Abundance, Distribution, and Mortality of Hydrobionts in the Section of the Gorky Reservoir Affected by Heated Waters of the Kostroma Hydroelectric Power Plant

This study has been conducted in the river section of the Gorky Reservoir in the zone affected by the heated water discharge from the Kostroma hydroelectric power plant during the growing season (May–November) of 2021. The reduced species richness is observed in the sites of the maximum water heating, which, however, quickly return to the initial values in the downstream section of the river. The lethal effect of warm waters on zooplankton is mainly manifested in the summer, when the natural warming of the river waters is influenced by the heated water discharge from the Kostroma hydroelectric power plant. In autumn, the warming effect of heated waters, on the contrary, promotes the better survival of organisms. Fish reach a high abundance in the study area by the end of summer, when the juvenile leave shallow waters. In the autumn period, the fish redistribution and a significant increase in the abundance occur in the area of heated waters compared to the background sites. The results indicate the absence of an environmentally significant lethal effect. Local plankton losses are quickly compensated for due to high reproduction rates and the short-cycle development of invertebrates. No lethal effect of the exposure to high temperatures on fish was found.