Study of the Synergistic Dynamics of Labile Phosphorus and Iron at the Water-Sediment Boundary of an Inland Reservoir

ZrO-Chelex diffusion gradient in thin film (DGT) probes and high-resolution peepers were used in this study for the synergy between phosphorus (P) and iron (Fe) at the water-sediment boundary (WSB) of the reservoir. A negative oxidation–reduction potential (ORP) across all sampling areas (SA) was observed, indicating hypoxic bottom waters. Positive flux values of Fe (41.75–63.50 mg/m2/day) and P (0.04–0.39 mg/m2/day) indicate their release from sediments into the overlying water column. A strong positive Pearson correlation existed between labile Fe and P across all sampling areas (p < 0.05), highlighting their coupled mobilization through reductive dissolution of iron oxyhydroxides (FeOOH). Spatial variations in the labile Fe:P slope revealed notable variations in P corelease efficiency, signifying how sediment iron dynamics impact P mobility. Strong correlation between dissolved organic carbon (DOC) and both labile Fe and P (0.7 ≤ r ≤ 0.9 for Fe-DOC and 0.6 ≤ r ≤ 0.9 for P-DOC, p < 0.05) indicated that organic carbon was a key driver of Fe and P mobilization. Ca2+ concentrations influenced P mobility by facilitating competing immobilization pathways, revealed by its negative correlation with P flux (r = −0.6). These findings provide a comprehensive framework for predicting nutrient fluxes and guiding water quality strategies in similar monsoonal ecosystems prone to black water formation.