Microcystis species, particularly M. wesenbergii and M. viridis but also M. aeruginosa, occurred in low densities in two subsequent summers after the most intensive removal of fish. A corresponding decline was not found for microcystin concentration. When intensity of fish removal was reduced, biomass of cyanobacteria started to increase at the expense of other taxa without a corresponding increase in nutrient concentrations or changes in mixing conditions of the water column. In spite of the increase in Microcystis biomass, microcystin concentration remained low, mainly due to the dominance of M. aeruginosa having a lower correlation with microcystin concentration. The more toxic M. viridis did not return in 2002-2003.
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