The problem of thermal water pollution is still relatively unknown in Europe. The development of industry, especially the power industry, causes the release of enormous amounts of heated water from cooling operations to aquatic ecosystems. The observations made here confirm that an increase in water temperature in a reservoir leads to changes in aquatic ecosystem function and affects reservoir animals. Water temperature is one of the factors that determine species occurrence. Changes in water temperature immediately change fauna distribution and, consequently, the trophic relationships in a given ecosystem. Changes in water temperature can create conditions suitable for introduced fauna and may exacerbate the activity of infectious disease and parasitic agents that can damage native fauna populations. Increasing water temperature can lead to more frequent infections in animals by affecting the animal’s immune system and by influencing pathogenicity. Thermal pollution of water bodies is not ecologically neutral and needs more attention from European scientists, especially ecologists.