Development of toxin-producing cyanobacteria during the water level manipulation in a shallow heavily modified lake

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^-3, the microcystin concentration was below 1 μg dm^-3, and three variants of microcystins were detected. After the water entrance from the canal, the biomass of cyanobacteria increased to 1.3-7.1 mg dm^-3 with the dominants Aphanizomenon gracile and Dolichospermum planctonicum. After the water discharge from the lake, Planktothrix agardhii reached the highest biomass (2.3-6.6 mg dm^-3). During the mass development of toxigenic cyanobacteria, the total microcystin concentrations were mostly higher than 5 μg dm^-3 and the number of MC variants increased. Both Pl. agardhii and D. planktonicum accounted for microcystin production. The higher NH4-N concentrations supplied with water from the canal supported the biomass increase of Nostocales, whereas Pl. agardhii mass development was due to the low light intensity and high TP concentrations. Our study revealed that the use of nutrient-rich waters for the water management in the hydromorphologically modified lake supported the persistent development of cyanobacteria species leading to increased amounts of MC and a higher number of their structural variants.