Comparative study of the variability and trends of phytoplankton biomass between spring and winter upwelling systems in the South China Sea

High phytoplankton biomass usually appears off the coastal waters southeast of Vietnam (SV) during summer and southwest of the Luzon Strait (SLS) in winter. Although this seasonal upwelling has been recognized in SV and SLS for more than half a century, there has been limited documentation comparing the characteristics and formation mechanisms of these two upwelling systems. To understand the dynamic features of phytoplankton growth in SV and SLS, seasonal and interannual variabilities of Chlorophyll-a (Chl-a) concentration were studied based on decadal-scale satellite-derived datasets (2003–2018). Bivariate wavelet coherence (BWC) and multiple wavelet coherence (MWC) were performed to evaluate the temporal variability of multiple controlling factors. The results demonstrated that there were different patterns of seasonal cycles and interannual variability of Chl-a concentrations in the two upwelling regions. In SV, high Chl-a concentrations during summer were primarily formed by the interactions of multiple factors, causing the mixed layer depth to increase during summer and sufficient nutrient concentrations on the surface (supplied by the mixed water column). In contrast, the high Chl-a concentrations in SLS are caused by a combination of deep mixing (induced by wind and buoyancy forcing) and enhanced upward advection (induced by strong eddy activities). These factors support intense levels of phytoplankton biomass during winter. The sea surface temperature (SST) was the best single factor to explain the Chl-a variance in both regions. The addition of more factors to the MWC analysis changed the main factors influencing the Chl-a variations. The two-factor combination of SST-Niño 3.4 in SV and the SST-mixed layer depth (MLD) in SLS controlled the Chl-a variations on an interannual scale. Moreover, the three-factor combination of SST-Niño 3.4-MLD was still meaningful for explaining the Chl-a variations. This indicated that the influencing the Chl-a variations differed in these two upwelling systems.