Decadal Shifts in Lake Macrophyte Communities Trace the Impact of Global Change in the Pyrenees

High‐mountain lakes are among the most sensitive ecosystems to global environmental change. Their remote locations and low anthropogenic disturbance make them ideal sentinels for detecting subtle, long‐term ecological responses. This study investigated decadal‐scale changes in aquatic macrophyte communities and water chemistry across a network of remote lakes in the Pyrenees. Our aim was to assess the extent to which regional macrophyte assemblages have shifted over the past 36 years and to identify the environmental variables—both global and local—associated with these changes.

We surveyed aquatic macrophyte communities in 30 high‐altitude lakes across the Pyrenees, southwestern Europe, comparing data collected in 1987–1988 with resurveys conducted between 2017 and 2023. Macrophyte assemblages were assessed through visual transects in clear, oligotrophic waters. Water chemistry was analysed from shoreline samples in 1987–1988, 2011 and 2023, focusing on conductivity, alkalinity, major ions, phosphorus and dissolved inorganic nitrogen. Temporal β‐diversity metrics were used to quantify community turnover and nestedness, with patterns interpreted in the context of long‐term climate trends, atmospheric deposition and lake‐specific characteristics.

While total macrophyte richness across the region remained relatively stable, we observed marked shifts in community composition. Bicarbonate‐using elodeid species (e.g., Potamogeton spp., Myriophyllum alterniflorum ) became more prevalent, particularly in lakes where alkalinity exceeded 150 μeq/L or conductivity surpassed 20 μS/cm over the study period. In contrast, mosses and floating‐leaved taxa declined, particularly in ultra‐oligotrophic systems. These biotic changes paralleled increases in alkalinity, conductivity and major ion concentrations, suggesting intensified rock weathering and dust deposition. Dissolved inorganic nitrogen, by contrast, declined over time, reflecting unbalanced nitrogen (N) and phosphorus (P) deposition in favour of P.

Our findings reveal that aquatic macrophyte communities in Pyrenean high‐mountain lakes have undergone significant compositional turnover over the past three decades, even in the absence of major local disturbances. These changes are best explained by regional‐scale biogeochemical shifts linked to climate change and atmospheric inputs, rather than by local stressors such as fish introductions. Chemical thresholds for alkalinity and conductivity appear to mark tipping points in macrophyte assemblage structure, emphasizing the vulnerability of ultra‐oligotrophic lakes to even modest changes in water chemistry.

Our results underscore the utility of macrophyte communities as ecological indicators and highlight the need for continued monitoring of high‐altitude lake systems worldwide, many of which face similar pressures from climate change, atmospheric deposition and biogeochemical alterations.