Numerical Study Of The Key Physical And Biological Processes In The Bohai Sea Ecosystem

Numerical models are used to represent key physical-biochemical processes influencing the Bohai ecosystem. In terms of physical process two principal dynamics are stressed in this study: first, features of BS tides and tidal currents which determine the transport and distribution of major nutrients are examined, the character of tidal mixing as well as evolving mechanism of tidal front is also analyzed and discussed with a stratification parameterization fit to shallow shelf-sea; secondly, the river plume development is simulated, and the influence of tide and wind stress on river plume evolution are further examined. Fundamental results are as follows:(1) M₂ constituent is predominant for both tides and tidal currents in BS. There are two amphidromic points for M₂ constituent: one near Qinhuangdao, another close to old Yellow River mouth; rectilinear currents dominate in narrow sea-routes and coastal areas, whereas rotation is the basic moving style in the central BS. Both relatively drastic depth transformation and intense nonlinear interaction of shallow water lead to the asymmetry of ebb/flood tidal current. Therefore, several strong M₂ residual current whorls are evident in BS.(2) Vertical stratification structure comes from the balance between heat flux through sea surface and vertical stirring in the water column. Heat flux increases remarkably in spring, so layering comes into being gradually and concentrates on the central BS where tidal mixing is weak; whether the range or intensity culminates during the Summer Solstice; stratification is weakened in accompany with the decrease of heat flux in the fall, and is still apparent in BS strait; the gale enhances vertical stirring of sea water, so no obvious stratification exists in winter.(3) Tidal front is the interface separating mixing area and stratification area,just like a retention or a dynamic obstacle preventing the exchange of water, particulate or dissoluble material.(4) The pure buoyancy river plume is a typical double-layer flow with light fresh water floating on dense sea water, and different flow direction is evident for the upper and lower current adjacent to estuary. Both tide-induced and wind-forced kinetic dissipation are conducive to weakening fresh water front.(5) Different wind stress determines distinct river plume features: offshore wind enhances seaward development of fresh water inducing strong upwelling in the estuary, whereas shoreward wind confines river discharge to a narrow band along the coast.As one of the important research fruits concerning the dynamic process of BS ecosystem, a NPZD type of biological model, which takes nitrate, ammonium and phosphate as major nutrients and includes another three biological variables -phytoplankton, zooplankton and detritus, is established and coupled with POM, Dobson and Smith irradiance model as well as Vassiliki river discharge model in this study. The biannual blooms of phytoplankton and yearly rise and fall in nutrient contents are successfully reproducing, which is reasonably consistent with true case in BS. In addition, the author also estimates the primary production in every sub-region and completely investigates the BS f-ratio in the first time. By comparison with observed data as well as pioneer work, numerical results presented here are reliable. The model used in this study is especially applicable to water area which has not been subject to eutrophication yet.