The Sensitive Ecological Dynamic Processes And Their Simulations Of Algal Growth Of Xiangxi Bay In The Three Gorges Reservoir

It has been almost 15 years since the impoundment of the Three Gorges Reservoir(TGR),with significant economic and social benefits.However,eutrophication and algal blooms in typical tributaries of the TGR,such as the Xiangxi River(XXR),continues to pose serious threatss to the residents' daily life and the sustainable development of the hydropower industry.Due to density current intrusion from the mainstream,and with the complex community composition of phytoplankton,the processes of algal bloom formation and dispersion in XXR are quite different from other lakes and rivers in the world.Hence the XXR was chosen as the principle study area in this thesis.By comprehensively analyzing the eco-hydrological field data from 2010 to 2011,the characteristics of phytoplankton community succession and environmental factors were systematically conducted.Through gradient analysis,the effects of environmental factors on phytoplankton community composition were studied.The sensitive ecological dynamic processes of algal growth were identified via correlation analysis,which include the water temperature variation,diurnal vertical migration,rainfall and the reservoir operation.Based on theoretical derivation and field monitoring data,the constitutive equations of different algal growth rate and water temperature were derived,the effects of water temperature variation on phytoplankton community succession were analyzed.The diurnal vertical migration equation which described the diurnal vertical distribution pattern of different species during algal bloom was coupled with the CE-QUAL-W2 model,to simulate the effect of vertical migration.The processes of algal formation and desperation during rainfall and the fluctuation of water level were simulated for different conditions,in order to reveal the mechanism of these processes.The main conclusions are summarized as follows:(1)The composition of phytoplankton community was complex,with seasonal succession of phytoplankton community.Various environmental factors tend to be stable in winter and fluctuate intensively in summer.The levels of nitrogen,phosphorus and silicone in water all exceeded the threshold of algae bloom.According to gradient analysis with CANOCO,the result showed that the surface water temperature had close relationships with the succession of phytoplankton community.Correlation analysis between algal growth rate and other environmental factors involving the rainfall intensity and water level fluctuation were used to recognize sensitive ecological dynamic processes the algal growth successfully.(2)A vertical two-dimensional hydrodynamic-water quality-ecological model(linked to the CE-QUAL-W2 model)of XXR in the TGR was established,which used to forecast the complex algal bloom.The model can accurately reflect the water temperature with vertical stratification and the density current intrusion from main river channel in different seasons.Through perturbation analysis,the key parameters impacting the algal biomass were identified,including the ratio of algal biomass to chlorophyll-a,maximal algal growth rate,algal respiration,morality and settling velocity.(3)Based on the characteristics of phytoplankton community succession under different temperature,the dominant species successions in Xiangxi Bay were generalized.Based on theoretical derivation,the constitutive equations of different algal growth rate and water temperature were put forward.A method for determining constitutive parameters of algal growth rate and temperature based on field monitoring data is proposed.The cultivated laboratory experiment of the typical algal,Microcystis,under different temperature was carried out.The curve between algal growth rates and temperature was consistent with that the above,which proved that the method is effective and reasonable.(4)Continuous monitoring of the vertical distribution formation of Peridiniopsis,Cyclotella,Microcystis during their algal bloom period revealed that Peridiniopsis,Microcystis are typical migration algal during both daytime and night.According to the vertical distribution patterns of different algal,the diurnal vertical migration effect can be concluded,instead of the conventional algal fixed sinking rate.This knowledge can be used to improve algal simulation work.(5)The processes of algal formation and disappearance under rainfall and different reservoir operation conditions were simulated.The results indicated that the CE-QUAL-W2 model can simulate the water temperature,velocity and the chlorophyll-a concentration variation well.The results revealed that both processes of the air temperature decreasing and the rainfall intruding from gas-water interface into water cannot break out the state of density current,which did little impact on algal bloom dispersion.The inflow in the upstream of XXR increased by rainfall trended to intrude water form bottom,made the sensitive area focused in the upstream and bottom water,which weaken the effect on surface chlorophyll-a concentration in XXR.The effect of continuous impoundment in the reservoir serves to prevent the algal bloom mainly by increasing mixed layer depth.Moreover,the greater the water storage intensity,the more obvious the effect of algal bloom control.