The concentration of organic nitrogen in mountain lakes is increasing as a result of reduced acid deposition and climate change.

The ionic and nutrient composition of mountain lakes recovering from atmospheric acidification is increasingly influenced by climate change (increasing air temperature and frequency of heavy rainfall events). We investigated the evolution of organic nitrogen (ON), dissolved organic carbon (DOC) and phosphorus (P) concentrations in alpine lakes in the Tatra Mountains (Central Europe) between 1993 and 2023, resulting from changes in climate and the ionic composition of atmospheric deposition. Our results suggest that the decreasing acidity of precipitation and the climatically induced increasing frequency of heavy rainfall events and air temperatures fluctuating around the freezing point have the potential to increase ON concentrations in alpine lakes despite decreasing deposition of inorganic N. The increasing ON involves its allochthonous and autochthonous sources: (1) increased co-export of ON with DOC from soils in dissolved organic matter due to less acidic precipitation and more frequent heavy rainfall events and (2) increased in-lake primary productivity (chlorophyll a) associated with higher P availability. Based on our previous studies, we hypothesize that P availability has increased due to (i) reduced adsorption of phosphate in precipitation to the metal hydroxides in the soil-adsorption complex as a result of increasing pH in precipitation and soil water and (ii) increased P production by weathering due to climate-induced increased mechanical erosion of rocks in unstable scree areas. The extent of these changes was related to the percentage cover of scree areas and meadow soils in the lake catchments. In addition, our results suggest that ON (besides chlorophyll a) may be a more sensitive indicator of increasing productivity of oligotrophic alpine lakes under changing air pollution and climate than generally low P concentrations and their poorly detectable trends.