Numerical Study On The Relations Between Seasonal Variation Of Phytoplankton And Hydrodynamic In The Bohai Sea

The algal bloom is the process that phytoplankton biomass accumulates rapidly. Studying the phytoplankton growth factors is of great significance to reveal the ecological mechanism of the algal bloom. In the paper, the hydrodynamic and water quality model COHERENS is adopted to simulate the phytoplankton growth seasonal changes and research the relationship between the phytoplankton and hydrodynamic environment in the Bohai Sea. The physical environment of the phytoplankton bloom is explored, focusing on temperature, salinity and turbulence effect.The 3-D hydrodynamic and water quality equations are applied to establish the actual model in which the weather conditions and runoff conditions are considered. The model is verified from the harmonic constants, tide level, temperature and salinity. The seasonal changes and stratification characteristics of temperature and salinity are simulated firstly. The results show that the thermocline gradually emerged from the spring, the strength of thermocline reaches its maximum in the summer. The temperature of the bottom existed asymmetric nuclear structure. After that, the thermocline decreases and finally disappeared in the autumn. The halocline only occurred in the summer, compared to the thermocline, the halocline relatively delayed. It can be found that the long term variation of temperature and salinity can be reasonably simulated with the hydrodynamic model.By adjusting zooplankton prey pressure, nitrate and ammonia nitrogen fluxes in the sediments, and other important parameters, the water quality model is established to simulate the chlorophyll, nitrate and ammonia nitrogen characteristics. The results show that the annual cycle of chlorophyll presents bimodal structure, the nitrate and ammonia shows the process of winter accumulation and summer consumption at time series. In the horizontal distribution, chlorophyll concentration, nitrate and ammonia always maintained a high concentration near coastal and estuarine due to the impact of water depth and runoff. It can be found that the year cycle characteristics of surface chlorophyll, nitrate and ammonia can be accurately simulated with the water quality model.Finally, the paper researches the relationship between the phytoplankton growth and the physical environment, such as temperature, salinity and turbulence effect. The surface distribution of turbulent viscosity in the Bohai Sea is simulated. The results show that the correlation coefficients of sea surface temperature and chlorophyll concentration in winter, spring, summer and autumn are -0.32,0.83,0.82 and -0.21. The correlation coefficients of sea surface salinity and chlorophyll concentration in winter, spring, summer and autumn are-0.54,-0.92,-0.86 and -0.53. In the horizontal distribution, in autumn and winter, the strong vertical turbulent mixing causes the surface nutrients adequate. However, the temperature and light limitation leads to lower chlorophyll content in the Bohai Sea. In spring and summer, the suitable light and temperature makes higher surface chlorophyll content. Nevertheless, the weaker turbulent mixing results in the stratification structure and reduced nutrient concentrations in the abyssal zone. Eventually, the chlorophyll content is low in the abyssal zone. The phytoplankton growth is the result of multiplex factors including temperature, salinity and turbulent effect.