TOP-DOWN OR BOTTOM-UP EFFECTS BY FISH - ISSUES OF CONCERN IN BIOMANIPULATION OF LAKES

A large-scale biomanipulation trial was carried out on Lake Vesijarvi in Finland during 1989-1993. Following the mass removal of coarse fish the biomass of cyanobacteria collapsed from 1.4 g/m(-3) to below 0.4 g/m(-3), while total phosphorus concentration declined from 45 mu g/L to 30 mu g/L. No relevant changes in zooplankton communities were obse rved. The results suggest that the success of food web manipulation as a tool for lake restoration is not necessarily dependent on the grazi ng rate of zooplankton. The effects of reduced fish-mediated internal loading and recycling of nutrients are in many cases stronger than tho se of reduced planktivory. Alternative stable states of water quality may also exist in lakes not covered by macrophytes, owing to the chang es in the behavior of fish stocks. Year-to-year variation in the litto ral zone may cause large oscillations in lake ecosystems-for example, through the recruitment of fish. In addition, the nutrients translocat ed by fish from the littoral zone may affect the nutrient dynamics of the pelagial plankton community. In terms of phytoplankton species com position and the ratio of phosphorus to chlorophyll a, the water quali ty in Lake Vesijarvi has improved in a stepwise fashion within the las t 10 years. This is probably due to the fact that the five-year mass r emoval of fish in Enonselka fulfilled the requirement of sustained man agement of fish stocks in order to maintain nonequilibrial conditions between alternate stable states. The prediction of the water quality d evelopment is obscured, however, by spatial and temporal within-lake v ariation, which sets high requirements for sampling programs.