The research concerned the Wapienica dam reservoir, which is located in the Polish Carpathians, near the city of Bielsko-Biala. The dam closes the flow of water from the 11.1 km 2 catchment. The forests cover about 90% of the catchment area, while the rest of the catchment constitutes grassland areas. The reservoir has a capacity of 1.05 million m 3. The collected water is intended for supplying a water supply system. The catchment is treated as semi-natural, because within its borders the only works related to forest management are carried out. The aim of the research was evaluation of the water quality in this reservoir, including trophic status, based on the analyses carried out in 2015. It was found that the water was of good quality in basic physicochemical terms (e.g. suspension ≤13 mg•dm-3 ,COD Cr ≤11 mgO 2 •dm-3 , conductivity ≤82 µS•cm-1 , phosphates <0.05 mg•dm-3 , nitrates ≤5.1 mg•dm-3 , dissolved iron ≤0.12 mg•dm-3). The contamination with heavy metals was not ascertained, with the exception of the zinc. The assessed sanitary indicators are worse-the bacteria of the Coli group and periodically also enterococci were observed in the reservoir water. The reservoir is not homogeneous in terms of biological production (the trophy degree). It is low in the upper part (oligotrophy generally) and increases in the part near the dam (mesotrophy, also eutrophy). The established presence of dangerous bacteria and symptoms of the reservoir eutrophication are disturbing in the context of the opinion on the purity of uninhabited mountain areas.

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In this publication the eutrophication level of Kozlowa Gora dam reservoir base on composition species of algae and biomass of phytoplankton, was defined. The evaluation of phytoplankton biomass was achieved by two methods (direct - for biovolume by comparing the shape of algae to geometrical figures and indirect - base on concentrations of chlorophyll a). The trophic estimation of reservoir water was achieved on the grounds of classifications, which included the boundary values of phytoplankton biomass and concentrations of chlorophyll a. These boundary values are characteristic for individual trophic types of water. The typical species for eutrophic water were observed in the samples of water taking from the Kozlowa Gora dam reservoir, also the highest average percentage share of Cyanophyta in whole biomass of phytoplankton was observed (it may indicate on the advanced eutrophication processes). The research concerning the average biomass of phytoplankton and average concentrations of chlorophyll a allowed classification water of this reservoir to eutrophic and hipertrophic.

STRESZCZENIEW artykule zaprezentowano wielkość biomasy oraz skład gatunkowy glonów planktonowych, które występowały w wodach zbiornika Tresna w czasie trwania badań. Badania prowadzono w sezonie wegetacyjnym 2012 r., a pró-by pobierano z dwóch stanowisk badawczych (T1 -rejon ujścia rzeki Soły do zbiornika oraz T2 -rejon zapory). Na podstawie wyżej wymienionych parametrów oraz dodatkowo stężeń fosforu ogólnego określono poziom zeutrofizowania wód zbiornika. Przy uwzględnieniu tych parametrów podjęto też próby określenia możliwości rekreacyjnego wykorzystania jego wód.Słowa kluczowe: biomasa fitoplanktonu, bioindykatory, eutrofizacja, zbiornik zaporowy. THE EUTROPHICATION OF THE TRESNA DAM RESERVOIR IN TERMS OF ITS RECREATIONAL USE ABSTRACTThe paper presents the values of phytoplankton biomass and species composition of the planktonic algae. These phytoplankton organisms occurred in the reservoir water during research. The research was conducted during the vegetative season in 2012 year and the samples were taken from two research points (T1 -the part of river Sola inflow to the reservoir and T2 -the part of the reservoir dam). On the basis of the above-mentioned parameters and addition the concentrations of total phosphorus, the eutrophication level of reservoir water was defined. Attempts were also made to determine the possibilities of the recreational use of its water, based on these parameters.