The effects of biomanipulation were studied in ten Finnish lakes to determine responses in fish and plankton communities and water quality after mass removal of cyprinids. From 1997 to 2001, the fish communities shifted from the dominance of large cyprinids to an explosion of small cyprinids and a higher proportion of piscivores in effectively biomanipulated lakes (>200 kg ha )1 3 yr )1 ). The biomass of cyanobacteria decreased, and the duration of the blooms shortened and shifted towards the autumn. Decreased concentrations and slower cycling of nutrients and increased grazing by cladocerans probably affected the declined biomass of cyanobacteria. Less intensive sediment disturbance and increased phosphorus-retention in fast growing fish biomass may have turned the role of the fish assemblage from 'nutrient recycler' to 'nutrient storage'. Increased potential grazing pressure, higher proportion of edible algae, and lower chlorophyll a:total phosphorus ratio indicated strengthened herbivore control. A high mass removal catch in relation to trophic state, low background turbidity, and bearable external loading favoured the successful biomanipulation, whereas intensive cyprinid reproduction, high nutrient loading and non-algal turbidity hindered the recovery. Three important issues should be noticed before biomanipulation in Finland: (1) careful selection of target lake, (2) well-planned, effective and long-lasting biomanipulation and (3) sustainable management of piscivores.

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A large scale biomanipulation of pelagic fish by trawling was started in the eutrophic Enonselka basin (26 km 2 ) of Lake Vesijdrvi to improve the water quality which had remained poor in spite of the termination of nutrient loading . The distribution and density of the fish were studied by hydroacoustics before and during the removal . The initial annual fish density varied between 13 000-21 000 fish ha -' in the study area in August 1984-89 . The mass-removal of the fish by pelagic trawling took place in 1989-1992 . The catch varied annually between 64 and 92 kg ha -1 . Roach (Rutilus rutilus (L.)) and smelt (Osmerus eperlanus (L .)) accounted for c . 85% of the weight of the catch . The mass-removal decreased the pelagic fish density in the Enonselka basin during the trawling . An increase in the density was observed after the trawling ceased in these years, and the initial density level was reached within one month . The density level after mass-removal remained high compared with oligotrophic lakes . The pelagic fish had a diurnal ascending trend with the decreasing light intensity in August, and the fish were significantly (p < 0 .01) higher in the water mass in temperature non-stratified water in autumn than in stratified water in summer. The fish were somewhat deeper in the years of mass-removal than before it . Mass-removal did not affect the individual length of the echosurveyed fish . Fish smaller than c . 15 cm (TS < -44 dB ; mainly smelt) were numerically dominant throughout the whole study period .

The possibility of utilizing biomanipulation to improve the water quality of Tallinn's drinking water reservoir (Lake Ülemiste) was analysed on the basis of water quality data, test fishing by different methods, and earlier studies on aquatic plants, light climate and sediments. Eutrophic, polymictic Lake Ülemiste is characterized by a prevalence of high filamentous cyanobacteria biomass, rotifers in the zooplankton community, mature bream in the fish biomass, and a high density of planktivores (YOY perch). Several prerequisites for being a potential target for biomanipulation were identified, including (i) decreased external total phosphorus (TP) loading; and (ii) a sufficient stock of piscivorous fish in the lake. Prohibition of fishing should favour biomanipulation efforts. The potential for recolonization of macrophytes and large Daphnia species in Lake Ülemiste could be judged from historical data. The in-lake TP concentrations are within the limits that facilitate successful restoration. As phytoplankton biomass was significantly limited by phosphorus in Lake Ülemiste, its water quality improvement will be driven primarily by reduced nutrient concentrations, and then by zooplankton grazing. Internal TP loading and a new phytoplankton species community were assessed as major challenges for successful implementation of biomanipulation as a means of improving the water quality of Lake Ülemiste.

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