Abrupt physico-chemical changes in the limnology of Pyrenean mountain lakes necessitate urgent management actions

Abstract Mountain lakes are rare ecological niches supporting unique species assemblages and are important sources of safe drinking water for wildlife, livestock and humans. Located at high altitude, these lakes are vulnerable to environmental stressors such as climate or land-use change, impacting their ecosystem functions. However, although trends in water temperature and physico-chemical composition have been fairly well studied in large mountain lakes, the same cannot be said for small- to medium-sized shallow lakes. Here, using measurements of the water chemistry of 25 Pyrenean mountain lakes and temperature records for 14 of them, we show that the environmental conditions which prevail in their littoral zones have undergone rapid changes between 2007 and 2023. Over this 16-year period, we found that, on average, the duration of the ice-cover period decreased by 58.35 days, and the average annual water temperature rose by 1.65 °C. We also found a significant overall decrease in pH, total organic carbon and nitrogen, as well as a significant increase in hardness and dissolved potassium. Our results demonstrate rapid abiotic environmental alterations in mountain lakes, with potential negative implications for humans, wildlife, livestock and ecosystem functioning. Likely consequences include declines in aquatic biodiversity, a degradation in the quality of drinking water and increased health risks from harmful algal blooms. Policy must urgently acknowledge the impacts of global change in mountains in order to take the necessary management actions.