Direct and indirect mechanisms behind successful biomanipulation

Lake Vesijarvi is a relatively large (length 25 km; total area 110 km(2)), shallow (mean depth 6 m), but stratified lake in southern Finland. The Enon selka basin (26 km(2)), surrounded by the city of Lahti, received its sewag e effluent, and changed from a clear water basin with flourishing fisheries from the 1940-50s to one of the most eutrophic lake systems in Finland the reafter. In 1976, the sewage effluent was diverted, resulting in a temporar y recovery of water quality. However, in the 1980s, massive surface scums o f cyanobacteria degraded the water quality and arrested the recovery of the lake. A restoration strategy providing an ecologically sound basis for the management of the lake was initiated in 1987. This strategy involved bioma nipulation (mass removal of coarse fish) together with conventional polluti on control measures on discharges to the lake. Biomanipulation was chosen i nstead of much more expensive chemical and/or technical methods, such as ch emical treatment or dredging of the profundal sediment. The large-scale bio manipulation trial was carried out in the Enonselka basin during 1989-93. F ollowing the mass removal of coarse fish (1000 metric tons of fish; mainly roach and smelt), the biomass of cyanobacteria collapsed concomitantly with a decline of total phosphorus concentration from 45 to 35 mg P m(-3), and with an increase of Secchi depth from 1 m to 3.5 m. These observed improvem ents in the water quality were matched with a large decline in roach-mediat ed phosphorus movement from littoral to pelagial, from 100 mg P m(-2) in 19 89 to 15 mg P m(-2) in 1993. Year-to-year variation within the littoral com munities, and in the recruitment of fish, could in this way cause large osc illations in the whole ecosystem. The involvement of local people (fisherme n, farmers etc.) in controlling non-point nutrient loading and fish stock d evelopment, is of prime importance for the long term success of lake restor ation.