The Nine year report: Acidification of surface water in Europe and North America. Long-term Developements (1980s and 1990s) (ICP Waters report)

The International Cooperative Programme on Assessment and Monitoring of Acidification of Rivers and Lakes (ICP Waters) is designed to assess the degree and geographical extent of acidification of surface waters. During the last ten years international emission reduction measures in Europe and Northe America have resulted in a decrease in atmospheric sulphur deposition of up to 50%. Nitrogen deposition has stayed almost constant. Trends in surface water chemistry at the ICP Waters sites were calculated by applying the Nonparametric Seasonal Kendal Test and, on a regional scale, by Trend Meta-Analyses. Empirical relationships between nitrogen deposition and stages of nitrogen saturation were used to assess the importance of nitrogen leaching at the sites. To show effects of acidification on aquatic fauna (geographic extent and long-term trends) presence/absence studies, acidification indexes and correlation analysis were used. Trends in water chemistry indicate that sulphate concentrations are decreasing at almost all ICP Waters sites, and in almost all cases the decreases in the 1990s are larger than in the 1980s. This is partly also reflected in a recovery of the invertebrate fauna at many sites. Decreasing sulphate concentrations emphasize the importance of nitrate as the second important acidifying anion. Besides nitrogen deposition, the overall nitrogen status of ecosystems, changes in climate extremes and hydrology can have strong influences on leaching of excess nitrate (and ammonium) from a watershed.Norwegian Pollution Control Authority-SF