Spatiotemporal Variability of Non-Point Source Pollution and Water Quality Response in a Typical Shallow Lake Basin

Shijiu Lake is a typical transboundary shallow lake connected to the Yangtze River in the lower Yangtze region, where water environmental management faces practical challenges due to complex pollution sources and difficulties in cross-jurisdictional coordination. This study systematically analyzed the spatiotemporal variability of non-point source (NPS) pollution loads in the Shijiu Lake basin and investigated the response relationships between major inflowing rivers and lake water quality at monitoring sections. The results indicated that: (1) the total annual total nitrogen(TN) load entering the lake was 2924.20 t and the total annual total phosphorus(TP) load was 109.68 t, of which trans-basin water conveyed through the Zhongliu River accounted for 69% and 67% of the total TN and TP inputs, respectively, representing the dominant source of pollutants entering the lake; (2) high TN load areas were mainly distributed in the eastern zones with dense population and industrial activities and in the southwestern agricultural areas, whereas high TP load areas were concentrated in the northeastern zones with intensive livestock farming and the southwestern agricultural areas; and (3) the Zhongliu River inflow significantly influenced water quality at the Shijiu Lake center, Gaochun, Dangtu, and Bowang monitoring sections, the Xinqiao River primarily affected the Lishui section, and the remaining inflowing rivers had limited influence confined to river-mouth areas.