North Atlantic Ocean nitrogen sources

Capone, D. G., Bums, J. A., Montoya, J. P., Subramaniam, A., Mahaffey, C., Gunderson, T., Michaels, A. F., and Carpenter, E. J. (2005). Nitrogen fixation by trichodesmium spp. An important source of new nitrogen to the tropical and subtropical North Atlantic Ocean. Global Biogeochem. Cycles 19, GB2024, doi 10.1029/2004GB002331. 

Lipschultz, P., and Owens, J. P. (1996). An assessment of nitrogen fixation as a source of nitrogen to the North Atlantic Ocean. Biogeochemistry 35, 261—274. 

Table 36.2 Sources of nitrogen to the continental shelves in major temperate-zone water regions of the North Atlantic Ocean...

Figure 12 Nitrogen isotopic values of suspended particulate matter and sinking particles (as collected by sediment traps) in the subtropical North Atlantic Ocean near BATS (31° 50 N, 64° 10 W see Figure 6). The profiles of suspended PN show the representative depth gradient in <5 N, with lower iS N in the surface ocean than at depth. The <5 N of the sinking flux shows a decrease with depth, which has also been observed in other regions. Reprinted from Altabet MA, Deuser WG, Honjo S, and Stienen C (1991) Seasonal and depth-related changes in the source of sinking particles In the North Atlantic. Nature 354 136-139.

Figure 13-13 Nitrogen budget for the North Atlantic, modified from Galloway et al. (1996), demonstrating the m or fluxes (xlO mol year ) across the boundaries of the hasin as well as sinks (denitrification) and new sources (nitrogen fixation nitrogen deposition (N dep.)) of reactive N. Fluxes into the m or physical realms of the North Atlantic (open ocean, shelf waters, and estuaries) are highlighted hy diagonal-line fills loss terms and exchange fluxes between realms are indicated by open arrows.

On balance, the shelves are not a net source of N to the open ocean. Instead, the North Atlantic has major exchanges with the Arctic Ocean and with the South Atlantic. Ganachaud and Wunsch (2002) estimate southerly nitrate fluxes of 2200 (+/- 3800) and 6600 (+/- 4700) x 10 mol N year- at 7.5°N and 4.5°S, respectively. We take their mid point value of 4400 (+/— 4000) X 10 mol N year as the net transport of nitrate from the North Atlantic to the South Atlantic. A significant uncertainty lies in the net meridional transport of DON in the basinwide N transport budgets in the Atlantic. Rintoul and Wunsch (1991) speculated that the imbalance they quantified in the poleward nitrate flux across subtropical sections may be compensated by unobserved fluxes of organic nitrogen. DON measurements are stiU too sparse and too imprecise to test this hypothesis. 

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