Environmental Engineering Reference
In-Depth Information
region, was about 2.7 billion barrels per day in 2002. Given the trend over the
past decade (the energy consumption rose by 64% from 1992 to 2002 [20],
reflecting rapid economic growth), it is expected that fuel consumption will
continue to increase steeply in the foreseeable future. Moreover, the ongoing
industrialization and urbanization are greatly influencing patterns of land use.
Because of these demographic and economic factors, the disproportionately
large continental-margin O
2
deficiency that exists in the North Indian Ocean
acquires an even greater significance and underscores the vulnerability of the
coastal O
2
deficient environment to human activities with potentially important
global implications.
In this chapter we describe the factors responsible for the occurrence of
suboxic/anoxic
1
conditions, their evolution over the annual cycle, and impact
on the biology and chemistry along a segment of the continental margin - off
the west coast of India - that not only experiences the most extreme conditions,
but has also been the best studied.
2. HYDROGRAPHY AND CIRCULATION
Surface currents along the west coast of India, as in other parts of the North
Indian Ocean, reverse every six months. During the summer or the southwest
monsoon (SWM; June-September), the West India Coastal Current (WICC)
flows toward the equator (Fig. 1b), carrying at its peak about 0.5 Sv (1 Sv
=10
6
m
3
s
−
1
) of water in the north and 4 Sv in the south [52]. As in other
eastern boundary environments, the equatorward surface flow is accompanied
by upwelling, which begins in the south (off the southwest coast of India and the
west coast of Sri Lanka) sometime in May. The presence of a cyclonic eddy,
the Lakshadweep Low (LL), located close to the islands it has been named
after, also contributes to low sea surface temperatures and shallow thermocline
extending beyond the continental shelf in this region [52]. Upwelling gradually
propagates toward the north, persisting until November-early December along
the northwest coast (e.g. off Mumbai), well after the collapse of the SWM
winds [4]. Thus, the local winds alone do not seem to account for upwelling
along the Indian west coast, and it is believed that the process is, to a large
extent, remotely forced [32].
Hydrography of coastal waters is also profoundly influenced by the SWM
rainfall. The heaviest precipitation (
>
3000 mm y
−
1
) occurs along the central
west coast (around 13-14
o
N latitude) decreasing northward to
<
1000 mm y
−
1
along the Gujarat coast (north of Mumbai), and more gently toward the south
(e.g.
∼
2500 mm at Trivandrum). The large freshwater inputs to coastal region
1
In order to classify the degree of O
2
-deficiency operationally, we use the following criteria:
hypoxic
- 0.1
<
O
2
(Winkler)
≤
0.5 mL L
−
1
,NO
2
−
=0µM, NO
3
−
>
0 µM;
suboxic (denitrifying)
-O
2
≤
0.1 mL L
−
1
,NO
2
−
,NO
3
−
>
0 µM;
anoxic (sulphate reducing)
-O
2
=0mLL
−
1
,NO
2
−
,NO
3
−
=0µM, H
2
S
>
0 µM.
