Effect of Acid Deposition on Chemistry and Biology of High-Altitude Alpine Lakes

Abstract

Trend analyses of the key parameters involved in acidification processes measured in 20 Alpine lakes during the period 1980–2004 revealed significant decreasing sulphate (15 out of 20) and increasing alkalinity trends (14 out of 20) in most studied lakes, while trends for base cations and nitrate were small and mostly insignificant. The average increase in alkalinity between 1980 and 2004 was 0.012 meq l⁻¹. Today two lakes out of 20 are still acidic (alkalinity < 0 meq l⁻¹), 13 are sensitive to acidification (0 meq l⁻¹ < alkalinity < 0.05 meq l⁻¹) and five have low alkalinities but are not at risk (0.05 meq l⁻¹ < alkalinity < 0.2 meq l⁻¹). Differently, in the 1980s four lakes were acidic, 14 were sensitive to acidification and two had low alkalinities. During the same time period the pH increased on average by 0.3 units.

Because of accelerated dissolution of aluminium minerals a pH value below 6 can become critical for the biology of lakes. Compared to seven lakes in the 1980s today only three lakes out of 20 exhibit an average pH below 6. A comparison between the populations of macroinvertebrates in lakes with different acidity showed that at average lake pH’s below 6 the population of macroinvertebrate changes. The taxa richness and the EPT index (= number of families of the order Ephemeroptera, Plecoptera, Trichoptera) decreases and acid-sensitive species disappear.