Atlantic Ocean denitrification

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. 

The authors found strong stabtiizing feedbacks in the northern Indian Ocean and in the North Atlantic that act to minimize changes in the marine N inventory over relatively short timescales ( 30—200 years) and concluded that rates of N2 fixation are tightly linked to N P ratios in surface waters and thus denitrification, locally for the Indian Ocean and more remotely for the Atlantic Ocean. ITowever, the strength of these stabilizing feedbacks was weaker in other regions mainly due to Fe... 

In order to expand the scope of comparison with the A S ratios, we also consider several sites in the equatorial Atlantic Ocean where denitrification consumes a significant fraction of the NOs released to pore waters by oxic decomposition. In these cases, we use models describing pore-water NO and O2 profiles to estimate the O2 NOs ratio during oxic decompostion. For example, we consider a site at 3,100 m water depth on the... 

Seitzinger, S. P, Giblin, A. E. (1995). Estimating denitrification in north Atlantic continental shelf sediments. In R. W. Howarth (Ed.), Nitrogen Cycling in the North Atlantic Ocean and Its Watersheds. Kluwer Academic Publishers, 5—260. 

Fig. 6.17 Typical nitrate concentration profiles of surface sediments from different productivity regions in the South Atlantic Ocean. The profiles from the continental slope of the Argentine and the Cape Basin indicate denitrification at about 3 cm. The profile from an oligo-trophic equatorial site shows no denitrification.

Paul and Clark (11), pp. 147-158. Denitrification is also a particularly effective way of removing that fraction of fertilizer nitrogen that reaches estuary and shelf waters see Nixon, S. W., et al. 1996. The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean. Biogeochemistry 35 141-180. Dissolved nitrate in ground waters with low... 

Interpretation of the N distribution in the Pacific and Indian oceans is more difficult because of the existence of strong denitrification sites adjacent to the places where one expects to find the N2 fixation signals. In the subtropical North Pacific, where N2 fixation is well documented (Karl et al., 1997), the N maximum exists just near the surface and does not extend into the thermocline as is the case in the subtropical North Atlantic. This is likely a result of the eroding effect by waters that obtain extremely low N values in the eastern Pacific and are then advected westwards in the main thermocline (Fig. 1.12). 

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.

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). 

Figure 34-4 Water column profiles of nitrate concentration (open symbols) and c5 N itrate (filled symbols) in the Eastern Tropical North Pacific (ETNP, coastal Baja California), Southern Ocean and North Atlantic (Sargasso Sea). The ETNP shows a large increase in c5 N i ate in the thermocline owing to local water column denitrification. The Southern Ocean shows little deviation from the global deep mean c5 N it te except at the surface, where partial NO3 assimilation leaves residual nitrate enriched in N. The North Atlantic profile shows low c5 N itrate in the thermocline owing to the nitrification of locally fixed N. Note that the ETNP profile also includes deep measurements from near Hawaii (diamonds) the smooth transition between samples at distant locations emphasizes the homogeneity of the deep Pacific.

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