It is a conventional view that nutrients in the euphotic zone are primarily sourced from the ocean depth through advective and diffusive processes in oceanic regimes where land-input is limited. While being essential to support the oceanic new productivity, such nutrient fluxes are practically difficult to constrain as neither the diffusion and advection rates nor high-resolution gradients of nutrients are easy to obtain. Based on simultaneous high-resolution profiling measurements of nutrients, dissolved inorganic carbon (DIC) and turbulence microstructure made at the South East Asian Time-series Study (SEATS) station in August 2012, this study examined the diapycnal fluxes of both nutrients and DIC in the oligotrophic basin of the South China Sea (SCS). A particular novelty of this study was that the diapycnal advective term commonly omitted in previous studies was included in our estimation. We showed that diapycnal advective fluxes accounted for ~16% of diapycnal nutrient fluxes at the base of the euphotic zone, while diapycnal diffusive fluxes accounted for the residual proportion, showing the importance of including advective term in calculating diapycnal nutrient fluxes in the upper ocean.
According to vertical structures of nutrients and their diapycnal fluxes, we categorized the euphotic zone into two distinct layers: a nutrient depleted layer from the surface to the top of nitracline, and a nutrient enriched layer from the top of the nitracline to the base of the euphotic zone. The nutrient depleted layer at SEATS was 0-50 m where maximum stratification was observed. Here, the vertical fluxes of the Dissolved Inorganic Nitrogen (DIN) and Dissolved Inorganic Phosphate (DIP) were estimated to be 1.83×10-4 and 4.41×10-4 mmol m-2 d-2 at 47 m, respectively, which were about 4-5 orders of magnitude lower than the nutrients needed for primary productivity, the upper limit of which were about 1.7 ×10-3 and 5.2 ×10-4 mmol m-2 d-1, respectively. This strongly indicated that the nutrients sourced from the depth only played a minor role for sustaining the primary production, and thus it is essentially a self-sustaining system.
