Lake response to anthropogenic impact: comparison of multi-proxy sediment record and historical evidence

Rapid anthropogenic eutrophication in lakes raises concerns about lake ecosystem tolerance capacity and ability to recover. During the last decades, restoration programmes have been initiated to improve lake ecosystems and re-establish their natural conditions. In this study we used 200-year dataset to describe 20th century anthropogenic impact on small, highly trophic Lake Verevi. Series of paleolimnological proxies were used to describe lake natural conditions and lake development also for the time period when monitoring data are missing. Increase in pigments and change in Cladocera composition around 1850s suggested that the lake trophic level raised already before direct urbanization on the lake shores which started around 1900. Simultaneous increase in erosion suggests that land-use around the lake on-going earlier and had clear effect on lake productivity. Second increase in algal production occurred around 1960s and it was connected with intensified sewage-contamination. Albeit, it didn’t remarkably influence the Cladocera population, pigments showed increase in cyanobacteria and sediment became enriched in authigenic carbonates. In 1980s, extremely high concentrations of sedimentary pigments, particularly those indicating the cyanobacteria blooms, corresponds with monitoring data by which the lake was under hypertrophic conditions. Rise in trophicity was concurrent with change in δ13Ccarb and δ18Ocarb values, what suggest that in highly productive lake also δ18Ocarb values are affected by lake productivity. New community structure of Cladocera and lower proportion of cyanobacteria at the end of 20th century after Lake Verevi underwent restoration management, could be a sign of improvement of the conditions in the lake.