The effect of two different reservoirs on some parameters of river water quality has been studied. The Antoninek Reservoir (wetland-type, mean depth 0.4 m, area 7.2 ha, mean residence time 0.5 day) and the Maltański Reservoir (also not stratified but plankton-type, mean depth 3.1 m, area 64 ha, mean residence time 34 days) are situated on the same river (mean discharge 0.67 m³ s^-1). Water of this river, which leaves the hypertrophic Swarzędzkie Lake upstream from the reservoirs, is rich in nutrients and plankton. As it flows through the Antoninek Reservoir it loses most of the phytoplanktonic organisms, measured as chlorophyll-a (70%), mostly due to the shading effect of emergent vegetation. The fraction autotrophic picoplankton (APP) also decreased considerably, both in numbers and biomass (about 68% and 54% on average, respectively). A similar reduction was recorded for the numbers of bacterioplankton (39% on average) and suspended solids (66%). Phytoplankton growth was observed again in the Maltański Reservoir, due to the longer residence time of water and lack of macrophytes. Chlorophyll-a increased by 37%, APP numbers by 120%, their biomass by 154%, while the numbers of bacterioplankton only by 31%. Changes in plankton abundance and biomass as a result of retention of water in a shallow reservoir influenced the quality of outflowing water. Their range strongly depended on the presence of macrophytes within the reservoir.