The studies were carried out in six humic lakes (Polesie Region, Eastern Poland) in order to present a role of phytoplankton in relation to physical and chemical parameters characteristic of particular processes. On the basis of physical and chemical properties of waters (SD, colour, pH, EC, Ca 2+ , DIC, DOC), these lakes are divided into three groups: dystrophic (Lakes Orchowe and Płotycze), humoeutrophic (Lakes Brudzieniec and Pereszpa) and mesoeutrophic (Lakes Moszne and Długie). The occurrence of dystrophication, humoeutrophication or eutrophication processes is clearly reflected in the phytoplankton composition and its abundance, as well as its biomass. In both dystrophic and humoeutrophic lakes, flagellates dominated in the phytoplankton. Among them, the invasive species Gonyostomum semen was very abundant. Mesoeutrophic lakes were characterized by other phytoplankton composition, mainly by non-flagellate species (green algae).

Species composition and quantitative structure of small-sized Cladocera community and their algal diet before, during and after cyanobacterial blooms were studied in highly eutrophic lake. The objective of the study was to investigate, how the mass development of toxin-producing cyanobacteria affect the abundances of small-sized Cladocera and their preferences within consumed algal cells. Cyanobacterial blooms were predominantly constituted by microcystin-producing genera Planktothrix, Dolichospermum, Microcystis. The concentration of intracellular microcystins in lake water ranged 0.0–23.61 μg dm−3. Bosmina longirostris, B. coregonii, Diaphanosoma brachyurum and Daphnia cucullata were dominant in Cladocera community. The highest abundances of B. longirostris occurred in periods without cyanobacterial blooms and B. coregonii during blooms and after them. The maximum abundances of D. cucullata were observed before and after the cyanobacterial blooms, while the abundance of D. brachyurum was the highest at the beginning of blooms. Small Bacillariophyceae, small Chlorophyceae and Cryptophyceae were the most abundant among identified algal cells detected in digestive tracts of the Cladocera dominants. Tracts of D. cucullata, B. longirostris and B. coregonii contained the highest number of Bacillariophyceae always before blooms. During cyanobacterial blooms, cells of small Chlorophyceae predominated in tracts of D. cucullata. After bloom, cells of Cryptomonas spp. were mainly consumed both by D. cucullata and by B. coregonii. Fragments of Dolichospermum spp., besides Bacillariophyceae and Cryptomonas spp. cells, were occasionally found in tracts of D. brachyurum. Our study indicated that blooms constituted by toxin-producing cyanobacteria may influence quantitative and qualitative structure of the small-sized Cladocera community.

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The development of cyanobacteria and microcystin variation was studied during two years in response to periodical water-level regulation in modified Lake Tomaszne. Before the water entrance from a canal, the biomass of toxigenic cyanobacteria was 0.001-0.33 mg dm

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