Phosphorus release flux and mechanism at the sediment-water interface of the Three Gorges Reservoir in the Yangtze River basin, China

PurposeThe Three Gorges Reservoir (TGR) is the largest water conservation project in the world but suffers from harmful algal blooms (HABs) currently. A large amount of phosphorus (P) has accumulated in the sediment due to the construction of the Three Gorges Dam. Phosphorus release from sediment may provide an important P source for overlying water that further triggers HABs. This study aimed to evaluate the contribution of sediment internal P and reveal the mechanisms controlling sediment P release.Material and methodsChemical sequential extraction approach and diffusive gradients in thin films (DGT) techniques were employed to determine the P fractions in sediments and the vertical distribution of P, iron (Fe), and sulfur (S) at the sediment-water interface (SWI).Results and discussionResults indicated that the total P content in the sediments of the TGR is high, with a mean content of 1368 mg kg(-1). The P concentration of different fractions in sediments followed the order HCl-P > NaOH-P > BD-P. The averaged P release flux at the SWI was estimated at 0.42 mg m(-2) day(-1), suggesting that sediment P release is a potential P source for the overlying water. Significant positive relationships between DGT-P and DGT-Fe concentrations from sampling sites Wushan County (WS), Zigui County (ZG), and Xiangxi River (XX) were observed with correlation coefficients (R-2) of 0.91, 0.39, and 0.29, respectively. Furthermore, DGT-P and DGT-S concentrations were also significantly positively correlated at the sampling sites WS, ZG, and XX, with R-2 of 0.71, 0.87, and 0.50, respectively.ConclusionThe internal P load is severe in the TGR. The reductive dissolution of Fe-P is likely one of the main mechanisms causing P release in the sediments. Furthermore, sulfate reduction associated coprecipitation with Fe promotes the release of Fe-P. These results provide important scientific and technical support for the mitigation of internal P pollution in large deep-water reservoirs.

Automated summary

This study evaluated the contribution of sediment internal phosphorus (P) and revealed the mechanisms controlling sediment P release in the Three Gorges Reservoir (TGR). The results indicated that sediment P release may trigger harmful algal blooms in the overlying water. These findings provide important scientific and technical support for mitigating internal P pollution in large deep-water reservoirs.