Frontal Control Mechanisms On The Phytoplankton Blooming Outside The Changjiang River Estuary

Phytoplankton is the main primary producer in the marine ecosystems and plays a significant role in global biogeochemical cycles.Its distribution determines marine productivity.The phytoplankton blooms and its biomass increase are physiologically codetermined by nutrient availability which provides basal nutrition,and water properties such as salinity,temperature and turbidity that affect the photosynthetic efficiency.Estuaries and coastal waters are generally widely known as the regions with high marine productivity.Phytoplankton bloom events frequently occur in global estuarine and coastal regions,where the waters are influenced by the input of terrigenous materials.The dynamic circumstance,which includes the confluence of riverine fresh water with ambient oceanic waters,is complicated in estuarine and coastal regions.It gives rise to strong gradients of hydrologic features and aquatic environmental elements between the riverine and oceanic water volume.The location of front is general where maximum substance gradients are in waters.The substance fronts existed in estuaries include plume front,turbidity front,nutrient front and so on.The strong substance concerntration gradients in estuaries bring up spatial differences of phytoplankton growth condition,and ultimately determines the spatiotemporal distribution characteristic of phytoplankton and even the complete aquatic ecosystem.Among the determinants,fluvial nutrient and turbidity are particularly prominent considering a eutrophic river plume system with strong tidal forcing.There are many river plume systems with energetic tidal forcing that are eutrophic due to severely influences by anthropogenic activities.The Changjiang River plume is a typical case.In winter,the Changjiang River plume propagates southward along the coast with strong horizontal salinity gradient against the ocean.It exhibits typical characteristics of bottom-trapped plume;In summer,the river plume extends northeastward offshore with strong vertical stratification against the ocean.It is typical characteristics of surface-trapped plume.Taking considerable concerns on the plume dynamic processes and its frontal effect,an understanding of the mechanisms that control phytoplankton blooms and its direct linkage with physical processes remain a great challenge,which is exactly the focus of this study.Based on historical statistical information of harmful algal blooming event from 2000 to 2014,observational hydrobiogeochemical data in May,2016,and improved three dimensional coupled hydrodynamic?sediment?ecosystem numerical model results,the seasonal spatiotemporal distribution patterns of phytoplankton outside the Changjiang River Estuary is analyzed.Phytoplankton blooms in spring and summer seasons.Its high value centers settle in longitude between 122 °E and 124 °E within the 30 m to 50 m isobaths,exhibits a sinuous and swinging strip-like pattern paralleling to the coastlines.On the view of hydrologic and aquatic environmental circumstances as well as the hydrodynamics,the high value centers of phytoplankton locate seaward of the surface suspended sediment front and shoreward of the surface nutrient front during spring bloom period.Likewise,the high chlorophyll concentration settles in the interspace between the locations of the bottom and surface plume fronts.The interspace between the bottom and surface plume fronts directly determines the sizes of the high chlorophyll concentration centers across the shelf.South of Changjiang River mouth,the river plume propagates as bottom-trapped plume with relatively less interspace between the bottom and surface plume fronts,the high chlorophyll concentration centers distribute as narrow strip-like pattern.East and north east of Changjiang River mouth,the river plume extends as surface-trapped plume.The surface plume water volunm could expand farfield under the completed estuarine dynamics.The scope of high chlorophyll concentration centers is relatively larger,distributing as large patch-like pattern in normal.On the view of dynamics,the outer brim of surface plume front determines the seaward limit of the nutrient-rich river-influenced water and set the outer boundary of algal blooming.The location of bottom plume front is currently the boundary separating the stratified and well-mixed water volunm.The bottom suspended sediment is transported to surface layers due to vertical mixing and thus inhibits phytoplankton blooming shoreward of the bottom plume front,while the effect is basically relieved due to the existence of pycnocline seaward of the bottom plume front.The terrestrial nutrient input directly determines the scale of high chlorophyll concentration centers,and the high tubidity zone inside the bottom plume front is the primary factor inducing the high value centers of phytoplankton migrating outside the bottom plume front in the Changjiang River Estuary.Inhibiting effect of aquatic environmental factors have seasonal spatiotemporal variable impact on the phytoplankton growth.On the view of seasonal horizontal distribution,the inhibiting effect of sediment on phytoplankton growth in the high tubidity limiting zone inside the bottom plume front is generally up to 60%,somewhere maximum could even exceed 80%.The inhibiting effect of phosphate is ubiquity outside the Changjiang River Estuary during the spring and summer blooming periods.The phosphate limitation peaks in summer with the shelf core zone occupying approximately 60%?On the contrary,the inhibiting effect of temperature surpassing 60% occurs in East China Sea shelf in winter.The temperature inhibition starts to decline in spring and almost disappearsd in summer.On the view of vertical distribution,the inhibiting effects of aquatic environmental factors distribute with an obvious horizontal gradient in the bottom-trapped plume region during the spring blooming periods.Seaward across shelf direction,the primary inhibiting effect in surface and subsurface area is in turn sediment inhibition,temperature inhibition,phosphate inhibition and dissolved inorganic nitrogen inhidition with unobvious vertical difference,respectively.However,in the surface-trapped plume case,the primary inhibiting effect is sediment inhibition nearshore,phosphate inhibition offshore within 5 m water depth and temperature inhibition offshore beneath 10 m water depth.