Deep ocean water, nitrification

Similarly, N02 rarely accumulates in oxygenated habitats (see below for exceptions), although N02 is an essential intermediate in several oxidation and reduction processes in the N cycle. Nitrate, the end product of nitrification, however, accumulates in the deep ocean, and seasonally in the deep water of lakes, where there is no demand for inorganic N by phytoplankton. 

The major N product of organic matter decomposition in seawater is NH4+, but NH4+ is present at trace or undetectable levels in the huge volume of the deep ocean. Rather, deep water contains NOs" at 20-40 pM concentrations, which would seem to imply that nitrification occurs mainly in the deep ocean. Nitrate concentrations in the surface ocean are usually maintained at low levels because phytoplankton assimilate N03 more rapidly than it can be suppHed by mixing or diffusion from the deep NOs" reservoir. Ammonium, which is produced in the photic zone by heterotrophic processes, is also usually immediately assimilated by phytoplankton and heterotrophic bacteria before it can be nitrified. The important physical and biological differences in the source functions of NH4+ and NOs" are... 

Rising CO2 in the atmosphere or purposeful deep ocean CO2 disposal may lead to decreases in oceanic pH. pH was not even considered as a factor that influences nitrification rate in the hst of potential factors mentioned earher, because ocean pH is rehably invariant over most of the ocean in today s world. But changes of 1—2 pH units in response to CO2 increases could inhibit nitrification rates by up to 90% (Huesemann et ah, 2002). Long term inhibition of nitrification may lead to the accumulation of NH4+ in the oxic waters, decreases in N03 available for denitrification, and changes in phytoplankton community composition as a result of nutrient preferences and competition. 

NH4+, N02 and DON are also characterized as nutrients, despite the fact that none of them has a nutrient-hke profile in the open sea (e.g., DON Fig. 16.6). The most important reason for this is that NH4+, N02 and selected compounds in the DON pool have much shorter residence times in the water column, in part, due to the fact that they are either partially or fully reduced and contain additional potential metabolic energy. Essentially aU of the fixed or reactive N that is exported from the surface ocean to greater depths, whether in dissolved or particulate form, is in the —3 valence state (R-NH3 or NH4 ), whereas essentially all of the fixed N in deep water is in the +5 valence state (NOa ). This indicates an important role for deep water nitrification (org-N/NH4 —> NOs ) as a key metabolic process even though we have not yet identified the site(s) where active nitrification occurs or characterized the diversity of deep water nitrifiers. 

Nitrogen exists in the ocean at oxidation states from -3 to +5. There are three forms of fixed inorganic N NOs", N02 and NH4+. Nitrate is the final oxidation product and is the dominant form of fixed N in the deep ocean. Nitrite generally occurs at very low concentrations because it is an intermediate in the processes of nitrification (NH4+ —> N02 —> NOs") and denitrification (N03 —> N02 —> NO —> N2O —> N2) and so seldom accumulates to a large degree. Concentrations of NH4+ are highly variable but tend to be near the limit of detection in open ocean surface waters. Each of the inorganic forms has a number of manual and automated methods of analysis. We discuss the most widely used below. 

In the anoxic zone of the Sea ammonia increases in a similar way as hydrogen sulfide (Fig. 4) and reaches maximum concentrations of about 90-100 pM in the bottom mixed layer. The losses of combined nitrogen due denitrification and the absence of nitrification in deep waters has transformed the Black Sea into an ammonium basin, where about 98% of total inorganic nitrogen stock is composed of NH4, while in the ocean 98-99% of it belongs to NO3 [23]. 

Because NH4+ contains N at the oxidation level of proteins, it is readily assimilated by both phytoplankton and bacteria, and is a preferred N source. Ammonia oxidizers may be in competition for NH4+ with other planktonic organisms. The different physiological requirements of phytoplankton and nitrifiers probably play a role in determining exacdy where in the water column NH4+ assimilation and NH4+ oxidation occur. As explained below, most nitrification occurs within or near the base of the euphotic zone in the upper 100 or so meters of the ocean. However, there is usually very little N03 in the surface ocean, due to utilization by phytoplankton, except in high nutrient low chlorophyll regions and when supplied by episodic events such as regional upweUing. The N03 in the deep water of the oceans has accumulated from nitrification because phytoplankton assimilation is essentially zero below the euphotic zone. It is because of nitrifiers... 

Bianchi, M., Fehatra, Treguer, P., Vincendeau, M. A., and Morvan, J. (1997). Nitrification rates, ammonium and nitrate distribution in upper layers of the water column and in sediments of the Indian sector of the Southern Ocean. Deep-Sea Research 44, 1017—1032. 

North Pacific and asserted that nitrification in aerobic deep waters is a minor source of oceanic N2O. As will be shown in Section 4.1, isotopic measurements reveal different compositions of N2O at the shallower and deeper maxima, suggesting the involvement of various formative mechanisms. 

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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