Temporal distribution and influencing factors of nitrogen and phosphorus in rivers and lakes of the Yuanhe River Basin, middle and lower reaches of the Yangtze River, China

Abstract Water quality monitoring data of rivers and lakes in the Yuanhe River Basin from 2021 to 2023 were collected to investigate the distribution characteristics and pollution influence factors of nitrogen (N) and phosphorus (P) in the water bodies. The results showed that most lake TP exceeded Class IV, river TP met Class I-Ⅲ, and the total nitrogen (TN) concentration in all samples was greater than 0.2 mg l −1 , indicating a risk of eutrophication in the river-lake water. The Water Quality Index (WQI) indicates that the water quality in the Yuanhe River Basin ranges from average to excellent. The average NH 4 + -N concentration in river and lake water accounted for 13.4% and 9.1% of TN, respectively. Principal Component Analysis (PCA) revealed that nitrogen and phosphorus concentrations in river water were mainly influenced by Point source pollution, non-point source pollution and organic matter decomposition. For lake water, the main controlling factors included Point source pollution, agricultural cultivation and livestock breeding (non-point source pollution), phytoplankton growth, and organic pollutant inputs from rainfall scouring. Results from the Absolute Principal Component Scores-Multiple Linear Regression (APCS-MLR) model further indicated: (1) non-point source pollution significantly affected river TP, with a contribution rate of 39.3%, while agricultural cultivation and livestock farming contributed up to 71.9% to lake TP; (2) Point source pollution had a notable impact on TN in both rivers and lakes, with contribution rates of 52.7% and 75.4%, respectively; (3) Unknown sources contributed the largest proportion to river and lake water at 53.3% and 18.7%, respectively, highlighting the need for further investigation.