The Coupling Mechanism Of Algal Swimming And Hydrodynamics In Density Currents And Its Numerical Simulation

Lakes and reservoirs are the main fresh water resources for the society,and are the essential support for the sustainable development of economy.Algal bloom in lakes and reservoirs becomes one of the most important water environmental issues in the world due to human activities.Therefore,the forming mechanism and control measures associated with algal blooms are hotspots in aquatic environmental research.The hydrodynamic mechanism of algal vertical distribution,researching on the influence of hydrodynamic force on algal swimming,is an important component of bloom forming mechanism.Density currents commonly exist in lakes and reservoirs,which are closely related to the formation of algal blooms.In present paper,the swimming characteristics of Chlamydomonas reinhardtii is intensively studied,and the swimming characteristic parameters are calculated.The vertical distributions of C.reinhardtii in density currents are studied through laboratory experiments.The mechanism of how algal swimming is affected by the hydrodynamic force is revealed,and based on which an algal vertical distribution model is established.Forthermore,Xiangxi Bay in Three Gorges is selected for a case studied to analyze the influence of the density currents on bloom forming and disappearing.Some suggestions on ecological regulation for bloom controlling are proposed.Main achievements have gained as follows:(1)Swimming characteristics of C.reinhardtii is intensively studied,and its swimming characteristic parameters are put forward for the first time.On the basis of eliminating the influences of illumination,temperature and nutriments on algal swimming,the velocity distributions of C.reinhardtii in still water are measured to obtain the swimming characteristic parameters.The results show that the averaged swimming velocity is 106?m/s,and the algae prefer swimming upward.The reorientation time B is 10.8s,representing its directionality in swimming.The rotational diffusion coefficient Dr is 0.05rad2/s,representing its randomness in swimming.(2)The algal vertical distribution and its hydrodynamic mechanism in density currents are revealed for the first time.The vertical distributions of C.reinhardtii in lock-release density currents are studied.The results show that the algal cells are concentrated below the interface between fresh water and saltwater,where the shear rate is intense.The concentrating mechanism of swimming alga in density currents is revealed based on algal gravitaxis.The concentrating phenomenon is observed in density currents.The threshold of shear rate for C.reinhardtii being trapped is 0.13?0.15s-1.(3)A numerical model is estabilished to simulate the vertical distributions of swimming alga in density currents.Base on the mechanism of how algal swimming is affected by the hydrodynamics,a two-dimensional laterally-averaged numerical model coupling with algal swimming is established.And the model is verified by experimental results and field observations.(4)The hydrodynamic mechanism of how the algal bloom forming is affected by the flow in Xiangxi Bay is revealed.The results indicate that alga cells are vertically uniform in the unstratified flow,so that the algal bloom is difficult to happen.In the middle-layer-backflow,swimming algal cells concentrate below the interface between the up and the middle layer.Thus the algal bloom can be prevented by the middle-layer-backflow.In the up-layer-backflow,algal cells are concentrated in the up layer,so the algal bloom is likely to occur.Higher the shear rate in the up layer,easier algal bloom to happen.The threshold of shear rate for alga being trapped is 0.01-0.03s-1 in Xiangxi Bay.(5)Occurrence principle of algal bloom in XiangxiBay is explained by the mechanism of how hydrodynamics affect algal swimming.Changing the vertical distribution of algae is more important than promoting the growth rate of algae for algal blooming and disappearing.The mechanical mechanism can better explain why algal blooms frequently occurred in the end of the back water,and why the changing rates of algal density in the surface water is higher than the algal growth rate during March to Spetember.(6)Suggestions on ecological regulation for bloom controlling in Xiangxi Bay are proposed.The area where the blooms frequently occur,is located at the end of the backwater.The flow in this area is affected by the water level of Three Gorges Reservoir and flow from upstream.Research results indicate that the influence of upstream flow is greater than that of downstream water level.Therefore,the specific strategies of regulating the upstream flow is proposed to prevent the algal blooms in Xiangxi Bay according to the variation of downstream water level.