Numerical Simulation Of The Spatial And Temporal Distribution Of Nutrients And Phytoplankton In The Changjiang Estuary And Its Adjacent Areas

Estuaries are the areas with the most complicated and strong land-sea interactions.They are also influenced a lot by human activites and closely related to the changes of coastal regions.In the past few decades,many dams were constructed in the Changjiang basin and affecte the sediment discharge.Moreover,nutrients increase a lot because of the large usage of fertilizer and the discharge of untreated sewage.All these factors make the ecosystem of Changjiang estuary and its adjacent areas change a lot.As regard to few quantitative studies on the impact of long-time changes in Changjiang Estuary,this paper quantifies the variations of nutrients and phytoplankton in Changjiang Estuary and identifies the underlying mechanisms which contribute to the observed changes in biogeochemistry,with the application of numeraical simulation.By coupling hydrodynamic model FVCOM and biogeochemical model ERSEM based on FABM,we develop a new coupling physical-biogeochemical model: FVCOM-FABM-ERSEM.With this model,we establish a one-dimention-vertical(1DV)model firstly to verify the well coupling of two models.Then East China Sea model is established to lay the foundation for the construction of Changjiang Estuary case.After combining the observational data and simulational results of Changjiang Estuary case,we study the main biogeochemical processes of Changjiang Estuary and quantify the short-term and long-term effects of Changjiang-inputs-changing.The main research conclusions include:1.The biogeochemical processes of Changjiang Estuary was simulated.The distribution of temperature,salinity,nitrate,phosphate and Chl-a fit the observation data and remote sensing data well.The characteristics of Changjiang Estuary fronts are also well represented.Benthic module of model acts well,the simulational results of full benthos processes are more consistent with observational data than no benthos case.2.Ecosystem of Changjiang Estuary and adjacent areas respond quickly to the variablies of changes of Changjiang inputs.The patterns of nutrients change in the ecosystem are similar with that of Changjiang runoff,which indicates that Changjiang is the main source of nutrients in the coastal areas.The distribution of nitrate reflects the nitrate dilution in coastal region.The pattern of phosphate is closely related to the consumption and vertical mixing of phytoplankton.Suspended sediment exerts little influence to the spatial variabilities of dissolved nutrients.The spatial pattern of chlorophyll-a is consistent with the pattern of sediment front.The phytoplankton in Jiangsu and Shanghai coastal areas has a longer bloom duration and higher concentration than that in Zhejiang coastal region.Due to the change of nutrients ratio,the percentage of dinoflagellates increases.3.Due to the intensive impact of anthropogenic activities on the Changjiang in last 60 years,the ecosystem of Changjiang estuary and its adjacent areas have undergone great changes.The concentration of nitrate and phosphate increases up by 4 times.Furthermore,the ratio of nutrients changes.The N/P ratio increases from 1.25 times the Redfield ratio to 3.125 times.The ratio of Si/N and Si/P both show a downward trend.Si/N is closer to the Redfield ratio and Si/P is still much larger than Redfield ratio.The increasing of nutrients makes algal bloom more frequent and the change of nutrients proportion makes the structure of phytoplankton community change.Since the mid-1990 s,the abundance of diatoms has decreased and that of dinoflagellates has increased.