Abstract
The structure of phytoplankton communities of Lake Jeziorak was presented for the first time. The objective of our research was qualitative and quantitative analysis and bioindices of phytoplankton during and outside the tourist season. Analysis of phytoplankton assemblages were performed in 2011-2012. A total of 96 taxa were identified in Lake Jeziorak, mainly: Cyanobacteria - 20, Bacillariophyceae - 49, and Chlorophyta − 19. Biomass of the phytoplankton varied from 10 mg l−1 in October to 29 mg l−1 in May. In the whole research period, Cyanobacteria dominated and represented up to 68% of the total biomass. The cyanobacterial blooms were constantly observed. Biomass in the summer period was composed of filamentous Aphanizomenon gracile, Limnothrix redekei, Planktothrix agardhii and Pseudanabaena limnetica. Outside the holiday season, i.e. in autumn and spring, filamentous Cyanobacteria accompanied cryptomonads in phytoplankton. The species composition, the biomass of phytoplankton, and TSI indicate the hypertrophic conditions of the lake. Phytoplankton in Lake Jeziorak was in a state of equilibrium for almost the entire study period. S1 was a dominant group and its abundance and biomass did not change by more than 10%. There was no significant direct effect of the seasonal tourism impact on the development of phytoplankton in Lake Jeziorak.