Meridional Fluxes of Dissolved Organic Matter in the North Atlantic Ocean

<p> Using bio&hyphen;optical estimates of gelbstoff and a few platinum measurements of dissolved organic carbon (DOC _pt ), a budget of the meridional flux of DOC and dissolved organic nitrogen (DON) across 36&deg;25&prime;N in the North Atlantic is constructed from previous inverse models of water and element transport. Distinct southward subsurface fluxes of dissolved organic matter (DOM) within subducted shelf water, cabelled slope water, and overturned basin water are inferred. Within two cases of a positive gradient of DOC _pt between terrestrial / shelf and offshore stocks, the net equatorward exports of O ₂ and DOC _pt from the northern North Atlantic yield molar ratios of 2.1 to 9.1, compared to the expected Redfield O ₂ /C ratio of 1.3. In the first case, 63% of the apparent oxygen utilization demands of the water column may be met by DOC, instead of only 14% in the second scenario, preserving a role for falling particles in the sea. With a DOC/DON ratio of 10, the larger net southward export of DON across 36&deg;25&prime;N balances the postulated net northward input of 1.7 &times; 10 ³ kg NO ₃ s ^&minus;1 of unutilized nitrate within the Gulf Stream. Without an enhanced supply of DOM from the shelves, a zero seaward gradient of DOM in the third case suggests that none of the poleward nitrate flux is returned southward as DON, but instead a net poleward flux of DON prevails as well. Our present estimates are confounded, however, by the seasonal and multiyear variability of sinking processes in the North Atlantic. Future active and passive remote sensors, field programs, and simulation models must now discriminate between particulate and dissolved components of surface color signals to verify the importance of both continental margins and DOM in global biogeochemical cycles.</p>