Mechanisms of phosphorus source-sink transformation across the sediment-water interface of a hilly and mountainous reservoir

This study focused on Duihekou Reservoir, a typical deep-water reservoir located in the southeastern hilly and mountainous areas of China. A one-year field investigation was conducted to collect columnar sediment column samples from the main water bodies. Detailed analyses were conducted on the phosphorus concentrations, fractions, interstitial water profile characteristics, and spatiotemporal changes of fluxes across the sediment-water interface. The results showed that the average total phosphorus (TP) concentrations in the surficial sediment of the reservoir ranged from 470.8 to 1012.3mg/kg, with an average of 688.4 ± 186.48mg/kg. An increasing trend of TP concentrations was discovered from the upstream to downstream. The most severe phosphorus pollution was discovered around the dam. The phosphorus fractions followed a descending order of Al-P (352.61mg/kg) > Fe-P (98.10mg/kg) > Res-P (88.77mg/kg) > Ca-P (72.42mg/kg) > Org-P (33.38mg/kg) > Ex-P (1.64mg/kg). Overall, the sediment in the entire reservoir acted as a source of phosphorus to the water, with the highest risk of release in autumn and most of the sediment in winter presenting as a sink of phosphorus. The average static release flux and diffusion flux of phosphorus were (0.81±2.34) and (2.15±3.47) mg/(m2·d), respectively. The annual phosphorus release amounts obtained by the two methods were 1.92 and 3.67 tons respectively, with the highest release amount in autumn. The conversion of Fe-P might be an important factor controlling phosphorus release from the sediment. The anaerobic dissolution and release of Fe-P caused by oxygen stratification will increase the phosphorus concentration in the lower layer of the water column, which would subsequently increase the phosphorus concentration in the whole water column after thermal stratification. After a large amount of phosphorus is released in autumn, more attention should be paid to the phosphorus concentration in the water influenced by the subsequent thermal inversion. © 2024 Chinese Society for Environmental Sciences. All rights reserved.