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Less acid rain led to biological recovery
At the end of the twentieth century acidification was a serious problem in Europe, thanks to international cooperation. The success of the Air Convention led to dramatic decreases in sulphur dioxide which has improved chemical conditions in freshwaters and a biological recovery of acid-sensitive organisms. In the 1980’s in Europe levels of sulphur dioxide were high and led to acid deposition (colloquially referred to as acid rain). This acid rain in turn led to acidification of rivers and lakes, which elevated hydrogen and aluminium ion concentrations. The toxic effects of hydrogen and aluminium ions are major drivers of the loss of acid-sensitive organisms.
The UNECE Convention on Long-range Transboundary Air Pollution (LRTAP Convention), established in1979, reduced air pollution using an effect-based approach, which was proven to be very successful for reducing sulphur dioxide. 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. In a new report, the biological recovery in lakes and rivers from the Czech Republic, Germany, Italy, Norway, Sweden, Switzerland and UK was studied. Sites clearly impacted by chemical factors other than air pollution (i.e. sites that were limed or were located close to mines or in deeply weathered soils) were excluded from the analyses of overall trends.
The research found that 47 per cent of all included rivers (21 sites over the period 1994–2018) and 35 per cent of all lakes (34 sites over the period 2000 to 2018) showed significant increases in species richness. Correlations between species diversity and water chemical components (acid neutralising capacity, pH and sulphate levels) were found, supporting the thesis that biological responses were related to chemical recovery. The report shows the usefulness of long-term biological monitoring data and demonstrates the effectiveness of international cooperation under the Air Convention.
