Environmental Engineering Reference
In-Depth Information
others) can change N
2
saturation state. Devol et al. [28] present four profiles
from the Indian Ocean south of the Arabian Sea oxygen deficient zone (Fig.
5), which are nearly identical to profile from other oxygenated waters of the
Atlantic, Pacific and Southern Ocean ( [44]; Uhlenhopp et al., unpublished).
Although the N
2
:Ar ratio increases slightly with depth, this increases is due
to the effects of bubble injection during water mass formation rather than any
biological alteration [44]. In contrast, the profile of (N
2
:Ar)
n
in the waters of
the ODZ contains a distinct maximum indicative of the excess of nitrogen gas
resulting from denitrification (Fig. 5). To quantify the excess, “background”
values obtained by averaging the four profiles outside the suboxic zone were
subtracted from the ODZ values. The resulting fractional N
2
excesses were
multiplied by the in situ equilibrium N
2
saturations to give the actual concen-
trations of excess N
2
(Fig 5; [28]). Excess N
2
values were zero at the top of
the ODZ, reached a maximum of about 22 µg-at/l at 250 m and then decrease
more or less exponentially to background values around 2000 m (Fig. 5). Inter-
estingly, the shape of the excess N
2
profile is similar to the shape of the NO
3
−
deficit profile determined by Codispoti et al. [19], but the absolute amount of
excess N
2
is about twice as much as the NO
3
−
deficit. The position of both the
excess N
2
and NO
3
−
deficit maxima near the top of the suboxic zone is likely
due to the fact that the rain rate of organic matter is greater there.
Reasons for the large discrepancy between the N
2
excess values and the
NO
3
−
deficits are unclear but the discrepancy suggests sources of combined ni-
trogen for N
2
production in addition to just NO
3
−
. It also suggests NO
3
−
deficits
underestimate the true denitrification rate, i.e. the conversion of combined N to
N
2
gas. Because NH
4
+
concentrations in the ODZ are uniformly low, part of the
difference is likely due to oxidation of the NH
4
+
liberated from organic matter
decomposition during denitrification to N
2
, possibly by anammox bacteria [48].
However, given Redfield organic matter and the denitrification stoichiometry
of Froelich et al. [38], NH
4
+
oxidation could only account for a 15% increase
over the nitrate deficit. Even the protein rich stoichiometry for denitrification
proposed by Van Mooy et al. [91] would increase the N
2
production by only
about 27%. Although, nitrogen gas production coupled to manganese (and io-
dine) cycling has been suggested for sedimentary environments [36, 57], these
oxidations are likely to be only minor contributors in the open Arabian Sea
(Codispoti et al. [19]). A potential source of excess N
2
is from denitrification or
anammox in the sediments in contact with the ODZ. The Arabian Sea is a semi-
enclosed basin with continental margin on three sides and radium distributions
show that sedimentary signals penetrate into the interior waters [89]. Given a
sedimentary denitrification rate of the same order as that off the West Coast of
the US (
1.5 mM m
−
2
d
−
1
4.5x10
11
m
2
[28], potential sedimentary denitrification could be 3.9 Tg N
∼
; [45]) and a sediment area in contact with the ODZ of
yr
−
1
.
