Dissolved organic nitrogen and phosphorus pools and fluxes in the central Atlantic Ocean

The variation in the concentration and the turbulent fluxes of dissolved organic nitrogen and phosphorus (DON and DOP) across the thermocline in the central Atlantic were studied along a quasi-meridional transect from the Canary Islands to Argentina (22°N to 31°S). In general, DON concentrations were high in surface waters and declined toward the thermocline, whereas DOP concentrations were less variable with depth. Vertical, gradient-driven fluxes of DON and DOP generally involved a downward flux, with a median DON : DOP ratio of 28 : 1. The downward flux of DON was closely correlated with the nitrate supply to the mixed layer and sufficed to remove an amount of nitrogen equivalent to that associated with the upward nitrate influx into the mixed layer (median = 104%). The downward flux of DON exceeded the supply of nitrate off the NW African coast, where Trichodesmium sp. was abundant. The downward flux of DOP was also very closely correlated with the phosphate supply to the mixed layer (r = 0.87; P = 0.0007), but the downward export of phosphorus as DOP accounted for only 9% of the upward phosphate supply to the mixed layer. There was also evidence of a deep upward flux of DON and DOP from below the thermocline, which, combined with the general downward flux in the upper waters, indicates the presence of a strong sink for DON and DOP within the thermocline, tentatively identified to be microplankton respiration. Our results point to a dominant role of downward DOM fluxes as a path for the removal of nutrients, and probably carbon, from the biogenic layer of the ultraoligotrophic central Atlantic.