Tropical Dominance of N₂ Fixation in the North Atlantic Ocean

To investigate the controls on N₂ fixation and the role of the Atlantic in the global ocean's fixed nitrogen (N) budget, Atlantic N₂ fixation is calculated by combining meridional nitrate fluxes across World Ocean Circulation Experiment sections with observed nitrate ¹⁵N/¹⁴N differences between northward and southward transported nitrate. N₂ fixation inputs of 27.1 ± 4.3 Tg N/yr and 3.0 ± 0.5 Tg N/yr are estimated north of 11°S and 24°N, respectively. That is, ~90% of the N₂ fixation in the Atlantic north of 11°S occurs south of 24°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 N₂ fixation rate of 30.5 ± 4.9 Tg N/yr north of 30°S, implying only 3 Tg N/yr between 30° and 11°S, despite evidence of P-bearing, N-poor surface waters in this region as well; this is consistent with iron limitation of N₂ 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 N₂ fixation can stabilize the N-to-P ratio of the global ocean. However, the calculated rate of Atlantic N₂ fixation is a small fraction of global ocean estimates for either N₂ fixation or fixed N loss. This suggests that, in the modern ocean, an approximate balance between N loss and N₂ fixation is achieved within the combined Indian and Pacific basins.