Near-surface ocean nitrification

Near-surface ocean nitrification at rates that are Hkely to exceed the upward eddy-diffusion of NOa from beneath the euphotic zone (e.g., Martin and Pondaven, 2006) has important implications for both the conceptual model of new vs. regenerated production (Dugdale and Goering, 1967) and for the... 

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... 

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. 

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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