Anthropogenically induced changes in the sedimentation processes in the littoral zone of Lake Verevi, South Estonia

Studies of four sediment cores from the littoral zone of a small Lake Verevi with a well- established history of lake hydrochemistry and hydrobiology were conducted. Observational data describe essential changes in the composition of macrophytes and the trophic state of the lake during the last century. Lithological composition (organic, siliciclastic, and carbonaceous matter) in the four sediment profiles taken within a distance of 20 m showed good lithostratigraphical correspondence but differences (up to two times) in the mass accumulation rates of various compounds. Most of the organic matter (75-85%) in the sediment originates from phytoplankton as demonstrated by low organic carbon/nitrogen ratios. Accumulation of organic matter is not directly related to changes in lake trophicity and primary production but rather seems to be dependent on the accumulation of siliciclastic matter, which engages the organic matter in the settling processes. This is supported by the extremely high correlations between the mass accumulation rates of siliciclastic and organic matter (R 2 > 0.8). Although the greatest changes in the lake environment took place after 1970 due to severe human impact on the lake, most of the changes in the sediment records started earlier. Precipitation of CaCO3 began already in the 1930s due to natural eutrophication processes and reached its highest values in the 1970s, which can be related to the anthropogenic impact. Also the share of carbon from algae, indicating a rise in the trophic status, started to increase in the 1930s. Macrofossil records reveal that also major changes in the vegetation may have started earlier than historically recorded. The rise in the trophic status of the lake had no direct effect on the accumulation of organic matter.