Long-term chemical changes in remote lakes as an effect of climate warming

The water chemistry of mountain lakes and its change in time is affected by variation in atmospheric deposition and climate. The effects of climate warming on mountain lakes are mainly indirect, and result from a combination of multiple stressors. Climate affects both biological and physico-chemical processes, such as weathering of rocks and soils. Particularly important is the indirect effect of change in snow and glacial cover in lake catchments. In this study we focused on the possible effect of climate change on chemical variation in the chemistry of alpine lakes over a 30-year period. We analysed long-term data (major ions and nutrients) available for about 40 lakes in the Western Alps and assess changes in relation to the main meteorological variables (temperature, precipitation, snow cover). High frequency monitoring data exist for a few sites, belonging to the LTER network, and allowed an analysis of short-term changes in relation to extreme meteorological events (e.g. heavy snowfall, heat waves). A general tendency towards increasing conductivity and solute content in lake water was detected. A change in the ionic composition was also observed, with an increasing contribution of sulphate to the total ionic content. Several lakes showed a positive trend of sulphate concentrations, in contrast with the decrease of sulphate deposition which has affected the study area in the last decades. A climate effect was hypothesized to explain these trends: reduced snow cover in the catchments and glacier and permafrost degradation were identified as the main drivers of change.