Study On Material Flux In The Changjiang Estuary And The Adjacent Waters Based On A Hydrodynamic Box Model

A traditional box model is a stoichiometrically linked steady state model forwater-salt-nutrient budget to solve nutrient flux and budget among waters withdistinct salinity difference. However, a traditional model cannot cope appropriatelywith those without distinct salinity difference parallel to coastline or in a complexcurrent system, as the results are highly affected by box division in time and space,such as the Changjiang (Yangtze River) estuary (CE) and adjacent waters(30.5°N-32°N,122°E-123°30′E).Therefore, we tried to construct a coupled box model with a hydrodynamicmodel unit on the traditional model and the regional oceanic modeling system model(ROMS)（named as′′the hydrodynamic box model′′）.The mass transport wasorthogonally decomposed to be the horizontal and the vertical fluxes, each dividedinto the advection and the diffusion one. The flow field data of the hydrodynamicmodel was matched to the boundary of the box model accurately, through which thehydrodynamic model and the box model was linked susccessfully through the Matlabprogramming.Horizontal, vertical and boundary water fluxes were calculated by thehydrodynamic box model. The characteristics, kinetic factors, mechanisms for waterflux and the relationship between water exchange and anoxia were discussed. Basedon the water flux and using data from four cruises in2005, horizontal, vertical andboundary nutrient fluxes were calculated, in which flux fields were depicted and themajor processes controlling the nutrient flux were discussed.It was found that horizontal water flux in upper layer in winter> that inautumn> that in summer> that in spring, while that in lower layer in summer> that in spring> that in winter> that in autumn. Horizontal water flux in lower layer wasabout one order of magnitude smller than that in upper layer in autumn and winter,and about1/2of that in upper layer in summer, with the smallest difference in springbetween the upper and lower layers. It was the monsoon that controlled the water fluxas a whole, making water being transported from north to south or from south to northseasonally; while it was the Taiwan Warm currents that played an important role in theprocess of lower waters being transported northwards in spring and summer，and theChangjiang diluted water(CDW), topography and the onshore wind all playedimportant roles in the spatial and temporal distribution of water flux. The upwellingwater flux was18.53m3/s in summer,12.55m3/s in spring,13.07m3/s in winter and14.25m3/s in aumtumn. The western boundary water flux flowed to the seathroughout the year, and the water flux across the southern and northern boundariesboth flowed with the monsoon on the whole.But there was no regulare pattern forwater flux across the eastern boundary. Water flux across the southern and northernboundaries was about one order of magnitude greater than that across the westernboundary. And water flux across the eastern boundary was obviously smaller than thatacross the western boundary. Water flux in upper layer across the southern andnorthern boudaries was about one magnitude greater than that in lower layer inautumn and winter, twice of that in lower layer in summer,and about1/2inspring.The Changjiang discharge was not dumped into the open seas east of123.5°Edirectly (5.41m3/s in summer), which was significantly smaller than water flux inacross the eastern boundary in spring and autumn(12.45m3/s、27.75m3/s).Water fluxwas transported out of the study area across the southern boundary first at the rate of187.73m3/s annually, and then to the open seas through the ocean current system.Under the strong monsoon, water exchange depended more on the horizontal waterflux in the monsoon direction, mainly in the same layer but not between the upper andlower layers. It was concluded that the ultimate cause of anoxia was the spring layerthat hindered the oxygen exchange between the upper and lower waters.Results showed that horizontal nutrient flux in upper layer in winter> that inautumn> that in summer> that in spring, and horizontal nutrient flux in lower layer in summer> in spring> in winter> in autumn. The spatical difference and seasonalchange of horizontal nutrient flux in upper layer was significantly greater than that inlower layer. Nutrient flux in the study area varied greatly in season and space,depending more on physical dilution than biochemical reactions. Similar to water flux,it was the monsoon that made nutrients being transported from north to south or fromsouth to north seasonally; while it was the Taiwan Warm currents that played animportant role in the process of nutrients in lower waters being transportednorthwards in spring and summer，being a limiting factor of the Changjiang dischargetransported eastwards to the open seas. Besides, vertical nutrient flux replenishment,open waters intrusion and primary production consumption all played important partsin the spatial and temporal distribution of nutrient flux.Upwelling flux outweighed upward diffusion flux in vertical direction.Upwelling flux and upward diffusion flux regions overlapped largely all the year.Vertical flux in spring and summer were much greater than that in autumn and winter.Vertical nutrient flux was a main source of DIP (dissolved inorganic phosphate). Themaximum vertical flux for DIP occurred in summer，but the most prominent affectfrom vertical flux was in spring. Vertical flux in spring was3-7times and3-5timessmaller than horizontal flux for DIN and DSI, respectively, while vertical flux for DIPcould match the horizontal flux. Vertical flux in summer was3-6times,5-10timesand5-8times smaller than horizontal flux for DIN, DSI and DSI, respectively. AndVertical fluxes in autumn and winter were both2orders of magnitude smaller thanhorizontal flux for DIN,DSI and DIP.Nutrient fluxes across the southern and northern boundaries were significantlygreater than those across the eastern and western boundaries in the same season.Nutrient flux across the eastern boundary was the smallest among the four boundaries,about1-2orders of magnitude smaller than that across the western boundary. Westernboundary nutrient flux flowes towards the sea throughout the year, and nutrient fluxesacross the southern and northern boundaries flowes with the monsoon on the whole,southward in autumn and winter and northword in spring and summer. But nutrientflux across the northern boundary in upper layer and lower layer in autumn was opposite, southward in upper layer and northward in lower layer. There was no regularpattern for eastern boundary nutrient flux.Nutrient flux from the western boundarywas mainly confined to coastal waters with little into the open seas east of123.5°E.The study area acted as a conveyer transferring nutrients from the Yellow Sea to theEast China Sea in the whole year at the rate of7149.82mol/s,4097.97mol/s and115.42mol/s for DIN, DSI and DIP respectively.Therefore, the hydrodynamic box model is superior to the traditional one inestimating nutrient fluxes in a complicated hydrodynamic current system and providesa modified box model approach to material flux research.