Numerical Simulation Of Pelagic Ecosystem's Seasonal Variation In The Northern East China Sea(ECS)

As an important part of biosphere,marine ecosystem plays a key role in linking atmosphere and lithosphere.It composes and transfers energy and supports material circulation around the oceans.The East China Sea(ECS)is a typical marginal sea in China,which is influenced by fresh water discharge and others ocean currents.Due to complicated hydrodynamic and biogeochemical environments of ECS,marine ecosystem research usually based on in-situ data and 3D physical-biogeochemical coupled model methods.However,there has been little research reported on the seasonal variability of the marine ecosystem in a focused area with high vertical and temporal resolution.We used a 1D coupled physical-biogeochemical model: GOTM-FABM-ERSEM driven by atmospheric forcing to dynamically simulate the seasonal and vertical variations of key pelagic marine ecosystem components in the northern ECS(125°E,31°N).In addition,combined with analysis of seasonal and vertical variation of plankton community structure,this thesis focused on physical and biogeochemical drivers that control seasonal changes in key pelagic marine ecosystem components,and analyzed the interactions and feedbacks of various components in pelagic marine ecosystem.Furthermore,it has been found that light and nutrients had significant impacts on seasonal changes in pelagic ecosystem,and this method developed foundation for furfure upgrade 3D coupled model.Simulations(2006-2007)indicated that the coupled model represented the physical and nutrients field in the northern ECS with high skill,tested through a comparison between model data and the observation data.We further discussed the seasonal and vertical variability of plankton productivity and biomass.Furthermore,we studied the interactions and feedbacks between different ecosystem components,and investigated the energy flow and material circulation of marine ecosystem.We also analyzed the community structure of phytoplankton,which showed diatoms(phytoplankton requiring silicate)acted as the dominant functional group on an annual basis.Nanophytoplankton and picophytoplankton were dominant species in after-phase of spring blooming and upper layer from June to September,which illustrated that they were more competitive compared to diatoms in lower nutrient environments.Main drivers that caused seasonal and vertical variability of phytoplankton community structure were light,temperature,zooplankton,cell size,nutrients and their vertical exchange.Moreover,zooplankton showed an alternation in dominant functional types: the dominant functional type in January-April was heterotrophic nanoflagellates,but in April-May was microzooplankton.Mesozooplankton was the dominant group from May to December except in the upper layer in summer(June to September).The main drivers that controlled seasonal and vertical variations of zooplankton community structure were temperature,phytoplankton,cell size,bacteria and dissolved oxygen.CCA analysis indicated that the relationship between phytoplankton functional types and environmental drivers changed seasonally.In winter,light was a key driver controlling diatoms growth,while nitrate was not significant.In addition,temperature was the main driver that controlled nanophytoplankton,picophytoplankton and microphytoplankton growth.In spring,nanophytoplankton,picophytoplankton and microphytoplankton were limited by nutrients supply and driven by temperature and light.In summer and autumn,light and nutrients were the key drivers that promoted phytoplankton growth.To study the effects of variability in solar radiation and surface forcing(wind,surface temperature,atmospheric pressure,humidity)to pelagic marine ecosystem,we provided ten years of three-hourly atmospheric forcing and solar radiation data to coupled model.Simulated results(2006 and 2012)indicated that the variation of surface forcing and solar radiation would cause the seasonal and vertical changes of components of pelagic marine ecosystem.Overall,the physical parameters(temperature,salinity,light,MLD,depth of euphotic layer,TKE)exhibited slightly variability between 2006 and 2012,except for some specific periods,particularly in surface layer in summer.The growth of plankton were roughly similar between 2006 and 2012.However,because of different distributions in nutrients,plankton biomass in 2012 were higher than that in 2006.However,Our 1D coupled model only considered local processes,without horizontal substance transport and circulation processes.This could be further developed for more comprehensive understanding marine ecosystem.Therefore,based on such discussion,a 3D coupled physical-biogeochemical model(FVCOM-FABMERSEM in our study)is suggested to be carried out to study the ECS ecosystem in the further work.