Tropical Dominance of N2 Fixation in the North Atlantic Ocean

To investigate the controls on N-2 fixation and the role of the Atlantic in the global ocean's fixed nitrogen (N) budget, Atlantic N-2 fixation is calculated by combining meridional nitrate fluxes across World Ocean Circulation Experiment sections with observed nitrate N-15/N-14 differences between northward and southward transported nitrate. N-2 fixation inputs of 27.1 +/- 4.3 Tg N/yr and 3.0 +/- 0.5 Tg N/yr are estimated north of 11 degrees S and 24 degrees N, respectively. That is, similar to 90% of the N-2 fixation in the Atlantic north of 11 degrees S occurs south of 24 degrees N in a region with upwelling that imports phosphorus (P) in excess of N relative to phytoplankton requirements. This suggests that, under the modern iron-rich conditions of the equatorial and North Atlantic, N2 fixation occurs predominantly in response to P-bearing, N-poor conditions. We estimate a N-2 fixation rate of 30.5 +/- 4.9 Tg N/yr north of 30 degrees S, implying only 3 Tg N/yr between 30 degrees and 11 degrees S, despite evidence of P-bearing, N-poor surface waters in this region as well; this is consistent with iron limitation of N-2 fixation in the South Atlantic. Since the ocean flows through the Atlantic surface in <2,500 years, similar to the residence time of oceanic fixed N, Atlantic N-2 fixation can stabilize the N-to-P ratio of the global ocean. However, the calculated rate of Atlantic N-2 fixation is a small fraction of global ocean estimates for either N-2 fixation or fixed N loss. This suggests that, in the modern ocean, an approximate balance between N loss and N-2 fixation is achieved within the combined Indian and Pacific basins.