Changes in air temperature, but not in precipitation, determine long-term trends in water chemistry of high mountain lakes of the Alps with and without rock glacier influence

Abstract

Climate change has strongly affected lakes around the world, but the relative effects of warmer air temperatures and changing precipitation on the water chemistry of alpine systems are not well understood. Here we tested the effect of monthly and seasonal climate on the water chemistry of six high mountain lakes located in the Alps. From 1982 to 2020, water samples were collected annually from different depths during the autumn mixing. We observed a simultaneous increase in electrical conductivity, ionic content, and pH with air temperature. In lakes with rock glacier influence, the increase in conductivity, ionic content, and especially in sulfate was even more pronounced, but accompanied by a strong decrease in pH. These differences are attributed to the direct influence of acidic meltwater from active rock glaciers in catchments with acidic bedrock. We then analyzed changes in lake chemistry, taking into account seasonal trends in air temperature and precipitation, using redundancy analysis. Temperature increase significantly affected water chemistry in five of the six lakes, especially at times of ice breakup. Increasing warming explained 17% to 32% of the changes in electrical conductivity, alkalinity, pH, major ions, and nitrogen. In contrast, precipitation had little effect on the changes of those parameters. Nevertheless, late spring snowfall and high snowfall in early fall, which result in prolonged ice cover, had a dampening effect on the impact of climate warming on lake chemistry. Our results confirm that climate warming remains a major driver of chemical changes in alpine lakes, but provide new evidence that late spring temperatures are the most important triggers.