Environmental Engineering Reference
In-Depth Information
Figure 7.
Surface N
2
O concentrations in July-August in the Arabian Sea along the cruise
track shown in the inset. The vertically hatched area is the area of coastal upwelling, while the
horizontally hatched area is the area of open-ocean upwelling during the southwest monsoon.
The area of downwelling with a deep mixed layer is shaded gray and the arrow represents the
Finlader atmospheric jet that drives the South-West Monsoon. (redrawn from [4]).
production of N
2
O in oxygen-poor (but not suboxic/anoxic) waters may result
from enhanced yield of N
2
O during nitrification at low oxygen levels [41],
denitrification [96] or a possible coupling between the two processes [69]. As
discussed below, isotopic measurements reveal different compositions of N
2
O
within the shallower and deeper maxima, suggesting the involvement of various
formative mechanisms.
The low N
2
O concentrations within the core of the ODZ are typical of other
open-ocean ODZs [16, 18, 22]. The N
2
O minimum coincides with the SNM,
which suggests a net consumption of N
2
O by denitrifiers. This is in contrast
with the trend seen in the more variable, shallower denitrifying systems [17, 18],
including that occurring seasonally over the Indian shelf (Naqvi et al., this vol-
ume), where denitrification can sometimes lead to enormous buildups of N
2
O
in the water column. Naqvi et al. [74] attributed this to the low N
2
O reduc-
tase activity in “young”, rapidly denitrifying systems since denitrifiers usually
have to synthesize this enzyme after the development of suboxic conditions
[18].
The proximity of the shallower N
2
O maximum to the sea surface sustains
high concentrations (in excess of the saturation values) in surface waters as
mentioned earlier. The energy needed for the entrainment of N
2
O-rich subsur-
