Spatiotemporal distribution characteristics of nitrogen and phosphorus nutrients in a river-reservoir overlapping system: a case study of the Liuchong River, the northern source of the Wujiang River

Abstract This study investigates the spatiotemporal variation of nitrogen (N) and phosphorus (P) nutrient concentrations across diverse ecological within a ‘river-reservoir’ hydrological overlapping system. Using monthly monitoring data of total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH 3 -N), and permanganate index (COD Mn ) collected from designated monitoring stations along the Liuchong River and the upper reaches of Wujiang River between 2020 and 2023, this study aims to understand the primary drivers and influencing mechanisms behind the observed variations in N and P concentrations. The findings revealed that from 2020 to 2023, the TP concentration exhibited a consistent decline, while the TN, NH 3 -N, and COD Mn concentrations displayed nonlinear fluctuations in both the river and reservoir sections. The average nitrogen-to-phosphorus (N/P) ratio frequently exceeded 55, indicating phosphorus limitation within water bodies. The reservoir areas demonstrated a retention effect on N and P, with TP retention rates exceeding those of TN. This differential retention significantly increases the risk of eutrophication in reservoir waters. Rainfall-induced runoff variations were identified as the primary driver of N and P distribution, while retention effects and internal loading functioned as dual regulatory factors influencing the nutrient cycling dynamics within the reservoir. This study elucidates the influence and underlying mechanisms of the river-reservoir overlapping system on the spatial and temporal distribution of N and P in water bodies. The findings offer a scientific basis for safeguarding groundwater ecosystems, particularly in the context of cascade reservoir construction in the Wujiang River basin.