Environmental Engineering Reference
In-Depth Information
extensive blooms of
Trichodesmium
occur on a regular basis in the eastern Ara-
bian Sea following the cessation of upwelling (during the NEM and SI periods).
Such blooms add new nitrogen to the system, partially compensating for the
loss through denitrification and contributing significantly to biogeochemical
cycles [15].
6. COMPARISON WITH OTHER AREAS
Of all the naturally-formed O
2
deficient zones, what distinguishes the one
over the western Indian continental margin is its pronounced seasonality. Within
the monsoon-affected North Indian Ocean itself, this shelf segment experiences
the most severe O
2
depletion. Milder reducing conditions have recently been
found to occur over the more productive, but better ventilated, Omani shelf (un-
published data collected during September 2004). However, such conditions
were confined close to the seafloor, and did not produce the extreme effects,
viz
.N
2
O and H
2
S accumulation, observed off India. On the other side of the In-
dian subcontinent, in the Bay of Bengal, despite enormous river runoff, coastal
waters do not turn suboxic for two reasons: (a) upwelling is much weaker in
the Bay of Bengal as compared to the Arabian Sea, and (b) contrary to expec-
tations, DIN loading by the South Asian rivers is not very large (
<
0.5 Tg N y
−
1
[39]).
In terms of the intensity of O
2
deficiency, the two other areas that are compa-
rable with the eastern Arabian Sea are the eastern boundary current systems off
Namibia (the Benguela Current [7, 10]) and Peru-Chile (the Humboldt Current
[9, 12-14, 18]). The extent to which suboxic conditions extend offshore in these
regions varies greatly owing to differences in the hydrography and respiration
rate. Unlike the Pacific and the Indian Oceans, the suboxic zone does not extend
much beyond the continental margin in the Atlantic Ocean, and O
2
content of
waters upwelling over the Namibian shelf is initially
>
100 µM [7]. Respiration
rates in this region therefore ought to be extremely high. In the eastern tropical
Pacific Ocean, off the coasts of Peru and Chile, the suboxic zone extends hun-
dreds of kilometers offshore from the shelf. Here too vigorous respiration [9]
fuelled by high PP appears to be the principal cause for the development of an
intense O
2
deficiency including episodes of naturally-caused SO
4
2
−
reduction.
In the Arabian Sea, the suboxic zone over the shelf is not contiguous with the
more extensive offshore suboxic zone with the WIUC flowing off the conti-
nental margin separating the two systems. However, the poleward undercurrent
is ubiquitous to all upwelling systems, and the fact that it retains its slightly
elevated O
2
content in the Arabian Sea implies low respiration rates. Given the
less intense upwelling, O
2
deficiency over the Indian shelf should be caused
more by restricted ventilation than by high productivity. Also, we postulate that
