Effect of sediment on concentration of dissolved phosphorus in the Three Gorges Reservoir

Eutrophication and algal bloom has been occurring in the Three Gorges Reservoir ever since the reservoir's impoundment in 2003. The concentration of dissolved phosphorus (DP) is one of the main controlling factors for algal bloom. Fine sediment adsorbs DP thereby affecting the concentration of DP. The effect of the sediment reduction on the DP concentration of the Three Gorges Reservoir is discussed in this paper. Samples were taken from the Yangtze River and its tributaries, i.e. the Minjiang River and the Tuojiang River. The DP concentration of the water samples and the available phosphorus, total phosphorus, and phosphorus-fixing capacity of the sediment samples were analyzed. The amount of phosphorus adsorbed by the sediment was about 3% of the total DP load in the Yangtze River. An experiment was done in the lab simulating the DP concentration, sediment concentration, and turbulence. The results showed that quite a high amount of DP was adsorbed by the sediment. The adsorption capacity of the sediment was 135 mg/kg and the highest rate of adsorption was 29 mg/kg·h. The Xiluodu Dam is located at nearly 1,100 km upstream of the Three Gorges Dam. The Xiloudu Dam will trap sediment and thence reduce the sediment load entering the Three Gorges Reservoir after impoundment in 2013. It is estimated that the sediment reduction due to the Xiluodu Dam may result in an increase of the DP concentration in the Three Gorges Reservoir by 6.8%.