| Because REE is abundant in China and is widely used, a lot of REE go into the water environment. Meanwhile low concentration REE can promote the growthism of hydrophytic vegetation which could cause waterbloom, then the aquatic ecosystem, and human healthy will be influenced greatly. In recent years, REE has been exploited and utilized blindly for agriculture and industry, which leads to the unauthorized mining and serious pollution. So it is necessary to investigate the REE content and eutrophication condition of the upper reaches of Yangtze basin again, which is the focus of this paper. The result is as follows: In high-water period, REE content, in main tributaries and part of trunk stream in the upper reaches of Yangtze basin, is in the range of 9.61-63.37μg/L; in low flow period,it is in the range of 12.11-90.20μg/L, which is 155 times as high as the REE content of world's fresh water (0.582μg/L) and in the whole year, it is in the range of 22.46-52.80μg/L, which is 90 times as high as the REE content of world's fresh water. The TP and TN content is both high;in high-water period, TN and TP content are in the range of 1.02-2.17mg/L and 0.01-0.32mg/L respectively; in low flow period, are 1.04—2.2mg/L and 0.01—0.39mg/L respectively. According to the research on nutritive salt, all rivers have the possibility to occur waterblooms. The most polluted rivers are trunk stream of Yangtze, Jialin-Jiang and Min-Jiang, whose water quality is close to the standardⅡ.There are many researches reported about the influnence on hydrophytic vegetation of REE, however most researchers focus on the range of REE concentration which affects one kind of hydrophytic vegetation, the critical concentration has not been reported. The research on the critical concentration of REE can provide scientific support for drawing up standard of security emissions of REE and the threshold value allowed of REE in the water environment. Spirodela polyrrhiza was used to be the material of test. The static test result is as follows: under the condition of low REE cerium concentration(0.05-0.65mg/LCe4+), the Chlorophyll a and fresh weight of Spirodela polyrrhiza are increasing, photosynthetic rate is enhancing, Spirodela polyrrhiza grows faster, high REE cerium concentration(0.80-1.00mg/L Ce4+) show different degrees of inhibition to the growthism of Spirodela polyrrhiza. The best promoted REE cerium concentration is 0.55mg/L. Curve linears about the relationship beteen REE cerium concentration and the max density outbreak time of Spirodela polyrrhiza is y=-74.272x3+138.854x2-74.612x+16.251.The best promoted REE cerium concentration is 0.55mg/L. Under this concentration, in static test, the growthism of Spirodela polyrrhiza has been promoted.Compared with CK, Spirodela polyrrhiza quickly absorb N, P and other nutrients faster, then COD in the water then persistently rises, DO reduces, the water quality had been accelerated deterioration.As building Three Gorges Dam in China, Yangtze has turned into a reservoir from a natural river, whose flow pattern has changed a lot. The possibility of Three Georges Reservoir to occur waterblooms is concerned by the community. The critical flow velocity becomes the hot topic. By dynamic test, we discoveried that the effection of the best promoted REE cerium concentration can be influenced by flow rate and the critical velocity equals to 0.1226m/s.
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