THE DEEP INDIAN OCEAN FLOOR
Amanda W.J. DEMOPOULOS, Craig R. SMITH and Paul A. TYLER
The Indian Ocean is the world’s third largest, stretching
9600 km from Antarctica to the Bay of Bengal and
7600 km from Africa to Australia. Compared to the
Atlantic and Paciﬁc, the Indian Ocean remained
poorly explored scientiﬁcally until the International
Indian Ocean Expedition (IIOE) in 1962–1965, the
results of which were published in the Atlasofthe
Indian Ocean (Wyrtki, 1971). Prior to the IIOE, the
voyages of the ‘heroic age’ of deep-sea exploration
had sampled relatively little in the Indian Ocean.
During the circumnavigation of the world’s oceans by
HMS Challenger (1872–1876), study of the Indian
Ocean was conﬁned to circumpolar waters south of
the Polar Front. The Valdivia expedition (Chun, 1900)
sampled the free-swimming larvae of the deep-water
brachiopod Pelagodiscus in surface waters of the Indian
Ocean, benthic adults being collected from depths
as great as 2490 m (Helmcke, 1940). In the 1930s,
the John Murray Expedition to the Indian Ocean
worked mainly in shallow water, but some deep-
water sampling provided material for monographs (e.g.,
Knudsen, 1967). The Swedish Deep Sea Expedition
of 1947–1948 on the Albatross sampled extensively
in all the oceans (Pettersson, 1957), taking bottom-
living specimens and cores from the deep sea (for a list
of publications see Menzies et al., 1973). The last of
the ‘heroic age’ cruises, on the Galathea (1950–1952),
sampled deep-water fauna between Sri Lanka (formerly
Ceylon) and the Kenyan coast, as well as along the
Mozambique Channel to South Africa (for a list of
publications see Menzies et al., 1973). The taxonomic
studies of these major expeditions were supplemented
by Soviet cruises to the Indian Ocean, particularly on
the Vityaz, again resulting in monographic treatments
(e.g., Pasternak, 1964, 1976). The IIOE also gave the
opportunity for quantiﬁcation of the bottom fauna of
the Indian Ocean (Neyman et al., 1973). A historical
perspective of the major early cruises, including those
to the Indian Ocean, has been given by Mills (1983).
Since the IIOE, deep-sea research in the Indian
Ocean has focused on the Arabian Sea. Because few
systematic sur veys or conceptually integrated studies
exist, deep-sea ecosystems in the Indian Ocean remain
poorly known (Banse, 1994).
The Indian Ocean is characterized by a number of
unusual oceanographic features. Patterns of circulation
are unlike those in any other oceans, owing to large-
scale monsoonal shifts in wind stress and current
directions north of 5ºS. In addition, the Indian Ocean is
landlocked to the north and lacks temperate and polar
regions north of the equator, further modifying oceanic
circulation and hydrology. The basin also harbors one
of the largest and most intense intermediate-depth oxy-
gen minimum zones in the world ocean (Kamykowski
and Zentara, 1990; Rogers, 2000). These unusual
features of the Indian Ocean substantially affect the
spatial and temporal variability of primary production,
the deep ﬂux of particulate organic carbon (POC) and
the oxygen concentration, profoundly inﬂuencing the
nature of the deep seaﬂoor habitats.
Deep-sea habitats studied with modern techniques in
the deep Indian Ocean include the Kenya slope, the
oxygen minimum zone of the Oman Margin, and the
abyssal Owen Basin in the Arabian Sea. In addition,
limited deep-sea data exist for continental-rise and
abyssal habitats in the central Indian Ocean and in
the Bay of Bengal. In this chapter, we ﬁrst describe
physical characteristics and key habitat variables of the
deep Indian Ocean; we then summarize the limited
deep-sea benthic data available from this ocean, and
highlight topics of interest for future research.
220 Amanda W.J. DEMOPOULOS et al.
PHYSICAL CHARACTERISTICS OF THE DEEP
The Indian Ocean, including adjacent seas (e.g., the
Arabian Sea, the Bay of Bengal, and the Southern
Ocean), covers 73 426 000 km
, roughly one-ﬁfth of
the total world oceanic area. It has an average depth
of 3890 m, which is approximately equivalent to the
average world-ocean depth.
The morphological features of the Indian Ocean are
similar to those of the Atlantic Ocean (see Chapter 6),
and include mid-ocean ridges, abyssal plains, and few
deep-sea trenches. The Indian Ocean has very few
seamounts and islands (see Rogers, 1994), but contains
numerous submarine plateaus and rises (Fig. 7.1).
Two types of continental margins are present in this
basin: divergent and convergent. Divergent margins
are the most common, and are characterized by wide
continental shelves, broad continental rises, and little
seismic activity. Such margins are found along East
Africa, the Arabian Peninsula, much of the Indian
subcontinent, and Western Australia. The Indian Ocean
contains only one convergent margin in its northeast
corner, the Java (Sunda) Trench, where oceanic crust
is subducted beneath a continental plate (Fig. 7.1). The
7500 m deep Java Trench is part of the Indonesian Arc,
which contains 14% of the world’s active volcanoes.
Because of large riverine inputs of terrigenous
sediment, par ticularly from the Indus and Ganges
Rivers, onto gradually sloping divergent margins, the
Indian Ocean has vast continental rises and abyssal
plains (Kennett, 1982). The rises are gradually sloping
plains of terrigenous sediment several kilometers thick.
Beyond the continental rises lie level abyssal plains; the
abyssal plain south of the Bay of Bengal is the ﬂattest
large area of the earth’s surface (Tomczak and Godfrey,
1994). Much of this plain has arisen from a turbidity
ﬂow down the northern slopes of the Bay of Bengal,
extending 3000 km southwards into the deep sea of the
Like the Atlantic Ocean, the Indian Ocean is
subdivided into a number of major basins by long
sections of mid-ocean ridge (Fig. 7.1). In the Indian
Ocean, some of these ridges (e.g., the Ninety-East
Ridge, the Mascarene Ridge and the Chagos-Laccadive
Ridge) are aseismic; they do not appear to be sites
of active seaﬂoor spreading. Active ridges include the
Carlsberg Ridge and the Mid-, Southwest and Southeast
Indian Ridges, the last two of which extend beyond the
limits of the Indian Ocean, connecting with the world
Mid-Ocean Ridge system. The abyssal Indian Ocean
is divided into several smaller basins by meridional
ridges. The West Australian Basin and the Mid-Indian
Basin are separated by the Ninety-East Ridge whilst
to the west of the Mid-Indian Ridge are a series of
basins including the Somali, Mascarene, Madagascar
and Natal Basins. The Carlsberg Ridge lies north of
the Arabian Basin (Fig. 7.1).
Three important factors make the circulation and
hydrology of the Indian Ocean different from those of
any other ocean: the closure of the Indian Ocean in the
northern subtropics; the seasonally-reversing Monsoon
Gyre; and the blocking effects of the equatorial currents
to the spread of water masses along the thermocline
(Fig. 7.2). Owing to seasonal heating and cooling of
the vast Asian landmass, winds vary seasonally north
of the equator, resulting in the Indian Ocean monsoons.
From November to March the Northeast Monsoon is
accompanied by the northeast trades, which are rein-
forced by the rapid winter cooling of air over Asia. As
a result, the westward-ﬂowing North Equatorial Current
from 8ºN to the equator is prominent in January
through March, generating a small anticyclonic gyre
north of the equator (Fig. 7.2). Very little upwelling
occurs during the Northeast Monsoon, and hydrological
effects are generally superﬁcial (Wyrtki, 1973). From
April to September, the Asian landmass warms faster
than the ocean, drawing moist air ashore from over the
ocean, and creating the Southwest Monsoon. During
this period, eastward surface currents north of the
equator combine with the Equatorial Countercurrent,
establishing the Southwest Monsoon Current between
15ºN and 7ºS (Fig. 7.2) and a strong westward-ﬂowing
South Equatorial Current around 5ºS. This reversal
of surface currents gives rise to the greatest seasonal
variation in hydrography of any ocean basin (Burkill
et al., 1993). Strong upwelling occurs off the Somali
and Oman coasts, resulting in substantial increases in
The South Equatorial Current forms a marked
hydrographic boundary between the monsoon-driven
circulation in the north and the Southern Hemispheric
Sub-tropical Anticyclonic Gyre to the south. The
circulation pattern to the south of 5ºS is analogous to
THE DEEP INDIAN OCEAN FLOOR 221
Fig. 7.1. Topography of the Indian Ocean including major ridges and basins. OM, Oman Margin. Modiﬁed from Tomczak and Godfrey
(1994). The 1000, 3000, and 5000-m isobaths are shown, and regions less than 3000 m deep are shaded.
the gyres of the South Atlantic and South Paciﬁc. In
the southern Indian Ocean, western boundary currents
include the East Madagascar, Mozambique, Agulhas,
and Zanzibar Currents (Fig. 7.2). All of these currents
extend to great depths, disturbing sediments with
their high velocity ﬂow to depths as great as 2500 m
(Tomczak and Godfrey, 1994). Off South Africa, the
Agulhas Current for ms a cyclonic loop, and, although
some water is lost to the Atlantic, most ﬂows eastward
forming the northern boundar y of the Circumpolar
A signiﬁcant consequence of the seasonally changing
circulation pattern in the north Indian Ocean is
pronounced upwelling along the western coastline
when the Southwest Monsoon produces strong Ekman
transport away from the coasts of Somalia and Arabia
(Swallow, 1984). During the Southwest Monsoon,
the strong winds from the northwest force intense
upwelling and deep mixing, which reduce coastal
sea-surface temperatures by approximately 5ºC, and
222 Amanda W.J. DEMOPOULOS et al.
North Equatorial Current
Equatorial Counter Current
South Equatorial Current
South Indian Ocean Current
South Equatorial Current
Fig. 7.2. Major surface currents of the Indian Ocean (A) during the Northeast Monsoon season (March–April), and (B) during the
Southwest Monsoon season (September–October). The circulation south of 20ºS remains unchanged. Abbreviations: STF, Subtropical Front;
SAF, Subantarctic Front; PF, Polar Front; SJC, South Java Current; ZC, Zanzibar Current; SC, Somali Current; EAC, East Arabian Current;
EMC, East Madagascar Current; WGB, Weddell Gyre Boundary. Modiﬁed from Tomczak and Godfrey (1994).
cause high primary productivity and a substantial ﬂux
of sinking particles in this region (Cushing, 1973;
Banse and McClain, 1986; Nair et al., 1989). Primary
production is exceptionally high and widespread dur-
ing this period, reaching levels of 547.5 g C m
(Krey, 1973; Nair et al., 1989). Because upwelling
is embedded in the swiftly moving wester n boundary
current, nutrient upwelling has a reduced effect on
local primary production. The strong current removes
much of the additional planktonic biomass from the
upwelling system before it can be utilized, with a
consequent enhancement of biomass and secondary
production in the open Arabian Sea. As a result,
zooplankton concentrations in the upwelling system of
THE DEEP INDIAN OCEAN FLOOR 223
the Arabian Sea are not as high as those of coastal
upwelling systems in the Paciﬁc and the Atlantic
Oceans. Upwelling also occurs off the southeast
margin of Oman, associated with enhanced biological
productivity (Hermelin and Shimmield, 1990). The
sediments of the Oman Margin and northwest Arabian
Sea vary laterally between facies rich in organic carbon,
in biogenic silica, or in carbonate, deposited under this
highly productive upwelling regime (Kennett, 1982;
Tomczak and Godfrey, 1994).
Upwelling along the eastern boundary of the Indian
Ocean is uncommon, because winds favorable for
upwelling are weak during the Northeast Monsoon
and absent during the Southwest Monsoon. This is in
contrast to the eastern boundary of the Paciﬁc, where
there is signiﬁcant upwelling in both the northern and
southern hemispheres (see Chapter 6). A small amount
of upwelling occurs along the coast of Java during the
Southwest Monsoon and weak upwelling also occurs
off southwest India (Wyrtki, 1973; Burkill et al., 1993).
Unlike typical eastern boundary currents, the Leeuwin
Current (Fig. 7.2) off the west coast of Australia ﬂows
poleward against the wind, and the undercurrent is
equatorward, instead of moving toward the continent, as
in the Peru–Chile Margin. For these reasons, upwelling
does not occur on the Western Australian shelf, and
thus overall biological productivity in this region is
Deep water masses
The hydrology of the deep Indian Ocean is much
less affected by the seasonal monsoon cycle than
near-surface currents. Monsoonal inﬂuence is restricted
to the surface mixed layer and western boundary
currents. Three mediterranean-type seas inﬂuence the
hydrographic properties of the Indian Ocean: the
Persian Gulf, the Red Sea, and the Australasian
Mediterranean Sea. In the Persian Gulf and Red Sea,
evaporation exceeds precipitation and warm, dense
water ﬂows out of the Persian Gulf and Red Sea into
the northern Indian Ocean forming North Indian Ocean
Intermediate Water. This intermediate water does not
reach the deep sea bed. In the southern Indian Ocean,
south of 10ºS, between the depths of ~500 and 1000 m,
the northward ﬂow of Antarctic Intermediate Water is
blocked by the equatorial current system (Tomczak and
Abyssal ﬂow in the Indian Ocean is divided
into three components associated with three western
boundary currents. Western basins are penetrated by
bottom waters derived from the Indian and Atlantic
Basins (Warren, 1981). Like the Atlantic and Paciﬁc
Oceans, the water masses in the Indian Ocean below
3500 m consist mostly of cold Antarctic Bottom Wa-
ter (AABW), with a potential temperature T of 0.3ºC.
Antarctic Bottom Water leaves the circumpolar current
to enter and ﬁll the Indian Ocean below a depth
of 3800 m at two locations: the Madagascar Basin,
and gaps in the Southwest Indian Ridge. Antarctic
Bottom Water then ﬂows across the Madagascar
continental slope and forms a deep western boundary
current (Swallow and Pollard, 1988). Recirculation of
Antarctic Bottom Water in the Madagascar Basin is
fast, bottom currents ﬂowing northward at a speed of
approximately 0.2 m s
. The bottom water continues
along the western pathway in the Somali Basin, where
it eventually enters the Arabian Basin and disappears
through gradual mixing into overlying Deep Water. In
the eastern Indian Ocean, Antarctic Bottom Water, also
called Circumpolar Water, enters the South Australian
Basin via the Australian-Antarctic Discordance. It ﬁlls
the Great Australian Bight and moves west then north,
forming a western boundary current along the Ninety-
East Ridge. The oxygen concentration follows the ﬂow
pattern, with the concentration in the bottom water
decreasing towards the north from 5 ml °
to 0.2 ml °
in the Arabian Sea and Bay of Bengal,
as the water mass increases in age and isolation.
This situation is in contrast to that in the Atlantic
and Paciﬁc, where the most notable regions of low
oxygen are on the east side around the equator. From
a depth of 1500 to 3800 m, the Indian Ocean is ﬁlled
with Indian Deep Water formed from North Atlantic
Deep Water (NADW) carried into the Indian Ocean
with the upper Circumpolar Current. It spreads north
in the western boundary current to the Arabian Sea
and the Bay of Bengal. The water properties consist
of a moderate oxygen concentration (4.7 ml °
temperature (2ºC), and high salinity (35.85), similar to
the North Atlantic Deep Water. The northern Arabian
Sea is closed by land mass in the north, and is semi-
enclosed altogether (Fig. 7.1). The Arabian Sea is
deeper than 3000 m, and the basin is closed in the south
by the Central Indian Ridge, the Carlsberg Ridge, and
the Chagos–Laccadive (or Maldive) Ridge (Fig. 7.1).
Therefore, bottom water must enter in the west, through
the Owen Fracture Zone, rather than from the south
(Tomczak and Godfrey, 1994).
224 Amanda W.J. DEMOPOULOS et al.
Clay or no Deposit Calcareous OozeSiliceous Ooze
Shelf and Slope DepositsDeep-Sea MudsGlacial Debris
0 1000 2000
Fig. 7.3. Distribution of surface-sediment types in the deep Indian Ocean. Modiﬁed from Berger (1974).
Substratum types in the deep sea can inﬂuence the
distribution patterns of benthic organisms. Speciﬁcally,
the organic-matter content of deep-sea sediments can
be correlated with the abundance, composition, and
diversity of benthos (Grassle and Grassle, 1994;
Rice and Lambshead, 1994; Snelgrove and Butman,
1994). In general, organic-rich sediments often are
low-diversity habitats, containing mostly tube-dwelling
polychaetes, whereas organic-poor sediments typically
contain a diverse community of deposit feeders (Levin
and Gage, 1998). Hard substrata in the deep sea provide
niches for a broad variety of sessile organisms (see
Most of the Indian Ocean seaﬂoor, particularly
that remote from land, is covered with calcareous
ooze (Fig. 7.3) (Berger, 1974). Calcium carbonate
concentrations are typically intermediate between those
of the carbonate-rich Atlantic and the carbonate-poor
Paciﬁc. The carbonate critical depth, CCRD, where
calcium carbonate drops to <10% of the sediment
mass, is deepest in the equatorial region, and shoals
to 3900 m between 50º and 60ºS. Carbonate sediments
are relatively poor in organic carbon (<1%) and have
coarse g rain size (Berger, 1974; Kennett, 1982). Nearly
all river discharge occurs in the northern par t of
the Indian Ocean adjacent to Asia, thick terrigenous
sediment being deposited in the northern and western
parts of the Indian Ocean, especially the Arabian Sea
and the Bay of Bengal. These sediments have high
concentrations (2–5% by weight) of organic carbon,
THE DEEP INDIAN OCEAN FLOOR 225
and are composed mostly of terrestrial plant material,
phytodetritus, and mineral grains transported by rivers
(Kennett, 1982; Levin and Gage, 1998; Levin et al.,
2000). In the Bay of Bengal, terrigenous sedimentation
from the Ganges is particularly extensive, reaching
depths of 5000 m. In the Arabian Sea, there is an
organic carbon maximum (4.9%) at 400 m, owing ap-
parently to preferential preservation and accumulation
of organic matter under low-oxygen conditions in the
bottom water (Levin et al., 2000). The particle ﬂux to
the deep Bay of Bengal is enhanced by the freshwater
input from the major rivers entering at the north of the
Bay (Ittekkot et al., 1991).
Sediments are at most very thin on the crests
of mid-ocean ridges, and essentially absent on the
ridge axes (Fig. 7.3). Thick siliceous oozes, composed
primarily of radiolarian and diatom tests, occur at
depths of ~5000 m south of the polar front and along
a few mid-ocean ridges, where there is high biological
productivity (Berger, 1974; Kennett, 1982). However,
because of the oligotrophic nature of the equatorial
Indian Ocean, siliceous sediments are rare in low
latitudes of the Indian Ocean compared to the Paciﬁc
Ocean (Kennett, 1982). Red clay is present mostly in
the eastern and southern Indian Ocean, near the equator
and high latitudes. It is composed of ﬁne-grained,
organic-poor sediments resulting from volcanic activity
at ridges (Berger, 1974; Pilipchuk et al., 1977; Kennett,
Hard substrata in the Indian Ocean consist of basalt
rocks, rock faces, and the surfaces of ferroman-
ganese concretions. The morphology of the central
Indian Ocean Basin is composed of abyssal hills and
seamounts, as well as valleys and abyssal plains.
The topographic highs, which are in the proximity
of three major fracture zones, are composed of hard,
massive basalts occurring at the crests, along the
slopes and on the foothills as talus deposits (Sharma
et al., 1997). Owing to strong geostrophic currents and
consequent scouring of the sediments, the Wharton
Basin, the southern Mascarene Basin, and parts of the
Southwest Indian and Australian–Antarctic Basins have
little or no sediment (Kennett, 1982). Sediment in these
areas, when present, is mostly brown clay. Along the
Southwest Indian Ridge, German et al. (1998) have
identiﬁed hydrothermal activity. The associated fauna
inhabiting these hydrothermal regions is dominated by
shrimps and anemones (T. Shank, pers. comm.). In
the southeast and southwest Indian Ocean, and in the
Mozambique Basin, there are extensive pavements of
manganese nodules at depths of about 4000 m (Kolla
et al., 1980). It has been suggested that their presence
at the sediment–water interface is a result of benthic
biological activity and strong bottom currents. Benthic
organisms may nudge the nodules upward, maintaining
them near the sediment–water interface, while strong
currents may limit the deposition of sediment, allowing
the nodules to grow (Berger, 1974; Paul, 1976; Kennett,
Currents in the deep sea inﬂuence sediment deposition
and the organisms that inhabit the seaﬂoor (Nowell and
Jumars, 1984). In regions where there are strong cur-
rents, sediment deposition is minimal. These currents
can smother sessile and suspension-feeding organisms
with sediment grains. Where currents are weak, sed-
iment chemistry and biology may be controlled by
diffusive processes. In these environments, suspension
feeders may suffer owing to the inadequate supply
of advected particles (e.g., Jumars and Gallagher,
1982). Thus, near-bottom currents not only may control
sediment deposition, but can also manipulate sediment
chemistry and the structure of the benthic communities
(Nowell and Jumars, 1984).
Both bottom and deep waters of the Indian Ocean
are derived from the Atlantic, and spread throughout
the deep sea by active ﬂow (Wyr tki, 1973). Antarctic
Bottom Water warms as it spreads north, and ﬁlls the
deep basins of the central Indian Ocean. Therefore, the
abyssal Indian Ocean is an area of active deep-sea cir-
culation. Antarctic Bottom Water supplies ﬁne-grained
sediments from the Africa–Madagascar source to the
marginal areas of the Mozambique Basin and generates
wavy bedforms in this region. In addition, eddies
appear to penetrate to great depths in the Arabian Sea,
and the circulation inﬂuences the bottom topography,
causing depressions and rises (Das et al., 1980). There
are strong bottom currents, with speeds approaching
10–20 cm s
, in the Wharton Basin and the southern
Mascarene Basin, and in parts of the Southwest Indian
Basin and Australian–Antarctic Basins, resulting in
minimal sediment deposition (Kennett, 1982; Gage and
Tyler, 1991). The active circulation ensures that the
abyssal bottom waters of the Indian Ocean remain
There is no evidence for benthic storms (see
Chapter 2) in the deep Indian Ocean although they are
226 Amanda W.J. DEMOPOULOS et al.
predicted to occur in the extreme southwest region of
the ocean (see Gage and Tyler, 1991).
Oxygen concentration in the bottom water can be
a controlling factor in the preservation of organic
carbon and benthic fauna assemblages. For example,
at depths between ~100 and 1000 m, oxygen minimum
zones may develop below productive waters and coastal
upwelling zones, where the average annual ﬂux of
organic matter to the seabed is high. The resulting
hypoxia can reduce abundance and biomass of many
benthic animals, altering species composition and
richness (Diaz and Rosenberg, 1995).
In the Northern Indian Ocean, an oxygen minimum
zone occurs between depths of 100 m and 1000 m,
where oxygen concentrations are <0.5 ml °
. This zone
results from a combination of high surface productivity
driven by upwelling, inﬂow of oxygen-poor waters
from the Persian Gulf, Red Sea, and Banda Sea, and
slow deep-water circulation (Wyrtki, 1973). Such a
hypoxic layer at intermediate depths has signiﬁcant
consequences for the quantity and quality of organic
matter reaching the deep sea from surface production.
Sinking ﬂux of particulate organic carbon (POC)
Benthic organisms are fueled by sinking organic matter
from surface waters. Therefore, it is important to
evaluate what controls the ﬂux of particulate organic
carbon to the deep sea. In general, regional ﬂux of
particulate carbon decreases with depth and distance
from continents, and is directly controlled by overlying
primary production, the depth of the water column
(Suess, 1980; Smith and Hinga, 1983; Jahnke, 1996),
and the freshwater supply (Ittekkot et al., 1991).
Therefore, along continental slopes, where primary
production is high and the water column is shallow,
the ﬂux of particulate organic carbon to the seaﬂoor
is high over annual periods. Speciﬁcally, over the
Oman Margin of the Arabian Sea, particle ﬂux is
strongly seasonal, with peaks during the Southwest
and Northeast Monsoons (Nair et al., 1989; Honjo
et al., 1999). High monsoonal primary production
(912.5 g C m
), resulting from wind-induced mix-
ing and nutrient injection into the euphotic zone, is
the main factor controlling the observed pattern of
particle ﬂux (Burkill et al., 1993). For example, off
the Oman coast during the Southwest Monsoon, the
rate of sedimentation is approximately 365 g C m
at depths between 100 and 500 m (Burkill et al., 1993;
Pollehne et al., 1993). At 1500 m, the total annual ﬂux
of particulate organic carbon drops to 53 g C m
decreasing with depth to 23 g C m
at 3500 m
(Honjo et al., 1999). In general, particle ﬂuxes during
the Southwest Monsoon are greater than during the
Northeast Monsoon (Honjo et al., 1999). Because of
the lack of upwelling, the spring inter-monsoon period
is the most oligotrophic season. Low sedimentation
rates are recorded during the inter-monsoon period,
corresponding to 6% of the total annual ﬂux (Nair et al.,
1989). Therefore, there is high seasonal variability in
the total ﬂux of particulate organic carbon, as a result
of monsoonal forcing.
Rates of primary production in the central abyssal
Indian Ocean, however, are similar to those of the
south Atlantic Ocean, which is characterized by low
production rates (Steeman Nielsen, 1975). For example,
in the oligotrophic gyre of the Indian Ocean, net
primary production is less than 109 g C m
corresponding organic carbon ﬂux in this region is
approximately 36.5 g C m
(Burkill et al., 1993;
Pollehne et al., 1993).
REPESENTATIVE DEEP INDIAN OCEAN HABITATS
Oxygenated slopes and basins on the Kenya
An oxygenated slope and basin region occurs along
the margin off the coast of Kenya. Preliminary
investigations of the benthic fauna have been conducted
here, making this the best-studied region of its type in
the Indian Ocean.
Habitat and community description
Several rivers discharge terrigenous material onto the
Kenya shelf, and during monsoon periods the outﬂow is
large. The continental-shelf region of Kenya is narrow,
and the ocean is very deep near the coastline. Although
knowledge of the benthic communities is limited, some
data exist for densities and biomass of macro- and
meiofauna from sediments collected by a boxcorer-
respirometer (belljar). Along the Kenyan Slope, the
densities and biomass of macrofauna (animals retained
on a 500 mm sieve) decrease with increasing depth to
1000 m. Macrofaunal densities decrease from 7590 in-
at 500 m to 2960 individuals m
1000 m, and the biomass decreases from 26.0 g C m
THE DEEP INDIAN OCEAN FLOOR 227
(500 m) to 4.9 g C m
(1000 m) (Duineveld et al.,
1997). The meiofauna (animals retained on a 32 mm
sieve) follow the same spatial patterns as the macroben-
thos, with densities ranging from 806 individuals m
(500 m) to 223 individuals m
(1000 m). Nematodes
are the dominant taxon among the meiof auna, but
foraminifera are also present, especially branching
forms (Duineveld et al., 1997).
Carbon sources and trophic types
The Kenya shelf lacks the pulses in primary pro-
ductivity driven by upwelling (Feldman, 1989), and
thus this area has a low rate of primary production,
ranging from 109.5 to 182.5 g C m
et al., 1997). The concentration of organic carbon
in the sediment is 0.4 to 1.6% and concentrations
of organic nitrogen range from 0.05 to 0.2% (Ev-
eraarts and Nieuwenhuize, 1995). Most of the or-
ganic matter present in the sediments originates from
pelagic production (Duineveld et al., 1997; Kromkamp
et al., 1997). Sediment pigment concentrations decrease
downslope along the Kenyan margin (Duineveld et al.,
1997), suggesting that benthic faunal distribution may
be inﬂuenced by the concentration of organic matter in
the sediment and/or the ﬂux of organic matter to the
Rates of key ecological processes
In order to understand the biological and chemical
processes of benthic communities, it is important to
estimate rates of key ecological processes including
respiration, production, bioturbation, and recoloniza-
tion following disturbance. On the Kenyan slope, cross-
shelf and downslope transport of particulate organic
carbon (POC) adds to the ﬂux of sinking particles on
the slope (Duineveld et al., 1997). These processes
supplying organic matter to the seaﬂoor are tightly
coupled with benthic metabolism (Duineveld et al.,
1997). On the Kenyan margin, oxygen consumption
by the sediment community (SCOC) ranges from
1 to 14.2 mmol m
, decreasing with increasing
water depth down to 1000 m. In addition, there appears
to be little temporal variation in the rate of oxygen
consumption from June to December.
Oxygen minimum zones
Oxygen minimum zones are found in the Arabian Sea
and Bay of Bengal (Fig. 7.4). The most studied of
these is on the Oman Margin of the Arabian Sea. The
initial systematic investigation of the entire Arabian
Fig. 7.4. Locations of major oxygen minimum zones in the
Indian Ocean. In the shaded areas, dissolved bottom-water oxygen
concentrations are less than 0.2 ml °
. Modiﬁed from Diaz and
Sea was conducted within the scope of the Indian
Ocean Expedition (IIOE), from 1959 to 1965 (Wooster,
1984; Banse, 1994).
Habitat and community description
Upwelling of nitrate-rich water along the south-
ern Arabian coastline during the Southwest Mon-
soon gives rise to high rates of primary production
(304 g C m
) based on the newly-supplied nutrients
(Burkill et al., 1993), making the basin one of the
most productive oceanic regions in the world (Nair
et al., 1989). High productivity during the upwelling
season affects more than one-third of the Arabian Sea
(Ryther et al., 1966; Wyrtki, 1971, 1973; Banse, 1973).
Sediments accumulating on the Oman Margin under
the oxygen minimum zone have a high content of or-
ganic matter, owing to the high settling ﬂux of organic
matter, supported by monsoon-driven upwelling and
redistribution of the organic material by hydrodynamic
inﬂuences after deposition (Pedersen et al., 1992).
Because of the semi-enclosed nature of the northwest
Arabian Sea and the resulting sluggish intermediate-
depth circulation of water from the Red Sea and Persian
Gulf, microbial decay of the high standing crop of
228 Amanda W.J. DEMOPOULOS et al.
organic matter promotes an intense oxygen minimum
zone between the water depths of 50 and ~1000 m.
Bottom-water oxygen concentrations within this zone
range from 0.5 ml °
near the boundaries of the zone
to ~0.02 ml °
within the core at depths of 400–
700 m (Fig. 7.5) (Smith et al., 2000). Concentrations
1000 2000 3000 40000
Water Depth (m)
Fig. 7.5. Proﬁle of oxygen versus water depth from the Oman Margin.
Modiﬁed from Smith et al. (2000).
of organic carbon in sediments within the zone reach
~4% (Levin and Edesa, 1997; Levin et al., 2000). In
the upper part of the oxygen minimum zone, the high
ﬂux of particulate organic carbon coupled with the low
oxygen concentration and free hydrogen sulﬁde appears
to modify the distribution of benthic organisms (Gage
et al., 2000). Within the core of the oxygen minimum
zone (i.e., at oxygen concentrations <0.3 ml °
macrofaunal assemblages are characterized by high
densities and low diversities (Levin et al., 1997).
Macrofaunal species diversity is limited within the
Oman oxygen minimum zone, apparently because only
a relatively small number of species can tolerate oxygen
concentrations below 0.2 ml °
(Levin et al., 2000).
Between depths of 400 and 700 m within the
Oman oxygen minimum zone, sediments are frequently
speckled with remarkable worm tubes created by
cirratulid polychaetes. These worms, in the genus
Tharyx, produce cigar-shaped mudballs, 4.5–25 mm
long, which protrude several millimeters above the
sediment–water interface. Mudball densities reach
~16,000 individuals m
and they provide a habitat for
a variety of benthic organisms, including cirratulids,
epizoic polychaetes, and agglutinated and calcareous
foraminifera. Polychaetes, nemerteans, and nematodes
are also found inside the tests. Mudballs appear to
inhibit colonization by certain tube-building taxa (two
polychaetes and an amphipod), possibly because tube-
building organisms compete for food and space. In
addition, the mudballs may provide effective refuges
from predation, both for the cirratulids inside, and
for nearby burrowing taxa (Levin and Edesa, 1997).
Distribution of mudball-building cirratulids appears to
be highly restricted in terms of depth and location;
they are abundant in at least two other margin settings
with low oxygen concentrations (the San Diego Trough
and the Santa Catalina Basin). Other biogenic features
within the Oman oxygen minimum zone include
sediment mounds and burrows. Burrow diameter and
the diversity of burrow types are positively correlated
with oxygen concentration in bottom water within the
depth range of the oxygen minimum zone (Smith et al.,
The ﬂux of small organic particles to the deep
Arabian Sea is the best documented source of carbon
to the benthos. The results of short-term and long-
term measurements with sediment traps indicate a
seasonal pulse of particulate organic carbon to the
Arabian Sea bottom (Nair et al., 1989; Passow et al.,
1993; Honjo et al., 1999). The seasonal ﬂuctuation
in particulate organic carbon is the result of intense
biological productivity in the surface waters driven by
the monsoon (Nair et al., 1989; Passow et al., 1993;
Honjo et al., 1999). Particle ﬂux is most intense during
the Southwest Monsoon (Honjo et al., 1999). However,
one study of sediment-community oxygen consumption
(SCOC) revealed no signiﬁcant differences between the
Southwest and Northeast Monsoon periods (Duineveld
et al., 1997). There are seasonal ﬂuctuations in
phaeopigment concentrations on the slope; however,
high concentrations occur during the non-upwelling
season (Duineveld et al., 1997). In general, the highest
vertical ﬂuxes of particulate organic carbon occur
during the upwelling season, so it is interesting that the
sediment phaeopigment concentration does not show a
peak thereafter (Duineveld et al., 1997). It is possible
that there is a delay in chloropigment degradation, thus
providing a pool of labile organic matter available to
the benthos over long time periods (Duineveld et al.,
Aggregates of phytoplankton detritus (phytodetritus)
occur within the sediments of the oxygen minimum
THE DEEP INDIAN OCEAN FLOOR 229
zone. Phytodetritus accumulations are usually thickest
in the fall, (up to 2 cm thick), but have also been
observed as a thin layer on the sediment surface during
the spring months (Prell and Murray, pers. comm.). The
deep oxygen minimum layer allows great quantities of
detrital material to sink to the deep sea, without being
recycled by mid-water consumers; this results in an
intense ﬂux of labile organic material to the deep-sea
benthos (Gage et al., 2000). Thus, metazoan densities
and biomass observed in November may be a response
to the abundant phytodetritus available on the surface
Most of the quantitative data available for deep-
water benthic metazoans in the oxygen minimum zone
were collected during November, which corresponds to
the onset of the Northeast Monsoon upwelling period
and is ﬁve months after the onset of the Southwest
Monsoon (Levin et al., 1997, 2000; Cook et al., 2000;
Gooday et al., 2000). Therefore, the abundance of
metazoans found during this period may result from
the seasonal pulse of organic matter. Sampling needs
to be conducted during other time periods to evaluate
any seasonal ﬂuctuation in metazoan abundance and
biomass as a result of monsoonal forcing.
Megafauna: On the highly productive Oman mar-
gin, in the most intense oxygen-deﬁcient layer, the
megafaunal assemblage has a high-biomass but low
diversity. Between 300 and 700 m, the community is
dominated by the spider crab Encephaloides arm-
strongi, a cocoon-dwelling mytilid (Amygdalum sp.),
and an ascidian (Styela gagetyleri) (Creasey et al.,
1997; Young and Vazquez, 1997). At depths between
900 and 1000 m, oxygen levels and megafauna diver-
sity increase, the community consisting primarily of
ophiacanthid ophiuroids, spider crabs, and galatheid
crabs, including an abundance of Munidopsis spp.
(Gage, 1995; Smallwood et al., 1999; Creasey et al.,
2000). The ophiacanthid Ophiolimna antarctica has a
density of 51 individuals m
(Smallwood et al., 1999).
Encephaloides armstrongi is abundant throughout the
oxygen minimum zone on the Oman Margin and
appears to tolerate low oxygen concentrations (Creasey
et al., 1997; Smallwood et al., 1999). At 1000 m, the
density of crabs in general is approximately ten times
greater than that at 800 and 1250 m, with spider crabs
averaging 47 individuals m
(Smallwood et al., 1999).
It is possible that this depth is a boundary between the
more signiﬁcant oxygen minimum zone above and the
increasingly oxic conditions below (Smallwood et al.,
The high densities of spider crabs and ophiuroids in
the oxygen minimum zone have implications for the
burial of deposited organic material. Spider crabs and
ophiuroids may be highly mobile, and may resuspend
ﬁne organic material from surface sediments; in
addition, phytoplankton-derived sterols are altered by
their digestive processes (Smallwood et al., 1999).
These megabenthic activities may inﬂuence the quality
of organic matter in organic-rich sediments on the
continental slope (Smallwood et al., 1999).
Macrofauna: The macrobenthos (for the Arabian
Sea, deﬁned as animals retained on a 300 mm sieve)
are represented in the oxygen minimum zone by an
abundant, low-diversity soft-bodied fauna. Polychaetes
are the dominant group within this zone, deﬁned by
the depth range 100–1000 m and oxygen concentra-
tions <0.5 ml °
(Herring et al., 1998). Spionids and
cirratulids are most common in the upper part of the
zone (400–700 m), where oxygen concentrations are
low (0.13 ml °
), and ampharetids and paraonids in the
lower portion (850–1000 m), where oxygen concentra-
tion increases to 0.29 ml °
. Between 400 and 1000 m,
macrofaunal density ranges from 5818 to 19 183
, the highest densities being found at
700 m (Fig. 7.6; Levin et al., 2000). Macrofaunal
0 5 10 15 20 25 0 20 40 60 80
Number of Macrofauna
(m x 10 )
Water Depth (m)
(g m )
Fig. 7.6. (A) Mean density and (B) biomass for macrofauna sampled
at six water depths within and beyond the oxygen minimum zone
on the Oman Margin. Error bars represent standard error. Modiﬁed
from Levin et al. (2000).
biomass at these depths ranges from 14.2 to 59.7 g m
and again the biomass is highest at a depth of 700 m.
Each taxon appears to have a threshold above which
oxygen concentration and organic matter supply are
high enough for the animals to survive (Levin and
Gage, 1998; Levin et al., 2000).
The families represented vary with depth as bottom-
water oxygen concentration varies within the oxygen
230 Amanda W.J. DEMOPOULOS et al.
minimum zone. At 400 m, where the oxygen min-
imum is most intense, the dominant taxon is the
tube-building spionid polychaete Prionospio (Minus-
pio) sp. A (63%), followed by the cirratulid polychaete
Aphelochaeta sp. A (27%) (Levin et al., 1997).
Macrofauna are most abundant in the upper 5 cm
of sediment, this layer accounting for 84% of the
individuals found and 77% of the biomass (Levin et al.,
1997, 2000). Overall, 10 species were found at this
depth. At 700 m, two species of spionid polychaetes
(Minuspio sp. A and Paraprionospio sp. A) dominate,
whereas at 850 m the most abundant species is a
paraonid polychaete, Aricidea sp. A (21.3% of the
total individuals in the macrofauna). The most abun-
dant species at 1000 m is an ampharetid polychaete,
Eclyssipe sp. B. Crustaceans between 400 and 1000 m
primarily consist of amphipods (Ampelisca sp., and
an unidentifed gammarid), together with a few tanaids
(Levin et al., 2000). In total, 28 species are found
at 700 m, indicating an increase in species richness
with increase in oxygen concentration within this zone.
The oxygen minimum zone in the Arabian Sea is
characterized by signiﬁcantly lower species richness
than in other oxygen minimum zones such as Walvis
Bay (Sanders, 1969) and that in the eastern Paciﬁc on
the summit of Volcano 7 (Levin et al., 1991) where
oxygen concentrations range from 0.08 to 1.3 ml °
Most (63%) of the taxa within the oxygen minimum
zone live in tubes, including Prionospio sp. A,
ampharetid polychaetes, and mudball cirratulids. It is
possible that these organisms use the structures to
provide channels for pumping oxygen from above the
seaﬂoor (Levin et al., 1997). Certain organisms have
adapted to their low-oxygen environment by altering
their morphology. For example, spionid and cossurid
polychaetes from the Oman Margin have enlarged
respiratory surface area, and larger and more branched
branchiae (Lamont and Gage, 2000). Within the oxygen
minimum zone at about 400 m, the most common
species have large numbers of long branchiae or
tentacles, which most likely assist in oxygen utilization
(Levin et al., 1997). The mussel Amygdalum, which
occurs in low abundance, has a thin shell. Echinoderms
and coelenterates are absent from the community. The
general lack of taxa other than polychaetes within the
oxygen minimum zone suggests that most molluscs,
crustaceans, and echinoderms are intolerant of low-
oxygen conditions; this results in the low diversity
in these regions (Levin et al., 2000). The general
community composition of the oxygen minimum zone
in the Arabian Sea conforms with the dysaerobic
facies described for bottom-water concentrations of 0.1
to 0.5 ml °
by Rhoads et al. (1991) and with the
oxygen minimum zone macrofaunal structure present
at Volcano 7 (Levin et al., 1991, 1997), where
oxygen concentrations ranged from 0.1 to 0.2 ml °
However, in the near-anaerobic conditions on the Peru
~0.02 ml °
), bur rowing oligochaetes are
the dominant taxa, and no tube builders are present
(Levin et al., unpubl.). In addition, at Volcano 7
there is a mixture of burrowing, epibenthic and tube-
building taxa (Levin et al., 1991). Therefore, it appears
impossible to make generalizations regarding dwelling
behavior of low-oxygen macrofauna (Levin et al.,
Below the oxygen minimum zone, between 1250
and 3400 m, macrofaunal densities range from 2485
to 3190 individuals m
and biomass ranges from ~2
(Fig. 7.6; Levin et al., 2000). At 1250 m,
amphipods, tanaids, and cumaceans are present (2.7%
of macrofaunal individuals). At 3400 m, amphipods,
tanaids, and isopods form 31% of the total macrofauna.
At 1250 m, the most abundant species was a syllid
polychaete, and at 3400 m, a tanaid. Molluscs appear
to be present only at depths 1000 m (1.9%), their
proportion in the fauna increasing as oxygen concentra-
tion increases with depth – 23% at 1250 m and 18% at
3400 m (Levin et al., 2000). In general, low pH and low
oxygen concentration create an unsuitable environment
for calciﬁed taxa (Levin et al., 2000). The taxa present
on the Oman slope are broadly distributed throughout
the deep sea (Smith and Demopoulos, Chapter 6, this
volume). In the Arabian Sea, outside of the oxygen
minimum zone, the macrofauna are not as abundant as
within this zone, suggesting that there is a threshold
for opportunistic organisms that can tolerate the low
oxygen conditions and utilize the abundant food supply
in this highly productive region (Levin and Gage, 1998;
Levin et al., 2000).
Metazoan meiofauna: Meiobenthos are abundant in
the oxygen minimum zone, and have been well studied
in the Arabian Sea. However, few of these studies used
comparable sampling and laboratory techniques. Ne-
matodes and foraminiferans are the major meiobenthic
taxa present, followed by harpacticoid copepods, poly-
chaetes, and turbellarians (Qasim, 1982). Data from
core samples (10 cm deep × 3.4 cm
area) taken from
grab samples indicate that meiobenthic biomass in the
oxygen minimum zone between 200 and 1000 m ranges
THE DEEP INDIAN OCEAN FLOOR 231
from 2.01 to 42.30 g m
, and at depths greater than
1000 m the biomass ranges from 16.55 to 119 g m
(Qasim, 1982). Since it has been documented that the
oxygen concentration in the bottom water increases
from 0.13 ml °
at a depth of 400 m to 0.27 ml °
1000 m (Smith et al., 2000), it appears that meiobenthic
biomass and abundance follow the same pattern as for
the mega- and macrofauna, increasing with increasing
Nematode abundance, estimated from sediment sam-
ples collected with a multiple corer using 25 cm
is positively correlated with macrofauna abundance.
Between 400 and 700 m, nematode abundance ranges
from 1700 to 2495 individuals m
, and the oxygen
concentration from 0.13 to 0.16 ml °
(Cook et al.,
2000; Smith et al., 2000). At the lower boundary of the
oxygen minimum zone (1250 m) and beyond (3400 m),
nematode abundance decreases, ranging from 860 to
494 individuals m
, respectively (Cook et al., 2000).
Bottom-water oxygen concentration does not appear to
be the controlling factor for the nematode population –
rather, food quality, as measured by the hydrogen
(Patience and Gage, unpublished), appears to be
the major predictor of overall nematode abundance in
the Oman slope region (Cook et al., 2000).
Protozoa: In the oxygen minimum zone between 200
and 600 m, the dominant foraminifera present include
Bolivina pygmaea, Bulimina sp., and Lenticulina iota
(Hermelin and Shimmield, 1990). At 400 m, corre-
sponding to the core of the oxygen minimum zone, the
foraminiferan taxa also include allogromiids, bathysi-
phonids (Bathysiphon spp.), hormosinaceans (mostly
Leptohalysis spp.), saccamminids (Lagenammina spp.),
spiroplectamminaceans, textulariaceans and trocham-
minaceans (Gooday et al., 2000). From 600 to 1000 m,
Ehrenbergina trigona, Hyalinea balthica, Tritaxia sp.,
and Uvigerina peregrina dominate the foraminiferan
assemblage. These taxa appear to be closely related;
they could be limited by low oxygen concentration, and
possibly by the organic-carbon concentration in the sed-
iment (Hermelin and Shimmield, 1990). Foraminiferan
taxa found in the oxygen minimum zone appear to be
smaller in size (92.9% were <500 mm) and have more
elongate tests (160 mm) than foraminifera collected out-
side the oxygen minimum zone at 3400 m, which had
an average test length of 120 mm (Gooday et al., 2000).
Below the oxygen minimum zone (3350 m), very large,
tubular, agglutinated species can be found, speciﬁcally
the genera Bathysiphon, Hyperammina, Rhabdammina
and Saccorhiza (Gooday et al., 2000). Foraminiferan
densities in the oxygen minimum zone of the Arabian
Sea are among the highest reported from an oxygen-
poor environment (Gooday et al., 2000). Foraminifera
from the Santa Barbara Basin (590 m, O
~0.1 ml °
and from the Peru margin (300–1200 m, O
to 1.6 ml °
), follow the same trend; in these areas,
however, soft-shelled monothalamous taxa are rare
and large agglutinated taxa are absent. Foraminifera
and metazoans show similar population responses to
oxygen stress: species dominance increases, diversity
decreases, and the relative abundance of major taxa
changes (Gooday et al., 2000).
The benthic ﬂagellates are signiﬁcantly more abun-
dant in the sediments during the non-upwelling season.
Although grazing rates are low, bacterivory at that
period has a signiﬁcantly greater impact on bacterial
standing stock in the bottom water than during
upwelling (Bak and Nieuwland, 1997). Microbes are
fueled by particle ﬂux from the surface waters, and
respond to seasonal sedimentation of organic matter
(e.g., Pfannkuche, 1993).
Nanobiota: Seasonal deposition of organic matter
in the Arabian Sea results in seasonality of benthic
microbial production. After the upwelling season,
for instance, bacterial abundance and production are
high (Ducklow, 1993). Bacterial density, biomass, and
cell volume are larger during the August upwelling
period than in the non-upwelling period (February).
There is no obvious relationship between the biomass
and abundance of microbes and the existence of
the intense oxygen minimum zone, which is equally
present in both seasons (Duineveld et al., 1997; Bak
and Nieuwland, 1997). There is a decrease in the
biomass and abundance of benthic bacteria and benthic
nanoﬂagellates with increase in depth in sediment, and
also with increasing ocean depth. Bacterial densities
in the Arabian Sea decrease from 1.5×10
surface sediments to 0.8×10
at a depth of
10 cm within the sediment (Bak and Nieuwland, 1997).
Within the core of the oxygen minimum zone (400 m),
average bacterial densities range from 25×10
the upper 0.5 cm of sediment, to 10 × 10
The hydrogen index has been suggested as a proxy for sediment food quality, and is a measure of the hydrogen content (and hence the
redox state) of the organic matter. Its units are (mg hydrocarbon)/(g total organic carbon).
232 Amanda W.J. DEMOPOULOS et al.
depth of 4.5 cm in the sediment (Levin et al., 1997).
Colonies of Thioploca reach densities of 22 117 m
(Levin et al., 1997). With increasing seaﬂoor depth,
bacterial densities range from 4×10
at 200 m to
0.6 × 10
at 5000 m (Bak and Nieuwland, 1997).
The most prevalent feeding mode among the macro-
fauna in the oxygen minimum zone is deposit feeding –
that is, the ingestion of sediment and associated
organic matter. For the depth range from 400 to
1000 m within the oxygen minimum zone, most of
the macrofauna (94%) are tentaculate, surface-deposit
feeders. The nemerteans are likely to be scavengers or
carnivores, and the mussel Amygdalum politum is a
ﬁlter feeder. Cossurid polychaetes (constituting 1.1%
of the fauna) may be the only subsurface deposit
feeders present in this region. Below 850 m, subsurface
deposit feeders constitute an increasing proportion of
the total fauna, the largest ﬁgure being recorded at
3400 m (Levin et al., 2000). Subsurface deposit feeders
are usually present in deep-sea or organically enriched
environments (Levin et al., 1997). However, in organic-
rich oxygen minimum zones, opportunistic species that
can survive oxygen stress are generally surface-deposit
feeders. Organic-rich sediments resulting from high
surface production probably contribute to the high
dominance of surface-deposit feeders in a relatively
dense faunal assemblage, which has been observed
(Levin et al., 1997, 2000; Levin and Gage, 1998).
Nematodes generally feed on detrital particles, sed-
iment, and/or bacteria, although some nematodes are
carnivorous (Gage and Tyler, 1991). Food availability
appears to govern foraminiferal abundance and biomass
(Altenbach, 1988; Altenbach and Sarnthein, 1989;
Herguera and Berger, 1991; Gooday et al., 2000).
Generally, foraminifera consume phytodetritus, the
bodies of small dead animals, bacteria associated with
sediment, particulate organic carbon, and potentially
dissolved organic carbon (Gooday et al., 1992). Where
food is plentiful, foraminifera succeed, b ut they also
must tolerate the reduced oxygen availability that is
concomitant with abundance of organic matter (Gooday
et al., 2000). The predominance of these organisms in
the oxygen minimum zone of the Arabian Sea suggests
that the meiofauna, of which they constitute the major
part, occupy low trophic levels.
Rates of ecological processes
Very few data exist estimating the rates of key
ecological processes in the oxygen minimum zone of
the Arabian Sea. Useful data exist for the oxygen con-
sumption of the sediment community (SCOC) in the
oxygen minimum zone, speciﬁcally from the sediment
below the Yemen–Somali upwelling region (~500–
800 m). During both the Southwest and Northeast
Monsoons, the oxygen consumption of the sediment
community ranged from 0.7 to 4.3 mmol m
the zone between 70 and 1700 m was covered with
water with a low oxygen content (10–50 mM) (Duin-
eveld et al., 1997). These values are 3–7 times higher
than reported for oxygenated slopes in the Paciﬁc
(Hammond et al., 1996; Smith and Demopoulos,
Chapter 6, this volume).
It may be expected that the bioturbation activities
of benthos are linked with bottom-water oxygen
concentration (Pearson and Rosenberg, 1978; Rhoads
et al., 1978; Diaz and Rosenberg, 1995; Smith et al.,
2000). In the oxygen minimum zone on the Oman
slope, rates and patterns of bioturbation have been
evaluated using proﬁles of
Pb and X-radiography
(Smith et al., 2000). The mixing depths for
the oxygen minimum zone, with oxygen concentrations
of 0.13–0.27 ml °
, were half of those on oxygenated
slopes in other oceans (mean depths 4.6 cm and 11 cm,
respectively). The reduction in the
Pb mixing depth
likely results from the prevalence of surface-deposit
feeders and tube builders within this oxygen minimum
zone (Levin et al., 2000; Smith et al., 2000). Unlike
oxygen minimum zones in other oceans, there does not
appear to be enhanced bioturbation at the boundary of
the Oman oxygen minimum zone, possibly because of
the gradual change in oxygen concentration from 0.13
to 0.27 ml °
over the breadth of the zone (Smith et al.,
The Western and Central Abyssal Indian Ocean
Habitat and community description
The deep abyssal zone of the Indian Ocean is an
area of active deep-sea circulation (Parulekar et al.,
1982). It is a habitat with rich benthic biomass.
Investigations on deep-sea benthos in the western and
central Indian Ocean, in the depth range of 1500 to
6000 m, have revealed abundant biota but low species
diversity (Parulekar et al., 1992).
Sediment samples have been collected by grab,
and macrofauna (retained on a 500 mm sieve) and
meiofauna (retained on a 44 mm sieve) from the deep
Arabian Basin and the Central Indian Basins have
THE DEEP INDIAN OCEAN FLOOR 233
been quantiﬁed (Par ulekar et al., 1982, 1992). The
fauna from these abyssal sediments (3600–5300 m) are
composed of 12 macrofaunal and 3 meiofaunal inver-
tebrate 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 proto-
zoans (which were not included in these percentage
ﬁgures). Macrofaunal densities range from 92 to
462 individuals m
, and biomass ranges from 0.47 to
13.32 g m
(Parulekar et al., 1982, 1992). Megafaunal
scavengers present include ophidiid ﬁsh (Lochte and
The abyssal plains of the Indian Ocean harbor rich
meiobenthic assemblages, meiofauna density ranging
between 50 177 and 232 912 individuals m
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
to 0.41 g m
todes are the most abundant group, accounting for
53.3% of the individuals, followed by foraminifer-
ans 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 be-
cause 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
) in areas of thick sediments,
compared to areas with thin sediments (3.5–5 kg m
(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
, dominated by nematoda. The
macrofaunal density ranged from 8–64 individuals m
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
(Parulekar et al., 1982). These sediments had almost
three times the macrofaunal biomass (5.16 g m
the yellow calcareous oozy sediments without nod-
ules (1.78 g m
The mean benthic population density (meiofauna
and macrofauna) from abyssal sediments varies from
233 322 individuals m
in the 1500–1999 m depth
zone to 50 269 individuals m
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
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 t ransporting 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 f auna 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
234 Amanda W.J. DEMOPOULOS et al.
and Moore, 1971; Ohta, 1984). During present sea-
level conditions, a majority of sediments from large
rivers (the Ganges and Brahmaputra) are reported to be
trapped in the subsiding deltas and on the inner shelf,
and thus little sediment and nutrients are transported by
the turbidity channels (Curray and Moore, 1971). The
Bay of Bengal experiences wind-generated upwelling
along the coast, promoting primary productivity. Aver-
age primary productivity in the Bay is 109.5 g C m
(Pant, 1992). The carbonate compensation depth in
these waters is ~4500 m; at shallower depths, the
calcium-carbonate sediments are covered with thin
greenish brown ﬂocculent material.
Limited data from this region come from sedi-
ments collected by grab samples; the total benthic
biomass (meiofauna and macrofauna) in the Bay of
Bengal ranges from 0.11 to 0.38 g m
Pasternak, 1962, 1964; Neyman et al., 1973). These
abundances appear to be low compared to the rest of
the Indian Ocean. Despite the relatively low biomass
of deep-sea benthic organisms in the Bay of Bengal,
distinct biogenic features can be observed on the
surface of the deep-sea ﬂoor. Speciﬁcally, star-shaped
feeding traces produced by echiuran worms can be
observed (Fig. 7.7). They live between the depths of
Fig. 7.7. Star-shaped echiuran feeding trace from ~4000 m in the Bay
of Bengal. Modiﬁed from Ohta (1984).
2635 m and 5025 m (Ohta, 1984). As the organism
feeds on surface detritus, its proboscis skims the
sediment surface of the deep-sea ﬂoor radially, leaving
a distinctive star-shaped feature. These features are also
found in the deep Paciﬁc and Atlantic Oceans (Gage
and Tyler, 1991; Gage, Chapter 11, this volume).
The surface productivity by phytoplankton is poor,
and therefore the zooplankton biomass is poor (Pant,
1992). As a result, total transport of organic matter to
the sea ﬂoor may be expected to be low. Thus, benthic
biomass and abundance in the Bay of Bengal appear to
reﬂect the low surface productivity.
CONCLUSIONS AND OUTSTANDING PROBLEMS
The deep Indian Ocean is composed of a variety
of habitat types, including abyssal plains, oxygenated
slopes and basins, oxygen minimum zones, seamounts,
and trenches. This chapter summarizes the available
data from a few of these habitats. The general
conclusions are that the deep Indian Ocean still remains
poorly known, and is waiting to be discovered and
understood. We have identiﬁed below speciﬁc areas that
need to be explored within the Indian Ocean, including
habitats and ecological rates.
(1) Complete benthic habitat descriptions for
seamounts, the Java Trench, and other oxygen
minimum zones (e.g., the Bay of Bengal) are not
available. In order to understand the productivity of
the Indian Ocean and compare it with other oceans,
extensive benthic surveys need to be conducted. In
addition, the acquisition of reliable data for biomass
and abundance from seamounts is important in ﬁsh-
eries. Current knowledge of the deep-sea organisms
constituting the Indian Ocean benthos is very limited.
(2) Total energy budgets and biomass estimates
for all size classes are not available for any deep-sea
habitat in the Indian Ocean. Complete estimates of
biomass production for benthic populations are scarce.
(3) Composition, variability, and ﬂux rate of
particulate organic carbon to the seaﬂoor is poorly
quantiﬁed throughout the Indian Ocean. The nature
and ﬂux of other food sources to the deep sea, (e.g.,
phytodetritus, nekton falls), is also unknown for the
Indian Ocean. Not only are these important food
sources for the deep-sea benthos – quantifying them is
necessary for calculating the global carbon budget.
(4) Data on ecological rates, including the
benthic response to the intense seasonal (monsoonal)
production cycle, are scarce. Bioturbation rates
have been evaluated in oxygen minimum zone of
the Oman slope, but data for other habitats in the
deep sea are very limited. Because the mining of
manganese nodules is becoming more important, more
intensive research involving the responses of benthic
communities to disturbance, both anthropogenic and
natural, is imperative.
(5) Chemosynthetic environments. There is recent
THE DEEP INDIAN OCEAN FLOOR 235
evidence of hydrothermal venting on the Southwest
Indian Ridge (German et al., 1998), and it would be
of great biogeographic interest to see if the dominant
fauna is more related to the Atlantic or to the Paciﬁc
vents. There is also evidence of reducing conditions in
sediments near the base of the oxygen minimum zone,
where the fauna may gain energy from chemosynthetic
The Indian Ocean remains an exciting area for
pioneering research. A complete understanding of
its habitats and processes will not be possible until
the outstanding problems mentioned above have been
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