The Remote Sensing And Modeling Study Of The Phytoplankton Dynamics In The Bohai Sea

In this paper, the MERIS-derived chlorophyll data and a coupledphysical-ecological model are applied to analyze the variation of phytoplanktonbiomass and the possible influencing mechanisms in the Bohai Sea. The response ofthe Bohai ecosystem to different biological and physical processes is investigated indetail, which lay a solid foundation for the further study of the interaction between theclimate change and the ecosystem evolution. The main conclusions are as follows:1) Based on the MERIS-derived chlorophyll data, this paper demonstrates thespatial and temporal variations of chlorophyll in the surface layer of the Bohai Sea.The annual cycle of chlorophyll concentration in most part of the study area ischaracterized by a two-peak pattern, with high concentrations occurred in spring andautumn, leaving low value in summer, while for the regions that are located in thenortheast of Yellow River Mouth and the southern part of the Laizhou Bay, highconcentration always occur in late summer or early autumn. On the basis of the lasttime, the summer/autumn blooms are clarified into two types: the seasonalsummer/autumn blooms and the occasional blooms. Correlation analysis indicates thatthe Yellow River discharge plays a relatively significant role in the occurrence ofsummer/autumn blooms in the northeast of Yellow River Mouth and the southern partof the Bohai Bay by means of the nutrient replenishment and the formation of riverplume. Seasonal increment of the wind speed can induce seasonal autumn blooms inthe east part of the Liaodong Bay, for the intensive vertical turbulence caused bystrong wind leads nutrient replenishment of the surface layer from the lower layers. Insummer, sporadically abrupt increases of chlorophyll concentration are alwaysaccompanied by stronger wind events, either with the conditions that wind speed ishigher than10m/s but has a short duration or with that wind speed is higher than6m/sbut lasts a longer time.2) FVCOM is used to simulate the current filed and the temperature variation of the Bohai Sea, taking the sea surface forces of wind and heat flux into account. Themodel results show that semidiurnal tidal current dominate the tidal current filed. Thetidal residual current is weak all over the Bohai Sea, with a clockwise circulationoccurred in the central Bohai Sea and an anticlockwise circulation occurred in thethree bays respectively. Multi-eddy is the most important feature for the circulationpatterns of the Bohai Sea. In winter, the vertical distribution of temperature is uniform,while for the horizontal distribution, temperature in deep water regions is higher thanthat in coastal waters, with the lowest temperature occurred in the top of the LiaodongBay and a warm water belt stretching form the north Yellow Sea to the Central BohaiSea through the Bohai Strait. In summer, the temperature reaches the highest value ofthe whole year with the strongest stratification occurred at the same time. Surroundedby cold waters, a warm water column is captured in the Central Bohai Sea, where thetemperature was nearly vertically uniform.3) The annual cycle of the Bohai ecosystem is simulated using a0-D ecologicalmodel, which has considered the biological processes as much as possible, toinvestigate the effect of different biological processes on the ecosystem dynamics. Asthe annual cycles of phytoplankton and nutrients are well consistent with the previousstudies in the Bohai Sea, the model results analysis indicates that it is light conditionthat controls the phytoplankton biomass accumulation in the early spring, whileafterwards, the nutrient limitation plays a dominate role in the phytoplankton variation.As for the nutrient limitation, the inorganic nitrogen acts as the limiting nutrient in thebloom periods. Due to the heave pressure of the zooplankton grazing and the failureto consider external nutrient supply, the summer bloom is not as strong as the springbloom, although the primary production is high both in spring and in summer.4) A simplified NPZD ecological model coupled with FVCOM is performed tocarry out the three-dimensional numerical simulation of the ecosystem dynamics inthe Bohai Sea. During the spring bloom period, the highest phytoplanktonconcentration occurs in the three bays and the coastal regions. In the Central BohaiSea, the seasonal development of stratification maybe account for the relatively lowphytoplankton concentration in summer and relatively high concentration in autumn. Due to the horizontal advection, the nutrient distribution is inhomogeneous. Theconcentration reaches the highest value in winter and reaches the lowest value insummer.5) Finally, a numerical experiment is conducted to highlight the effect of strongwind events on the development of the occasional blooms. The results indicate thatstrong winds make a significant contribution to the nutrient replenishment in theupper layer via destroying the stratification, which can trigger the summer blooms.