| After impounding, water environment of the Three Gorges Reservoir (TGR) has been significantly changed due to dramatically increasing water level, lower flow velocity, poorer diffusion ability and longer retention time in its tributaries. XiangXi Bay is the biggest tributary which is near the TG Dam, and algal bloom takes place every year after TGR impounding. But the time, place and degree of algal bloom are different in the bay. Algae swims in the water and the nutrient substance transports and transforms in the water too, so the change of motionstate of water body may play an important part on the algal bloom. Based on the data about the hydrodynamic and water environment situations monitored from Dec,2009 to Nov,2010, this dissertation analyses the features of the hydrodynamics and divides the bay into four different hydrodynamic sections, and then discusses the the main influence factors of algal bloom at different sections in order to provide some theory evidence for algal bloom prevention and control in the bay. The following conclusions are include in this dissertation.1 Based on the field data of XXB from Dec, 2009 to Nov, 2010, the results showed that the inflow from upstream entered into XXB as an underflow all the time, while the TGR main stream intruded into XXB as reverse density currents most of the time. And the conductivity of water which has conservative nature can prove this hydrodynamic feature. The reverse density currents occurred from underflow, interflow to upperflow in different time. In dry season, the underflows occur, in discharge water period the interflows occur and in flood season the interflows and upperflows occur. The thickness of reverse density current is always thickness which changes from 20.31m to 58.74 m, and the relative thickness changes from 0.28 to 0.67. The average flow velocity of the reverse density current changes from 1.94 cm/s to 5.53 cm/s.2 Water temperature is the primary contributor to density difference driving reverse density flows, whereas sediment is the secondary in the creation of these flows. Although the sediment concentration of water is higher in flood season, but the change trend of water total density is still the same to other periods. The main factors which affect the thickness of reverse density current are the density difference of surface layer between mainstream of reservoir and water in Xiangxi Bay and water level of TGR. And the main factors which affect the distance the reverse density current can influence are water level and water level daily variation.3 Through clustering analysis of the hydrodynamic probability characteristic value, such as slowly rate, index of stability, flow backward rate and cumulative rates, the Xiangxi bay can be divided into the following four parts: XX00-XX02, XX03-XX05, XX06-XX08, XX09-XX10. And sign them asâ… ,â…¡,â…¢,â…£.4 Based on the water environment field datas, the result showed that the concentration of TN is high, which changes from 0.05 mg/l to 2.54 mg/l, and the annual concentration is 1.11 mg/l. The concentration of NO3-N is 63.06% of TN. And the concentrations of N are different in different seasons, from winter, summer to spring, autumn the concentration getting lower and lower. And fromâ… toâ…£, the concentration also gets lower and lower. Also, the concentration of TP in Xiangxi bay is high, changing from 0.03 mg/l to 0.78 mg/l, and the annual concentration is 0.15 mg/l. The concentrations of P change in different seasons, from spring, winter to summer, autumn the concentration getting lower. But fromâ… toâ…£the concentration gets higher. The concentration difference of D-Si is not too much, and not significant in different areas either.5 The vertical distributions of water temperature are different obviously. In the downstream of the bay, the vertical stratification is not significant due to the reverse density currents, and the temperature difference between the upper lay and the bottom water is tiny relatively. What's more, the mixing depth is deep in summer and shallow in winter. The water vertical stability coefficient and mixing depth are both different significantly in different areas.6 The vertical light attenuation coefficient (Ksd) in XXB runs like a parabola which first increases, and then decreases, and it positively correlates to Chl-a, and the correlation coefficient is 0.74. So it can be said that the mass of phytoplankton is the main influence factor to the vertical light attenuation coefficient. And the reciprocal of transparency positively correlates to Ksd, and the correlation coefficient is 0.75, so the transparency can reflect the extent of vertical light attenuation coefficient in certain degree. What's more, the depth of euphotic layer and the transparency have positive correlation, the correlation coefficient is 0.76. The depth of euphotic layer and the transparency are both deep in winter and shallow in spring and summer, and the difference in different areas is significant. Zeu /Zmix which indicates how the water mixing in vertical influences the algae growth is small in autumn and winter and big in spring and summer. So in spring and summer the light in water is enough for the algae growth. And the difference in different areas is not significant.7 In our monitoring period, algae bloom took place seriously except autumn in which the reservoir is impounding water. And the peak value of Chl-a can reach to 227.84 mg/m3. In the whole XXB, fromâ… toâ…£the algae bloom was more and more serious. According to correlation analysis, in algae growth period the inorganic nutrients NO3-N, PO4-P and D-Si decrease significantly. Inâ… (XX00-XX02), the main impact factors were water level, vertical stability coefficient, water temperature, mixing depth, the depth of euphotic layer and Zeu/Zmix. Inâ…¡(XX03-XX05), the main impact factors were water level, water level daily variation, vertical stability coefficient, mixing depth and the depth of euphotic layer. And inâ…¢(XX06-XX08), the main impact factors were vertical stability coefficient, mixing depth, the depth of euphotic layer and Zeu/Zmix. Inâ…£(XX09-XX10), the main impact factors were the flow coming from the upper reaches, mixing depth and Zeu/Zmix. |
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