8000-year history of palaeoproductivity in a large boreal lake

An 8000-year record of palaeoproductivity, based on the chemical and chiron omid stratigraphies from Lake Paijanne, S. Finland, was assessed with respe ct to known morphometric, climatic and anthropogenic events. A gradual tren d of dystrophication and an associated decrease in aquatic production was d etected during the Holocene, with the following exceptions: (1) high diatom and chironomid production around 8000-6000 cal yr BP, (2) eutrophication a round 2000 cal yr BP, and (3) an anthropogenic signal during the last few d ecades. The changes in chironomid assemblages, before the past few decades, have ma inly been shifts in concentration, but not in species composition. Variatio n in chironomid production was mainly explained by the accumulations of bio genic silicon, carbon and organic matter. Nutrient availability seems to be important in controlling biogenic silicon, which we use to infer past diat om production. The high production ca. 8000-6000 cal yr BP and the fluctuat ion in chironomid influx after ca. 2000 cal yr BP, however, were probably c aused by the proposed warm/dry and cold/wet conditions during these times, respectively. These results highlight the sensitivity of boreal shield lake ecosystems to climatic forcing. In contrast, the pronounced change in the morphometry of the basin around 7000 cal yr BP had little effect on the tro phic state of the lake. The human-induced trophic change during the past fe w decades has affected the Lake Paijanne ecosystem to an extent never exper ienced before during the last 8000-years.