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
−2
(Levin et al., 1997). With increasing seafloor depth,
bacterial densities range from 4×10
9 cm
−3 at 200 m to
0.6 × 10
9 cm
−3 at 5000 m (Bak and Nieuwland, 1997).
Trophic types
The most prevalent feeding mode among the macrofauna 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 organicrich 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, sediment, 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, but 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 consumption 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
−2 d
−1 when
the zone between 70 and 1700 m was covered with
water with a low oxygen content (10–50 mM) (Duineveld 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
210 Pb and X-radiography
(Smith et al., 2000). The mixing depths for
210 Pb within
the oxygen minimum zone, with oxygen concentrations
of 0.13–0.27 ml °
−1 , were half of those on oxygenated
slopes in other oceans (mean depths 4.6 cm and 11 cm,
respectively). The reduction in the
210 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 °
−1 over the breadth of the zone (Smith et al.,
2000).
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
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
−2
(Levin et al., 1997). With increasing seafloor depth,
bacterial densities range from 4×10
9 cm
−3 at 200 m to
0.6 × 10
9 cm
−3 at 5000 m (Bak and Nieuwland, 1997).
Trophic types
The most prevalent feeding mode among the macrofauna 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 organicrich 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, sediment, 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, but 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 consumption 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
−2 d
−1 when
the zone between 70 and 1700 m was covered with
water with a low oxygen content (10–50 mM) (Duineveld 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
210 Pb and X-radiography
(Smith et al., 2000). The mixing depths for
210 Pb within
the oxygen minimum zone, with oxygen concentrations
of 0.13–0.27 ml °
−1 , were half of those on oxygenated
slopes in other oceans (mean depths 4.6 cm and 11 cm,
respectively). The reduction in the
210 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 °
−1 over the breadth of the zone (Smith et al.,
2000).
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