A paleolimnological assessment of the effects of post-1970 reductions of sulfur deposition in Sweden

Analysis of diatoms in sediment cores from 10 acidic (pH < 6) lakes in southern Sweden shows that eight of the lakes have acidified after 1950, while two lakes have not significantly acidified. However, since the 1970s, sulfur deposition has decreased by 50%, and lake water chemistry monitored since 1983 shows an initial reversal of acidification. However, the diatom data do not indicate that a general recovery in pH has occurred yet. The diatoms show that a small recovery has occurred in only one lake (pH increase from 4.7 to 4.9). The 10 lakes vary in total organic carbon content from 2 to 17 mg.L-1. According to the diatoms, the lakes with high total organic carbon (>9 mg.L-1) have not acidified as much as the lakes with lower total organic carbon (<7 mg.L-1). We ascribe this difference in response to acid deposition to the buffering capacity of organic acids. Knowledge of the role of organic acids when combined with anthropogenic acid deposition is important in predicting responses to decreasing acid deposition and the time scales required for recovery. Paleolimnological methods are shown here to provide valuable information for these purposes as well as to provide a long-term perspective on lake acidity changes needed for the evaluation of recovery.