Internal phosphorus loading and regulatory mechanism in China's largest urban lake: Implications for eutrophication...

Internal phosphorus (P) loading in sediments can cause or exacerbate lake eutrophication. Limited information is available on the coupling of P, sulfur (S) and iron (Fe) in the sediments of eutrophic urban lakes and the mechanisms regulating P release. Field samples from Lake Tangxun, the largest urban lake in China, were collected quarterly over one year from three sites with varying levels of P loading. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) were employed to determine soluble reactive phosphorus (SRP), soluble Fe, DGT-labile P/Fe/S, as well as SRP fluxes. The findings indicated that the trends of DGT-labile P and S during the warm seasons were synchronized and significantly (p < 0.05) positively correlated in all sites, indicating that P remobilization was mainly controlled by sulfur cycling. Strong dissimilatory sulfate reduction (DSR) contributed significantly to the generation of high SRP fluxes at the sediment-water interface (SWI) in the warm seasons. The SRP fluxes demonstrated significant (p < 0.05) variability across the study sites, ranging from 0.18 to 49.41 mg m^-2 d^-1, with the highest SRP fluxes in areas with higher sediment P content. SRP fluxes and DGT-labile P/Fe/S were significantly (p < 0.05) higher in summer and autumn than in winter. The statistical results indicate that dissolved oxygen (DO), and sediment available P content were crucial controlling factors for P flux. Our results highlight the dominant role of S cycling in internal P dynamics in this urban lake. We suggest problematic SRP fluxes that lead to eutrophication can be reduced by covering or removing sediment in areas of high P loading to reduce available P pools, or implementing artificial oxygenation during critical warm periods to inhibit Fe/S reduction.