Floodplain lakes are important elements of landscapes with large rivers. In this study we compared planktonic communities of two oxbow lakes of the Vistula River. We investigated how the inflow of the river's water affected their physicochemical and biological conditions including water temperature, water transparency, oxygen concentration, and macrophyte coverage of the bottom. These parameters in turn affected plankton community. The average phytoplankton abundance in the isolated lake was over two times lower than in the lake connected to the river. Cryptophyta dominated in the phytoplankton community in the isolated lake and diatoms – in the lake supplied with water from the river. The average abundance of zooplankton in the isolated lake was more than twice as high as that in the lake which was connected to the river. The first lake proved to be more attractive for zooplankton due to its stable living conditions (similar to the conditions observed in ponds), higher temperature in summer, and nutrient availability due to the high abundance of small phytoplankton. The results of our research indicate that species composition, plankton abundance, and Chl-a concentration depended on whether there was water exchange between the particular lake and the Vistula River. Hydrological conditions shaped the relationships between the components of the biota.