Recent acidification and biological changes in Lilla Öresjön, southwest Sweden, and the relation to atmospheric pollution and land-use history

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Abstract
Palaeolimnological techniques were used to study the recent acidification history of Lilia Oresjon in southwest Sweden, and its relation to the deposition of airborne pollutants and land-use. The sediment analyses suggest that water quality began to deteriorate at the beginning of the 20th century and resulted in an acute acidification phase in the 1960s. An indifferent (circumneutral) diatom flora with some planktonic taxa was replaced by a non-planktonic acidophilous and acidobiontic flora; diatom inferred pH decreased from 6.1 in the 19th century to the present value of about 4.6. The history of acidification and of major biological change in the lake is reinforced by the analyses of chrysophyte scales and cladocera and chironomid remains, which show that alterations of species composition and an impoverishment of faunal communities took place. There is close stratigraphic agreement between these biological changes and indicators of the deposition of atmospheric pollutants. The concentration of Pb, Zn, Cu and S increased from the beginning of the 19th century to peak values during the 1960s and 1970s. Spheroidal carbonaceous particles, polycyclic aromatic hydrocarbons (PAH) and ‘hard’ isothermal remanence, indicative of oil and coal combustion, peaked during the 1970s and 1980s, respectively. The increased deposition of airborne pollutants from fossil fuel combustion and industrial processes is suggested as the main cause of the acidification of the lake, although vegetation changes, such as a recent expansion of spruce-pine forest, have also occurred during the 200—300 year period studied.