Nitrate deficits

The first quantitative estimates of denitrification in the eastern tropical North Pacific were made by Codispoti and Richards (1976). Codispoti and Richards used apparent oxygen utilization, AOU, and phosphate data to stoichiometricaUy extrapolate back to the nitrate concentration present when a given water mass was previously at the surface. In this way they were able to develop a relationship between pNOj and sigma-f for waters of the ETNP-ODZ, from which they calculated nitrate deficit as outlined in Eq. (6.4). 

A second method used to calculate nitrate deficit takes advantage of the semiconservative water-mass tracer NO (Broecker, 1974). StoichiometricaUy, every mole of O2 respired wiU remineralize approximately 1/9 of a mole of NOJ so that NO, defined as... 

Again, one can then calculate the nitrate deficit from Eq. (6.4). This method has been used primarily to estimate denitrification rates in the Arabian Sea (HoweU et ah, 1997 Mantoura et ah, 1993 Naqvi, 1987). 

Figure 6.9 Nitrate deficit in the ODZ of the Arabian Sea. Soiid cricles are Codispoti et al. (2001) modified N calculation, while the open circles are calculated from sigma theta relationships of Naqvi and Sen Gupta (1985). Shaded zone approximates the depth zone of the ODZ. (Redrawn from Devol etal, (2007).

Figure 6.10 Excess nitrogen gas in the Arabian Sea. Excess Nj gas values are presented in units of pg-at 1 so there is a 1 1 stoichiometry with nitrate anomaly. Compare the maximum values presented here, 25 pg-at with the values of about 13 pg-at derived from nitrate deficit calculations shown in Fig. 6.9. (Redrawn from Devol etal., 2007).

However, given a Redfield stoichiometry for organic matter decomposition this could only be a transient situation because with the estimated nitrate deficit the nitrite concentration would need to be much greater than observed. In contrast to the ETSP, NichoUs et al. (2007) found fittle evidence for anammox in the Arabian Sea ODZ based on conversion of to N2. They also concluded that... 

The isotopic fractionation is easily seen in 8 N03 and 8 N2 distributions in the major open-ocean denitrification zones (Altabet et al., 1999 Brandes et al., 1998 Cline and Kaplan, 1975). Typical open ocean values ofsub-euphotic zone nitrate are about 5%o (Lehmann et al., 2005 Sigman et al., 2000 Wu et al., 1997) but within the ODZ they climb to upwards of 15%o. Concomitant with this increase is a decrease in the 8 N2 from about 0.6%o to 0.2%o (Fig. 6.15). The large enrichment of N-N03 and the mirror image decrease in N-N2 is undoubtedly due to fractionation during denitrification. It is also possible to derive a fraction factor, , from the isotope distributions in the ODZ if one makes some assumption about the amount of nitrate that has been removed by denitrification, i.e., the nitrate deficit. For the eastern tropical North Pacific Brandes et al. (1998) assumed a Raleigh fraction mechanism and both open (advection-reaction) and closed (diffusion-reaction) systems to derive fractionation factors from the data, in Fig. 6.15. (Raleigh fractionation 8 N03 = where 8 N03 is the isotopic composition... 

Sigman et al. (2003), and Voss et al. (2001) have similarly used observed isotopic values and nitrate deficits to estimate from ODZs (Fig. 6.16) and all of these estimates cluster around an of 25%o. 

Figure 6.16 Solutions to a horizontal ventilation model of denitrification for waters of the eastern tropical North Pacific for d NOa vs J, the fraction of nitrate remaining, i.e., the measured nitrate minus the nitrate deficit. Nitrate deficit was calculated as NOa deficit) = M.8 x PO4—NO3, where NO3 and PO4 are the measured concentrations.The two equations describing the steady-state nitrate isotope distrihution are 0[ NO3]/0t—J[ N03] + 0 [ NO3]/0a and0[ NO3]/0t—aQ/[ N03] + ri0 pNO3]/0x where is the eddy diffusion coefficient in the x direction, J is the denitrification rate, a is the fraction factor, and Q is a N N ratio that makes the system non-linear. (See Voss et at, 2001 for solution details). Solutions for three different values of are given ( = [l-0(] x 1000).

Nearly all quantitative estimates of open-ocean water column denitrification are based on some type of nitrate deficit calculation. In the Arabian Sea, Naqvi (1987) have used nutrient-potential temperature relationships to estimate a denitrification rate of 30 Tg/y. In a later study Naqvi and Shailaja (1993) arrived at a... 

Denitrification rates in the other two main ODZs, the ETNP and ETSP appear to be about 25 Tg a each Codispoti et al. (2002). In the ETNP denitrification was estimated by the geostrophic flux of nitrate deficit out of the ODZ area, with nitrate deficit determined from nutrient-density relationships (Codispoti, 1973 Codispoti and Richards, 1976). Volumetric estimates of denitrification rate based on ETS activity are in concert with this rate (Codispoti and Richards, 1973 Devol, 1975). The ETSP the denitrification rate is based on measurements of ETS activity (Codispoti and Packard, 1980). As with the Arabian Sea these estimates are again for canonical denitrification. It is likely that processes identified in the Arabian Sea are also occurring in the ETNP and ETSP. Thus, the rate of 25 Tg a might be raised to 50 Tg ar. Given a denitrification rate between 30 and 50 Tg a for each of the major ODZ s, global water-column denitrification would appear to between 90 and 150 Tg a. ... 

Lehmann, M. F., Sigman, D. M., McCorkle, D. C., BruneUe, B. G., Hoffmann, S., Kienast, M., Cane, G., and Clement, J. (2005). Origin of the deep Bering Sea nitrate deficit Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate flrtxes. 

Sea, the estimate was 0.3molNirf2y-1 (Simpson Rippeth, 1998). These estimates are in line with newer field measurements of denitrification (Lohse etal., 1996 Trimmer etal., 1999). Hydes etal. (1999) used the NERC-NSP data from January 1989 to show that the concentration of P predicted, from a concentration-salinity relationship, at the salinity of source waters at the shelf break, matched the observed concentration. However, the predicted concentration of nitrate was significantly lower than at the shelf break. The nitrate deficit in the southern North Sea was equivalent to a denitrification rate of 0.25 mol N m-2 y-1, assuming a flushing time of one year for the North Sea. 

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