Regulation of the development and species dominance of summer phytoplankton in Lake Vesijärvi: predictability of enclosure experiments

Abstract

Lake Vesijärvi has a long history of sewage-induced eutrophication and partial recovery. In 1976 the municipal sewage loading was diverted from the lake. Initially, the recovery of the lake was rapid but after some years it slowed down and summer blooms of cyanobacteria (Aphanizomenon, Microcystis) increased again. Field experiments in four enclosures with different roach densities, and the nearby lake area (2–3 m depth zone) were carried out to evaluate the contribution of fish, Zooplankton, water turbulence, phosphorus and nitrogen to the development of summer algal biomass and species dominance. In general, phytoplankton biomass increased during the field experiment (21 June-1 August) with the fish density. The artificially induced turbulence, as well as phosphorus enrichment alone, had little effect on algal biomass and species composition, but combined with nitrogen enrichment considerably increased the algal biomass and the numbers of cyanobacteria in three of the four enclosures. However, at the end of the experiment there was wide variation in the species composition and dominance between different enclosures. Grazing by Zooplankton was of minor importance in all enclosures and also in the lake.