L - Cochin University of Science and Technology

The distribution of oxygen in the ocean is controlled through the exchange
with the atmosphere and the bioiogical processes of photosynthesis and respiration.
Oxygen from the atmosphere dissolves in seawater at the sea surface. The amount
that can be dissolved decreases gradually with increasing temperature and to a lesser
extent, with increasing salinity. Amount of organic matter present in the system also
influences the availability of oxygen. Particulate organic matter sinks down and
accumulates on the density gradient generated by the thermocline. Bacteria
breakdown this debris and consume oxygen in the process, thereby producing oxygen
minimum layers (Levinton, 1982). Almost all eUkaryotic organisms require oxygen
for metabolism. The continued absence or even depletion of dissolved oxygen (DO)
results in lowering of metabolic activity. Active species consume more oxygen than
inactive species. Sponges, ascidians and most bivalves consume much less oxygen
than decapods, cephalopods and teleosts. Species actively feeding during day require
more oxygen. Oxygen dissolved in water plays a significant physical as well as
biochemical role in the life of aquatic organisms. The oxygen - hydrogen sulphide
system is responsible for the development of oxidation-reduction potential. This
system begins to operate when the oxygen is depleted, mostly due to the presence of
large amount of organic matter associated with effective vertical separation of the
water masses. Under anaerobic condition, bacteria, which use the oxygen bound in
sulphide for oxidation of their organic nutrients, develop, with concomitant formation
of gaseous H2S, which dissolves in the seawater. As H2S is a powerful biological
poison, normal plant and animal life can no longer be sustained in such regions. In
certain fine sediments, anaerobic conditions may develop and effectively exclude
many species requiring a good supply of oxygen (Fincham, 1984). However, many of
the meiobenthic forms thrive in this deoxygenated condition.
Arabian Sea (AS) is unique among the low latitude seas because it is land
locked in the north by Asian landmass and has marked continental influence. It
experiences seasonal reversal of atmospheric forcing, and consequently the upper
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