THE DEEP INDIAN OCEAN FLOOR
233
been quantified (Parulekar et al., 1982, 1992). The
fauna from these abyssal sediments (3600–5300 m) are
composed of 12 macrofaunal and 3 meiofaunal invertebrate taxa (Parulekar et al., 1982). Specifically, the
abyssal macrofauna consists primarily of polychaetes
(41.6%), followed by peracarid crustaceans (31.7%),
ophiuroids (12.2%), Echiura and Bryozoa (9.7%),
molluscs (4.8%), and agglutinating rhizopod protozoans (which were not included in these percentage
figures). Macrofaunal densities range from 92 to
462 individuals m
−2 , and biomass ranges from 0.47 to
13.32 g m
−2 (Parulekar et al., 1982, 1992). Megafaunal
scavengers present include ophidiid fish (Lochte and
Pfannkuche, 2000).
The abyssal plains of the Indian Ocean harbor rich
meiobenthic assemblages, meiofauna density ranging
between 50 177 and 232 912 individuals m
−2 . These
densities are many times greater than densities reported
for meiofauna of the central North Pacific and the
east and west Atlantic (Wolff, 1977), but are only
one-tenth of that observed in the bathyal depths of
the northwest Indian Ocean (Thiel, 1966). Meiofaunal
biomass ranges from 0.02 g m
−2 to 0.41 g m
−2 . Nematodes are the most abundant group, accounting for
53.3% of the individuals, followed by foraminiferans at 17.6% and harpacticoid copepods (16.8%).
Kinorhynchs, ostracods, and turbellarians were also
present in small quantities (Parulekar et al., 1992). The
density of meiofauna decreases with increasing water
depth (Parulekar et al., 1982).
In general, manganese nodules from the 3000–
4000 m depth range in the abyssal zones are relatively
barren with respect to benthic biomass, possibly because of the oligotrophic feeding conditions (Neyman
et al., 1973; Parulekar et al., 1982). Benthic tunicates
represented 20% of the invertebrate species collected
with a trawl in an area with polymetallic nodules
(Monniot and Monniot, 1985). Nodules occur at low
abundance (1–2 kg m
−2 ) in areas of thick sediments,
compared to areas with thin sediments (3.5–5 kg m
−2 )
(Sharma et al., 1997). Meiofauna and macrofauna have
been quantified from sediment cores collected from
nodule areas. Meiofaunal density ranges from 0.3 to
4.5 individuals cm
−2 , dominated by nematoda. The
macrofaunal density ranged from 8–64 individuals m
−2 ,
generally dominated by polychaetes (Sharma et al.,
1997). In the abyssal region of the central Indian
Ocean in the 3500 to 4500 m depth range, in brown
oozy sediments with polymetallic nodules, meiofaunal
densities range from 0.4 to 1.5 individuals cm
−2
(Parulekar et al., 1982). These sediments had almost
three times the macrofaunal biomass (5.16 g m
−2 ) of
the yellow calcareous oozy sediments without nodules (1.78 g m
−2 ).
The mean benthic population density (meiofauna
and macrofauna) from abyssal sediments varies from
233 322 individuals m
−2 in the 1500–1999 m depth
zone to 50 269 individuals m
−2 in the 5500–5999 m
depth zone (Parulekar et al., 1992). The abundance
of both macrofauna and meiofauna decreases with
increasing water depth. However, the proportions of
meiofauna and macrofauna are reversed with increasing
depth; the proportion of meiofauna increasing with
depth. Benthic biomass in the abyssal Indian Ocean
is relatively poor, ranging from 0.11 to 12.75 g m
−2 ,
compared to other deep sea habitats in the Indian Ocean
(Parulekar et al., 1982, 1992).
It has been suggested that the supply of organic
material to the abyssal plains of the deep Indian Ocean
results from deep-water circulation transporting organic
matter from the shelf and slope to abyssal depths
(Parulekar et al., 1982). It is possible to find terrestrial
plant debris at depths of 4500 m in the abyssal central
Indian Ocean (Parulekar et al., 1982).
The diverse benthic fauna and the high values
of standing crop in the western and central Indian
Ocean are dependent on high organic production in the
overlying water column. The correlation in the abyssal
Indian Ocean between the total oxidizable organic
content of the water column and the benthic standing
crop is statistically significant (Parulekar et al., 1992).
In addition, microbial activity and biomass show
significant linear correlations with the vertical flux of
particulate organic carbon (Lochte and Pfannkuche,
2000). The high biomass on the central Indian Ocean
abyssal plain is probably a result of high biological
productivity in surface waters (Humphrey, 1972).
A close relationship between primary productivity and
benthic standing crop was observed (Parulekar et al.,
1982), which is not consistent with the suggestion that
the deep-water supply of organic matter from the shelf
and slope to abyssal depths promotes high standing
crops of deep-sea benthos (c.f., Gage, 1978).
The oligotrophic abyss
The seafloor of the Bay of Bengal is characterized by
a deep-sea fan cut by many distribution channels of
turbidity current fanning out from the north (Curray
233
been quantified (Parulekar et al., 1982, 1992). The
fauna from these abyssal sediments (3600–5300 m) are
composed of 12 macrofaunal and 3 meiofaunal invertebrate taxa (Parulekar et al., 1982). Specifically, the
abyssal macrofauna consists primarily of polychaetes
(41.6%), followed by peracarid crustaceans (31.7%),
ophiuroids (12.2%), Echiura and Bryozoa (9.7%),
molluscs (4.8%), and agglutinating rhizopod protozoans (which were not included in these percentage
figures). Macrofaunal densities range from 92 to
462 individuals m
−2 , and biomass ranges from 0.47 to
13.32 g m
−2 (Parulekar et al., 1982, 1992). Megafaunal
scavengers present include ophidiid fish (Lochte and
Pfannkuche, 2000).
The abyssal plains of the Indian Ocean harbor rich
meiobenthic assemblages, meiofauna density ranging
between 50 177 and 232 912 individuals m
−2 . These
densities are many times greater than densities reported
for meiofauna of the central North Pacific and the
east and west Atlantic (Wolff, 1977), but are only
one-tenth of that observed in the bathyal depths of
the northwest Indian Ocean (Thiel, 1966). Meiofaunal
biomass ranges from 0.02 g m
−2 to 0.41 g m
−2 . Nematodes are the most abundant group, accounting for
53.3% of the individuals, followed by foraminiferans at 17.6% and harpacticoid copepods (16.8%).
Kinorhynchs, ostracods, and turbellarians were also
present in small quantities (Parulekar et al., 1992). The
density of meiofauna decreases with increasing water
depth (Parulekar et al., 1982).
In general, manganese nodules from the 3000–
4000 m depth range in the abyssal zones are relatively
barren with respect to benthic biomass, possibly because of the oligotrophic feeding conditions (Neyman
et al., 1973; Parulekar et al., 1982). Benthic tunicates
represented 20% of the invertebrate species collected
with a trawl in an area with polymetallic nodules
(Monniot and Monniot, 1985). Nodules occur at low
abundance (1–2 kg m
−2 ) in areas of thick sediments,
compared to areas with thin sediments (3.5–5 kg m
−2 )
(Sharma et al., 1997). Meiofauna and macrofauna have
been quantified from sediment cores collected from
nodule areas. Meiofaunal density ranges from 0.3 to
4.5 individuals cm
−2 , dominated by nematoda. The
macrofaunal density ranged from 8–64 individuals m
−2 ,
generally dominated by polychaetes (Sharma et al.,
1997). In the abyssal region of the central Indian
Ocean in the 3500 to 4500 m depth range, in brown
oozy sediments with polymetallic nodules, meiofaunal
densities range from 0.4 to 1.5 individuals cm
−2
(Parulekar et al., 1982). These sediments had almost
three times the macrofaunal biomass (5.16 g m
−2 ) of
the yellow calcareous oozy sediments without nodules (1.78 g m
−2 ).
The mean benthic population density (meiofauna
and macrofauna) from abyssal sediments varies from
233 322 individuals m
−2 in the 1500–1999 m depth
zone to 50 269 individuals m
−2 in the 5500–5999 m
depth zone (Parulekar et al., 1992). The abundance
of both macrofauna and meiofauna decreases with
increasing water depth. However, the proportions of
meiofauna and macrofauna are reversed with increasing
depth; the proportion of meiofauna increasing with
depth. Benthic biomass in the abyssal Indian Ocean
is relatively poor, ranging from 0.11 to 12.75 g m
−2 ,
compared to other deep sea habitats in the Indian Ocean
(Parulekar et al., 1982, 1992).
It has been suggested that the supply of organic
material to the abyssal plains of the deep Indian Ocean
results from deep-water circulation transporting organic
matter from the shelf and slope to abyssal depths
(Parulekar et al., 1982). It is possible to find terrestrial
plant debris at depths of 4500 m in the abyssal central
Indian Ocean (Parulekar et al., 1982).
The diverse benthic fauna and the high values
of standing crop in the western and central Indian
Ocean are dependent on high organic production in the
overlying water column. The correlation in the abyssal
Indian Ocean between the total oxidizable organic
content of the water column and the benthic standing
crop is statistically significant (Parulekar et al., 1992).
In addition, microbial activity and biomass show
significant linear correlations with the vertical flux of
particulate organic carbon (Lochte and Pfannkuche,
2000). The high biomass on the central Indian Ocean
abyssal plain is probably a result of high biological
productivity in surface waters (Humphrey, 1972).
A close relationship between primary productivity and
benthic standing crop was observed (Parulekar et al.,
1982), which is not consistent with the suggestion that
the deep-water supply of organic matter from the shelf
and slope to abyssal depths promotes high standing
crops of deep-sea benthos (c.f., Gage, 1978).
The oligotrophic abyss
The seafloor of the Bay of Bengal is characterized by
a deep-sea fan cut by many distribution channels of
turbidity current fanning out from the north (Curray