Environmental Engineering Reference
In-Depth Information
Nitrite (NO
2
−
), nitric oxide (NO) and nitrous oxide (N
2
O) are obligatory
intermediates that sometimes escape to the environment where they can either
accumulate and be exported or be re-assimilated and reduced. These features
are clearly reflected in vertical profiles through the ODZ (Fig. 2). Although the
NO
3
−
concentrations begin to increase below the mixed layer, they have a local
minimum near 250 m that reflects NO
3
−
removal by denitrification. More or
less coinciding with this minimum is a maximum in NO
2
−
, an intermediate of
denitrification; this feature is often referred to as the secondary nitrite maximum
(SNM). Nitrous oxide typically shows local maxima on either side of the SNM
also reflecting its nature as an intermediate. The oxygen deficient zone in
the Arabian Sea generally thickens from south to north; NO
2
−
concentrations
first increase and then decrease in the same direction with the highest values
occurring in the central Arabian Sea (Fig. 3). This high NO
2
−
concentration is
frequently interpreted as indicating increased denitrification rates [20, 63]. In
the northwestern corner of the Arabian Sea, the input of Persian Gulf water is
clearly evident as a southward penetrating tongue of slightly more oxygenated
water (
>
5 µM).
The rate of water column denitrification in the Arabian Sea is generally
believed to be globally significant. However, published estimates differ widely
(from 0.1 to 44 Tg N yr
−
1
) with a majority falling between 21 and 44 Tg
Nyr
−
11
[19, 31, 46, 59, 66, 71]. These rates are strictly for canonical deni-
trification (N
2
produced from NO
3
−
) but new information suggests that such
estimates may be too low due to inappropriate stoichiometries and under-
appreciated pathways (see below). In addition to nitrogen oxides, oxides of
Fe, Mn and I can also serve as oxidants under suboxic conditions, but the im-
portance of these electron acceptors in suboxic oxidation of organic matter in
the Arabian Sea water column is not known.
3. DENITRIFICATION RATE IN THE ARABIAN SEA
Various methods have been used to estimate denitrification rate in the Arabian
Sea including stoichiometric relationships, measurements of the enzymatic
activity of the electron transport system (ETS), and direct measurements of
the amount of excess nitrogen gas. In combination with residence times or
mass transport calculations these measurements yield areal denitrification
rates.
Stoichiometric calculations involve calculation of an “expected NO
3
−
” con-
centration (
sensu
Codispoti and Richards [15]), i.e., the NO
3
−
concentration
in a parcel of ODZ water expected before denitrification took place. Given
an expected NO
3
−
concentration and an analytical determination of the actual
inorganic nitrogen concentration (NO
3
−
+NO
2
−
+NH
4
+
), the measured can be
subtracted from the expected to give a “NO
3
−
deficit” or the amount of NO
3
−
