Environmental impacts on freshwaters: acidification as a global problem

Abstract

Nutrient and acid loads are the two main global impacts on freshwaters. Recognition of problems began in 1960–1970 both for eutrophication and acidification, but changes in lake ecosystems depending on nutrient inputs started 10 years earlier, whereas acidification of sensitive lakes began approximately 100 years ago. Freshwater acidification is no longer restricted to the northern hemisphere but it affects industrialized regions of the whole world. Reduction of eutrophication in large lakes of developed countries was partly successful, but effective measures against acidification are still missing, although SO 2 emissions and deposition of sulfuric acid are decreasing: sulfate in acid rain has largely been replaced by nitrogen compounds. Moreover and in contrast to chemical viewpoints, recent studies have shown that biological effects of acidification start very early (approx. pH 6.5); for sensitive organisms the chemical threshold of acid water, i.e. pH 5.5 or zero alkalinity, represents the end point rather than the onset of acidification. The application of models and the reconstruction of the pH history of high altitude lakes in the Alps showed that very sensitive lakes can acidify even at low acid inputs and that climate changes can strongly interact with anthropogenic emissions of acids.