284
Chapter 10: The Indian Ocean
the Southwest Monsoon it increased to 30–200 mgC m
−2 . Copepod diversity responds
appropriately: of a total of about 185 copepod species that occur frequently in the pelagos, 57 species were encountered near-surface under the Findlater Jet in the Southwest
Monsoon, but only 43 during the Northeast Monsoon. In the central gyre, smaller
species dominate the reproducing copepod populations: Paracalanus aculeatus, Undinula vulgaris, Cosmocalanus darwinii, Undinula vulgaris, Calanus minor, and others. The
large Calanoides carinatus is restricted to the near-shore upwelling regions of the ARAB
province. According to Madhupratap and Haridas (1986) the Indian Ocean copepod
fauna has closer affinities to the Pacific than to the Atlantic fauna: this what one might
expect, given the relative permeability of the Indo-Pacific boundary.
The deep oxygen minimum (at about 200–500 m) of the Arabian Sea extends over
much of the northern Indian Ocean, though it apparently has little impact on the vertical
distribution of mesoplankton beyond the central Arabian Sea itself (Madhupratap and
Haridas, 1990). Elsewhere in this province the vertical distribution of typical tropical
genera closely follows the pattern we shall encounter in the eastern Pacific and as was
described for the region north of the Findlater Jet by Luo et al. (2000). However, further
south, in the presence of the subsurface strongly anoxic layer (O 2 concentration is near
zero below 160 m and H 2 S may be present) has major consequences for the distribution
of plankton in all size classes; the classical investigations of this phenomenon were those
of the Soviet academicians Vinogradov and Voronina (1961). The oxygen-deficient layer
suppresses the diel vertical migration that would otherwise be expected to occur here,
the diel migrants remaining in the surface layers. On the other side of the anoxic barrier,
the deep-living copepod Lucicutia sp. is restricted below the lower layer of the zone; it
occurs, therefore, only deeper than 400 m. Here, it consumes sinking particles—detritus,
fecal pellets, clumped bacteria, phytoplankton and microheterotroph cells, and so on.
Whole cells form a smaller proportion of its diet during the spring intermonsoon period,
when more intense reworking of organic material within the surface microbial web must
occur (Gowing and Wishner, 1998).
However, the ocean always reserves surprises for us: during the JGOFS investigations at the central mooring site, where very strong anoxic conditions obtained at all
seasons below 200 m, Mincks et al. (2000) reported that abundant crustacea occur in
this zone and perform limited diel migrations into the surface layers above. Thus,
pelagic decapod shrimps (Gennades, Sergia, and Eupasiphae spp.) and portunid crabs
(Charybdis smithii) live largely within the anoxic layer and must have major respiratory modification for that habitat. Feeding on mesoplankton occurs principally within
the anoxic layer, and is continuous day and night. The population of pelagic crabs is
at times very dense (< 13 g m
−2 , 0–500 m, has been observed) and the species appears
to have an annual life cycle in which the pelagic phase is returned passively to the
shelf regions for reproduction. Should this transport fail, very dense sedimentation of
immature crabs may ensue in the open ocean; in one such fallout, up to 1 crab m
−2
was observed on the deep ocean floor. Such falls may represent 20–30% of the total
regional POC sinking flux and create regions of intense microbial activity in the sediments as the carcasses are integrated into the deep-sea food web (Christiansen and
Boetius, 2000).
Locations of high abundance of higher trophic level organisms, exemplified by tuna
that are capable of long-distance migrations, respond to the relative productivity of
different parts of this province. Yellowfin, skipjack, and bigeye all avoid the Bay of Bengal,
except the area of the coastal current along the eastern coast of India and Sri Lanka,
but are abundant in the eastern equatorial part of this province. The central Arabian Sea
is largely avoided, perhaps because of the oxygen minimum layer, and only yellowfin
penetrate the Arabian Sea itself in large numbers, concentrating mostly in the Omani
upwelling region.
Chapter 10: The Indian Ocean
the Southwest Monsoon it increased to 30–200 mgC m
−2 . Copepod diversity responds
appropriately: of a total of about 185 copepod species that occur frequently in the pelagos, 57 species were encountered near-surface under the Findlater Jet in the Southwest
Monsoon, but only 43 during the Northeast Monsoon. In the central gyre, smaller
species dominate the reproducing copepod populations: Paracalanus aculeatus, Undinula vulgaris, Cosmocalanus darwinii, Undinula vulgaris, Calanus minor, and others. The
large Calanoides carinatus is restricted to the near-shore upwelling regions of the ARAB
province. According to Madhupratap and Haridas (1986) the Indian Ocean copepod
fauna has closer affinities to the Pacific than to the Atlantic fauna: this what one might
expect, given the relative permeability of the Indo-Pacific boundary.
The deep oxygen minimum (at about 200–500 m) of the Arabian Sea extends over
much of the northern Indian Ocean, though it apparently has little impact on the vertical
distribution of mesoplankton beyond the central Arabian Sea itself (Madhupratap and
Haridas, 1990). Elsewhere in this province the vertical distribution of typical tropical
genera closely follows the pattern we shall encounter in the eastern Pacific and as was
described for the region north of the Findlater Jet by Luo et al. (2000). However, further
south, in the presence of the subsurface strongly anoxic layer (O 2 concentration is near
zero below 160 m and H 2 S may be present) has major consequences for the distribution
of plankton in all size classes; the classical investigations of this phenomenon were those
of the Soviet academicians Vinogradov and Voronina (1961). The oxygen-deficient layer
suppresses the diel vertical migration that would otherwise be expected to occur here,
the diel migrants remaining in the surface layers. On the other side of the anoxic barrier,
the deep-living copepod Lucicutia sp. is restricted below the lower layer of the zone; it
occurs, therefore, only deeper than 400 m. Here, it consumes sinking particles—detritus,
fecal pellets, clumped bacteria, phytoplankton and microheterotroph cells, and so on.
Whole cells form a smaller proportion of its diet during the spring intermonsoon period,
when more intense reworking of organic material within the surface microbial web must
occur (Gowing and Wishner, 1998).
However, the ocean always reserves surprises for us: during the JGOFS investigations at the central mooring site, where very strong anoxic conditions obtained at all
seasons below 200 m, Mincks et al. (2000) reported that abundant crustacea occur in
this zone and perform limited diel migrations into the surface layers above. Thus,
pelagic decapod shrimps (Gennades, Sergia, and Eupasiphae spp.) and portunid crabs
(Charybdis smithii) live largely within the anoxic layer and must have major respiratory modification for that habitat. Feeding on mesoplankton occurs principally within
the anoxic layer, and is continuous day and night. The population of pelagic crabs is
at times very dense (< 13 g m
−2 , 0–500 m, has been observed) and the species appears
to have an annual life cycle in which the pelagic phase is returned passively to the
shelf regions for reproduction. Should this transport fail, very dense sedimentation of
immature crabs may ensue in the open ocean; in one such fallout, up to 1 crab m
−2
was observed on the deep ocean floor. Such falls may represent 20–30% of the total
regional POC sinking flux and create regions of intense microbial activity in the sediments as the carcasses are integrated into the deep-sea food web (Christiansen and
Boetius, 2000).
Locations of high abundance of higher trophic level organisms, exemplified by tuna
that are capable of long-distance migrations, respond to the relative productivity of
different parts of this province. Yellowfin, skipjack, and bigeye all avoid the Bay of Bengal,
except the area of the coastal current along the eastern coast of India and Sri Lanka,
but are abundant in the eastern equatorial part of this province. The central Arabian Sea
is largely avoided, perhaps because of the oxygen minimum layer, and only yellowfin
penetrate the Arabian Sea itself in large numbers, concentrating mostly in the Omani
upwelling region.