Nitrogen in the Atlantic Ocean

Atmospheric deposition and possible contribution to excess nitrate 

The location of the North Atlantic (between Europe and North America) has made it the focus of considerable study some of the most important advances in marine nitrogen dynamics have been developed in the region (Lipschultz et al, 2002). The South Atlantic, in contrast, has undergone expeditionary surveys, but fewer process studies have been specific to that basin. 

This feature suggests that the Atlantic Ocean may be a source of fixed nitrogen to the global ocean, thus balancing the nitrogen removal due to denitrification occurring elsewhere. 

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Only occasionally has the N content of solid phase extracts been reported. At a site in the Atlantic Ocean the carbon to nitrogen ratio (C N) of XAD 8 and XAD 2 extracts fell in the range of 40-57 (57 0.9 and 41.1 3.3, respectively DrufFel et ai, 1992). In contrast, at the same site XAD 4, when used as the second resin in series with XAD 8 or XAD 2, extracted compounds with lower C N ratios - 19—24 (21.0 2.4). These values are only slighdy higher than ratios reported for total DOM (see below). McKnight and Aiken (1998) reported a C N value of 37 for DOM extracted by XAD 8 at one site in the Pacific Ocean at other sites in the N. Pacific Ocean XAD 2 was found to extract DOM with a C N ratio between 32 and 46.5 (Druffel et al, 1992 Meyers-Schulte and Hedges, 1986). Bronk (2002, Table III) compiled various literature values and arrived at an average C N ratio of 32.8 19.5 for total humic substances isolated from a variety of aqueous environments (see McCarthy and Bronk, this volume). 

Table 3.1 Average carbon (6 3C) and nitrogen (6 5 ) isotope signature of HMWDOM isolated from several different sites in the Atlantic Ocean. Pacific Ocean, and the Gulf of Mexico...

MiUward, N. J., Rees, A. P., Joint, I., Law, C. S., Owens, N. J. P., Tyrrell, T., and UpstiU-Goddard, R. (2005). Nitrogen fixation in the Atlantic Ocean. Meeting of the American Society of Limnology and Oceanography. Santiago de Compostela, Spain, 19-24 June 2005. 

Mahaffey, C., Williams, R. G., Wolff, G. A., and Anderson, W. T. (2004). Physical supply of nitrogen to phytoplankton in the Atlantic Ocean. Global Biogeochem. Cycles 18, GB1034, doi 10.1029/ 2003GB002129. 

Coles, V. J., and Hood, R. R. (2007). Modeling the impact of iron and phosphorus limitations on nitrogen fixation in the Atlantic Ocean. Biogeochemistry 4, 455—479. 

Figure 15.7. Stoichiometric correlations among nitrate, phosphate, oxygen, sulfide, and carbon. The correlations can be explained by the stoichiometry of reactions such as equation 3 concentrations are in micromolar, (a) Correlation between nitrate nitrogen and phosphate phosphoms corrected for salt error in waters of the western Atlantic, (b) Correlation between nitrate nitrogen and apparent oxygen utilization in same samples. The points falling off the line are for data from samples above 1000 m (Redfield, 1934, p. 177). (c) Correlation between nitrate nitrogen and carbonate carbon in waters of the western Atlantic, (d) Relation of sulfide sulfur and total carbonate carbon in waters of the Black Sea. Numbers indicate depth of samples. Slope of line corresponds to AS /AC = 0.36. (From data of Skopintsev et al., 1958, as quoted in Redfield et al., 1966.) (e) Correlation of the concentration of nitrogen to phosphate in the Atlantic Ocean (GEOSECS data). The slope through all the data yields an N/P ratio close to 16.

All the GEOSECS data on the correlation of the concentration of nitrogen to phosphate in the Atlantic Ocean are given in Figure 15.7e. 

Gorzelska, K., and J. N. Galloway, Amine Nitrogen in the Atmospheric Environment over the North Atlantic Ocean, Global Biogeochem. Cycl., 4, 309-333 (1990). 

Mino, Y., Saino, T., Suzuki, K., and Maranon, E. (2002). Isotopic composition of suspended particulate nitrogen in surface waters of the Atlantic Ocean from SON to SOS. Global Biogeochemical Cycles doi 10.1029/2001GB001635. 

Semeneh, M., Dehairs, F., Elskens, M., Baumann, M. E. M., Kopczynska, E. E., Lancelot, C., and Goeyens, L. (1998). Nitrogen uptake regime and phytoplankton community structure in the Atlantic and Indian sectors of the Southern Ocean. J. Mar. Syst. 17, 159-177. 

Varela, M. M., Bode, A., Moran, X. A. G., and Valencia, J. (2006). Deissolved organic nitrogen releae and bacterial activity in the upper layers of the Atlantic Ocean. Microh. Ecol. 51(7), 487—500. 

The availability of nitrogen in the euphotic zone is an important, potentially limiting factor for productivity and the biological sequestration of carbon in the ocean. There are three principal routes by which new nitrogen makes its way into the euphotic zone of the Atlantic Ocean Physical transport of nitrate, nitrogen fixation by diazotrophic organisms, and aeolian transport and deposition. 

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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