The comparison of species richness, abundance and diversity of phytoplankton blooms, which developed both in winter and summer seasons as an effect of lake hypertrophy, was the aim of this study. In the ice-covered lake (0.30 mg PO₄ -P L⁻¹, 1.35 mg NH₄ -N L⁻¹; TSISD = 64; TSIchl = 93), the algal bloom, responsible for high concentration of dissolved oxygen in water, consisted mainly of the centric diatom Stephanodiscus minutulus (3.9 × 10⁷ ind. L⁻¹) accompanied by Limnothrix redekei (Cyanobacteria), Koliella longiseta (Chlorophyceae), > 1.1 × 10⁶ ind. L⁻¹ each, as well as Mallomonas sp. (Chrysophyceae) and Monoraphidium komarkovae (Chlorophyceae), >5.0 × 10⁵ ind. L⁻¹ each. The toxic cyanobacterium Planktothrix agardhii of extremely long trichomes (up to 0.93 mm) and potentially toxic dinoflagellate Peridinium aciculiferum f. inerme occurred also in high numbers (1.9 × 10⁵ ind. L⁻¹ and 7.7 × 10⁴ ind. L⁻¹, respectively). In summer (0.05 mg PO₄ -P L⁻¹; 0.42 mg NH₄ -N L⁻¹; TSISD = 78; TSIchl = 102), the phytoplankton bloom consisted of P. agardhii (average total abundance 49.4 × 10⁶ ind. L⁻¹) and ten other taxa of Cyanobacteria, Bacillariophyceae, Chlorophyceae and Cryptophyceae (average total abundance 17.9 × 10⁶ ind. L⁻¹). The total phytoplankton abundance was 1.5 times higher in summer than in winter and the total biomass of the most abundant species was approximately 4 times higher in warm (139.8 mg L⁻¹) than in cold season (32.5 mg L⁻¹). The values of the Shannon-Weaver diversity index were very low, however, over 2 times higher in summer (0.60) than in winter (0.31). The obtained results revealed that in the hypertrophic lake the very high nutrient concentrations (especially NH₄ -N and PO₄ -P), found both in winter and summer, were responsible for year-long mass development of phytoplankton. The winter phytoplankton was composed mainly of very small centric diatoms, whereas summer blooms were created by filamentous cyanobacteria (mainly Oscillatoriales; 98%).
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