Environmental Engineering Reference
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the Arabian Sea is shown in Fig. 6. Surface waters are generally supersaturated
[4, 29, 56, 69, 80, 93]. Below the mixed layer, concentrations increase to
about 10 times the saturation values forming a local maximum. This feature
usually coincides with the lower boundary of the oxycline [69]. Within the
core of the ODZ, N 2 O concentrations decrease to very low values, but at the
lower transition from suboxic back to oxic conditions the concentrations again
increase to highly supersaturated values. Below this deeper maximum, N 2 O
concentrations fall steadily with depth but remain well above saturation at least
down to 2500 m [69, 72, 73].
Figure 6.
Vertical profiles of N O (open circles, dotted line),
(line only
NO
2 (solid circles, solid
line)
2
and
O 2
) .
The ODZ depth interval is shown by crosshatching. Composite redrawn
from figures in
[72] .
In the oxygenated waters N 2 O concentrations are positively correlated with
the apparent oxygen utilization (AOU), albeit with separate relationships in
the upper and deep waters [69]. The linear relationships between N 2 O and
AOU observed in various oceanic areas have been interpreted to reflect N 2 O
production through nitrification [34, 78, 97]. However, as the oxygen concen-
trations approach suboxia at the boundaries of the ODZ, N 2 O accumulates to
levels much above those predicted by the N 2 O-AOU regression line. Enhanced
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