Environmental Engineering Reference
In-Depth Information
Benthos.
At the time of the most intense upwelling, the entire continental
shelf off India is enveloped by subsurface/bottom waters with O
2
<
0.5 mL
L
−
1
[38], and this has long been known to have a profound impact on benthic
fauna. Observations by Carruthers et al. [8] off Mumbai led them to conclude,
“Scarcely any benthic animals were found in the depths where the oxygen
content of the bottom water was less than 0.5 ml per litre. In shallow water,
benthos consisted mainly of hermit crabs,
Jibia curba
shells and a few nereid
and polychaete worms.” The shift to anoxic conditions has since affected the
benthic population even further as indicated by observations off Goa where
surface sediments within the zone presently affected by bottom-water SO
4
2
−
reduction contain plenty of bivalve shells, but no live animals [B. Ingole,
personal communication]. Sulphur bacteria such as
Thiomargarita
,
Thioploca
and
Beggiatoa
have not been reported from the coastal anoxic zone so far,
perhaps due to the restriction of anoxic conditions to a season and also low free
sulphide concentrations in interstitial waters.
Thioploca
mats have, however,
been observed at greater depths along the continental slope off Pakistan [47].
Fisheries.
The impact of near-bottom O
2
deficiency on fisheries has at-
tracted much attention [3, 4, 8, 26]. The historical data, reviewed by Banse
[4], could be taken to be representative of a pristine environment (i.e. before
the increase in fertilizer consumption in South Asia). It is obvious that sea-
sonal O
2
deficiency over the shelf had always affected fisheries along the west
coast of India. According to Banse [4], “Off Cochin, the deoxygenation of
near-bottom waters results in the regular disappearance of demersal fishes and
in unprofitable trawling in a belt between the aerated water nearshore and the
relatively new bottom water on the outer shelf. For the dominant fish,
Synagris
japonicus
, oxygen concentrations of 0.25-0.5 ml/l seemed to be critical (Banse,
1959). . . The fact that off Bombay, as off Calicut, the oxygen content dropped
to zero in near-bottom water suggests that there may possibly be a vast area
on the outer shelf (and perhaps also on the middle shelf) approximately from
Cochin to Karachi that is devoid of commercially important concentrations of
demersal fishes during the southwest monsoon.” Given the recent observations
showing a decrease in the O
2
levels and an appearance of H
2
S, the environment
could only have become more hostile to demersal fishes (including prawns). On
numerous cruises undertaken during the period August-October since 1997 we
observed practically no bottom trawling activity between Cochin and Mumbai
to a depth of at least 50 m and unusually intense trawling over the outer shelf.
The zone of 'aerated water nearshore' referred to by Banse [4], when present,
is generally no more than a few hundred meters wide as indicated by the re-
sults of our sustained observations off Goa. It usually disappears sometime in
August when the O
2
deficient layer moves shoreward. This is associated with
greatly enhanced fish catch, including that of the highly valued 'solar prawns'
