THE DEEP INDIAN OCEAN FLOOR
231
from 2.01 to 42.30 g m
−2 , and at depths greater than
1000 m the biomass ranges from 16.55 to 119 g m
−2
(Qasim, 1982). Since it has been documented that the
oxygen concentration in the bottom water increases
from 0.13 ml °
−1 at a depth of 400 m to 0.27 ml °
−1 at
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
oxygen concentration.
Nematode abundance, estimated from sediment samples collected with a multiple corer using 25 cm
2 tubes,
is positively correlated with macrofauna abundance.
Between 400 and 700 m, nematode abundance ranges
from 1700 to 2495 individuals m
−2 , and the oxygen
concentration from 0.13 to 0.16 ml °
−1 (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
−2 , 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
index
1 (Patience and Gage, unpublished), appears to be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, corresponding to the core of the oxygen minimum zone, the
foraminiferan taxa also include allogromiids, bathysiphonids (Bathysiphon spp.), hormosinaceans (mostly
Leptohalysis spp.), saccamminids (Lagenammina spp.),
spiroplectamminaceans, textulariaceans and trochamminaceans (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 sediment (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 outside 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 oxygenpoor environment (Gooday et al., 2000). Foraminifera
from the Santa Barbara Basin (590 m, O 2 ~0.1 ml °
−1 )
and from the Peru margin (300–1200 m, O 2 = 0.02
to 1.6 ml °
−1 ), 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 abundant 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
9 cm
−3 in
surface sediments to 0.8×10
9 cm
−3 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
7 cm
−3 for
the upper 0.5 cm of sediment, to 10 × 10
7 cm
−3 at a
1 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).
231
from 2.01 to 42.30 g m
−2 , and at depths greater than
1000 m the biomass ranges from 16.55 to 119 g m
−2
(Qasim, 1982). Since it has been documented that the
oxygen concentration in the bottom water increases
from 0.13 ml °
−1 at a depth of 400 m to 0.27 ml °
−1 at
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
oxygen concentration.
Nematode abundance, estimated from sediment samples collected with a multiple corer using 25 cm
2 tubes,
is positively correlated with macrofauna abundance.
Between 400 and 700 m, nematode abundance ranges
from 1700 to 2495 individuals m
−2 , and the oxygen
concentration from 0.13 to 0.16 ml °
−1 (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
−2 , 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
index
1 (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, corresponding to the core of the oxygen minimum zone, the
foraminiferan taxa also include allogromiids, bathysiphonids (Bathysiphon spp.), hormosinaceans (mostly
Leptohalysis spp.), saccamminids (Lagenammina spp.),
spiroplectamminaceans, textulariaceans and trochamminaceans (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 sediment (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 outside 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 oxygenpoor environment (Gooday et al., 2000). Foraminifera
from the Santa Barbara Basin (590 m, O 2 ~0.1 ml °
−1 )
and from the Peru margin (300–1200 m, O 2 = 0.02
to 1.6 ml °
−1 ), 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 abundant 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
9 cm
−3 in
surface sediments to 0.8×10
9 cm
−3 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
7 cm
−3 for
the upper 0.5 cm of sediment, to 10 × 10
7 cm
−3 at a
1 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).