The Study On Nonlinear Stochastic Dynamics Of HABs Ecosystem In Bohai Sea

With the China's economy developing at very high-speed, the marineeutrophication state is becoming serious day by day. The emergence frequency andinfluencing scale of HABs(harmful algal blooms) are increasing in the coastal watersof China. Undoubtedly probing into the evolving course of HABs ecosystem is one ofthe most important ways of exploring the mechanism of HABs.In this dissertation, the evolving process of HABs is researched with nonlinear stochasticdynamics methods. According to the characters of the typical algae and their environment inBohai this dissertation finished tasks as follow.Firstly, a method for numerical calculation of nonlinear stochastic dynamicalsystem called stochastic increment-dimensional precise integrate method was createdand presented. The method can be applied in numerical calculation and numericalsimulation in nonlinear stochastic dynamic system.Secondly, a stochastic multi-population noctiluca scintillans-diatom-nutrientmodel and was created and presented. After simplifying the model through stochasticaveraging method, four theories were applied in analyzing the bifurcation of stabilityand the Hopf bifurcation of the stochastic system. The four theories were the theoryof boundary theory of diffusion process, invariant measure theory, Lyapunovexponent theory and FPK theory. The critical value of Hopf bifurcation parameterwas obtained and the pattern of the position of Hopf bifurcation drifting with theparameter. The stochastic system was simulated with stochastic increment-dimensional precise integrate method, the result verified the conclusions oftheoretical analyzing.Thirdly, other ecosystem nonlinear dynamical food chain model was presentedwith consideration of the concentration of typical nutrient brine, the density ofphytoplankton(the harmful algae), the density of zooplankton and density of detritus.Adding random factors, a nonlinear stochastic model was established. Theconcentrarion of nutrient brine below mixed layer was chosen as the bifurcationparameter. The variety of stability was analyzed under the default vlaues of otherparameters. The condition of occurrence of high-dimensional Hopf bifurcation wasdiscussed subsequently. As to the stochastic system, the stochastic system wassimulated with stochastic increment-dimensional precise integrate method, the resultverified the conclusions of theoretical analyzing. In conclusion, random factors(excitations) is one of the main reasons causingHABs. The possibility of HABs is in direct ratio to the intensity of the randomexcitations. The results obtained in this dissertation can be applied as theoreticalmethod and numerical simulation technique in studying the nonlinear stochasticdynamical mechanism of HABs, HABs prediction and early-warning system.The work reported in this dissertation was supported by Ministry of Educationimportant item foundation of science and technology and also supported by TianjinDevelopment Plan of Science and Technology Foundation.