Environmental Engineering Reference
In-Depth Information
maximum predicted deficit is about 30% larger (12 vs 9 µM) and remains
larger and extends about 1000 m deeper than the deficits calculated from
NO
-θ
relationships. The observation that the denitrification signal penetrates well
below the ODZ, to almost 2000 m is supported by isotopic data (See section
5).
Estimation of denitrification rate from NO
3
−
deficit requires knowledge of
the time over which the deficit has accumulated. Typically, this is taken as
the residence time of intermediate waters within the ODZ. Mantoura et al.
[59] vertically integrated NO
3
−
deficits calculated from
NO
-θ relationships,
extrapolated them to the area of the denitrification zone, and then combined
them with a CFC derived residence time of 10 years to arrive at a denitrification
rate of 11.9 Tg N yr
−
1
for the entire ODZ volume. Besides the uncertainty
involved in the nitrate deficit calculation, this estimate also compounds the
uncertainties in the area of the denitrification zone and the residence time. In an
effort to circumvent these uncertainties, Naqvi [66] took a different approach
and determined NO
3
−
deficit on an east-west section across the Arabian Sea.
He calculated the diffusive and advective losses of NO
3
−
deficits from the ODZ,
the latter from geostrophic transport of intermediate water across this section.
The rate so obtained was 29.5 Tg N yr
−
1
, approximately three times as great
as Mantoura et al.'s estimate. Howell et al. [46] combined data on NO
3
−
deficit
and CFC ages to compute a rate of 21 Tg N yr
−
1
.
Another technique that has been used to quantify denitrification in the Ara-
bian Sea is the measurement of ETS activity [71]. This method utilizes crude
enzyme extracts of material filtered from ODZ waters to which electron donors
(NADH and NADPH) are added and the rate of electron passage to an artifi-
cial electron acceptor is measured. This technique measures only to canonical
denitrification. Using this technique, Naqvi and Shailaja [71] determined the
denitrification rate in the Arabian Sea to be 24-33 Tg N yr
−
1
; Although this
estimate is independent of any residence time calculation, it is still dependent
on the estimate of the area of the denitrification zone (when adjusted to an
area of 1.5 x 10
12
m
2
, this range expands to 31.9-43.9 Tg N yr
−
1
; [28]). More-
over, it also involves assumptions concerning the conversion of ETS activity to
denitrification rate.
A final way investigators have attempted to quantify denitrification in the
Arabian Sea is to measure the end product of the process, N
2
. Devol et al. [28]
report measurements of nitrogen gas in waters from the Indian Ocean, espe-
cially the Arabian Sea, made by isotope ratio mass spectrometry. Results are
presented as normalized nitrogen:argon ratios, (N
2
:Ar)
n
=(N
2
:Ar)
s
/(N
2
:Ar)
e
where (N
2
:Ar)
s
is the sample ratio and (N
2
:Ar)
e
is the atmospheric equilib-
rium ratio, such that at equilibrium with the atmosphere (N
2
:Ar)
n
=1.00. Once
subducted below the euphotic zone the noble gas Ar should maintain its equi-
librium saturation but biological processes, such as denitrification (and possibly
