Nitrogen in the Atlantic

The meridional distribution of nitrate in the Atlantic generally follows the major water masses, with a clear and dominant signature of North Adantic Deep Water (NADW) formed from northern high latitude, nutrient depleted waters (Fig. 13.2A). 

Nitrate-rich signatures of Antarctic Bottom Water (AABW) and Antarctic Intermediate Water (AAIW) extend northward, reflecting the high surface concentrations where these water masses are formed. These water masses are also evident in the zonal structure of the North and South Atlantic, and nitrate-rich ( 30 iM) AAIW is evident in the South Atlantic (Fig. 13.3) at mid-depth, along with a hint of AABW with elevated nitrate concentrations in the deep western part of the basin. There is 

In the South Atlantic, with similarly low surface nitrate concentrations, DON is consistently 5 pmol kg, or 2 pmol kg lower than the North Atlantic. In fact, the western Sargasso Sea (near Bermuda) also has relatively low surface DON values (Hansell and Carlson, 2001), similar to values found in the South Adantic. The relatively elevated DON concentrations in the low latitude North Adantic are suggestive of unique inputs there (such as atmospheric deposition, N2 fixation, or upweUed nitrate with horizontal transport) compared to the South and western North Atlantic. 

DON concentrations are also elevated in the Adantic coastal and shelf regions (Bode et al., 2001 Doval et al., 1997) where freshwater inputs are significant (Bates and Hansell, 1999 Lopez-Veneroni and Ciflxentes, 1994). This material may be transported off-shelf and photo-oxidized, releasing ammonium to the phytoplankton community (MoreU and Corredor, 2001). 

BATS— Bermuda Atlantic Time-series Study  

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Lee, C., and C. Cronin. 1982. The vertical flux of particulate organic nitrogen in the sea decomposition of amino acids in the Peru upweUing area of the equatorial Atlantic. Journal of Marine Research 40 227-251. 

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

Figure 3.6 Wet versus dry deposition of oxidized nitrogen in the larger North Atlantic...

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. 

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. 

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. 

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. 

Nitrate is the largest pool of combined nitrogen in the ocean, with deep water concentrations around 20 to 30 pmol L in the Atlantic and up to 45 pmol in the Pacific. The isotopic composition of the NOs" pool is affected by a variety of processes that move N in and out of the ocean or its biota (Fig. 29.3), and subsurface N03 acts as a critical isotopic end member for biological production in the upper water column. Of the processes shown in Fig. 29.3, pelagic denitrification and N2-fixation are generally viewed as the major, long-term controls on the size and isotopic composition of the oceanic pool of NOs" (Brandes and Devol, 2002). 

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. 

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