Long-term divergence of nitrogen and phosphorus dynamics in small lakes across China

Small lakes are numerically dominant and play key roles in regional nutrient cycling globally, yet their long-term nutrient dynamics remain understudied. This study aimed to construct a national-scale dataset for total nitrogen (TN) and total phosphorus (TP) in Chinese small lakes (<1 km2), and characterize spatiotemporal patterns and quantify the dominant drivers of TN/TP ratios. Using 220,832 water quality records from 208 national automatic monitoring stations and 17 auxiliary variables, we applied a Random Forest model to predict monthly TN and TP concentrations for 18,210 small lakes from 2000 to 2020. We showed that 33.7% of those lakes showed significant decline trend, with national TN/TP ratio of 18.7±4.2 and average decline trend of 0.99/decade (p < 0.05). Eastern Plains lakes had the highest mean TN/TP ratios (48.2 ± 13.7) and the fastest decrease (-1.565/decade). Additionally, the TN/TP ratios increased with lake area and decreased with water depth. Climate variables and soil characteristics dominated TN/TP variation, with precipitation, soil pH, and temperature being the most impactful factors. Notably, anthropogenic activities influenced TN/TP ratios indirectly by modifying lake morphology and soil properties, rather than through direct inputs to the water column. This study fills critical data gaps in the nutrient dynamics of Chinese small lakes and highlights significant spatiotemporal heterogeneity in TN/TP ratios, provide a scientific basis for developing context-specific eutrophication management strategies for small lakes in China.