Acidification of Swedish freshwaters

Summary

Estimates indicate that at least 20% of plant and animal species have died out in the 15,000–25,000 European lakes where the pH has fallen by more than 0.5 units as a result of anthropogenic acid deposition. Severe depletion of species is also to be found in watercourses in several areas of Europe, but there is a lack of data upon which to base quantitative assessments.

No species has yet been reported extinct as a result of acidification, but it is important to bear in mind that few data are available upon which to determine whether or not a species has died out or is under threat.

In Sweden, at least 20% of species has probably disappeared from 5,000–7,000 acidified lakes and thousands of acidified watercourses. The number of lakes would probably have been higher (7,000–9,000) without liming. Significantly acidified lakes and watercourses can be found almost throughout the country. The most severely affected waters are to be found in south-western Sweden where about 80% of waters would have been significantly damaged by acidification had they1 not been limed.

Acidified lakes and watercourses are often characterised by a sharp increase in filamentous algae and Sphagnum moss, and a significant decline in the number of species of phytoplankton. The number of animal species is declining in all groups, in lakes and watercourses alike, mainly among molluscs, bivalves, crustaceans and mayflies. At a pH of below 5, only a few species remain in these groups. In acidified lakes, organic material decomposes more slowly on the surface of the sediment, but it is unclear whether this also applies within the sediment.

Lakes and watercourses where the critical load for acid deposition has been exceeded can be found throughout Sweden. This indicates that there is a clear risk of acidification affecting more waters than it already has done. Estimates show that if the acidifying load remains unchanged, the number of significantly acidified lakes and watercourses in Sweden may rise by about 50% within the next 20 years. In order to prevent Swedish surface waters from deteriorating, emissions in Europe must be reduced by 60%–70%. This can be compared with 30% reduction in emissions which will result from current international agreements and the action which has been planned at a national level.

In order to prevent increasing depletion of fauna and flora and rising mercury levels in fish, large-scale liming operations to prevent acidification of Swedish freshwaters must continue for at least the next 30–50 years.