Seasonal Variations of Nitrogen and Phosphorus in Urban and Non-Urban River Reaches: A Case Study of the nAnchong Section of the Jialing River, China

The process of urbanization exerts an increasingly pronounced impact on river water quality; however, existing studies have predominantly focused on large cities, with insufficient attention devoted to small and medium-sized cities (SMCs) undergoing rapid development. This study targeted the Nanchong Segment of the Jialing River as the research subject. Water samples were collected from 12 sampling sites in summer (August) and winter (January of the subsequent year) 2024 to analyze the spatial distribution and seasonal variation characteristics of nitrogen, phosphorus, and dissolved oxygen (DO) in urban versus non-urban reaches. Additionally, the trophic state index (TSI) and nutrient pattern classification method were employed to identify pollution source types. Results indicated that total phosphorus (TP) concentrations in urban reaches were significantly higher than those in non-urban reaches across both seasons (64% ~ 79% higher), while total nitrogen (TN) concentrations were 25% ~ 35% higher—suggesting that phosphorus exhibits a more sensitive response to urbanization. TP concentrations in urban reaches were 0.076 mg/L in summer and 0.078 mg/L in winter, consistently maintaining a eutrophic state (mean TSI: 52.8 ~ 53.4), whereas non-urban reaches remained in a mesotrophic state (TSI: 47.5 ~ 48.0). Regarding seasonal variations, DO in non-urban reaches displayed typical natural fluctuations (significantly higher in winter than in summer), whereas DO recovery in urban reaches was inhibited in winter, indicating continuous input of oxygen-consuming substances. Nutrient pattern classification further revealed that TP in urban reaches presented a "non-seasonal" pattern, dominated by point source inputs; in contrast, TP in non-urban reaches exhibited a "phase-synchronous" pattern, with non-point source (NPS) control as the primary driver. This study demonstrates that urbanization fundamentally alters the seasonal dynamics of river nutrients, shifting the dominant pollution source from seasonal NPS to continuous point sources. For rapidly developing SMCs, it is recommended to prioritize the strengthening of point source control (especially phosphorus removal) and implement a seasonal management strategy that integrates targeted supervision during the winter low-flow period with NPS interception in summer.