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.

小型湖泊在数量上占据主导地位，在全球范围内的区域养分循环中发挥关键作用，但针对其长期养分动态的研究仍较为匮乏。本研究旨在构建中国面积小于1平方千米的小型湖泊总氮（Total Nitrogen, TN）与总磷（Total Phosphorus, TP）的国家级数据集，解析其时空分布特征并量化总氮总磷比（TN/TP）的主要驱动因子。本研究依托208个国家自动监测站点的220832条水质记录与17项辅助变量，采用随机森林（Random Forest）模型对2000至2020年间18210个小型湖泊的月度总氮与总磷浓度进行预测。研究结果显示，其中33.7%的小型湖泊总氮总磷比呈现显著下降趋势；全国小型湖泊的总氮总磷比均值为18.7±4.2，平均下降速率为0.99每十年（p<0.05）。东部平原地区的小型湖泊总氮总磷比均值最高（48.2±13.7），下降速率也最快（-1.565每十年）。此外，总氮总磷比随湖泊面积增大而升高，随水深增加而降低。气候变量与土壤特征是总氮总磷比变化的主导因子，其中降水、土壤pH值与气温的影响最为显著。值得注意的是，人类活动并非通过直接向水体输入污染物，而是通过改变湖泊形态与土壤特性间接影响总氮总磷比。本研究填补了中国小型湖泊养分动态研究领域的关键数据空白，揭示了总氮总磷比显著的时空异质性，可为中国小型湖泊制定因地制宜的富营养化管理策略提供科学依据。