Environmental Engineering Reference
In-Depth Information
nitrification or denitrification and the N is depleted in the heavy isotope, while
the O is enriched.
Keywords:
Arabian Sea, oxygen-deficient zone, nitrogen cycling, nitrogen isotopes, nitrous
oxide
1.
INTRODUCTION
mary
carbon fixation in vast areas of the world's oceans [14, 32, 35]. Furthermore,
through its control of marine primary production, it has been suggested
that changes in the atmospheric concentration of carbon dioxide (CO
2
) during
the last glacial-interglacial transition were linked to, or driven by, changes in the
marine nitrogen cycle [1, 10, 60]. Thus, it is important to understand nitrogen
cycling in the contemporary ocean not only because it influences present day
nitrogen availability and the amount and rate of primary production, but also
Nitrogenhaslongbeenlongrecognizedas majornutrientlimitingpri
a
to gain insights into past climate and possible future global change.
Despite nitrogen's importance, its biogeochemical cycling in the oceans is
not well understood. This is perhaps best exemplified by the current controversy
over the state of the combined nitrogen budget. The dominant input of com-
bined nitrogen to the ocean is believed to be through fixation of atmospheric
di-nitrogen (N
2
) by cyanobacteria, while the principal canonical removal term
is reconversion of combined nitrogen to N
2
by denitrification. Some investi-
gators believe the contemporary budget is more or less in balance [25, 43],
while others have proposed marine nitrogen budgets that are severely out of
balance with losses due to denitrification far exceeding inputs from nitrogen
fixation [19, 26, 62]. The recent discovery that anaerobic ammonium (NH
4
+
)
oxidation to N
2
(anammox) may be common in suboxic zones of the oceans
further complicates understanding of the marine nitrogen cycle [23, 27, 54].
Denitrification and anammox take place in suboxic environments after virtu-
ally all the oxygen has been depleted, as do several other respiratory processes
that can potentially produce N
2
(Codispoti et al. [21] define suboxic as waters
in which dissolved oxygen is vanishingly small but sulfate reduction has not
been initiated). These suboxic environments are found in certain water-column
areas and in the sediments of most shelf and upper slope environments. In
today's oceans there are three major suboxic water masses: the intermediate
waters of the eastern tropical North and South Pacific and the Arabian Sea
[19].
2.
THE ARABIAN SEA OXYGEN DEFICIENT ZONE
(ODZ)
Oxygen deficient conditions (defined as O
2
≤
2-4 µM; [21]) in the inter-
mediatewaters(
∼
200-1000m)
of
the Arabian Sea were first
noticed
during
was not
the 1933-34 John Murray Expedition
[41]; but their spatial extent
