Forty years of studying effects of sulphur and nitrogen deposition in freshwaters in Europe and North America

Sammendrag

The International Cooperative Programme on Assessment and Monitoring of the Effects of Air Pollution on Rivers and Lakes (ICP Waters) was established in 1985 under the UNECE Convention on Long-range Transboundary Air Pollution (CLRTAP). An important basis for the work of ICP Waters is the long-term data submitted by participating countries, comprising over 500 sites in Europe and North America. The most recent trend analysis (1990-2020) showed declining sulphate and nitrate concentrations and increasing acid neutralising capacity (ANC) and pH, as expected given the general decline in sulphur and nitrogen deposition. There were clear regional differences, with the largest changes occurring in the more heavily affected regions. Trend trajectories of sulphate, ANC and pH indicated that chemical recovery has decelerated in Europe and accelerated in North America, which can be attributed to differences in the timing of regulatory measures. Nitrate declined to a lower extent than sulphate. This can be related to continued nitrogen retention within the catchments. Another ICP Waters analysis, targeted at nitrogen, showed that the strongest decline in nitrate concentrations occurred at sites with high nitrogen deposition. As a large share of sites show decreasing nitrate trends and an increase in the dissolved organic matter carbon to nitrogen ratios, there are few indications of widespread nitrogen saturation following long-term nitrogen deposition. Total organic carbon concentrations have increased, in line with the established connection between declining acid deposition and increased organic matter solubility. The increase has decelerated in Europe. Calcium concentrations declined less rapidly than sulphate concentrations, contrary to expectations based on charge balance. In low deposition regions, even an increase in calcium is observed. It is hypothesized that enhanced vegetation growth and longer growing seasons at less acid-sensitive sites can increase chemical weathering. Further chemical recovery was modelled at selected ICP Waters sites using the MAGIC model with different deposition scenarios. Current legislation is predicted to give considerable improvements in water chemistry in acidic lakes across Europe by 2040. Additional improvement from proposed revised legislation is limited, owing to relatively small additional deposition reduction and that sulphur deposition is already at a low level given current legislation.