<p>In recent years, the interplay between emerging human activities (extensive flood prevention, water environment management, and ecological restoration) and traditional human activities (industrialization, urbanization, and intensive agricultural practices) has posed challenges to the conventional understanding of phosphate source structures and phosphate loads in watersheds. This study focuses on the Fuyang River, a small watershed in Northern China, to investigate the current impact of complex human activities. By utilizing phosphate oxygen isotopes (δ¹⁸O_P), we delineate the contributions of various phosphate sources and loads across different seasons in the Fuyang River watershed. The results indicate that the total phosphate load at the watershed scale is higher during the rainy season (241.42&#xa0;kg/day) compared with the arid season (154.93&#xa0;kg/day). During the rainy season, tributary inflows contribute the most to the phosphate load (93.78&#xa0;kg/day, 38.8%), followed by sediment (56.39&#xa0;kg/day, 23.4%). In contrast, during the arid season, the main contributors are tributary inflows (55.62&#xa0;kg/day, 35.9%) and effluents from wastewater treatment plants (39.1&#xa0;kg/day, 25.2%). Significant differences in phosphate source structures and load contributions are observed across different administrative regions, reflecting the varied industrial, urbanization, and agricultural activities. Urban and suburban sections display distinct phosphate source structures and load patterns owing to differences in human activities and environmental management practices. This study advances the application of δ¹⁸O_P at the watershed scale and provides valuable insights into the multiscale phosphate source structures and loads under the influence of complex human activities.</p> Graphical Abstract <p></p>