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The South China Sea (SCS) is the largest semi-enclosed marginal sea in the western Pacific. The alternation of East Asian monsoon causes a significant seasonal pattern of chlorophyll, primary productivity, and export flux of sinking particles. However, the source and sink of dissolved organic carbon (DOC) pools with different bioavailability are less studied. Here we evaluated the seasonal production of DOC in labile, semi-labile and refractory forms using a coupled physical-biogeochemical model. This study aims to understand the dynamics and budgets of organic matters in the SCS. Model results show that the production of labile, semi-labile and refractory DOC is highly correlated with the net primary productivity (NPP) which is higher in winter and lower in summer, reflecting a dependence of DOC on the NPP. The seasonal variation in Pearl River discharge dominates the DOC production in the northern coastal region. In the northeast, the Kuroshio intrusion associated frontal system is attributed to cause high winter production. The DOC production in the southwest is controlled by both winter mixing and summer upwelling. The production of refractory DOC with the least bioavailability favors carbon sequestration. Its annual mean production is 1.8±0.5 mg C m−2 d−1, equivalent to 26% of the export flux of particulate organic carbon at 1000 m. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is strongly influenced by the East Asian monsoon system with seasonal reversal. Measurements from a 7-year continuous sediment trap located in the central SCS showed a clear seasonal pattern. The particulate organic carbon (POC) export flux at the depth of 1200 m was considerably higher in monsoon seasons. The driving dynamics leading to this seasonal variability of POC export, however, remains inadequately understood. Here, a one-dimensional physical-biogeochemical coupled model was developed to simulate the temporal variability of a lower-trophic planktonic ecosystem. The modeled POC export flux compared reasonably well with the 7-year time series from moored sediment trap. Model results showed that the POC export flux at 1200 m is highly correlated with the 0–100 m integrated primary productivity and with the export flux at 100 m, implying that the seasonal variability of sediment trap data could be induced by changes in phytoplankton production and its vertical export. Further model analysis suggested that the annual mean export ratio (e-ratio) at 100 m and transfer efficiency at 1200 m in the central SCS were 0.19 and 0.07, respectively, which are lower than those in high latitudes. The winter monsoon favors not only surface carbon fixation but also export to the deep ocean. The heat flux is the dominant factor regulating the seasonal cycle of mixed layer depth, nutrient supply, and the growth of phytoplankton in this region. The wind-driven mixing can further facilitate upward nutrient transport to the surface and amplify the seasonal amplitude of the POC export flux. [ABSTRACT FROM AUTHOR]