Diapycnal nutrient fluxes and a practical assessment of the biological nutrient trap in the north and south Atlantic subtropical gyres

Diapycnal mixing supplies nutrients to the euphotic zone, which in oligotrophic regions may substantially support rates of new production. However, the consensus view that diapycnal nutrient fluxes support new production within the entire euphotic zone is challenged by deep living autotrophs that likely consume some, if not all, of the diapycnal flux at depth. Quantifying how much of the diapycnal nitrate flux is trapped by biological consumption immediately above the nitracline remains challenging and the implications of nutrient trapping for comparisons of cross-nitracline diapycnal fluxes with euphotic zone integrals of new production remains unclear. It is increasingly important therefore to determine where in the euphotic zone the diapycnal flux has impact. In this study, a simple assessment is presented of the strength of the “nutrient trap,” which is attributed to picoeukaryotes, a widely distributed group of autotrophic picoplankton found in the subtropical and tropical ocean. This study finds significant potential for the total consumption of diapycnal nutrient fluxes within a few meters of the nitracline, thus largely negating the significance of vertical diffusive fluxes for processes occurring at shallower depths. These results suggest that the significance of diapycnal nutrient fluxes for integrated productivity estimates is lower than generally assumed. Yet, although diapycnal fluxes cannot be entirely discounted from nutrient budgets due to seasonality in the consumption of such fluxes at depth, this likely makes harder current modeling efforts to constrain future ocean productivity where predictions of increased stratification generally favor greater reliance upon the diapycnal pathway to support production.