EN
The biomass and species composition of blue-green algae in a new lowland dam reservoir was analysed from 4th till 6th year after filling. In summer, the massive abundance biomass of blue-green algae (0.93 mg dm^3 on average, max. 6.92 mg dm^3), chiefly of the genera Aphanizomenon, Microcystis and Anabaena causes the high trophy level in the reservoir, de-creasing water quality and aesthetic values of the studied water body. The conditions, which favour the development of blue-green algae, are mainly the result of the natural traits of water in the peat-forest catchment basin.