Table 1_Unravelling phosphorus adsorption characteristics and release potential in estuarine sediment under runoff regulation.docx

Endogenous phosphorus (P) release and its complex biogeochemical transformation pose ongoing challenges for effective P management in estuarine ecosystems. However, the understanding of how these processes respond to runoff regulations remains rarely constrained. This study investigated the spatiotemporal distribution of P forms in bottom water and sediment of the Liao River Estuary under anthropogenic runoff regulations characterized by low runoff in summer and normal runoff in winter. The sediment P adsorption capacity and release potential were studied through adsorption kinetics and thermodynamic experiments. Soluble reactive P (SRP) and total dissolved P (TDP) concentrations in bottom water were low in summer but increased markedly in winter (p<0.01). The average sediment total P (TP) concentrations were 495.26 mg/kg in summer and 399.62 mg/kg in winter, with higher TP levels mainly observed in the nearshore muddy areas, likely attributable to the high proportion of fine particles. Equilibrium P concentration generally exceeded the SRP concentration in bottom water under both conditions, indicating that the sediment served as a P source. The sediment P eutrophication risk index (ERI) further showed that release risk was predominantly moderate in summer, whereas it was low in winter. Both PLS-PM and correlation analysis showed that the concentrations of Fe, Al, organic matter, and the proportion of fine particles served as key determinants influencing the content of aluminum-bound P (NaOH-rP), organic P (NaOH-nrP), and calcium-bound P (HCl-P). Furthermore, elevated SRP levels combined with reduced salt in winter likely constrain P accumulation in the sediment. This study offers novel insight into the migration and transformation of P in the estuarine environment under anthropogenic runoff regulations.