Acidification in high mountain lakes in Central, Southwest, and Southeast Europe (Alps, Pyrenees, Pirin)

A comparison of lakewater chemistry in the Alps, Pyrenees and Pirin ranges showed the different acidic status of high mountain lakes in the areas studied. The most acidic lakes were situated in the Alps, i.e. Ticino (67% of lakes with pH lt 6), and Tyrol, Carinthia, and South Tyrol (42%). Lakes in the Pyrenees (19%) and Pirin(18%) were intermediate, whereas the lakes in Aosta had the highest pH and alkalinity (4%). The chemical composition of precipitation in the Alps and Pyrenees showed a similar concentration of inorganic nitrogen (49 vs. 42 mu-eq l-1), but a lower concentration of chloride (9 vs. 19 mu-eq l-1) and sulfate (37 vs. 50 mu-eq l-1) in the Alps. In both the Alps and the Pyrenees the mean chloride concentration in high mountain lakes was lower than in precipitation. This may be explained by the flushing effect of melting water during the thaw, an effect which makes it difficult to calculate evaporation factors. The most remarkable differences in precipitation in the Alps and the Pyrenees were found for calcium (30 vs. 89 mu-eq l-1) and potential acidity (45 vs. - 17 mu-eq l-1). Besides inorganic nitrogen, calcareous dust was the main feature in both areas as regards precipitation chemistry, and led, on a yearly basis, to acidic depositions in the Alps but alkaline precipitations in the Pyrenees. The different alkalinity of lakes in the two ranges is explained by the degree of acidity in precipitation and weathering rates. Statistical and conceptual models for the calculation of alkalinity changes, however, showed a similar loss of alkalinity for high mountain lakes in the Alps and the Pyrenees, ranging between 26 and 65 mu-eq l-1, depending on the model. A factor analysis showed four main sources of chemical variability: weathering, marine influence, anthropogenic pollution, and eutrophication. For acid sensitive lakes, weathering of Ca-2+ was the first cause of variation in the Alps, whereas in the Pyrenees it was marine influence.