Unconventional seasonal variation and mechanisms of sediment phosphorus release in an Urban lake

Urban lakes suffer from eutrophication fueled by excessive phosphorus (P) accumulation; however, comprehensive understanding of P dynamics under intense anthropogenic pressure remains unclear. We elucidate these processes in a typical heavily utilized urban lake (XWL), in Nanjing City, China. Using a year-long monthly sampling campaign, we integrated DGT and HR-Peeper with sediment P fractionation analysis. This approach allowed for the systematic assessment of Fe, S, and P speciation and mobility within the sediments and porewater, thereby elucidating the mechanisms governing P transformation. We observed distinct temporal patterns and strong correlations among the concentrations of C_DGT-Fe, C_DGT-P, and C_DGT-S. These findings confirm that P cycling in XWL is predominantly governed by Fe redox processes, with the S cycle playing a significant secondary role. Critically, we identified an anomalous seasonal pattern for internal P loading: SRP flux across the SWI varied annually between -1.10 and 5.25 mg/(m²·d), exhibiting a seasonal switch from a net P source in summer and autumn to a net sink in winter. Statistical analysis indicated that the SRP flux correlated positively with T_W and porewater SRP, but negatively with sediment Fe-P and Org-P. Corroborating evidence from DGT-induced flux in sediments model simulations demonstrated increased sediment P resupply capacity in autumn. We emphasize the critical need to address the amplifying effect of synergistic Fe-S interactions on internal P loads in urban lakes, providing new insights for dynamic P management strategies.