Ocena stanu troficznego wód zbiornika Tresna na podstawie stężenia chlorofilu

The aim of this research was to assess the state of eutrophication of the Tresna dam reservoir. Knowledge of the degree of eutrophication of the reservoir water is very important, because water with a high degree of eutrophication is often unsuitable for use (especially recreational and water supply). The trophic state was determined based on chlorophyll a concentrations. In addition, the species composition of planktonic algae in the reservoir water was also analyzed in the summer period. The water samples for research were taken during the vegetation season in 2023 year during 5 months (from May to September) from two research points – T1 (the beach area in Zarzecze) and T2 (the area of the dam in Tresna). Chlorophyll a concentrations in the area of the beach in Zarzecze exceeded the lower limit values for mesotrophy for most of the study period. In turn, in May and August, they indicated the eutrophic nature of the reservoir water. The average concentration of chlorophyll a in the study period in this region (7.24 μg/dm3) indicates the mesotrophic character of the water. In turn, chlorophyll a concentrations at the research site in Tresna (near the reservoir dam) were more variable during the research period and they indicated a state from oligotrophy to eutrophy. The average chlorophyll a concentration (5,27 μg/dm3) while allowed to classify the water of this region as mesotrophic. The dominant taxa of algae in the water samples were Fragilaria crotonensis, Asterionella formosa (diatoms), Pediastrum duplex and Gloeotil-la (green algae) and Ceratium hirundinella (dinoflagellates). The numerous presence of Fragi-laria crotonensis and Asterionella formosa in the samples may indicate an increase in the level of eutrophication, because they live in large numbers in eutrophic water and sometimes creating blooms. In turn, the presence of the dinoflagellate Ceratium hirundinella may confirm a more temporarily moderate the level of fertility in the reservoir water and a small amount of organic compounds.