Study On Eutrophication Charactertics, Mechanism And Its Countermeasures With Artificial Floating Beds In The Backwater Of The Linjiang River

The construction and operation of the three Gorges Reservoir (TGR) will generate tremendous benefits in economy, flood control, hydropower and etc. Meanwhile, the reservoir built also has significant impact on ecological environment, especially on tributaries.The Linjiang River is located in the upper reach of the Yangtze River. Before the construction of the TGR, the flow velocity of the river was considerably quick. When the TGR started impounding, parts of the Linjiang River became a bay, similar to a lentic reservoir due to the backwater from the Yangtze River, and the hydrodynamics in the bay changed substantially. Equally, the average flow velocity dropped sharply, both the river width and water depth increased significantly. Besides the nutrient concentration in the river is great, together with an appropriate illumination, which provided considerable conditions for algal growth.Field observations show that the algal bloom occurred in May and between in the summer and in the autumn but neither in the summer nor in the winter, in the backwater reach of the Linjiang River. Therefore, we took the backwater reach of the Linjiang River as an example to study its eutrophication charactertics,mechanism and influencing factors; in addition, the artificial floating bed technique used for eutrophication control as well as its removal mechanisms were also studied.Firstly, based on one-year monitoring data and field investigation, we studied the eutrophcation charactertics in the backwater reach of the Linjiang River, and then the trophic state of the reach was assessed using trophic state index and principal component analysis. The study indicated that the trophic state of the backwater reach was consitently eutrophic, and exhibited a aggravated trend. And the trophic state was the most serious in the middle reach of the backwaters in terms of the spatial variety.The Chlorophyta and diatiom were the predominat species in the reach, and followed by Cyanophyta.Additionally, we found that the temperature, the flow velocity, phosphorus and silion were the critical factors inducing the eutrophication in the reach, and the multivariate regression model exhibited the best accuracy among statistical models.Secondly, the influencing degree of related factors associated with the eutrophcation in the backwater reach was studied by gray relational analysis, and results revealed that the temperature has the greatest impact on the eutrophication, followed by SiO₃-Si, and TN ranked the last.Through the simulation test, the algal growth and death was studied. The study indicated that: The algal growth exhbibed a quadratic relationship with the water temperature, light intensity, the flow velocity, the TSS, the nitrogen leval, but a logarithm realationship with phosphorus and silicon, respectively. The semi-saturation concentration of nitrogen, phosphorus and silicon was 0.326 mg/L, 0.163 mg/L, 0.095mg/L, respectively. The optimal condition for algal growth was at 22℃in temperature, 1.98mg/L in silicon, 4000～4200lx in light and 60 r.p.m.In addition, the artificial floating bed was taken as an alternative to control the eutrophication in the backwaters, and the study demonstrated that the floating bed vegetated with Acorus calamus s for Canna indica exhibited a great potential to control the eutrophication, especially on the control of the algal growth.Also, we studied the effect of the temperature, the coverage of the floating bed and, the plant growth on the removal effect of artificial floating beds, and found that the optimal temperature and coverage were 25～29℃and 45%～60%, respectively, and the removal effect exhbibed a linear realationship with the plant growth.Some measures such as artificial aeration, the increase in the coverage etc., were carried out to enhance the removal effect and the results revealed susspendion of the fiber filler was the best enchancement for eutrophication.In addition, the control mechanism was studied in the field of the hydraulic characteristics, DO characteristics and mass balance and results indicated that more than 60% of phosphorus and nitrogen were removed by sedimentation, the actual flow pattern of the floating bed systems was intermediate between the ideal plug flow and a perfectly mixed flow and the floating bed exhibited a better DO gradient distribution. The plants played a critical role in eutrophication control through plant absorbition, allelopathic effects, shading and etc.