Water-column denitrification rates

Various methods have been used to estimate water-column denitrification rates including stoichiometric relationships, measurements of the enzymatic activity of the electron transport system (ETS), and N03 incubation experiments. In combination with residence times or mass transport calculations these measurements yield areal denitrification rates. 

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. ... 

There is also water column denitrification in Black Sea, Cariaco Trench and smaller anoxic basins and fjords, as well as within the more transient hypoxic zones over the continental shelves. The recent work by Kuypers et al. (2005) indicated that the denitrification rate off south west Africa is about 1.5 Tg a . Codispoti et al., 2002 have estimated the total denitrification in these smaller/more transient sites to be 10 Tg a. Finally, denitrification has been reported to take place in microenvironments within the oxygenated water column such as organic aggregates Aldredge and Cohen, 1987 Wolgast et al, 1998). However no quantitative estimates of the global importance of microsite denitrification are available. 

Since nearly aU subsurface respiration on the global scale is supported by oxygen, there is no clear a priori reason that water column denitrification should have been similarly important at aU times in the past increases in the rate of oxygen supply relative to consumption could have nearly eliminated it. However, the strong sensitivities of water column denitrification, while rendering this process susceptible to climate-driven variability, probably also make it more responsive to stabilizing negative feedbacks (see Section 2.2.3). 

Because the isotope fractionating processes are not evenly distributed throughout the ocean, temporal variations at fixed points will produce predictable changes in the (5 N of local nitrate pools. As examples (1) an increase in denitrification rates within the water column on the Oman margin would increase the <5 Nni ate of the Arabian Sea thermochne (Altabet et al., 1995 Schafer and Ittekkot, 1993), (2) an increase in N2-fixation in surface waters of the eastern Mediterranean would cause a decrease in d Nnitrate io the eastern Mediterranean thermochne (Stmck et al., 2001), and (3) an increase in relative nitrate consumption in Antarctic surface waters would cause an increase in local near-surface d N itrate (Francois et al., 1997). However, it must be kept in mind that these local signals are imprinted upon any variation in the baseline of mean ocean nitrate. A similar principle operates at finer scales, with... 

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