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

The variation in the concentration and the turbulent fluxes of dissolved or ganic nitrogen and phosphorus (DON and DOP) across the thermocline in the c entral Atlantic were studied along a quasi-meridional transect from the Can ary Islands to Argentina (22 degrees N to 31 degrees S). In general, DON co ncentrations were high in surface waters and declined toward the thermoclin e, whereas DOP concentrations were less variable with depth. Vertical, grad ient-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 corr elated with the nitrate supply to the mixed layer and sufficed to remove an amount of nitrogen equivalent to that associated with the upward nitrate i nflux into the mixed layer (median = 104%). The downward Bur of DON exceede d the supply of nitrate off the NW African coast, where Trichodesmium sp. w as abundant. The downward Bur of DOP was also very closely correlated with the phosphate supply to the mixed layer (r = 0.87; P = 0.0007), but the dow nward export of phosphorus as DOP accounted fur only 9% of the upward phosp hate supply to the mixed layer. There was also evidence of a deep upward fl ux of DON and DOP from below the thermocline, which, combined with the gene ral downward flux in the upper waters, indicates the presence of a strong s ink for DON and DOP within the thermocline, tentatively identified to be mi croplankton respiration. Our results point to a dominant role of downward D OM fluxes as a path for the removal of nutrients, and probably carbon, from the biogenic layer of the: ultraoligotrophic central Atlantic.