| Summary: | Harmful cyanobacteria and their cyanotoxins may contaminate drinking water resources and their effective control remains challenging. The present study reports on cyanobacterial blooms and associated cyanotoxins in the Obrzyca River, a source of drinking water in Poland. The river was examined from July to October 2019 and concentrations of microcystins, anatoxin-a, and cylindrospermopsin were monitored. The toxicity of water samples was also tested using an ecotoxicological assay. All studied cyanotoxins were detected with microcystins revealing the highest levels. Maximal microcystin concentrations (3.97 μg/L) were determined in September at Uście point, exceeding the provisional guideline. Extracts from Uście point, where the dominant species were <i>Dolichospermum flos-aquae</i> (August), <i>Microcystis aeruginosa</i> (September), and <i>Planktothrix agardhii</i> (October), were toxic for <i>Dugesia tigrina</i> Girard. Microcystin concentrations (MC-LR and MC-RR) were positively correlated with cyanobacteria biovolume. Analysis of the chemical indicators of water quality has shown relationships between them and microcystins as well as cyanobacteria abundance.
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