Human activities subtly reshape the nitrogen-to-phosphorus stoichiometry of large lakes across China

Driven by both human activities and climate change, shifts are occurring not only in nutrient concentrations but also in nitrogen-to-phosphorus stoichiometry (TN/TP ratios), with far-reaching consequences for aquatic ecosystem stability. While seasonal fluctuations in TN/TP ratios are often striking, their interannual changes are comparatively subtle. Yet, long-term studies on TN/TP dynamics and the evolution of their dominant drivers across different timeframes remain limited. Here, in this study, we analyzed multi-decadal data from 12 large Chinese lakes and quantified the contributions of human activities and climate factors to the TN/TP mass ratio dynamics across different time periods. Our results reveal that TN/TP ratios in most lakes have risen significantly over recent decades, initially under single-driver dominance primarily associated with human activities, but gradually shifting toward a regime characterized by the coexistence of multiple drivers. Although human activities have remained the main driver of TN/TP increases, their influence has weakened over time. In contrast, climatic factors currently account for the smallest share of variance but represent the greatest source of uncertainty, and their influence is steadily intensifying. Considering that nutrient concentrations in Chinese lakes remain high and most TN/TP ratios lie within the co-limitation range, dual nitrogen-phosphorus control is essential. In addition, strengthening the resilience of aquatic ecosystems is critical to better cope with the challenges posed by future climate change.