(^) This Arabian Sea anoxic layer does, however, serve as daytime habitat for an
abundant fauna of mid-water fish (particularly the myctophids Benthosema pterotum
and Diaphus arabica) preyed upon in the layer (and at night at the surface) by
abundant stenoteuthid squid (Sthenoteuthis oualaniensis). There are also euphausiids
(Euphausia sibogae) and shrimp with similar lifestyles. Food is acquired and oxygen
debt is paid off at night by migration into surface layers. Benthosema pterotum is
believed to be extremely abundant, with acoustic studies indicating a stock of 100 Mt
(megatonnes) (Gjøsæter 1984).
(^) During the upwelling season, the western Arabian Sea at sites inshore of ∼400 km
is one of several tropical, Indo-Atlantic areas supporting abundant populations of the
large copepod Calanoides carinatus (Smith et al. 1998). It is also found during the
annual, trades-driven upwelling season along the northern Gulf of Guinea coast and
occurs less prominently in upwelling intervals off West Australia and Brazil. It is not
found in the Pacific. Just as the southwest monsoon starts, it appears in surface layers,
runs through multiple and rapid life cycles during the whole season, and then the fifth
copepodites store copious lipid and descend to depths offshore below the anoxic layer.
This stage is a principal food source for deep-living predators in the Indian Ocean.
The rest of the mesozooplankton are a fairly standard set of tropical–subtropical
species.
(^) During the winter monsoon, the Arabian Sea becomes a prominent site for
Trichodesmium blooms and certainly for nitrogen fixation (Wajih & Naqvi 2008).
Dust off the deserts of Pakistan provides iron to support this production, and enhanced
denitrification (NO 3 − to N 2 ) in the relatively shallow oxygen-minimum zone leaves an
excess of phosphate in upwelled and upwardly mixed waters. Together that makes
ideal conditions for diazotrophy. The blooms become sufficient to color the ocean red
in large patches. Similar blooms occur in the Red Sea and are reputed to be the source
of its name.
Bay of Bengal
(^) This huge gulf also has a monsoonal circulation, anticyclonic (northward along the
Indian coast) from January to October, cyclonic in fall, but much weaker. It is subject
to severe cyclones (equivalent to hurricanes) that cause flooding in low-lying sectors
of Bangledesh. The whole gulf is strongly stratified by the great freshwater influx (a
salinity gradient of several PSS above 20 m from 20° to 16°N, deepening slowly
southward), keeping nutrients at depth. The river plumes carry a great deal of
terrestrial turbidity, making a very shallow euphotic zone. Together these make for
very low primary productivity, mostly about 200 mg C m−2 d−1 year round
(PrasannaKumar et al. 2006). Higher-trophic-level biomasses, at least
mesozooplankton, tend to be surprisingly great, 0.2 to >2 g C m−2, given the low