A ten-year period of monitoring of the Rímov reservoir was chosen to demonstrate the impact of fishstock manipulation on phytoplankton biomass and on species composition of the phytoplankton assemblage. Biomass of planktivorous fish decreased substantially under various forms of biomanipulative pressure. The proportion of large individuals of Daphnia galeata (retained on the 0.71 mm mesh sieve) increased from nearly 0 to 9% of the total zooplankton biomass during the period of 1985-1988 and remained nearly constant during subsequent years. Change in zooplankton structure had no significant response in the abundance of phytoplankton. Species composition of phytoplankton assemblage, however, changed considerably after the large daphnids had reached a biomass of more than 10 mg.m−2 of protein-N. Aphanizomenon flos-aquae var. flos-aquae formed about 8% of phytoplankton biomass before 1985, while its share increased to 40-65% during 1988-1990. Later, Aphanizomenon was replaced by other large forms of algae. The factors possibly influencing Aphanizomenon blooms are discussed with respect to the situation in other water bodies.

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