Environmental Engineering Reference
In-Depth Information
pected from the Redfieldian stoichiometry [19], and this could contribute to the
observed excess of N
2
over the NO
3
−
deficit.
Nitrogen fixation in the Arabian Sea could also elevate N
2
yields during
denitrification because N-fixers have N:P ratios much greater than Redfield (up
to 100; [43, 51]). Capone et al. [11] have shown that blooms of the nitrogen
fixing organism
Trichodesmium
can at times cover 20% of the Arabian Sea and
that N-fixation at times can supply all the nitrogen for new production. Further,
Brandes et al. [8] have recently estimated that 20% of the primary production
in the Arabian Sea is fueled by nitrogen fixation (see following section). Given
an N:P ratio of 60 for nitrogen fixers and a 20% contribution to new production
in the Arabian Sea, N-fixation would increase the N:P ratio of the sinking flux
to
24.8 , similar to that Karl et al. [51] observe at station ALOHA, 22.4. This
would be a 50% increase and thereby increase the N
2
excess (up to 50% if all
N
2
production were driven by this vertical flux
It appears that there is no single mechanism that could explain the discrep-
ancy between NO
3
−
anomaly and excess N
2
in the Arabian Sea ODZ; instead,
there are a number of identifiably processes that could supply the extra N
2
over that derived from the reduction of NO
3
−
. Consequently, it is likely that
the discrepancy results from some combination of these processes. In any case,
the N
2
/Ar data clearly indicate that the current estimates for water column
denitrification in the Arabian Sea may be too low with respect to those derived
from the nitrate deficit or the ETS activity (which also involved lower ratios
of N
2
production to NO
3
−
consumption). Codispoti et al. [19] scaled up the
current “best estimate” (30 Tg N yr
−
1
) of canonical denitrification by a factor
of 2, taking into account both the underestimation of nitrate deficits by earlier
workers and the likelihood of other sources of N
2
production in addition to
NO
3
−
reduction, to suggest that that the rate of water column denitrification
in the Arabian Sea may be as high as 60 Tg N yr
−
1
. It may be noted that this
estimate pertains to the perennial, open ocean suboxic zone, and it does not
include denitrification in the seasonal suboxic system over the Indian shelf.
However, the latter is an order of magnitude smaller in magnitude even though
the specific rates are much higher (Naqvi et al., this volume; [28]).
∼
4. N
2
O AND THE ODZ
Nitrous oxide is a by-product or an intermediate of both nitrification and
denitrification and each processes likely plays a role in determining the N
2
O
distributions in the Arabian Sea suboxic zone. Although N
2
O is quantitatively
of only minor significance as a combined nitrogen species in seawater, it is
nevertheless an important gaseous constituent because of its high greenhouse
efficiency - on a per molecule basis it is 200-300 times as effective as CO
2
in
trapping infrared radiation [58]. A typical profile of N
2
O in the suboxic zone of
