Phosphorus release from sediments of a drinking water reservoir under the influence of extreme water-level drawdowns

This study investigated sediment phosphorus (P) release and transformation in a drinking water reservoir under extreme water-level drawdowns in southern China. We characterized the daily changes in surface water quality of Tianbao Reservoir from the normal year 2019 to the extreme drought year 2021. Field sampling was performed in summer and winter during different water level periods, and the temporal dynamics of P fractions, dissolved oxygen (DO), and P distributions in sediments were analyzed using sequential extraction, high-resolution planar optode and diffusive gradients in thin films methods. Results revealed that during the extreme drought year, the weakened thermal stratification was more susceptible to disruption, leading to different conditions at the sediment-water interface. Storm runoff temporarily destroyed summer stratification, triggering resuspension of anoxic sediments, which resulted in a synchronized increase in nitrogen (N) and P concentrations and short-term hypoxia in the surface water. Total P and phosphates contents decreased in surface sediments, while iron-bound P fraction increased. Positive diffusion fluxes of P indicated that reservoir sediment consistently served as a P source even during the low-water periods with sediment DO penetration depth over 50.0 mm. It was concluded that extreme drawdowns transformed P limitation to N and P co-limitation, and endogenous P release induced by summer sediment resuspension and winter overturn continuously stimulated primary production in this reservoir. Therefore, this study provides a scientific basis for managing water quality in reservoirs undergoing extreme drought.