Environmental Engineering Reference
In-Depth Information
Nitrous Oxide Cycling and Fluxes.
Perhaps the most important aspect of
nitrogen cycling in the eastern Arabian Sea is the unprecedented accumulation
of N
2
O frequently observed during the late SWM - early FI period over the
inner- and mid- shelf regions north of 12
◦
N. The highest concentration (765
nM) recorded in the region is about four times the highest values reported from
the eastern tropical South Pacific [9, 12]. As noted previously (in Section 3),
the greatest accumulation of N
2
O occurs in denitrifying waters associated with
the buildup of NO
2
−
the concentration of which can reach up to 16 µM. This is
in sharp contrast to N
2
O distribution in the perennial suboxic zone of the open
ocean where N
2
O profiles invariably exhibit a minimum associated with the
NO
2
−
maximum [16, 40]. However, the denitrifying waters sometimes contain
N
2
O in very low concentration as well, presumably due to its reduction to N
2
(Fig. 4). These observations strongly point to transient production of N
2
O from
NO
3
−
through a reductive pathway.
For denitrification to be a net producer of N
2
O, it is imperative that the
activity of N
2
O reductase be suppressed. There are two characteristics of this
enzyme that might allow this to happen. First, N
2
O reductase contains copper,
and the non-availability of this element in a suitable form or quantity could
lead to the denitrification sequence being terminated at N
2
O. Such an effect
has been demonstrated by culture experiments carried out under trace-metal
clean conditions [23]. However, it is hard to conceive copper limitation in a
shallow coastal environment even though the possibility of the metal being
non-bioavailable due to speciation cannot be completely ruled out. Secondly,
N
2
O reductase is not readily available with denitrifiers and is synthesized as and
when needed [21]. This enzyme also appears to be more sensitive to O
2
than
the other denitrification enzymes [6]. Experimental evidence for the sensitivity
of N
2
O production to O
2
levels in a natural environment has been provided by
Castro Gonzalez and Farıas [9]. It has been speculated that frequent incursions
of O
2
into suboxic waters might deactivate N
2
O reductase and its recovery
would take some time after the reestablishment of suboxic conditions. This
would allow N
2
O build up in the water column. This hypothesis appears to be
supported by results of incubation experiments [38].
Even when a net consumption of N
2
O occurs in near-bottom waters, surface
concentrations (5-436 nM, mean 37.3 nM,
n
= 241) during the upwelling period
are almost always far in excess of the corresponding saturation values. Using
the individual data and employing two different models of air-sea gas exchange
[30, 56], N
2
O flux to the atmosphere has been computed to range from -1.2
µmol m
−
2
d
−
1
(the only negative value indicating absorption of atmospheric
N
2
O by the ocean) to 3243.2 µmol m
−
2
d
−
1
at wind speeds ranging from 5 to
10 m s
−
1
. The average flux varied from 39.1 to 263.8 µmol m
−
2
d
−
1
. Since high
surface N
2
O levels prevail even during the early phase of upwelling [42], this
flux has been extrapolated over a period of six months and an area of 180,000
