Study On The Formation And Disappearance Mechanism Of Algal Bloom In The Xiangxi River Bay At Three Gorges Reservoir

Constructed of stream way reservoirs will bring with large-scale backwater regions in the branches of upstream, namely River Bay, which would lead to slow flow in the branches. Under the conditions, the nutrient such as nitrogen and phosphor from upstream will settle down in the River Bay, resulting in the pollutant increment and water quality deterioration. It is easy for the formation of eutrophication and even algal bloom with the suiTab. environmental condition. Taken the Xiangxi River Bay of Three Gorges Reservoir as an example, the paper discussed the mechanism of formation and disappearance of algal bloom through field observation and simulation indoor.The impoundment of Three Gorges Dam has important impact on the water environment of the Xiangxi River Bay. After the impoundment, the phosphor concentration in the water dropped off and nitrogen risen. The concentration of nitrogen and phosphor in the sediment of the bay was high. The nutrient will be liberated from the sediment under the different conditions and be the risk of eutrophication and algal bloom. The evaluation of eutrophication showed that it was in the hyper-eutrophic status, and would transform to algal bloom formation, which was related to the hydrodynamic condition after the impoundment.The formation and diasppearance of algal bloom in Xiangxi River Bay was influenced by the integrated actions of enough nutrient, tardy flow and appropriate environment conditions.There were spatial difference of water environmental indices in Xiangxi River Bay, according to that, the bay was partitioned to six regions from estuary to the backwater terminal. The dissolved oxygen, pH, concentration of nitrogen and velocity were high in the estuary, at the same time, the turbidity and concentration of phosphor were low. Inversely, water temperature, concentration of nitrogen were low, and the turbidity and concentration of phosphor were high in the backwater terminal. The flow was slowly in the middle part of the bay.There was three times of algal bloom formation during the monitoring period. The first one was in Pingyikou at Feb.25,2007. The second, most severe one, was in the all parts of the bay from Mar.25,2007 to Apr.9,2007, which endured a long time, and involved from estuary to bay, from the branches to the main stream. Formation and diasppearance appearred alternately at the partly regions. The third one was at May6,2007, and disappeared in the middle of the month.The factor's analysis showed that there were three factors including nutrient conditions giving priority to the nitrogen, phosphor and silicon, the environmental conditions such as water temperature, and the hydrodynamic conditions such as flow velocity, which had influence on the time and spatial distribution of the algal bloom in Xiangxi River BayRelativity analysis showed that there was prominent negative correlation between discharge and chlorophyll-a, water level and chlorophyll-a, turbidity and chlorophyll-a, total nitrogen and chlorophyll-a, and nitrate and chlorophyll-a. Spatial autocorrelation analysis indicated that the indices such as chlorophyll-a had obvious autocorrelation, even conductivity, orthophosphate and nitrate had more autocorrelation. Multiple regression analysis indicated there was distinct linear relation between chlorophyll-a and nutrient and velocity at the formation of algal bloom in Xiangxi River Bay. Taken the nutrient and velocity as independent varialbes, chlorophyll a as dependent variable, the spatial regression was carried out. The two species regression results showed that the relative error was less than 10%.The physical model simulation experiments showed that the pH value to be eight would accelate the formation of algal bloom with the appropriate other conditions. During the formation process, the pH with different original value went to be 8.6 to 9.9. There was quadratic equation between growth rate of algal and water temperature. The growth rate was the biggest with the water temperature to be 34℃, and irradiance to be 5000lx. Longer the irradiant period, higher the maximal biomass with the irradiance to be 3000 to 3200lx. Dissolved total phosphor and nitrogen were to be 0.14 to 0.62mg/L, and 1.82 to 4.08mg/L separately, algal multiplicated fastly, and the algal bloom occurred rapidly,with the maximal growth rate of algal to be 0.667 and 0.900 separately, half-saturation constants to be 0.019mg/L and 0.751mg/L separately. Fartherly, the ratio of nitrogen and phosphor to be 20 was the perfect condition for the growth of algal and formation of bloom, and the maximal growth rate to be 0.896. Orthogonal experiment results showed that total phosphor, total nitrogen, water temperature and irradiance to be 0.8mg/L, 3.6mg/L, 29-30℃and 3300-3400lx separately led to the algal growth and bloom formation. Among the four factors, irradiance was the most important and dominant.Field monitoring results showed that velovity and concentration of chlorophyll a had sigmoid fit. At the velocity range of 0.05 to 0.18m/s, risen the velocity, fallen the chlorophyll-a, and inhibited the bloom formation. Experiment indoor found there was little influence on the bloom formation when the velocity was lower than 0.1m/s, and when the velocity was 0.1 to 0.3m/s, the flow would speed the bloom formation,as well as the bloom was inhibited when the velocity was higher than 0.4m/s. Although there was difference between the field monitoring and experiments indoor, the results indicated that the change of hydrokinetic condition was the main reason for the algal bloom occurrence frequently in Xiangxi River Bay after the impoundment of Three Gorges Reservoir.On the basis of field observation and physical model experiment, the numerical simulation was carried out used the ecological dynamic model of WASP. The results showed that the model was proper to simulate the algal bloom in Xiangxi River Bay.