Characteristics and influencing factors of nitrogen and phosphorus pollution in the Uyuer River Wetland

The assessment of nitrogen (N) and phosphorus (P) pollution in river wetlands serves as a critical tool for diagnosing the ecological health. To explore the spatiotemporal characteristics and driving factors of water pollution and sediment pollution in river wetlands, the core wetland area within the Uyuer River National Nature Reserve in Heilongjiang Province - the Uyuer River Wetland - was selected as the main study area. Based on long-term water quality monitoring data collected from 2018 to 2024, the Water Quality Index (WQI) was employed to comprehensively assess the overall water quality status of the Uyuer River wetland, while Principal Component Analysis (PCA) was used to identify the dominant factors governing the spatiotemporal variations in water quality. In addition, sediment monitoring data obtained in 2024 were analyzed to assess the pollution levels and potential risks associated with nitrogen and phosphorus accumulation in wetland sediments. The results indicated that the overall water quality status of the Uyuer River Wetland remained at a generally good level during the study period, with WQI values ranging from 76.49 to 82.65. Nevertheless, pronounced spatiotemporal heterogeneity was observed including annual variation, seasonal variation, and spatial distribution. Annually, the annual mean WQI values exhibited an increasing trend from 2018 to 2020, followed by a subsequent declining toward, suggesting a shift from initial water quality improvement to renewed degradation pressure in recent years. Seasonal analysis revealed that water quality was optimal in autumn (WQI_ave=82.62), while summer exhibited the poorest water quality conditions (WQI_ave=76.05). The spatial distribution of comprehensive water quality status was consistent with that of total nitrogen (TN) and total phosphorus (TP) in the water body, both exhibiting a trend of first increasing and then decreasing from upstream to downstream. Sediment analysis further revealed substantial nutrient enrichment, with total nitrogen (TN) and total phosphorus (TP) concentrations ranging from 240.77 mg/kg to 5 354.67 mg/kg and from 152.6 mg/kg to 4 881.1 mg/kg, respectively. Both TN and TP exhibited a clear gradient from slight pollution in upstream sediments to severe pollution downstream. Principal component analysis (PCA) results demonstrated that fertilizer application intensity and agricultural irrigation were the primary drivers of spatiotemporal heterogeneity in TN and TP concentrations, highlighting the dominant influence of agricultural non-point source pollution within the basin. Climatic factors indicated a regulating role in water quality. Based on these findings, it was necessary to formulate a comprehensive and multi-level pollution prevention and control strategies on the basis of enhancing the integrated "space-aerospace-land" water quality monitoring. Priority should be given to precision fertilization and zoned agricultural management to reduce the external input of pollutants such as nitrogen and phosphorus in upstream areas from the source. Relatively, targeted ecological restoration measures, including wetland vegetation restoration and sediment dredging, are needed for downstream wetlands to reduce internal nutrient loads and enhance purification capacity Overall, this study provided a comprehensive assessment of water and sediment pollution dynamics in the Uyuer River Wetland and provided valuable scientific insights for water quality evaluation, nutrient management, and ecological restoration in river wetlands and similarly fluvial wetland systems.