Environmental Engineering Reference
In-Depth Information
are clearly demonstrated by numerous repeat sections [33], including those
presented here. These changes must be related, albeit in a complex way, to
the inter-annual changes in the strength of the SWM, which affects both the
demand and supply of O
2
in subsurface waters. The severity of O
2
-deficiency
during and just after the SWM is probably determined by variations in pro-
ductivity (through fertilization of the euphotic zone) and stratification (through
freshening of the surface layer) in a given year.
4. RECENT INTENSIFICATION OF O
2
DEFICIENCY
Superimposed on the inter-annual variability one can detect a longer-term
change when recent measurements are compared with previous observations
that go back to the 1950s [3, 4, 8]. The time-series measurements made by Banse
[3] off Cochin are worthy of special mention although extended observations
were also made in shallow waters off Mumbai (Bombay) by Gogate [22] and
off Calicut by Subrahmanyan [53]. Banse sampled two stations over the shelf
having water depths of 25 and 55-60 m from August 1958 to January 1960,
and another deeper station over the continental slope (depth
∼
2000 m) from
November 1958 to May 1959. Combining these measurements with all other
data then available, he provided a comprehensive description, on a seasonal
basis, of hydrography along various segments of the west coast of the Indian
subcontinent [3-5]. The time series records of temperature, salinity and O
2
at the
mid-shelf station off Cochin [4] are very similar to the corresponding records
off Goa presented here. For example, the lowest temperature (
21
o
C) and the
lowest bottom-water O
2
occur during September-October in both records. It is
apparent from Banse's data that even in 1958-59 bottom-water O
2
sometimes
fell below 10% of the saturation value off Cochin. Nitrite was occasionally
present in concentrations up to 4 µM [3]. It is not clear if its production
occurred through dissimilatory NO
3
−
reduction in the water column, but if this
process did take place it would have been on a limited scale. H
2
S was never
present in concentrations high enough to be noticed by its odor [Karl Banse,
personal communication].
A more extensive set of hydrocast data for salinity, temperature and O
2
along
numerous cross-shelf sections off the Indian west coast, was generated under
the UNDP/FAO sponsored Integrated Fisheries Project (IFP) during 1971-
1975. The sections were repeated in different seasons. This data set serves
as an excellent reference for evaluating the extent of shift in O
2
distribution
over the western Indian shelf. We utilize here data only along the section off
Karwar (extending offshore from 14
o
49'N, 74
o
03'E) and at stations shallower
than 60 m that were occupied between August and October. These data are
compared with our own measurements made in the same region and during the
same season (August-September) between 1997 and 2004 (Fig. 6, Table 2). As
∼
