The paleolimnological development of the twin lakes Etujärvi and Takajärvi in Askola, southern Finland - implications for lake management

The twin lakes Etujärvi and Takajärvi in Askola, southern Finland, are closely interconnected mesotrophic headwater lakes with a relatively small catchment area. Both of the lakes have suffered from eutrophication and its consequences. Remediation activities such as oxygenation and biomanipulation have not resolved the problems. In this study a large set of paleolimnological techniques (radiometric AMS dating, spherical carbonaceous particles analysis, sediment lithology, grain-size analysis, phosphorus fractionation, and diatom analysis) were applied to put together the development of the basin and its water level fluctuations during the Holocene. The age for observed Trapa natans - horizons was determined, and lake management options were discussed. The studied lakes dried up after isolation from the Ancylus Lake at about 9500 cal. B.P. and remained at very low water level until ca. 8700–8500 cal. B.P. The mid-Holocene rise in water level resulted in fluctuating water levels, and led to the most recent rise starting about 2500 cal. B.P. as wet and cool climate conditions prevailed. The pronounced water level fluctuations led to the extensive growth of peat deposits surrounding the lake and probably also forced T. natans to disappear from lake flora. The unstable, erodable peat rims impact the lakes, causing heavy load of humic substances to the lake and presenting additional deterioration to their recreational value.