Multi-decadal ecosystem stoichiometric changes across high-mountain Pyrenean aquatic ecosystems driven by reduced acid deposition and climate change

The relative supply of carbon (C), nitrogen (N), and phosphorus (P) to aquatic ecosystems is a key regulator of productivity, nutrient cycling, and food-web dynamics. Several environmental changes in high-altitude regions can directly or indirectly influence carbon (C), nitrogen (N), and phosphorus (P) cycling, retention, and availability through terrestrial, atmospheric, and in-situ aquatic processes, thereby regulating their export to lakes, rivers, and headwater streams.

While increasing concentrations of dissolved organic carbon (DOC) have been widely documented in high-latitude surface waters and increasingly reported for many high-elevation lakes and streams, concurrent long-term trends in nitrogen (N) and phosphorus (P) availability—and associated shifts in elemental stoichiometry—remain poorly constrained, particularly across heterogeneous high-mountain aquatic ecosystems. In these regions, declining atmospheric deposition can directly reduce external nutrient inputs but also indirectly alter soil chemistry and biogeochemical processes, for example by enhancing microbial mineralization of soil organic matter following reductions in soil acidity. At the same time, rapid climate warming and elevated atmospheric CO₂ are promoting increased alpine and subalpine plant productivity and upslope vegetation expansion, potentially enhancing nutrient sequestration in biomass and soils while increasing soil DOC production. Climate-driven shifts in seasonality, including earlier snowmelt, longer growing seasons, and warmer autumns and winters, further influence the timing and magnitude of nutrient uptake, transformation, and mobilization along terrestrial–aquatic flow paths. Finally, fundamental differences in hydrological residence times, internal processing, and network connectivity between lakes and rivers may drive divergent long-term trends in carbon and nutrient stoichiometry, but such cross-ecosystem assessments within high-mountain river networks remain scarce.

Here, we analyzed decadal-scale changes (from 2005 to 2025) in dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN), and soluble reactive phosphorus (SRP) across 35 sites spanning lakes (n = 14) and rivers and streams (n = 21) within the Pyrenees mountain range. Dissolved organic carbon (DOC) increased consistently across sites, while dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) showed widespread declines, largely independent of catchment type or aquatic system. Declines in dissolved inorganic nitrogen (DIN) were most pronounced during the growing season and, together with increasing dissolved organic carbon (DOC) at several sites, suggest enhanced retention of nitrogen by alpine vegetation and soil microbial communities, potentially reinforced by long-term reductions in atmospheric nitrogen deposition. In contrast, declines in soluble reactive phosphorus (SRP) occurred primarily during late autumn and winter, indicating that key biogeochemical controls operate during the non-growing season, potentially linked to reduced physical weathering inputs, altered hydrological pathways, increased sediment retention, and changes in atmospheric deposition

Linking nutrient trends with rising DOC concentrations revealed a consistent shift in elemental ratios across the majority of sites, characterized by increasing carbon availability relative to limiting nutrients. Collectively, these patterns indicate a co-ocurrance of increased DOC (or browning) and oligotrophication of high-mountain lakes and running waters, with likely consequences for primary production, microbial metabolism, and food-web structure in alpine and subalpine aquatic ecosystems under continued climate change.