AbstractLimnological investigations of the river reservoir Mavčiče (the Sava River, Slovenia) were carried out between May and October at regular three‐week intervals. The scope of our investigation was to determine physico‐chemical parameters, species composition, abundance and changes in the plankton community during summer stratification. The dominant taxa of phytoplankton were Chlorophyceae and Dinophyceae, and less so Cyanophyceae and Bacillariophyceae, while Rotatoria and less so Ciliata, Heliozoa and Cladocera were common zooplankton taxa. Eutrophication is in progress and primarily results from the high input of municipal and industrial wastewater from the nearest town. The concentration of total phosphorus and nitrates regulates the extent of primary production in the reservoir. The result is oxygen depletion in the hypolimnetic layer and anoxic conditions in the sedi