Abstract:

Nutrients from depth have been hypothesized as a primary source of new nutrients that sustain new productivity in oligotrophic oceans; however, the flux is challenging to quantify. Here we show for a first time in the oligotrophic South China Sea an extremely low diapycnal dissolved inorganic nitrogen (DIN) flux as 1.8 × 10^-4 mmol m^-2d^-1 in the nutrient-depleted layer (NDL) above the nutricline, where other nutrient supplies sustain the new production. Here higher phosphate and silicate fluxes relative to DIN than Redfield stoichiometry further indicate N-limited biological productivity and additional removal of DIN by diatoms. Below the NDL across the nutricline to the base of euphotic zone, termed as nutrient replete layer, the DIN flux is three orders of magnitude larger and sufficient in supporting the export production therein. Here higher DIC flux relative to DIN than Redfield stoichiometry further infers DIC excess in the upper ocean.

Figure. Profiles of the potential density (σ_θ), squared buoyancy frequency (N²), NO₃^-, effective diapycnal flux of NO₃^- (F_{e_}NO₃^-), PP, and Chl α within the euphotic zone. In the upper 47 m, NO₃^- and F_{e_}NO₃^- were estimated from discrete water samples with nanomolar level sensitivity, while those below 53 m were estimated from continuous sensor measurements. Also shown are the surface mixed layer (SML), the nutrient-depleted layer (NDL), and the nutrient replete layer (NRL)

Key Points:

l  The commonly ignored diapycnal advection induced by turbulent mixing is important in calculation of diapycnal nutrient fluxes

l  The euphotic zone should be categorized into two distinct layers in terms of vertical nutrient fluxes in oligotrophic oceanic regimes

l  The modulation of diapycnal nutrient fluxes on biological carbon pump in the upper ocean and air-sea CO₂ exchange was inferred

Full text: http://onlinelibrary.wiley.com/doi/10.1002/2017GL074921/full.

Citation: Du, C., Liu, Z., Kao, S.-J. & Dai, M., 2017. Diapycnal fluxes of nutrients in an oligotrophic oceanic regime: the South China Sea. Geophysical Research Letters, 44. doi：org/10.1002/2017GL074921.