Lake Vesijärvi

Lake Vesijärvi

Lake Vesijärvi is situated in southern Finland (61° 00´ N, 25° 30´ E). The to­tal surface area of the lake is 110 km2and the mean depth 6.0 m. Originally, L. Vesi­järvi was a clear-water lake, but cultural eutrophication began already in the first dec­ades of the 20th century due to growing industrial wastewaters and domestic sewage. In the 1960´s, L. Vesijärvi had become one of the most eutrophic in the large lakes of Finland. Especially the Enonselkä basin (area 26 km2, mean depth 6.8 m) had became severely eutrophicated. The sewage loading was diverted from the lake in 1976 and the loading of phosphorus and nitrogen into the basin fell to 8 % of the previous level. The lake started to recover.

In the 1980's, however, the recovery faded and massive cyanobacterial blooms (Oscillatoria agardhii Gomont, Microcystis spp., Aphanizomenon flos-aquae (L.) Ralfs) occurred again. To restore the lake, biomanipualtion through mass removal of planktivorous and benthivorous fish was performed during 1989-1994. More than 1000000 kg of fish was removed from the Enonselkä basin. As a result, cyanobacterial blooms disappeared, water clarity increased and submerged macrophytes colonized larger areas. Following predictions by fish population analysis, moderate fish removal was continued. The biomanipulation project was based on the experimental work and fish stock assessment conducted by J. Horppila as the main researcher.

During the 2000’s the water quality development has again turned to an unwanted direction. The biomass of cyanobacteria has been increasing, although massive blooms have not occurred yet. Especially the development of the oxygen concentration has been alarming. Large areas of the hypolimnion have become anoxic during summer stratification period.

To improve the water quality and reduce internal phosphorus loading, it has been decided that large scale aeration will be started in the autumn 2009. Before that, numerous studies will be conducted. Our research group will perform a large scale sedimentation study, Bosminacalculate the phosphorus budget of the Enonselkä basin and estimate the magnitude of internal phosphorus loading. During the following years, the data are used to evaluate the effects of aeration. Additionally, the present state of the food web is examined through fish stock assessment (echosounding and experimental trawling), and by studies on the planktonic assemblages. Our research group focuses especially on planktonic invertebrate predators.