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The use of natural or artificial water reservoirs by thermal power plants for cooling purposes alterswater ecosystems. Water ecosystems are mainly subjected to the abiotic influences of higher temperature and increased mixing. This study presents the thermal and oxygen conditions in a lake used for cooling water as well as the chemical changes occurring in it. There is a spatial differentiation in the temperature of the water surface over time periods (in summer and winter).The vertical water temperature changes were definitely different from those of natural, unaffected lakes. Apart from a higher average water temperature, typical mixing and stratification periods were not observed. This apparently affected the dissolved oxygen conditions. The observed absence of a dissolved oxygen deficit resulted from the lack of a thermocline, which positively affected water quality. As a result, the two anthropogenic factors, i.e., the discharge of warm water and increased mixing, have flattened the negative effects on the lake. A new characteristic equilibrium for this lake was established. Single factors have been compensated for by this equilibrium. A relationship between temperature and biomass concentration was observed. The trophic state index (TSI) values permit classifying Rybnik Reservoir as a eutrophic lake.

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