The decrease of air pollution achieved since the 1990s thanks to measures adopted under the Air Convention is leading to ongoing biological recovery from acidification in freshwater environments in Europe. This is the main finding of a new report published in the framework of the International Cooperative Programme on Assessment and Monitoring Effects of Air Pollution on Rivers and Lakes (ICP Waters) under the UNECE Convention on Long-range Transboundary Air Pollution (Air Convention).
Air pollution peaked in Europe during the 1980s, leading to acidification of rivers and lakes, with knock-on effects on biological diversity as a result of deposition of pollutants (e.g. sulphur dioxide) and acid rain. Measures introduced by the Air Convention in the 1980s lead to gradual decrease of air pollution from the 1990s onwards. The reduced pollution has resulted in improved chemical conditions in freshwaters, as indicated by increasing pH, and a reduction in the concentration of other components, such as labile aluminium, sulphate and, to a lesser degree, nitrate. This chemical recovery is a precondition for biological recovery, i.e., the return of acid-sensitive organisms.
Since 1985, ICP Waters assesses, on a regional basis, the degree and geographical extent of the impact of atmospheric pollution on surface waters as an indicator for assessing the effectiveness of emission reduction policies. ICP Waters collects and assesses lake and stream monitoring data from an extensive network of monitoring sites. The monitoring includes both chemistry and biology. Biological data is derived from monitoring of fish populations, invertebrates and algae which show different tolerance to water chemistry conditions, among others. Data on changes in these populations can thus be important indicators of water quality. Chemical and biological data are used to assess temporal trends and spatial patterns, as well as evaluations of dose/response relationships.
As a result of the analysis of a call for data on biological recovery from acidification in 2021, ICP Waters found that 47% of all included rivers (21 sites, for the period 1994-2018) and 35% of all lakes (34 sites, for the period 2000 to 2018) showed significant increases in species richness. Correlations between species diversity and water chemical components (acid neutralizing capacity, pH, sulphate) were found, supporting that the biological responses were related to chemical recovery. Further analyses are necessary to consider the impacts of other components of the biological community and other potential drivers, such as catchment properties and climate change.
The report shows the usefulness of biological monitoring data and demonstrates the effectiveness of emission reduction measures under the Convention.