Climate-driven pH control of remote alpine lakes and effects of acid deposition

Abstract

DESPITE the attention given in recent years to the issue of lake acidification by anthropogenic emissions, the question of how climate change might influence the acid-base equilibria of lakes has been little explored. Here we use palaeolimnological data to show that the acidity of two soft-water, high-altitude lakes in the central Alps was correlated with regional temperature during the entire nineteenth century, colder years being associated with lower pH. Our findings of high concentrations of organic matter and elevated Fe/Mn ratios in the corresponding sediment layers support the view that this relationship is dictated primarily by the temperature dependence of in-lake processes of alkalinity generation. The onset of anthropogenically derived acid precipitation at the beginning of the present century led to a breakdown of the pH-temperature relationship, with pH dropping steadily to values of 5.6-5.8. Nevertheless, temperature maxima around 1950 still coincide with peaks in pH, and some pH minima before 1900 fell as low as as the levels around 1970. It is an open question whether the very recent levelling off that we observe in the decline of pH is due to rising temperatures or to decreasing precipitation acidity.