Paleoecological Evaluation of the Recent Acidification of Susceptible Lakes in Finland

The aims of this study were to reveal the causes of lake acidification and to study the acidification history and areal distribution of acidification of Finnish lakes. The 30 lakes selected for this study were mainly oligotrophic forest lakes. They do not represent statistically all lakes in Finland but are the most interesting, sensitive lakes. Estimates of inferred pH and alkalinity from sediment diatom assemblages were based on a calibration set of 92 lakes with surface sediment diatom assemblages and the corresponding chemistry of autumnal water samples. Of the studied 30 lakes, 43% had a significant recent decrease in alkalinity, 20% were naturally acidic, and 27% were with no acidification trends. Lakes with sediment with poorly preserved diatom frustules were 10% of the whole material. At all sites where recent acidification was recorded and the lake was deep enough, the proportion of planktonic diatoms decreased remarkably. In naturally acidic lakes, abundances of diatom plankton have also declined during the last century. In this study the chemical models used showed both larger and smaller acidification than the estimates from the palaeoecological approach. The recent acidification of small forest lakes is restricted to southern Finland where almost all acidic lakes have acidified since 1900. In central and northern Finland, on the other hand, the proportion of naturally acidic lakes is higher.