| The algal bloom, happened frequently and covered a large area in some tributaries of the Three Gorges Reservoir （TGR）, has done bad effects on water ecosystem, industry and agriculture, and is now the most serious environmental problem in the Three Gorges Reservoir Region. In order to improve the water quality in the TGR, it is very urgent and important to find useful methods to control and eradicate the algal bloom. However, up to now, the mechanism of the algal bloom is unclear. To solve this environmental problem in a short time is also very difficult.In this thesis, some work has been carried out to research the mechanism of the algal bloom and its controlled method in Xiangxi Bay （XXB） of the TGR. Firstly, the characteristics and mechanism of the special bidirectional density currents （BDDCs） in the XXB were analyzed based on a five year’s monitoring data from2008to2012. A mathematical model was also established to study the water cycle processes and their patterns in the XXB. Secondly, the spatial distribution and temporal changes of the nutrient in the bay have been monitored and analyzed. The tracer method, based on the major ions （Cl-and Na+） and hydrogen and oxygen isotopes (δD and δ18O), were used to estimate the contribution rates of the Yangtze River （CJ） and upstream of the XXB to the XXB. Thirdly, the Critical Depth Hypothesis was developed to be an applicative theory to clear the mechanism of the algal bloom. The relationships between the phytoplankton succession and water stratification were also analyzed based on the CDH theory. Finally, a "tide-type" water level fluctuation method was suggested to control the algal bloom in the tributaries of the XXB. Summary, followed6important results were shown below.（1） The sediment concentration was decreasing gradually in the CJ, which was less than0.5g/L in the area near the Three Gorges Dam （TGD）. The water flowed as a one-dimensional pattern from the upstream of the TGR to the TGD directly. The water residence time in the TGR was less than20days in summers and more than80days in winters, while in the XXB, which was more than200days in summers and900days in winters most of the time. It is shown that the complex BBDCs were the dominated hydrodynamics in the XXB all through the years, which was more caused by the water temperature deference between the CJ, the XXB and the inflows of the XXB. Exactly, the BBDC could be divided into5different patterns in different season: reverse bottom-layer cuneal flow （RBCF）, reverse bottom-layer density current （RBDC）, reverse middle-layer density current （RMDC） and reverse surface-layer density current （RSDC） from the CJ, and downslope bottom-layer density current （DBDC） from the inflow of the XXB. Those5flow patterns were changed from one to another with the changes of the water temperature difference in different water body of TGR. （2） In the CJ, the lowest water temperature happened in February and was more than9℃, while the highest one was less than27℃and happened in August. The differ between the surface and bottom layer was less than3℃. So, water body in the CJ was shown as a complete mixing pattern （river-type） most of the year, except in March when a weak stratification happened. The CJ was suitable to the Hutchinson’s typical classifications of the lakes in the north hemisphere. In XXB, the water stratification started to develop in March, peaked in summers, and disappeared in the impounding periods, and completely mixed in winters. Water body in the XXB was more shown as the lake-type pattern. However, the stratification in the XXB was not fit the Hutchinson’s typical classifications. It was very unusual, in deep water area in the downstream of the XXB, there was a weak stratification as a "double mixolimnion-metalimnion "pattern, but in shallow area near the end of the XXB, the stratification became more stronger and was shown as a "right half of U" pattern. Those particular stratification patterns were in turn affected by the BDDCs and changed gradually with the development of the BDDCs.（3） The hydrodynamic-temperature mathematical model, based on the CE-QUAL-W2, could simulate the unusual characteristics of BBDCs and water temperature. The boundary conditions of the water temperature in the simulation processes covered all the situations happened in different hydrodynamic patterns. As a result, the simulated flow, during the periods of the one-dimensional pattern, the RBCF, RBDC, RMDC, RSDC and DBDC, could well fit the measured features.5different flow circulation patterns were summarized through the flow lines charts, which could be very significant and useful to research the mechanism of the algal bloom and its control methods in the XXB.（4） In the inflow water of the XXB, total phosphorus concentration （TP） was relatively very high, but total nitrogen （TN） and dissolved silicon （D-SiO2） were much lower than CJ. The linearly dependent coefficient between Na+and Cl-was very high （R2=0.926）, and the Na+and Cl-could be effective tracer ions to estimate the contribution rates of different nutrient sources in the XXB. There were remarkable isotope （δD and δ18O） difference between the CJ and inflow of the XXB. The CJ and inflows of XXB were the main sources of the water and nutrient in the XXB. However, estimated results of constant ion tracer method and isotope tracer method were both shown that the contribution rates of the C J, both water and nutrient, always exceeded that of inflow of the XXB. The nutrient supplied processes-in the euphotic zone could be classified as five ways:horizontal transportation, middle-up in summers, overturn in winters, organic decomposition and points and no-points sources. Only in every March, euphotic zone could be more affected by the inflows of the XXB, which nutrient was almost mainly from the CJ in the other time.（5） There were serious and intermittent algal blooms both in springs and summers from2010to2011, while the summer algal bloom was much more serious. ************In fact, the critical depth could be estimated by the euphotic zone because the values of them were closed. There were three patterns, to affect the algal blooms, about the relationships between the mixing depth、compensation depth and critical depth according to Critical Depth Theory:in a water with a sufficient nutrient, the algal blooms could happened when mixing depth was less than composition depth, which could be restrained when the mixing depth exceeded the critical depth and develop when the mixing depth was less than the critical depth but deeper than the compensation depth. Those three inferences were proved to be credible in the XXB through a stratification experiment in winter. The algal blooms happened in2009-2010could also be illuminated by the critical depth theory. In XXB, the particular water stratification, caused by the BDDC, could be the main factor to trigger algal blooms. Conditions of water light intensity and stratification were also the main factors to affect phytoplankton succession.（6） The water level fluctuation of TGR could change the RMDC into RSDC, through which the stratification before the fluctuation would be destroyed. As a result, the mixing depth in the XXB increased and phytoplankton biomass decreased. Based on this situation, an eco-environmental friendly operation （EEFO） of the TGR, named"tide-type" water level fluctuation method, was suggested to control the algal bloom in those tributaries, consisted of a tide-type operation in the water releasing stage （TTOR）, a tide-type operation in flood control stage （TTOF）, and a stair-type impounding during the impounding stage （STII）. The demonstration indicated that the EEFO could do great effects on algal blooms in the XXB. The algal blooms could be restricted in summers and autumn trough using the EEFO. What’s more, the Critical Depth Theory and the Intermediate Disturbance Theory gave a theoretical supports to this EEFO to control the algal blooms in the tributaries of TGR. |
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