In a small polytrophic and meromictic lake (Fischkaltersee, Iffeldorf lake district, Upper Bavaria, FRG) artificial destratification was installed to increase water quality. During this procedure, viable biomass of algae in the water column almost doubled. This increase was not paralled by an adequate increase in primary production. Artificial destratification caused a total and enduring decrease of cyanophytes after one species, Oscillatoria limnetica, bloomed for a very short period of time, probably due to elevated concentrations of several chemical compounds after destroying the monimolimnion. The cyanophytes were replaced by various species of coccal and tetrasporal chloro-phytes, centric diatoms and small cryptophytes. Two kinds of mechanisms, responsible for this structural shift, are discussed: advantages for green algae and diatoms as well as disadvantages for cyanophytes.