Nutrient Distributions And Retention Of The Three Gorges Reservior

The Three Gorges Project (TGP) on the Yangtze River, the largest hydropower- complex project ever built in the world, completed its Phase-II construction and Phase-I water storage in 2003, finally elevating the pool level to about 135 m above mean sea level at the dam. The potential eco-environmental problems induced by the TGP are widely concerned in the world. Material transport research in particular is one of the most important concerns about the TGP. Reservoir effect will affect the biogeochemical processes of the Changjiang River. It is necessary and urgent to examine the reservoir effects of the TGR water storage on the Yangtze River material transport, which has important practical and theoretical significance.The main results in the thesis were drawn as follows:After the first stage of the TGR impoundment (135m), parameters of nutrient and water quality showed a uniform distribution, indicating that dynamics was the major fator to control nutrient and other parameters. However, sediment retention in the TGR had great influence on different phosphorus forms. Concentration of exchangeable P, iron-bound P, and organic P increased greatly in particles after across the TGR, while Detrital P decreased largely in particles. The retention effect of the large particles in TGR was the key factor controlling different phosphorus forms.In the TGR, nitrate was the main component of DIN, and DIN was the major parts of total nitrogen, while the contribution of DON and particle N is only of minor importance. Additional, phosphate was the major part of total phosphrous in the common period.During June to September in 2007, more than 60% fluxes of nutrient were transferred to downstream at Guizhou and Fuling station.Collected the history dates of nutrients in the past ten years in the Changjiang River of the Three-Gorge areas from the pubic documents and combined our studies in this region, a preliminary nutrient budget for the TGR was proposed to be used as first attempt to quantify the behavior of nutrients entering this reservoir after the second impoundment. As control, a budget was analyzed before the impoundment of the TGR. The nutrient budget showed that the Three-Gorge Reservoir was a net sink of total nitrogen and phosphorus after impoundment. The dam was responsible for removing around 18% of nitrogen and 15% of phosphorus entering the reservoir, which would be have significant downstream ecological and social implications. The deposition of particle nutrients and absorbed by phytoplankton would be responsible for this case of reducing nutrient as a result of a new dam.After the third filling of the TGR, more than 36% for phosphate, 4% for silica, 16% for total nitrogen and 7% for total phosphrous were retented by TGR, which would reduce nutrient fluxes of the Changjiang River to the East Sea (18% for phosphate, 2% for silica, 8% for total nitrogen and 4% for total phosphrous). So the TGR influenced phosphate thansport great than other nutrients.Inorganic ions and nutrients were measured in different layers of the Xiangxi and Daning Rivers to explore the mixing processes of representative bays in the TGR. Bicarbonate and calcium are the dominant ions. Carbonate weathering is the most important mechanism controlling the water chemistry. However, important differences exist between the main channel and its tributaries. Major ion levels in the TGR bays depend on hydrological mixing. Results show that the major ions of calcium, magnesium, sodium, potassium, strontium, sulfate, and chloride demonstrate chemically conservative behaviors during transit throughout the bays of the TGR. This means the ions can be used as tracers in the same way that salinity is used in estuaries to explore other non-conservative elements and to indicate specific source waters. These tracers could then be used to analyze nutrients in the mixing zone, where they are not conserved The reverse effect of the main channel on tributaries'backwater reach was the key factor controlling nutrient distribution in the Xiangxi and Daninghe Bays, with biological utilization acting as a secondary factor.