Environmental Engineering Reference
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(WIUC) may be identified from the distribution of temperature [upward sloping
of isotherms at the top of this feature and downward tilt close to its bottom
just off the continental shelf (Fig. 2a)], and even more clearly from those of
salinity (Fig. 2b) and O 2 (Fig. 2c). Note that the water derived from the south
has lower salinity and slightly higher O 2 content. As judged by the 35.400
salinity contour, the influence of the WIUC, at its peak, extends vertically
down to approximately 400 m depth and horizontally up to 200 km from the
continental slope at 15 o N latitude (Goa transect, Fig. 2b). The WIUC plays
an important role in determining the redox status of subsurface waters off
the western Indian continental margin where its relatively higher O 2 content
inhibits denitrification, probably as far north as 17 o N latitude, as reflected by
the distribution of nitrite (NO 2 , Fig. 2d). This is why the seasonal suboxic
zone over the Indian shelf is not contiguous with the perennial suboxic system
found offshore [36].
3. EVOLUTION OF O 2 DEFICIENCY
Subsurface waters over the entire North Indian Ocean are characterized by
very high nutrient contents (e.g. NO 3 concentrations exceeding 20 µM are
often found at depths shallower than 100 m [58]). Large quantities of nutrients
from this layer can be brought up easily to the euphotic zone by upwelling and
vertical mixing, resulting in high rates of phytoplankton production (PP). Over
the Indian continental shelf, except off Gujarat in the north, such fertilization
of the euphotic zone only occurs during periods of upwelling because, as
already pointed out, the winter circulation is associated with downwelling and
the absence of convective overturning. This imparts pronounced seasonality
to PP (Table 1). The slow rate of upwelling in this region not only allows an
efficient utilization of upwelled nutrients locally, but it also results in greater
consumption of O 2 in the upwelled water that has low O 2 content to begin
with (at the shelf break; Fig. 2). The presence of a low-salinity lens during the
periods of upwelling further contributes to subsurface O 2 depletion.
Table 1. Primary Production (PP) over the Western Continental Shelf of India during the
Southwest and Northeast Monsoons.
PP (g C m 2
d 1 )
Period
Number of
Range
Mean ± Std Dev
Observations
July 1998
0.18-3.96
1.29 ± 1.51
5
August 1998
0.87-7.29
3.39 ± 2.16
8
December 1999
0.29-0.56
0.35 ± 0.14
3
 
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