The aim of this research was to characterize the species structure and biomass of planktonic algae in the ponds of the Habsburg Park in Żywiec. This research was conducted in the 2018 vegetation season. The water samples for research were taken from two research points. The first research point was located in the north-west part of the park, in a sunny place, while the second research point was located in the south-eastern part of the park, in a shaded place. The phytoplankton biomass (given in wet mass) was calculated by comparing the phytoplankton organisms to geometric figures, and then measuring their volume. A special converter was used to convert volume to mass. Algae keys were used to identify planktonic algae species. The results of the conducted analyzes showed the variability of the phytoplankton biomass in the research season. The highest values of total planktonic algae biomass were recorded in the summer period, in turn, the lowest values were recorded in the spring and autumn periods at both research points. The following groups of planktonic algae were found on both research points: Bacillariophyceae, Chrysophyceae, Chlorophyceae, Cryptophyceae and Euglenophyceae. The conducted research showed that diatoms (59.6% share) and green algae (22.5% share) had the highest average percentage share in the total phytoplankton biomass on the first research point, whereas on the second research point the diatoms (66% share) and euglenins (15.3% share) had the highest average percentage share in the total phytoplankton biomass. The greatest species diversity was found among diatoms and green algae, in the remaining algae groups only two or three species of algae dominated. During the research, algae that prefer low fertile water were observed (e.g. Encyonema minutum (Hilse) D.G. Mann, Pinnularia sp.) and that prefer more fertile environments (e.g. Coelastrum astroideum De-Not., Aulacoseira granulata (Ehr.) Simonsen. Some of them belonged to ubiquitous algae, typical for various environments, e.g. Cryptomonas erosa and Ulnaria ulna (Nitzsch) Ehrenberg.
