Distribution of dissolved organic nutrients and their effect on export production over the Atlantic Ocean

A synthesis is provided of dissolved organic nitrogen (DON) and phosphorus (DOP) distributions over the Atlantic Ocean based upon field data from eight recent transects, six meridional between 50 degrees N and 50 degrees S and two zonal at 24 degrees and 36 degrees N. Over the entire tropical and subtropical Atlantic, DON and DOP provide the dominant contributions to total nitrogen and phosphorus pools for surface waters above the thermocline. Elevated DON and DOP concentrations (>5 and >0.2 mu mol L-1, respectively) occur in surface waters on the eastern side of the North Atlantic subtropical gyre and equatorial sides of both the North and South Atlantic subtropical gyres, while particularly low concentrations of DOP (<0.05 mu mol L-1) occur over the northern flank of the North Atlantic subtropical gyre along 36 degrees N. This distribution is consistent with organic nutrients formed at the gyre margins supporting carbon export as they are redistributed via the gyre circulation. The effect of DON and DOP transport and cycling on export production is examined in an eddy-permitting, coupled physical and nutrient model integrated for 40 years: organic nutrients are produced in the upwelling zones off North Africa and transferred laterally into the gyre interior, facilitated in part by the mesoscale eddy circulation, as well as fluxed northward from the tropics as part of the overturning circulation. Inputs of semilabile DON and DOP to the tropical and subtropical Atlantic Ocean play an important role in sustaining up to typically 40 and 70% of the modeled particulate N and P export, particularly on the eastern and equatorward sides of the subtropical gyres.