Analytical And Numerical Study On Subsurface Chlorophyll Maximum In The Northern South China Sea

The vertical distribution of marine phytoplankton is highly heterogeneous. Abell-shape vertical profile of phytoplankton in mass concentration was frequentlyobserved in coastal seas and open oceans. The corresponding subsurface chlorophyllmaximum (SCMs) has aroused a very high scientific interest. SCMs can becharacterized by three parameters, i.e., depth, thickness and intensity of the subsurfacechlorophyll maximum layer (SCML). These parameters are determined by marinehydrology, nutrient concentrations and phytoplankton species, etc. Identifying controlfactors to spatio-temporal variability of these key characteristics of SCML, anddeveloping vertical chlorophyll distribution function are essential for accurateestimation of marine primary production.In this thesis, impact of key physical processes (e.g. light attenuation, verticalturbulent mixing and phytoplankton sinking) and bio-chemical processes (e.g. growthand loss of phytoplankton) on SCML characteristics are systematically summarized.Previous studies show that depth of SCML was mainly a function of light andturbulent mixing in oligotrophic ocean. Its intensity was determined by light,turbulent mixing and biogenic processes associated with zooplankton grazing.However, relationship between these biogenic processes and SCML characteristic arenot well quantitatively expressed. And few studies about SCMs are found in coastalseas.In the northern South China Sea (SCS) case study, based on field observations,analytical modeling and numerical simulation are applied to indentify key factorsinfluencing depth, thickness and intensity of SCML. The results show thatSignificant spatial variation was observed in the northern SCS during autumns.SCML Depth is gradually increasing from coastal waters to open ocean, but SCMLintensity exhibits a decrease trend, SCML thickness was patchy. Significantly, SCMLintensity negatively relates with SCML depth and thickness. Positive correlationexists between thickness and depth.Based on Gaussian distribution function, a new piecewise function was proposed tofit the vertical chlorophyll profile in stratified water. A simple universal chlorophyll-nutrient model for one single species is used to build the general functionrelationships between the three key characteristics of SCML and their control factors.It is shown that (1) vertical turbulent mixing coefficient and phytoplankton sinkingvelocity beneath the upper layer (approximately the mixed layer) are key physicalfactors influencing SCML characteristics;(2) SCML depth logarithmically increaseswith surface irradiance, but decreases with irradiance half-saturation constant. Thethickness of SCML is hardly affected by surface irradiance, irradiance half-saturationconstant, and phytoplankton remineralization rate. SCML intensity and relative rate ofphytoplankton remineralization are in an inverse proportion; and (3) when sea watersis poorly mixed, depth and thickness of SCML are mutually independent, but intensityis inversely proportional to thickness of SCML; linear relationship between intensityand logarithmic thickness of SCML when sinking of phytoplankton is not considered,in the case that concentration of chlorophyll in surface is lower, depth and theintensity of SCML are mutually independent, but intensity is inversely proportional tothickness of SCML.After assessing a large number and wide range of environmental parameters in thenorthern SCS, the general relationships between the three parameters and theirinfluencing factors were applied to the northern SCS. The process-oriented numericalmodeling experiments suggest that (1) light attenuation coefficient significantlyeffects SCML depth and thickness;(2) SCML thickness is effected by maximumspecific growth rate; and (3) maximum specific growth rate, mixed layer depth, lossrate of phytoplankton, and vertical turbulent mixing coefficient beneath the upperlayer are determinants to SCML intensity.A1-D physical-biological coupled model was applied to analyze seasonalvariability of the three key characteristics of SCML and their influencing factors innorthern SCS. The simulated temperature, salinity and vertical chlorophylldistribution fit well with field observations at Station SEATS (South East Asia TimeSeries). The sensitivity analyses demonstrate that (1) the wind stress modulates thenutrient transport through wind-induced mixing. The fact that the depth of the SCMLremains unchanged during variable wind forcing suggests that the wind brings morenutrients and chlorophyll to the upper layer while the nutricline remains relativelystable. The thickness of SCML thickens with increased wind stress, and the intensityof SCML display a negative correlation with the magnitude of the wind stress.(2) Thedepth of the SCML is negatively correlated with light attenuation coefficient and the nutricline, which indicate the depth of SCML determining the maximum nitrategradient.(3) The nutrient level controls the seasonal variability of SCML intensity,but only has minor effect on the depth and thickness of SCML.