Nutrient Status and Its Influencing Factors in Fengjiangkou Reservoir

[Objective] Identifying the main areas and driving factors of eutrophication in Fengjiangkou Reservoir can provide guidance for water quality protection in the online regulation reservoir of the water resource allocation project in northern Hubei Province. [Methods] Taking Fengjiangkou Reservoir as the study area, water quality monitoring and meteorological and hydrological data collection were conducted. The comprehensive nutrient status index method and the partial least squares regression method were used to analyze the spatiotemporal variation characteristics of nutrient status in Fengjiangkou Reservoir and its influencing factors. [Results] There were significant spatiotemporal differences in indicators such as permanganate index, ammonia nitrogen, total phosphorus, total nitrogen, and chlorophyll-a. These indicators were higher during the irrigation period than during other periods, and higher in the estuary areas of the Fengjiangkou River and Shahedian River than in other waters, making them the key periods and water areas for pollutant control in Fengjiangkou Reservoir. In the estuary area of Fengjiangkou River, chlorophyll-a concentration was significantly positively correlated with TN concentration (P=0.02), TP concentration (P=0.05), and diffuse radiation (P=0.04). In the estuary area of the Shahedian River, chlorophyll-a concentration was significantly positively correlated with TN concentration (P=0.02) and TP concentration (P=0.02), indicating that TN concentration, TP concentration, and diffuse radiation intensity might be the main factors affecting the nutrient status in the estuary area of Fengjiangkou River, while TN concentration and TP concentration were the main factors affecting the nutrient status in the estuary area of Shahedian River. Although indicators such as air temperature and wind speed showed no significant correlation with chlorophyll-a concentration, each indicator exhibited a clear correlation with chlorophyll-a concentration under given conditions. During the irrigation period, the nutrient status of the Fengjiangkou River and Shahedian River estuary areas was mildly eutrophic. Contribution analysis results showed that permanganate index (20.6%), total nitrogen (11.0%), and nitrogen-to-phosphorus ratio (9.3%) were the main water quality indicators affecting eutrophication in the estuary area of Fengjiangkou River, while total phosphorus (15.6%), ammonia nitrogen (12.3%), pH (10.1%), and total nitrogen (8.3%) were the main water quality indicators affecting eutrophication in the estuary area of Shahedian River. Both normal radiation and diffuse radiation contributed over 10% to the occurrence and decline of eutrophication in the estuaries of Fengjiangkou River and Shahedian River, making them the main meteorological driving factors of eutrophication in Fengjiangkou Reservoir. [Conclusion] To synergistically ensure the water quality safety in Fengjiangkou Reservoir, recommendations are proposed, including reducing watershed non-point source pollution during the irrigation period, regulating algal biomass in the reservoir bay, and implementing water environment monitoring and intelligent management.