Environmental Engineering Reference
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km 2 to arrive at a total N 2 O efflux of 0.05-0.38 Tg N 2 O from the study region.
This is roughly of the same magnitude as the most recent estimate of N 2 O
efflux (0.33-0.70 Tg N 2 Oy 1 ) from the entire Arabian Sea [1].
The available data from the Indian coastal waters do not go back far enough
in time to evaluate the extent to which N 2 O concentrations might have been
enhanced as a result of intensification of coastal O 2 deficiency. However, in
view of the well known non-linear response of N 2 O production to changes
in ambient O 2 levels in the low range [12], we believe that the effect may
be substantial. And, should this also apply to the other coastal hypoxic sites,
then the human activities may be bringing about a significant increase in the
emissions of N 2 O from the ocean [38].
Stoichiometric Relationship. Degradation of marine organic matter with
various oxidants is believed to follow approximately constant stoichiometries
[46]. The data set from the Arabian Sea, representing all the three types of
redox environments [oxic/hypoxic, suboxic (denitrifying) and anoxic (SO 4 2
reducing)], provides a rare opportunity to test the constancy of these ratios in
the same region. For this purpose, data from 294 stations located over the shelf
(depth
200 m) were pooled and the DIN (NO 3 +NO 2 +NH 4 + ) was plotted
against DIP (dissolved inorganic phosphate, PO 4 3 + HPO 4 2 +H 2 PO 4 ). The
diagram (Fig. 11) may be visualized in terms of the spatio-temporal evolution of
anoxia in a parcel of initially-oxygenated water as it upwells over the continental
shelf and undergoes sequential changes in redox conditions. First, the decay
of organic matter in the presence of O 2 increases the concentrations of both
DIN and DIP in general accordance with the Redfield stoichiometry (with a
P of 13.74, represented by Line I in Fig. 11) 2 until the O 2 concentration
falls below the threshold for denitrification to set in. Once this threshold is
crossed (at this point DIN concentration is on an average
N:
24 µM), NO 3 is
used by the bacteria, and the concentration of DIN decreases rapidly, but that
of DIP increases slowly. The computed
P for denitrifying water (-79.1,
represented by Line II in Fig. 11) is slightly different from Richards' [46] value
(-94.4) for the case when NH 3 is oxidized by NO 3 but it is close to the value
(-84.8) for the case when such an oxidation does not take place. When the
system turns anoxic (after the complete loss of NO 3 that seems to take place
when DIP reaches an average concentration of 2.15 µM), the relative changes
in DIN and DIP exhibit complete departure from the theoretical value. That
is, as NH 3 released from organic matter cannot be oxidized by SO 4 2 , one
would expect the DIN and DIP values to fall along Line III in Fig. 11. Such is
not the case, though; instead, all data points are located to the left and above
N:
2 The slightly lower N:P remineralization ratio than the Redfield value (16) may reflect mixing between the
oxic/hypoxic and suboxic/anoxic waters
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