Denitrification water-column

Table 2.1 NjO yields of water-column nitrification and water-column denitrification...

In anoxic waters, such as the deep basin of the central Baltic Sea or parts of the shallow Po River delta, N2O is consumed by water-column denitrification. 

Figure 6.5 Dissolved oxygen on the 26.5 sigma theta surface. Note the three main pelagic water column ODZs, the eastern tropical North Pacific, the eastern tropical South Pacific and the Arabian Sea. Althouth oxygen levels are very low in the Bay of Bengal, this area does not appear to be a location of water column denitrification (Rao et al., 1994).

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. 

Arabian Sea show large changes bought about by microbial consumption/produc-tion of these dissolved nitrogen species (Altabet et al, 1999 Brandes et al, 1998 Naqvi et al, 1998a,b Yoshinari et al, 1997). The most pronounced mass-dependent fractionations appear to occur through water column denitrification. Preferential... 

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). 

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.

It seems safe to assume that N2 fixation, water column denitrification and sedimentary denitrification have been the dominant input/output terms of the marine N budget throughout, at least, the past 2 Myr. With this assumption in place, one can begin to consider how potential changes would have altered the of marine nitrate, both within a given region and at the scale of the whole ocean. 

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