Active and Passive Fluxes of Carbon, Nitrogen and Phosphorus in the Northern South China Sea

This study is the first time to simultaneously investigate the active and passive fluxes of carbon (C), nitrogen (N) and phosphorus (P) in the upper layer of the northern South China Sea (NSCS) where is generally regarded as tropical and oligotrophic. Other than the tropical storm condition, both active and passive fluxes of CNP are rather low in the summer season. The averaged active fluxes of C, N and P through 200 m are about 25.6 mg C m^-2 d^-1, 2.68 mg N m^-2 d^-1 and 0.37 mg P m^-2 d^-1, respectively. The averaged passive fluxes of C, N and P are generally less than 150 mg C m^-2 d^-1, 20 mg N m^-2 d^-1 and 1.0 mg P m^-2 d^-1, respectively, through the 100-m depth. However, carbonate dominates the deep fluxes of biogenic components and appears to play an efficient ballast for Corg transport to deep water. Both active and passive fluxes are significantly enhanced in winter with deepened mixed layers and extreme conditions such as typhoons, eddies and internal-waves events. The active fluxes may contribute to the biological pump up to 35% if all individual fluxes via zooplanktons (0.2-5 mm in size) are considered. The diffusion flux of DOM that is estimated from the distribution of DOM in water columns and calculated by the Fick’s first law is also lower in summer than in winter and extreme conditions. The contribution of DOM diffusion flux to the biological pump in the upper layer, however, is insignificant. The passive CNP fluxes remain play a crucial role in determining the biological pump in the tropical marginal sea.