230
Amanda W.J. DEMOPOULOS et al.
minimum zone. At 400 m, where the oxygen minimum is most intense, the dominant taxon is the
tube-building spionid polychaete Prionospio (Minuspio) 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 abundant 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 °
−1 .
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 lowoxygen 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 °
−1 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 °
−1 .
However, in the near-anaerobic conditions on the Peru
margin (O 2 ~0.02 ml °
−1 ), burrowing 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 tubebuilding taxa (Levin et al., 1991). Therefore, it appears
impossible to make generalizations regarding dwelling
behavior of low-oxygen macrofauna (Levin et al.,
2000).
Below the oxygen minimum zone, between 1250
and 3400 m, macrofaunal densities range from 2485
to 3190 individuals m
−2 and biomass ranges from ~2
to 10 g m
−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 concentration 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. Nematodes and foraminiferans are the major meiobenthic
taxa present, followed by harpacticoid copepods, polychaetes, and turbellarians (Qasim, 1982). Data from
core samples (10 cm deep × 3.4 cm
2 area) taken from
grab samples indicate that meiobenthic biomass in the
oxygen minimum zone between 200 and 1000 m ranges
Amanda W.J. DEMOPOULOS et al.
minimum zone. At 400 m, where the oxygen minimum is most intense, the dominant taxon is the
tube-building spionid polychaete Prionospio (Minuspio) 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 abundant 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 °
−1 .
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 lowoxygen 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 °
−1 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 °
−1 .
However, in the near-anaerobic conditions on the Peru
margin (O 2 ~0.02 ml °
−1 ), burrowing 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 tubebuilding taxa (Levin et al., 1991). Therefore, it appears
impossible to make generalizations regarding dwelling
behavior of low-oxygen macrofauna (Levin et al.,
2000).
Below the oxygen minimum zone, between 1250
and 3400 m, macrofaunal densities range from 2485
to 3190 individuals m
−2 and biomass ranges from ~2
to 10 g m
−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 concentration 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. Nematodes and foraminiferans are the major meiobenthic
taxa present, followed by harpacticoid copepods, polychaetes, and turbellarians (Qasim, 1982). Data from
core samples (10 cm deep × 3.4 cm
2 area) taken from
grab samples indicate that meiobenthic biomass in the
oxygen minimum zone between 200 and 1000 m ranges