The Study On The Eutrophication Threshold And Its Response Relationship With Outside Phosphorus Input Of Miyun Reservoir

Eutrophication is a prominent problem encountered in the field of water environment. Even though a great many studies on the concept of eutrophication, evaluation methods, the mechanism and prediction models have been developed both at home and abroad, the research on the threshold for eutrophication is still insufficient, affectting the evaluation and control of eutrophication. In this paper, from the response relationship of eutrophication and nutrients, the risk assessment method system of eutrophication based on the nutriment threshold is found, the flow water AGP test simulatting eutrophication of target water is designed, Miyun Reservoir, the important surface water source in Beijing,is selected as the study case.First, this paper defines the concept of eutrophication threshold: in the process of water eutrophication, some restrictive nutritional factors reach a certain value,and resulted in the transition behavior of the supply rate of organic matter(plankton). Eutrophication threshold denote the value of the critical effect threshold. From the view of threshold value, the definition of eutrophication in this paper is: when the restrictive trophic factor of the water body reaches a certain threshold conditions, primary productivity of the water body increase heavenly, resulting in the process of deterioration of water quality.Through the analysis strongpoint and shortcoming of some evaluatation methods, such as the water quality parameters method, the composite index method, the fuzzy mathematical method,and algal species diversity method, the evaluation method system is proposed in this paper based on eutrophication threshold, the main contents include: (1)identification eutrophication limit factors of target waterbody;(2) identifying the target water eutrophication threshold; (3) division water body evaluation criteria based on threshold; (4) division the risk level, and estimate risk.A significant correlation among the transparency, nitrate nitrogen, conductivity, total phosphorus, total nitrogen and chlorophyll-a is found after Multiple regression analysis, and total phosphorus is considered as eutrophication limiting factor in Miyun Reservoir. Monitoring sites in Miyun Reservoir are divided into four groups in accordance with the similarity of index in each monitoring sites, using hierarchical clustering analysis. The first group includes the kuxi, Baihe main dam;the second group includes Shuijiu, the main chaohe dam, Henghe, Taoli;the third group includs Jingou; the fourth group includes the Kudong. The main factors that affect the spatial and seasonal distribution of the growing algae in Miyun Reservoir can be summed up as two categories: nutritional factors represented by nitrate nitrogen and total nitrogen. The second category are environmental factors represented by water temperature, conductivity, pH value. Water quality from good to poor condition of the order as follows:Group1>Group2>Group4>Group3. After selecting kudong as the site for analysis of the time Distribution factor, the main factors affecting the temporal distribution of algal growth can be summarized into three categories: the first category is nutrient factors mainly comprised of nitrate-nitrogen, total nitrogen, total phosphorus, The second is under a integrate function of the total phosphorus, water depth , dissolved oxygen, dissolved oxygen, pH and other environmental factors and the nutritional factors. The third is determined by the ammonia. It is concluded that phytoplankton structure changes significantly during the year in Miyun Reservoir.Through the design of AGP-test equipments and methods which could simulate the impact of input of exogenous nutrients on eutrophication, the evolution of algae and aquatic species extinction process in Miyun Reservoir under different environmental and nutritional conditions are simulated. Then the ratio growth rate in the three simulation stages are simulated and analyzed combining the numerical analysis software MATLAB 7.0. At last, the conclusion that in the three designed stage, the total phosphorus threshold which leads to a water bloom in Miyun is respctively 0.053mg/L, 0.062mg/L, 0.064 mg/L is educed. Based on the regression relationship of total phosphorus,chlorophyll-a, total nitrogen, and the transparency,we compartmentalized the evaluation standard level of eutrophication in Miyun Reservoir.WASP ecological dynamic model is used for the Miyun Reservoir chlorophyll-a numerical simulation. And the results revealed that the model is preferable to reflect the eutrophication process of Miyun Reservoir. Finally, the probability analysis of prediction value as well as the zoning of the risk of eutrophication in Miyun Reservoir is carried out using the statistical software SPSS13.0; the paper provides a comprehensive and forward-looking way preventing against eutrophication of waterbody.