L - Cochin University of Science and Technology

opined that limited mixing. high organic production. sinking and decomposition of
large amount of organic matter were the reasons for this oxygen depletion in higher
latitudes. Ivanenko and Rozanov (1961) have reported the presence of H 2S in the
oxygen deficient zones of AS and BOB. Nejman (1961) observed sinking of high
saline. high temperature. oxygen poor water in the Persian Gulf and Gulf of Aden
and spread into the subsurface layers which may have its influence on the low
oxygen and high saline water observed in the northern transects. Rao and Jayaraman
(1970) suggested that the oxygen minimum is because of near stagnant conditions in
the north and central parts of the AS. According to Wyrtki (1973) the oxygen
minimum layer is due to the isolation and stagnation of the intennediate water.
limited horizontal advection and high primary productivity.
The distribution of DO in the northern Indian Ocean is different from most of
the other open ocean areas in that the surface layer is well mixed down to the
thennocline and oxygen maximum could occasionally be observed within this layer,
especially during pre-monsoon season. The intensity of the incident solar radiation is
very high during this period. which causes maximum primary production to occur a
few meters below the sea surface (Qasim. 1977). This together with high vertical
stability may result in the observed oxygen maximum in the shallow depths. The
strong density gradient prevent any significant exchange of DO from the euphotic
zone to layers below the thennocline. and the horizontal advection is poor due to the
semi enclosed nature of the region. These features in conjunction with a high rate of
supply of organic matter from the surface result in a severe depletion of DO below
the thermocline throughout the northern Indian Ocean. a feature recognjzed by
several workers (Nejman. 1961, Wyrtki, 1971. 1973. Gupta et al .• 1976a, 1976b,
Naqwi et al .• 1982).
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