Environmental Engineering Reference
In-Depth Information
established untill the 1962-1965 International Indian Ocean Expedition [95].
Although the complete depletion of dissolved oxygen with the concomitant ap-
pearance of hydrogen sulfide (H
2
S) has been reported [50] these conditions do
not appear to be common in the open-ocean oxygen deficient zone in the Ara-
bian Sea. However, H
2
S has been observed during the occurrence of seasonal
anoxia over the Indian shelf [74] and also off Peru [20, 33].
Circulation in the northern Indian Ocean is governed by two distinctly op-
posite monsoon cycles that are known to produce some of the greatest seasonal
variability observed in any ocean basin [88]. During the Southwest Monsoon
(June-September) strong southwesterly winds produce offshore Ekman trans-
port resulting in coastal upwelling off the Arabian Coast and open-ocean up-
welling due the wind stress curl [64]. The combination of upwelled nutrients,
high aeolian dust flux and near constant, intense irradiance results in high
primary production in a section extending about 1000 km off shore from the
Arabian coast (1.5 g C m
−
2
d
−
1
; [6]). The Northeast Monsoon (December-
February) is characterized by moderate winds from the Himalayas that send
cool, dry air over the region, that combined with reduced winter solar radiation
promote convective mixing. This raises surface nutrient concentration but not to
levels as high as during the SW Monsoon, which again stimulates high produc-
tivity [6]. Comprehensive studies of primary production, long-term sediment
trap deployments and detailed studies of the water-column distributions docu-
ment the seasonality and interannual variability of the production cycle [75].
The high primary productivity creates high oxygen demand in intermediate
waters and is one of the two primary factors responsible for the development
of oxygen deficient conditions in the Arabian Sea, the other being the limited
supply of oxygen to the intermediate layers (150-1000m). The oxygen deficient
zone is supplied with water advecting in from three major areas: the southern
Indian Ocean, the Persian Gulf, and the Red Sea. The majority of water enter-
ing the ODZ is ultimately derived from the southern Indian Ocean. This water
outcrops around 45
◦
S, where the surface oxygen concentration is high, but by
time it reaches the southern border of the ODZ (
14
◦
N) it loses almost all of its
dissolved oxygen (Fig 1; [77]). Thus the combination of high oxygen demand
and low oxygen source waters results in the strong and persistent ODZ in the
Arabian Sea.
Upon oxygen depletion a suite of oxidized compounds serves as terminal
electron acceptors for organic matter oxidation [38]. By far the most abundant
suboxic electron acceptor in the open ocean is nitrate (NO
3
−
). Thus, denitrifi-
cation is believed to be the major mode of respiration in the oxygen deficient
zone of the Arabian Sea [19, 67]. Canonically, denitrification proceeds via the
series of reductions described by:
∼
NO
3
−
→
NO
2
−
→
NO
→
N
2
O
→
N
2
(1)
