Spatial And Temporal Variations Of Nutrients In The Upper South China Sea:Physical-biogeochemical Modulations And Their Comparisons With The Gulf Of Mexico And Caribbean Sea

N,P,and Si are essential nutrients for marine biota's growth.New production is primarily fueled by nutrient-rich deep water via upwelling or diapycnal mixing.New production is balanced by carbon export fluxes under steady state thereby determining the net efficiency of biological carbon pump and the ocean's capability of sequestering atmospheric CO2.Multiple physical and biogeochemical processes control the spatial and temporal variations of nutrients in the ocean.As a classic oceanographic theme,elucidating the the processes modulating the variatiblity of nutrients in space and time remain challenging.It is even more challenging to quantify these processes.This dissertation sought to examine the spatial-temporal pattern of nutrients and their physical-biogeochemical controls in the South China Sea(SCS),a nutrient depleted marginal sea in the surface.This dissertation also attempted to compare the SCS with the Gulf of Mexico and Caribbean Sea(GoM-CS),a largest marginal sea system of the Atlantic in terms of nutrient dynamics and their controls in order to better understand nutrient cycles in tropical-subtropical marginal seas under the influence of western boundary currents.Scientific questions this thesis aims to address include:(1)How big is the vertical nutrient turbulent flux in the SCS?How can we better quantify this flux?(2)How does the Kuroshio intrusion affect the nutrients in the upper SCS?How can this effect be quantified?(3)How do nutrients vary in the SCS at seasonal and interannual timescales?(4)What is the climatology of the nutrient distribution in the SCS?What are the key physical-biogeochemical processes controling their distribution?(5)Are there any similarities and differences of the nutrient physical-biogeochemical modulations between the SCS and GoM-CS?Based on high resolution and frequency profiling measurements of NO3-and turbulence microstructure made at the South East Asian Time-series Study(SEATS)station in August 2012,this study seeks to examine the turbulent dififusive fluxes of NO3-in the SCS.In addition,a one-dimensional advection-diffusion model associated was applied to calculate the advective fluxes of NO3-.The summation of the diffusive and advective fluxes gives the total vertical fluxes of NO3.It is found that the total vertical NO3-fluxes in the upper 50 m were(1-50)×10-4 mmol m-2 d-1,implying that the externally supplied nutrients from subsurface play a minor role,while the internal recycling of nitrogen could be of dominant contribution.At the bottom of the euphotic zone(75 m),the vertical flux of NO3-was 0.105 mmol m-2 d-1;equivalent to a new carbon production of 0.69 mmol C m-2 d-1 production in the euphotic zone,which only compensated?11%of the export production.Comparing with other impact factors,our analysis indicated that the deficit of the NO3-inventory in the euphotic zone mainly provided the extra nutrients,which was the main factor that determined the export production.In additional to the in situ NO3-sensor measurments,we also took discrete samples for NO3-,NO2-and NH4+,dissolved inorganic phosphate(DIP)and Si(OH)4,based on which fluxes of these nutrient species were also determined at the SEATS station.According to the flux ratios,the nitrogen limitation probably occurred in the upper 50 m,while the silicate limitation was between 50-75 m.Due to the corse resolution,the discrete samples based vertical fluxes are subject to a larger range of uncertainty(upper to 40%)as compared to that from the continuous nutrient profilings.Based on four cruises covering a seasonal cycle in 2009-2011,we examined the impact of the Kuroshio intrusion,featured by extremely oligotrophic waters,on the nutrient inventory in the central northern SCS(NSCS).The nutrient inventory in the upper 100 m of the water column in the study area ranged from?200 to?290 mmol m-2 for N+N(nitrate plus nitrite),from?13 to?24 mmol m-2 for DIP and from-210 to-430 mmol m-2 for silicic acid.The nutrient inventory showed a clear seasonal pattern with the highest value appearing in summer,while the N+N inventory in spring and winter had a reduction of?13%and?30%,respectively,relative to that in summer.To quantify the extent of the Kuroshio intrusion,an isopycnal-mixing model was adopted to derive the proportional contribution of water masses from the SCS proper and the Kuroshio along individual isopycnal surfaces.The derived mixing ratio along the isopycnal plane was then employed to predict the genuine gradients of nutrients under the assumption of no biogeochemical alteration.These predicted nutrient concentrations,denoted as Nm,are solely determined by water mass mixing.Results showed that the nutrient inventory in the upper 100 m of the NSCS was overall negatively correlated to the Kuroshio water fraction,suggesting that the Kuroshio intrusion significantly influenced the nutrient distribution in the NSCS and its seasonal variation.The difference between the observed nutrient concentrations and their corresponding Nm allowed us to further quantify the nutrient removal/addition associated with the biogeochemical processes.We revealed that the nutrients in the upper 100 m of the water column had a net consumption in both winter and spring but a net addition in fall.By using the model output of the China Sea Multi-Scale Ocean Modeling System(CMOMS)operated at the Hong Kong Unveristy of Sceince and Technology led by Dr.Jianping Gan,this study diagonosed the temperature,salinity,DIP and Chla in the SCS.At the same time,we quantified the Kuroshio water proportion(Kw)at SEATS using the the isopycnal mixing model as described above.During 1988-2012,the amplitude of the inter-annual variation in Kw exceeded the seasonal variation.After removal of the seasonal signal,the inter-annual variation of Kw showed asymmetric response to the Multivariate ENSO index(MEI).As the Kuroshio carries the most oligotrophic water of the world's ocean,its intrusion into the SCS has a dilution effect on the nutrients within the SCS,which subsequently influences the Chla.We thus compared Kw,wind stress and vertical DIP flux with the DIP inventory in the upper 100 m in the SCS.It showed that the DIP inventory was significantly and negatively correlated with the Kw(R2=0.68),which was much more significant than that with the wind stress(2W=0.05)or the vertical DIP flux(R3=0.22).In addition,the DIP inventory was positively correlated with the Chla inventory in the upper 100 m(R2=0.62).Our study thus suggested that the Kuroshio intrusion was the most important determinant in the interannual variation of the DIP and Chla inventory in the upper 100 m at SEATS.We compiled literature data of temperature,salinity,and nutrients from both the SCS and GoM-CS basins and established empirical algorisiums between nutrients and temperature&salinity,which were used to predict the nutrient concentrations in these regions with the temperature and salinity data output from the HYbrid Coordinate Ocean Model(HYCOM).The predicted nutrient concentrations showed a tight linkage with the regions dynamics including large-scale circulation,upwelling,meso-eddy processes,the Loop Current(in the GoM-CS),and western boundary current intrusions.Diagnosis of nutrient budget analyzed term-by-term showed that the inflow of oligotrophic open-ocean water diluted nutrient concentrations in both basins.Vertical advection was the main nutrient source for both basins,with higher concentrations where upwelling or/and eddies were active.The residual term,ie.,source-minus-sink that,quantified biological alterations showed maximum nutrient uptake rates at?70 m in the SCS and at?100 m in the GoM-CS.The uptake-remineralization N:P ratio was 15.0 in the SCS and 20.6 in the GoM-CS.Our study indicated 0.5(1.44)mmol m-2 d-1 new nitrate,corresponding to 3.7(9.5)mmol m-2 d-1 of new production in the upper SCS(GoM-CS).The SCS and GoM-CS showed similar nutrient concentrations,but different depths of maxima.As the oligotrophic north Atlantic water flow directly through the GoM-CS,it significantly diminished the nutrient concentration and deepened the nutricline,inducing variations at deeper layers in GoM-CS as compared to the SCS.The coupled physical and biological processes were responsible for the nutrient sources and sinks in these oligotrophic marginal seas.In summary,the nutrient distribution in the SCS and GoM-CS showed a tight linkage with the regions' dynamics including large-scale circulation,upwelling,meso-eddy processes,the Loop Current(in the GoM-CS),and the western boundary current intrusions.In the NSCS,the Kuroshio intrusion significantly influenced the nutrient distribution and its seasonal variation.In addition,the Kuroshio intrusion was the main factor that influenced the interannual variations of the DIP inventory and Chla inventory in the NSCS.Similar nutrient physical-biogeochemical modulations are observed in the SCS and GoM-CS;it shows that the vertical advection and diffusion are the main nutrient sources for the euphotic zone.