Are Swiss Alpine lakes recovering from acidification?

Background/Question/Methods

During the last three decades, European countries have gone through considerable effort to reduce emissions of acidifying sulfur (S) and nitrogen (N) compounds. For example, at the continental scale, S emissions have been reduced by ≥ 60% since the end of the 1970s, while emissions of oxidized-N have been reduced by ≥ 30% since the 1990s. What are the effects on acid-sensitive ecosystems? Drawing from long-term monitoring data, we evaluated trends in the chemistry of 20 high-elevation lakes in Canton Ticino, Switzerland, an Alpine region that was severely impacted by acid deposition in the past. We report on the trends of the main acid anions (sulfate and nitrate), alkalinity, and pH in the lakes from 1986 to 2011, and compare these trends to changes in regional atmospheric deposition during the same period.

Results/Conclusions

 Since 1986, sulfate and nitrate concentrations have declined in the majority of the lakes monitored, although nitrate declined to a lower extent (range= 0.2-1.4 µeq l^-1yr^-1  vs. 0.2-0.7  µeq l^-1yr^-1 ) and later (from ~2006 vs. the onset of monitoring) than sulfate. These declines were consistent with regional reductions in S and N deposition. Alkalinity and pH have increased in most lakes, with potentially positive consequences for acid-sensitive organisms. If in 1988 40% of the lakes had an average pH < 6, which is harmful to certain aquatic organisms, such conditions now persist in only 15% of the lakes. Our results suggest that the reduction in acidifying emissions is contributing to the recovery of Alpine high-elevation lakes from acidification. However, the recovery is not complete, and further improvements will probably require a more substantial reduction in nitrate concentrations. Since lake-water nitrate depends not only on N deposition, but also on climatic factors, recovery trajectories in the future remain uncertain.