| Aquatic eutrophication is a prevalent ecological problem. The Three Gorges Reservoir started water storage in June 2003, and water level of the dam fluctuated around 135 meters. Since then, part of the Yangtze River became a typical channel reservoir, consequently, flow velocity of water in this area sharply decreased as hydraulic condition changed. Moreover, water level of the dam would finally reach up to 175 meters in 2009, and water flow velocity would decrease from 2 m/s to 0.25-0.5 m/s, which would seriously impair its transport and diffusion capacity for contaminants. With the same load of nitrogen and phosphor as well as the same discharge method, water quality of the Three Gorges Reservoir would definitely deteriorate. As another consequence, a series of factors resulted from damming, such as lower water flow velocity, higher transparence, depositon of suspended solid, higher water temperature, fewer foggy day and increased sunlight in winter, would be optimal for photosynthesis of aquatic plant, especially algae, which could potentially lead to eutrophication. Therefore, the structure and function of local aquatic ecosystem would be influenced by changing hydraulic condition, however, few studies on eutrophication have been reported since the water storage of the Three Gorges Reservoir. As a matter of fact, eutrophication phenomenon has already occurred in backwater area of some tributaries, which indicated that low velocity of water flow might lead to eutrophication. Therefore, to investigate the distribution characteristics of nutrient and biomass of the Three Gorges Reservoir at the initial stage has significant meaning to further study on nutrient change characteristics in the process of water storage and its correlation between the growth of phytoplankton, forecast the potential trend of eutrophication in reservoir area, as well as protect the water quality of the Three Gorges Reservoir.By means of field examination and sampling, 16 major tributaries in the Three Gorges Reservoir area and 35 large reservoirs in Chongqing were studied. As a result, the relationship between chlorophyll a and phosphorus was obtained, and concentrations of chlorophyll a in both main stream of the reservoir and estuary of its major tributaries when water level reaches 175 meters were also predicted. Based on the QUAL2E model, distribution characteristics of nutrients and chlorophyll a in the Wu River after the implementation of 175 meters water level were predicted. Meanwhile, large scale investigation of nutrients and organic pollutants were carried out at 6 monitoring sites along the Yangtze River from January 2004 to December 2004, consequentially, distribution characteristics of nutrients, organic pollutants and chlorophyll a were obtained.After the water level reached up to 135 meters, no significant differences between nutrientsand organic pollutants in the horizontal distribution were detected except BOD, and so did nutrients and organic pollutants in the vertical distribution in upriver (Zhutuo, Cuntan) and backwater area (Fuling). While in reservoir area (Wanzhou), BOD appeared to be significantly varied along vertical direction. High concentrations of nitrogen and phosphor were detected in the Three Gorges Reservoir, for instance, the concentration of NH3-N ranged from 0.12 to 0.17 mg/L, and that of NO2-N, NO3-N ranged from 0.018 to 0.036, 1.19 to 1.42 mg/L respectively. The contents of TN (total nitrogen) and TP (total phosphor) varied from 1.60 to 1.82, 0.11 to 0.15 mg/L, respectively, while that of COD and BOD ranged from 2.60 to 4.0, 0.80 to 1.28 mg/L, respectively. Obviously, concentrations of TP and TN exceeded the eutrophication limits, and the ratio between TN and TP was right in the range which could boost algae growth. System clustering indicated significant regional distribution of nutrients and organic pollutants, namely the upriver area and backwater area, which were different from the reservoir area. Based on current situation, provided the discharge method of nutrients in the upriver stays the same, nutrient loads at different water levels were predicted. When the water level reaches 135 meters, TP load would be 788010 tons per year, and that of TN would be 89088 tons. The annually input loads of TP, TN would decrease to 696630 and 83199 tons respectively when the water level reaches 156 meters, whereas 403920, 57306 tons respectively at the water level of 175 meters.Relatively low concentrations of chlorophyll a, ranged from 0.50 to 3.81 mg/m3, were detected in the main stream to the reservoir. According to the OECD assessment standard, most of the data indicated oligotrophic condition. However, high chlorophyll a concentrations in the Daning River, ranged from 7.53 to 27.3 mg/m3, exceeded the limit for eutrophication. Meanwhile, 8 categories, 43 families, 99 genus, and 301 species (including mutations) of phytoplankton were found in this area. Diatom and green algae were the major species, which made up to 76.1% of all. Besides, 3 categories, 35 families, 76 genus, and 106 species of zooplankton were also discovered. Similar conclusions from the above 2 assessment method suggested all monitoring sites were slightly polluted.Based on chlorophyll a concentration, 12 monitoring sites of all 39 were classified as eutrophication category. Among the tributaries, no significant correlations between phosphorus concentration, transparence and chlorophyll a were exhibited. During the eutrophication period, concentrations of chlorophyll a in the Wu River ranged from 11.1 to 29.6/^g/L. Each part of the Wu River could be classified as eutrophication according to the OECD standard. On the other side, 22 out of 35 large reservoirs were in the state of eutrophication, which consisted of 62.9% of all. Moreover, 5 reservoirs were categorized as hyper eutropher (14.3% of all), 10 as middle eutropher (28.6%), 7 as light eutropher (20%), and another 12 as mestropher (34.3%), only 1 (2.9%) was...
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