| The exit section of Jialing River is located in the edge of the Jialing River and the library postlude of the Three Gorges Reservoir. Its hydrology and stream current conditions are very complex, and its water environment condition has great impact on the main stream. So it influences the water environment of the Three Gorges Reservoir directly. Currently water pollution in this area is very serious. In the spring of 2005, Algae on the right of four dock of Jialing River proliferated and its amount reached 106×104cell/L. It exceeded the algae quantitative criterion of lake eutrophication. Therefore, in order to take timely and effectual countermeasure to safeguard the whole water environment of the Three Gorges Reservoir, it is quite important to study eutrophication forecast of the exit section of Jialing River.Presently, the simulation and forecast by eutrophication model in the Three Gorges Reservoir is limited. One kind of algae needs a model, so there are so many models to establish for all kinds of algae. So it is difficult to forecast. Thus, it is necessary to find a new approach to establish eutrophication model. If a special kind of algae(parameter algae) which can reflect the quantity change of the total algae and its growth model also can reflect the growth process of the total algae can be found, then the difficulty of model building can be reduced. And a new eutrophication model can be established to do dynamic analogies on the algaes growth process in the Three Gorges Reservoir. Consequently, the key of model building is whether this kind of algae exists.In this paper, the author try to seek and define parameter algae which quantity is big and frequency of occurrence is high as well as can reflect the quantity change of the total algae according to the analysis results of Phytoplankton community composition and changing of Jialing River by correlation analysis method and so on. And the conclusions are as follows:①The annual dominant algae in the exit section of Jialing River is Fragilaria brevistriata Grun.②Analysis on the Fragilaria brevistriata Grun. and the total algae by ratio and correlation methods in all the sampling position, from March to July their ratio is comparatively stable and has significant correlation in tranquil-flow water body. So select Fragilaria brevistriata Grun to reflect the quantity change of the total algae in tranquil-flow water body in the exit section of Jialing River. ③T he occurrence of parameter algae has close relation with the velocity of flow, and the stability of algae composition has negative relationship with the velocity of flow.④Eutrophication happened in tranquil-flow water body of the right of four dock of Jialing River in March,2005. From March to September is suitable for algae growth, but it is disadvantageous to algae activity in August and September which are rising-water time.Therefore, tranquil-flow water body in the exit section Jialing River is sensitive water body of eutrophication and from March to July are sensitive time of eutrophication.⑤As the study of water body is limited, the definition of parameter algae needs to expand the scope of water body to make the further sampling investigation to determine it step by step and make certain the relations of parameter algae with water quality and hydrology. Had determined the parameter algae, we can unify the variation of concrete temperature, rainfall amount, flow rate, silt quantity and so on, to make a function rating to Fragilaria brevistriata Grun's proportion.According to ratio stability, we might determine comprison expression of algae growth rate and correlation factors and construct the new eutrophication model to forecast nutrition state of the Three Gorges Reservoir.
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