measured using natural bacterial and algae communities in the zooplankton density manipulation experiments. Algae biomass (average${pm}$SD: $377{pm}306{mu}gC;L^{-1}$, n=5) was always higher than bacterial biomass ($36.7{pm}9.9{mu}gC;L^{-1}$, n=5). Total zooplankton biomass varied from 19.8 to $137{mu}gC;L^{-1}$. Total mean biomass of zooplankton was $59.9{pm}52.5{mu}gC;L^{-1}$ (average${pm}$SD, n=5). Average MICZ biomass ($40.2{pm}47.6{mu}gC;L^{-1}$ n=5) was nearly twofold higher than MACZ biomass ($19.6{pm}20.6{mu}gC;L^{-1}$ n=5). Total zooplankton CIRs on algae (average${pm}$SD: $56.6{pm}26.4{mu}gC;L^{-1};day^{-1}$) were $sim$fourfold higher than that on bacteria $(12.7{pm}6.0{mu}gC;L^{-1};day^{-1})$. MICZ CIRs on bacteria $(7.0{pm}2.8{mu}gC;L^{-1};day^{-1})$ and algae $(28.6{pm}20.6{mu}gC;L^{-1};day^{-1})$ were slightly higher than MACZ CIRs. On average, MICZ accounted for 55.6 and 50.5% of total zooplankton grazing on bacteria and algae, respectively. Considering the MICZ and MACZ CIRs, the relative role of transferring carbon to higher trophic levels were nearly similar between both communities in the lake-river ecosystem.