Responses of zooplankton and fish to restoration in eutrophic Lake Tuusulanjaervi in southern Finland

摘要

Lake Tuusulanjaervi (surface area 6 km~2, mean depth 3.2 m, maximum depth 10m) is the largest lake of theVantaa river basin, located in a densely populated area of southernmost Finland, 30 km north of Helsinki. Symptoms of eutrophication of this originally slightly dystrophic lake were first recorded in the 1930s, and since the mid-1950s rapid eutrophication has taken place due to increased nutrient loading from settlement and agriculture. Winter aeration of the lake was started in 1972, and sewage discharge stopped in 1979, but the lake remained hypertrophic due to high diffuse pollution and internal loading (Pekkarinen 1990). After positive experiences from some restoration projects of large Finnish lakes, such as L. Vesijaervi (Horppila et al. 1998) and L. Koeylioenjaervi (Sarvala et al. 2000), biomanipulation by means of mass removal of planktivorous fish in Tuusulanjaervi was started in 1997. For details of methodology, see Sammalkorpi (2000). The annual mass removal catches during 1997-2003 were 24-113 metric tons, 40-190 kg ha~(-1), totalling close to 420 metric tons and 700 kg ha~(-1) (Table 1).