[Distribution and Water Quality Impact of Sediment Phosphorus Fractions in Tributaries of the Three Gorges Reservoir].

In recent years, algal blooms have occurred frequently in the backwater areas of tributaries of the Three Gorges Reservoir. The retention and release of sediment phosphorus might be a key factor, so it is necessary to conduct in-depth research on its impact. This study took the four important tributaries of the Three Gorges Reservoir, the Xiangxi River, Daning River, Caotang River, and Xiaojiang River, as the research object to evaluate the potential release of sediment phosphorus in these tributaries and its impact on water quality, using investigation and incubation experiments. The results showed that the total phosphorus content in sediments at the mouth of the Xiangxi River, Daning River, Caotang River, and Xiaojiang River were 634, 434, 629, and 689 mg·kg-1, respectively. The active phosphorus （NaOH-P） contents in sediment of those four rivers were 69, 100, 103, and 115 mg·kg-1, respectively, which were at medium and lower levels compared with those in major domestic reservoirs. The phosphorus contents in sediments in the river mouth area, where tributaries flowed into the main stream of the Three Gorges, were higher than that of the upstream reaches of the corresponding tributaries. The incubation experiments showed that the sediments in the river mouth area of each tributary had higher phosphorus release potential than sediments in the upstream reaches. Within the four tributaries, the sediments in the Xiaojiang River had the highest phosphorus release ability. The changes in water environmental conditions, such as increased temperature and reduced dissolved oxygen concentration, could significantly promote the phosphorus release rates from sediments. The phosphorus release rate of sediments in Xiaojiang River was the largest, at 0.861 mg·（m2·d）-1, which made the dissolved phosphorus concentration in bottom water much higher than in surface water during the algae bloom period. Overall, the sediment in the tributaries of the Three Gorges Reservoir, especially in the Xiaojiang and Xiangxi Rivers, currently played the role as phosphorus "source" during the algal bloom periods, in which phosphorus release from sediments could accelerate the phosphorus cycle in the water body. The results of this study offer a scientific basis for the control of internal phosphorus in river-type reservoirs and the prevention and control of algal blooms.