Modeling The Algae Bloom Based On The Vertical Migration Of Phytoplankton In Xiangxi Bay And Its Control Methods By Scheduling The Three Gorges Reservoir's Operation

Systematic studies were carried out to research the mechanism of the phytoplankton bloom in Xiangxi Bay (XXB) of the Three Gorges Reservoir(TGR) using the theory of the environmentalism, ecology and hydrodynamics with the method of field monitoring, experimentation and mathematic modeling.The main aim of this study is to analyse the characteristic of the vertical migration of phytoplankton , light intensity , water temperature,nutrients,hydrodynamics and the ecological zone,modeling the vertical migration of phytoplankton and the emergence and disappearance of the algae bloom ,and give the ecological operation of the TGR to meet the emergent algae bloom.The main results are as follows:1.Continuous monitoring was conducted in the Xiangxi Bay of the Three Gorges Reservoir to study the diurnal vertical migration of Peridiniopsis sp.and Cyclotella. The results indicated that the vertical distribution of Peridiniopsis sp. biomass changed regularly throughout day and nightwithin the upper 12 m but the Cyclotella's was ruleless.The algae bloom was the phenomenon of phytoplankton's migration upto the surface water layed.The underwater light intensity, temperature and the physiology were the main factors to affect the vertical distribution of Peridiniopsis sp.2.The underwater light intensity was monitored continuancely to analyse the characterastics of the underwater light intensity's distribution and its control factors.It was indicated that the light intensity in suface water layed was suitable to phytoplankton's proliferation in spring,which was more over 100000 lux in summer but very weak in winter.Thre were significant positive correlation between underwater attenuation coefficient and turbidity,chlorophyll a (Chl.a)and transparency, and the biomass of the phytoplankton was the main factor to influence the underwater extinction coefficient.The euphotic depth(Zeu) was changed rulenessly inregularly in summer but high in winter and low in spring and autumn.The Zeu was controlled by phytoplankton in upstream areas but suspended solid from the Changjiang River with the reverse density current.3.The distribution of water temperature was monitored to analyse the water stratification and its mechanism.The results indicated that there was a weak stratification from March to April in TGR, the vertical temperature difference was only limited within about 1~2℃.There was a weak stratification in downstream of XXB because of the reverse density current flowing from the main stream of TGR and a strong stratification in upstream because of the process of downslope-bottom density current from the incoming flow with low water temperature.4.A model was established to model the light intensity both above and under water.The models based on biotic force were established to model the vertical migration of phytoplankton and the emergence and disappearance of the algae bloom,respectively.The results showed that the biotic force was a existing, specific,limited force offerd by living bodys aimed to get more resources or avoid some dangerous,the process of which meeted the fanction of the GuassAmp.The motion ability and proliferation ability of the phytoplankton and the mixing depth(Zmix) were the main factors to contrul the vertical distribution of the phytoplankton.Faster migration rate, thicker euphotic depth and thinner mixing depth make algae bloom emergenced more easily, nutrients was the reasons why the algae bloom in the mainstream of TGR was slighter than XXB.The reason why different species have different distribution of biomass was the different motion abilities among them.5.The hydrodynamic and the nutrient characteristics was analysed to research the ecological zone in XXB.The results indicated that the hydrodynamics of the XXB could be generalized as a density-stratified flow , and could not be simply simulated by one-dimension model. The upstream water mainly flowed out of the bay in a process of downslope-bottom density current, meanwhile, the TGR mainstream water entered into the XXB in a reverse density current. The density current of the XXB was mainly caused by the water temperature difference and turbidity difference between the TGR mainstream and the bay, and the former played a dominant role.The XXB could be divided into five different ecological zone:exchange zone,transition zone,stagnant zone,fluctuation zone and stream zone.In the stagnant zone, the water stratification was strong and the water exchanged up-and-down weakly,the nitrogen comed from the mainstream of TGR while the phosphorus and solicon from the incoming water upstream and domestic wastewater,and the algae bloom occurred frequently.6.The mechanism of emergence and disappearance of the algae bloom in XXB and its control methods by scheduling the Three gorges reservoir 's operation was discussed.The results indicated that the uplift rate of the water level was more than 1.0-1.5 m/day when the water density was different between mainstream and XXB,which could force the density current to occur and influence the whole XXB.The water exchange reduced the differences of the water quality parameters (for example, temperature, turbidity, nutrient level, pH, etc.) between the mainstream and the XXB during the impounding process. Meanwhile, the water stability index in the mainstream remained small but it was lowered in the XXB. The main causes included the dilution by water of lower concentration of the chlorophyll a (Chl. a) entering from the mainstream, the decline of the water stability, and the increase of suspended silt, which induced the Chl.a to decrease in the XXB during the impounding process. The grade of the water eutrophication changed from middle eutrophic level to mesotrophic state in the process.It was suggested that to use the'tide-type'operation in spring,'tide-type'operation in summer and the'impoundment ahead'operation of the TGR to relieve the algae bloom in spring,summer and autumn respectively.