RÉSUMÉ

The parameters of bacterio-, phyto-, and zooplankton and the chemical composition of water were studied to characterize the ecological sanitary state of the cooling reservoir of the Ladyzhin thermal power station in 1993. The results were compared with those obtained in 1974, i.e., during the early operation of the reservoir. The quantitative parameters of plankton did not change over this period. However, the water quality got worse with respect to the phosphate concentration, dichromate oxidation, and biochemical consumption of oxygen in five days (BCO₅) most likely as a result of fish farming. By these parameters, water was qualified as polluted or even muddy. Other parameters characterized water as satisfactorily pure or pure.