Nitrogen and phosphorus dynamics during medium and small flood regulation in the Xiangxi River Bay of the Three Gorges Reservoir: implications for tributary eutrophication.

Sediment serves as a critical nutrient carrier in aquatic ecosystems. Reservoir regulation disrupts natural hydrological and sediment transport processes, altering nutrient dynamics. This study investigates the impacts of medium and small flood regulation (MSFR) on nutrient distribution and transport in Xiangxi Bay (XXB), a tributary of the Three Gorges Reservoir (TGR), during flood seasons (2020-2021). Hydrological data and spatiotemporal variations in nitrogen and phosphorus were analyzed. During MSFR events, water levels fluctuated from 145 to 160 m, with suspended sediment concentrations (SSC) ranging from 0.003 to 1.202 kg/m. Particulate phosphorus (PP) accounted for 23% to 93% of total phosphorus (TP) in both mainstream and tributary systems. Mainstream TP (0.046-0.417 mg/L) and soluble reactive phosphorus (SRP, 0.019-0.051 mg/L) concentrations were significantly higher than those in XXB (P < 0.05). Temperature-induced density currents drove the reverse flow of mainstream water into XXB through mid-upper layers, which modified vertical TP distribution and enhanced stratification. Mainstream inputs contributed 81.25% of SRP and 74.02% of dissolved inorganic nitrogen (DIN) fluxes to the tributaries. Elevated water levels during floods showed no significant correlation with TP and chlorophyll a (chl-a) concentrations in XXB. Long-term trends (2010-2021) reveal rising TN/TP ratios in XXB, coinciding with increased flood-season algal bloom risks. These findings highlight the need for targeted monitoring of nutrient transport to mitigate PP deposition and tributary eutrophication during flood periods.