On the Potential Role of Dissolved Organic Nutrients in Sustaining Export Production in the Subtropical Atlantic Ocean

One mechanism by which export production in subtropical gyres may be partially sustained is the advection of organic matter from surrounding upwelling regions within the surface ocean. Observations of total organic nitrogen (TON) and phosphorus (TOP) concentrations from three transects (2 meridional and 1 zonal) of the oligotrophic Atlantic Ocean suggest that gradual meridional gradients in these species occur. Concentrations at the equator and in the North African upwelling are approximately 5 mmol m-3 TON and 0.2 mmol m^{- 3} TOP and decline uniformly towards the gyre interior to 4 mmol m-3 and 0.03 mmol m-3 at 30°N. Residual TOP values in the N and S Atlantic gyres are different, with lower values in the N Atlantic consistent with enhanced N2 fixation in that region. Estimates of the meridional advection of these species (14 mmol-TOP m-2 y-1 and 0.07 mol-TON m-2 y-1) derived by combining the meridional concentration gradients observed in the N Atlantic with northwards displacement derived from drifter data, suggest organic nutrients play a potentially significant role in sustaining carbon export. Estimated advection indicate a potential contribution of 30% from TOP and 10% from TON to export production rates estimated at Bermuda. Observations at 24°N suggest that this conclusion may be valid for much of the North Atlantic subtropical gyre.