THE DEEP PACIFIC OCEAN FLOOR
211
sediment–water interface (Levin and Thomas, 1989).
In another set of studies of contrasting sedimentmobility regimes, Levin et al. (1994) found that, on
Fieberling Guyot in the eastern Pacific at depths of 585
to 635 m, sediment mobility was associated with higher
macrofaunal densities (1870 m
−2 versus 1489 m
−2 ) and
lower species diversity than in a quiescent setting. Tube
building, surface-deposit feeding, and filter feeding
were more common in the stable substrata, whereas
subsurface burrowers were more common in shifting
sands. Colonization rates were also faster in the
shifting-sand habitat on Fieberling Guyot, apparently
because bedload transport of juveniles and adults was
the dominant recolonization mode over small spatial
scales at this site (Levin and DiBacco, 1995). In
fact, the macrofaunal community in the shifting-sand
site retained features of early successional stages,
suggesting that ripple migration constituted a significant macrofaunal disturbance (Levin and DiBacco,
1995). All of these studies suggest that substrate
mobility may exert substantial control over community
structure and colonization rates on Pacific seamounts,
and that the structure of the benthic community on
seamounts is controlled by a complex suite of variables
(e.g., hydrodynamic regime, sediment mobility, grain
size, presence/absence of xenophyophores) which vary
dramatically over space and time within and among
seamounts.
Other features of the ecology of deep seamounts,
such as rates of organic carbon flux and mineralization,
broad-scale patterns of species diversity and sizeclass structure of benthos, are either too poorly
studied or are too heterogeneous to allow useful
generalizations to be drawn. In general, we expect
that the specific conditions in seamount habitats (in
terms of hydrodynamic regime, sediment type and
mobility, horizontal and vertical fluxes of particulate
organic carbon) overwhelm the broader regional and
depth patterns discussed above for the level, sedimentcovered deep-sea floor. Nonetheless, a number of
insights into deep-sea biology can be gained from the
study of seamounts, and Rogers (1994) has provided a
detailed overview of current knowledge of the biology
of seamounts, including their potential importance as
sites of speciation and for commercial fisheries.
Deep ocean trenches
The hadal zone – that is, deep-ocean trenches with
depths ranging from 6000 to 11 000 m – covers about
2% of the Pacific Ocean floor. The most dramatic
environmental characteristic of the hadal zone is
extremely high hydrostatic pressure which exceeds
that in any known metazoan habitat. In the deepest
portions of the hadal zone (9000 to 11 000 m), the
pressures of 900 to 1100 bar have profoundly affected
the composition and zoogeography of the benthos.
Habitat and community description
The bottom waters of Pacific trenches range in
temperature from 1.1 to 3.3ºC. Despite their great
depth, trench floors receive relatively high fluxes
of particulate organic carbon and sediments, as a
result of their proximity to coastal productivity and/or
the focusing effects of their steep, narrow walls
(Belyaev, 1972). In addition, the high supplies of
sediments from coastal waters, combined with steep
slopes, topographic enhancement of bottom currents,
and frequent seismic activity, are likely to make
turbidity flows and sediment slumps common at the
bottom of trenches (Jumars and Hessler, 1976). As a
consequence, trench-floor sediments often are poorly
consolidated (or “soupy”) clayey sediments rich in
organic carbon compared to surrounding abyssal areas
(Belyaev, 1972; Jumars and Hessler, 1976; Hessler
et al., 1978). In addition, trench walls often contain
substantial areas of rocky substratum exposed by
erosive currents and/or sediment slumping (Hessler
et al., 1978). In general, trench habitats are thought to
be food-rich but physically unstable compared to the
most of the abyssal seafloor.
The benthic ecology of Pacific trenches has not been
well studied in recent decades, so little specific can
be said about carbon sources and ecological rates.
Most trench data are derived from older, semiquantitave
studies with trawls and grab samples. These studies
reveal a “hadal” (or trench) fauna distinct from that
of shallower depths in the deep sea (Vinogradova,
1979). Based on trawl samples from 27 trenches in
the Pacific Ocean, Belyaev (1972) reported that the
hadal fauna from depths exceeding 6000 m contains
broad taxonomic diversity, and is missing only a
few higher-level marine taxa (for example, decapods
and brachiopods). However, species diversity declines
dramatically from 6000 m to depths exceeding 8500 m.
Holothurians dominate megafaunal abundance and
biomass in trenches, especially at depths greater than
7000 m. Bivalves and polychaetes also are important
components, with ophiuroids, sipunculans, asteroids,
and non-decapod crustaceans occurring frequently
211
sediment–water interface (Levin and Thomas, 1989).
In another set of studies of contrasting sedimentmobility regimes, Levin et al. (1994) found that, on
Fieberling Guyot in the eastern Pacific at depths of 585
to 635 m, sediment mobility was associated with higher
macrofaunal densities (1870 m
−2 versus 1489 m
−2 ) and
lower species diversity than in a quiescent setting. Tube
building, surface-deposit feeding, and filter feeding
were more common in the stable substrata, whereas
subsurface burrowers were more common in shifting
sands. Colonization rates were also faster in the
shifting-sand habitat on Fieberling Guyot, apparently
because bedload transport of juveniles and adults was
the dominant recolonization mode over small spatial
scales at this site (Levin and DiBacco, 1995). In
fact, the macrofaunal community in the shifting-sand
site retained features of early successional stages,
suggesting that ripple migration constituted a significant macrofaunal disturbance (Levin and DiBacco,
1995). All of these studies suggest that substrate
mobility may exert substantial control over community
structure and colonization rates on Pacific seamounts,
and that the structure of the benthic community on
seamounts is controlled by a complex suite of variables
(e.g., hydrodynamic regime, sediment mobility, grain
size, presence/absence of xenophyophores) which vary
dramatically over space and time within and among
seamounts.
Other features of the ecology of deep seamounts,
such as rates of organic carbon flux and mineralization,
broad-scale patterns of species diversity and sizeclass structure of benthos, are either too poorly
studied or are too heterogeneous to allow useful
generalizations to be drawn. In general, we expect
that the specific conditions in seamount habitats (in
terms of hydrodynamic regime, sediment type and
mobility, horizontal and vertical fluxes of particulate
organic carbon) overwhelm the broader regional and
depth patterns discussed above for the level, sedimentcovered deep-sea floor. Nonetheless, a number of
insights into deep-sea biology can be gained from the
study of seamounts, and Rogers (1994) has provided a
detailed overview of current knowledge of the biology
of seamounts, including their potential importance as
sites of speciation and for commercial fisheries.
Deep ocean trenches
The hadal zone – that is, deep-ocean trenches with
depths ranging from 6000 to 11 000 m – covers about
2% of the Pacific Ocean floor. The most dramatic
environmental characteristic of the hadal zone is
extremely high hydrostatic pressure which exceeds
that in any known metazoan habitat. In the deepest
portions of the hadal zone (9000 to 11 000 m), the
pressures of 900 to 1100 bar have profoundly affected
the composition and zoogeography of the benthos.
Habitat and community description
The bottom waters of Pacific trenches range in
temperature from 1.1 to 3.3ºC. Despite their great
depth, trench floors receive relatively high fluxes
of particulate organic carbon and sediments, as a
result of their proximity to coastal productivity and/or
the focusing effects of their steep, narrow walls
(Belyaev, 1972). In addition, the high supplies of
sediments from coastal waters, combined with steep
slopes, topographic enhancement of bottom currents,
and frequent seismic activity, are likely to make
turbidity flows and sediment slumps common at the
bottom of trenches (Jumars and Hessler, 1976). As a
consequence, trench-floor sediments often are poorly
consolidated (or “soupy”) clayey sediments rich in
organic carbon compared to surrounding abyssal areas
(Belyaev, 1972; Jumars and Hessler, 1976; Hessler
et al., 1978). In addition, trench walls often contain
substantial areas of rocky substratum exposed by
erosive currents and/or sediment slumping (Hessler
et al., 1978). In general, trench habitats are thought to
be food-rich but physically unstable compared to the
most of the abyssal seafloor.
The benthic ecology of Pacific trenches has not been
well studied in recent decades, so little specific can
be said about carbon sources and ecological rates.
Most trench data are derived from older, semiquantitave
studies with trawls and grab samples. These studies
reveal a “hadal” (or trench) fauna distinct from that
of shallower depths in the deep sea (Vinogradova,
1979). Based on trawl samples from 27 trenches in
the Pacific Ocean, Belyaev (1972) reported that the
hadal fauna from depths exceeding 6000 m contains
broad taxonomic diversity, and is missing only a
few higher-level marine taxa (for example, decapods
and brachiopods). However, species diversity declines
dramatically from 6000 m to depths exceeding 8500 m.
Holothurians dominate megafaunal abundance and
biomass in trenches, especially at depths greater than
7000 m. Bivalves and polychaetes also are important
components, with ophiuroids, sipunculans, asteroids,
and non-decapod crustaceans occurring frequently