Short-code paleolimnological investigation of Lake Pihlajavesi in the Saimaa Lake complex, eastern Finalnd: assessment of habitat quality of an endemic and endangered seal population

A short-core paleolimnological investigation was carried out to acquire kno wledge of the recent history of Lake Pihlajavesi, Saimaa Lake complex, and define its natural state before cultural disturbances. In the natural state , with negligible human interference, the basin was oligotrophic and oxygen -rich. The bioproductivity in the littoral zone was low and the profundal b enthic quality was good according to Wiederholm's Benthic Quality Index. Based on diatom analyses, no significant changes have taken place in the ph ytoplankton communities during the past decades. Sedimentary chironomid com munities reveal, however, slight changes on profundal life in the Pihlajave denselka basin, near the main pollution sources, whereas there were no sign ificant changes in the outer basin. Three stages could be distinguished in the pollution history of Pihlajavedenselka: (1) 'The natural state' up unti l the 1960s, (2) 'the period of increasing loading' during the 1960s and 19 70s, and (3) 'the recovery of the basin' during the past two decades. Untreated municipal waste water from the town of Savonlinna was partly resp onsible for the commencement of eutrophication in the Pihlajavedenselka bas in in the 1960s. The paper and pulp industry in Varkaus, some 40 kilometres upstream from Lake Pihlajanvesi, has also increased eutrophication, especi ally during the worst period of water quality in the 1960s and 1970s. More effective waste water purification has markedly reduced effluent loading an d led to a general recovery of the area. Neverthless the effects of slight nutrient loading can still be seen, especially in littoral bioproductivity. Our paleolimnological data indicate that the present ecosystem will offer l iving conditions for the endemic Saimaa ringed seal population that are sim ilar to those that existed prior to human disturbances.