Cyanobacterial blooms produce various highly toxic substances (hepatotoxins, neurotoxins, endotoxins, etc.) whose mutagenicity and cancerogenicity is well known, therefore it is necessary to determine the factors that affect the yield of cyanobacterial blooming and the effectivity of the biosynthesis of toxins as well as to understand the mechanisms whereby they are released. Continuing earlier studies in the area of the Sulejów Reservoir, further research was carried out in the season 2002 into the effects of various environmental factors on the bloom yields of cyanobacteria and the synthesis of cyanobacterial hepatotoxins – microcystins. The first part of the paper focuses on physicochemical factors such as water temperature, pH and hardness, and the concentrations of dissolved oxygen and soluble and total organic carbon. It was confirmed that water temperature in the season belongs to the more important factors influencing bloom yields and microcystin production. Oxygen concentration in water is negatively correlated with bloom yields and the amount of synthesised toxins. Total and soluble organic carbon contents are positively correlated with bloom yields and the amount of microcystin-LR in the cells of cyanobacterial colonies.

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