Secondary nitrite maximum

Depth profiles from the eastern tropical North Pacific (Figure 24.8) show the effects of nitrogen metabolism under 02-deficient conditions. The thermocline is characterized by a sharp decline in O2 concentrations that coincides with increasing nitrate and phosphate concentrations. The oxycline is produced by the respiration of sinking POM under vertically stagnant conditions. Below the oxycline, in depths where O2 concentrations are suboxic, phosphate concentrations continue to increase, but at a slower rate. In contrast, nitrate concentrations decline and reach a mid-water minimum that coincides with a nitrite maximum. The latter is referred to as the secondary nitrite maximum. (At this site the primary nitrite maximum is located at 50 m.)... 

At this site in the eastern tropical North Pacific, denitrification is responsible fiar the midwater loss of nitrate and production of nitrite. The size of the secondary nitrite maximum is dependent on the relative rates of its production from NO3 and its loss via dissimilatory reduction to N2. The amount of nitrate lost to denitrification is shown as the difference between the measured nitrate and the calculated nitrate. The latter was estimated by multiplying the observed phosphate concentrations by the average nitrate-to-phosphate ratio in the three deepest samples (11.9 1.6pmolN/L). Note that the zone of denitrification is restricted to mid-depths, i.e., the depths of the OMZ at this site. 

In suboxic waters, a secondary ammonium maximum can also be present. It typically lies just above the secondary nitrite maximum. This secondary maximum is supported by high rates of ammonification. Because the waters are suboxic, nitrification rates are slow permitting the buildup of ammonium. 

Secondary nitrite maximum Subsurface concentration maximum in nitrite found in the oxygen minimum zone. Mainly caused by denitrification. 

Figure 14.3 Vertical sections of (B) temperature, (C) salinity, (D) O2, (E) N02 and (F) NOs", extending from Oman to India, constructed using data collected by the Research Vessels Sagar Kanya andThomas G. Thompson during June-September, 1995. Locations of stations comprising the section are shown in (A) with reference to the secondary nitrite maximum zone (after Naqvi, 1991) and the zone of minimum (Winkler) O2 at 300 m as demarcated by the 0.25 ml L contour (dotted curve) (Wyrtki,1971).

Garfield, P. C., Packard, T. T., Friederich, G. E., and Codispoti, L. A. (1983). A subsurface particle maximum layer and enhanced microbial activity in the secondary nitrite maximum of the northeastern tropical Pacific Ocean. J. Mar. Res. 41, lAl-ld. 

Structure and dynamics of the primary and secondary nitrite maximum layers... 

Nitrite-N (N02 ) has an intermediate redox position between that of NH4 and N03, thus N02 often accumulates in selected depth strata when active N transformations are occurring (Rakestraw, 1936 Vaccaro and Ryther, 1960). In the global ocean, two such zones have been identified and studied (Fig. 16.11) (1) the primary N02 maximum (PNM) zone that is usually located near the base of the euphotic zone worldwide and (2) the secondary N02 maximum (SNM) zone that is most prominent in oxygen depleted waters (Codispoti and Richards, 1976 Fiadeiro and Strickland, 1968). 

Dinitrogen trioxide reacts with the unshared pair of electrons on unprotonated secondary amine by a nucleophilic substitution reaction to form nitrosamines. The rate of nitrosation of secondary amines in a weakly acidic aqueous solution is proportional to the concentration of the amines and to the square of the nitrite concentration. The concentrations of these two precursors depend on the pH of the medium. While the concentration of unprotonated amines increases when pH increases, the concentration of nitrous acid increases when the pH decreases. Hence, the pH rate profile for the nitrosation of amines shows a maximum resulting from the interaction between these two opposite... 

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