EUTROPHICATION AND RECOVERY OF LAKE VESIJARVI (SOUTH FINLAND) - DIATOM FRUSTULES IN VARVED SEDIMENTS OVER A 30-YEAR PERIOD

Lake Vesijarvi was loaded by sewage from the City of Lahti for 60 year s until 1976 when the discharge was diverted. Paleolimnological analys es of the varved bottom sediment indicate that the sedimentation rate within the Enonselka basin, the most eutrophic part of the lake, has b een as high as 2 cm yr(-1), and total phosphorus accumulation was 20-4 0 g P m(-2) yr(-1) during the last 20 years. Within the less eutrophic Laitialanselka basin, the sedimentation rate did not exceed 1 cm yr(- 1) and the formation of varved sediment only began at the end of the 1 960's, i.e. about 10 years later than in Enonselka. Planktonic diatom production was highest in the Enonselka basin. The most abundant diato ms in the sediment between 1970-1985 were Asterionella formosa, Aulaco seira islandica and Stephanodiscus spp. Fragilaria crotonensis and Tab ellaria fenestrata had low abundances in the middle of the 1970's but increased again at the end of the 1970's. Asterionella formosa and Dia toma elongatum reached their maxima between 1979-1984 when the hypolim nion of the Enonselka basin was aerated artificially. In the Laitialan selka basin, the production of planktonic diatoms has been lower and t he species composition of the diatom community differed from that in E nonselka. However, at the end of 1980's the total accumulation of diat oms in Laitialanselka approached levels which were observed at the end of 1950's in Enonselka, prior to the rapid eutrophication period. The production and thereby the sedimentation of diatoms has decreased tow ards the end of the 1980's in Enonselka, indicating reduced nutrient a vailability in the lake water. This reduction was due to the decreased external loading of phosphorus as well as to the decreased release of phosphorus from the sediment as a result of improved oxygen balance i n the hypolimnion.