204
Craig R. SMITH and Amanda W.J. DEMOPOULOS
slope, the scavenger community of equatorial Pacific
sediments does not include epibenthic species (e.g.,
ophiuroids, onuphid polychaetes) which walk to baitfalls.
The xenophyophores, which constitute 90–95% of
the megafaunal abundance at both eutrophic and
mesotrophic EqPac sites, can be considered as deposit
feeders that primarily digest organic material from
detrital particles (Levin and Gooday, 1992; Gooday
et al., 1993). It is also quite possible that these giant
protozoans take up dissolved organic matter, prey on
small metazoans, and cultivate bacteria (Levin and
Gooday, 1992); they thus may simultaneously occupy a
number of trophic levels. Because of their low biomass,
the flux of energy through xenophyophores is likely to
be small compared to the remainder of the benthos,
even when xenophyophores are abundant (Levin and
Gooday, 1992).
Suspension-feeding glass sponges in the genus
Hyalonema dominate the metazoan, epibenthic megafauna at eutrophic and mesotrophic stations along
the EqPac transect, constituting 55% to 87% of the
metazoan megafaunal abundance (Hoover et al., 1994);
this contrasts sharply with California slope habitats
where mobile deposit feeders or omnivores dominate
the megafauna. The remainder of the megafaunal
epibenthos (13% to 45%) at eutrophic stations is composed of surface/subsurface deposit feeders including
irregular urchins that plow through surface sediments,
and presumably holothurians feeding selectively on
the surface deposits (Hoover et al., 1994; Smith and
Hoover, unpublished data). Fresh spoke traces (or
rosettes) formed by echiurans and large polychaetes
are quite common in both eutrophic and mesotrophic
settings, attaining densities (0.06 to 0.2 m
−2 ) comparable to that of the megafaunal epibenthos. These traces
indicate that the burrowing megafauna is also likely
to contain a relatively high abundance of selective
surface-deposit feeders.
Thus far, trophic analyses of macrofauna in the
equatorial abyss have been restricted to the polychaetes, which constitute more than 60% of community
abundance and biomass (see above). Studies in the
Clipperton–Clarion Fracture Zone (Paterson et al.,
1998) and at the DISCOL site (Borowski and Thiel,
1998) indicate that more than 58% of the total
polychaete abundance falls into families considered to
be deposit feeders in the deep sea (e.g., Kukert and
Smith, 1992), with the cirratulids, paraonids, sabellids
and spionids accounting for most of the abundance.
In both areas, surface-deposit feeders predominate,
comprising at least 37 to 56% of polychaete abundance.
Subsurface deposit feeders, consisting primarily of
paranoids, make up only 8.5 to 22% of the polychaetes.
In the Clipperton–Clarion Fracture Zone, predators and
omnivores constitute a surprisingly high percentage of
the polychaete community, accounting for 18 to 28% of
polychaete abundance. Based on the limited data thus
far available, there do not appear to be any marked
differences in polychaete trophic composition between
eutrophic and mesotrophic equatorial sediments (Paterson et al., 1998). It should be noted that the trophic
structure of polychaetes in the equatorial abyss differs
substantially from that on the oxygenated California
margin, where subsurface deposit feeders typically
constitute at least 50% of community abundance (see
above).
The Nematoda are also strongly dominated by
deposit feeders. Brown (1998) found that 59 to 76%
of individuals, and 53 to 68% of species, of nematodes
from the top centimeter of sediment along the EqPac
transect (0º, 2º, 5º, and 9ºN along the 140ºW meridian)
were deposit feeders. Selective deposit feeders predominated (57 to 68% of total numbers), and their absolute
abundance was strongly correlated with microbial
abundance in the sediments along the transect. Predatory and/or scavenging nematodes were very rare in
the equatorial Pacific, accounting for less than 10% of
the total number of individuals at each station (Brown,
1998). Very low predator/scavenger abundance is a
typical feature of abyssal nematode communities when
compared to shallow-water sediment assemblages, and
is thought to reflect a lower relative availability of prey
items and carrion in the abyss (Brown, 1998).
Rates of key ecological processes
A number of key ecological rates have been evaluated in the equatorial Pacific, including sediment
community respiration, bioturbation, and, to some
extent, recolonization following disturbance. These rate
data come primarily from the EqPac study and the
DISCOL experiment.
Studies with benthic incubation chambers and sediment porewaters indicate that, in January 1992,
seafloor oxygen consumption was fairly constant
along the equator from 103ºW to 140ºW, with rates
of 0.6 to 0.8 mmol m
−2 d
−1 (equivalent to roughly
2 g C m
−2 y
−1 ) (Hammond et al., 1996). Seafloor respiration rate declined roughly symmetrically with
distance from the equator along the 140ºW meridian
Craig R. SMITH and Amanda W.J. DEMOPOULOS
slope, the scavenger community of equatorial Pacific
sediments does not include epibenthic species (e.g.,
ophiuroids, onuphid polychaetes) which walk to baitfalls.
The xenophyophores, which constitute 90–95% of
the megafaunal abundance at both eutrophic and
mesotrophic EqPac sites, can be considered as deposit
feeders that primarily digest organic material from
detrital particles (Levin and Gooday, 1992; Gooday
et al., 1993). It is also quite possible that these giant
protozoans take up dissolved organic matter, prey on
small metazoans, and cultivate bacteria (Levin and
Gooday, 1992); they thus may simultaneously occupy a
number of trophic levels. Because of their low biomass,
the flux of energy through xenophyophores is likely to
be small compared to the remainder of the benthos,
even when xenophyophores are abundant (Levin and
Gooday, 1992).
Suspension-feeding glass sponges in the genus
Hyalonema dominate the metazoan, epibenthic megafauna at eutrophic and mesotrophic stations along
the EqPac transect, constituting 55% to 87% of the
metazoan megafaunal abundance (Hoover et al., 1994);
this contrasts sharply with California slope habitats
where mobile deposit feeders or omnivores dominate
the megafauna. The remainder of the megafaunal
epibenthos (13% to 45%) at eutrophic stations is composed of surface/subsurface deposit feeders including
irregular urchins that plow through surface sediments,
and presumably holothurians feeding selectively on
the surface deposits (Hoover et al., 1994; Smith and
Hoover, unpublished data). Fresh spoke traces (or
rosettes) formed by echiurans and large polychaetes
are quite common in both eutrophic and mesotrophic
settings, attaining densities (0.06 to 0.2 m
−2 ) comparable to that of the megafaunal epibenthos. These traces
indicate that the burrowing megafauna is also likely
to contain a relatively high abundance of selective
surface-deposit feeders.
Thus far, trophic analyses of macrofauna in the
equatorial abyss have been restricted to the polychaetes, which constitute more than 60% of community
abundance and biomass (see above). Studies in the
Clipperton–Clarion Fracture Zone (Paterson et al.,
1998) and at the DISCOL site (Borowski and Thiel,
1998) indicate that more than 58% of the total
polychaete abundance falls into families considered to
be deposit feeders in the deep sea (e.g., Kukert and
Smith, 1992), with the cirratulids, paraonids, sabellids
and spionids accounting for most of the abundance.
In both areas, surface-deposit feeders predominate,
comprising at least 37 to 56% of polychaete abundance.
Subsurface deposit feeders, consisting primarily of
paranoids, make up only 8.5 to 22% of the polychaetes.
In the Clipperton–Clarion Fracture Zone, predators and
omnivores constitute a surprisingly high percentage of
the polychaete community, accounting for 18 to 28% of
polychaete abundance. Based on the limited data thus
far available, there do not appear to be any marked
differences in polychaete trophic composition between
eutrophic and mesotrophic equatorial sediments (Paterson et al., 1998). It should be noted that the trophic
structure of polychaetes in the equatorial abyss differs
substantially from that on the oxygenated California
margin, where subsurface deposit feeders typically
constitute at least 50% of community abundance (see
above).
The Nematoda are also strongly dominated by
deposit feeders. Brown (1998) found that 59 to 76%
of individuals, and 53 to 68% of species, of nematodes
from the top centimeter of sediment along the EqPac
transect (0º, 2º, 5º, and 9ºN along the 140ºW meridian)
were deposit feeders. Selective deposit feeders predominated (57 to 68% of total numbers), and their absolute
abundance was strongly correlated with microbial
abundance in the sediments along the transect. Predatory and/or scavenging nematodes were very rare in
the equatorial Pacific, accounting for less than 10% of
the total number of individuals at each station (Brown,
1998). Very low predator/scavenger abundance is a
typical feature of abyssal nematode communities when
compared to shallow-water sediment assemblages, and
is thought to reflect a lower relative availability of prey
items and carrion in the abyss (Brown, 1998).
Rates of key ecological processes
A number of key ecological rates have been evaluated in the equatorial Pacific, including sediment
community respiration, bioturbation, and, to some
extent, recolonization following disturbance. These rate
data come primarily from the EqPac study and the
DISCOL experiment.
Studies with benthic incubation chambers and sediment porewaters indicate that, in January 1992,
seafloor oxygen consumption was fairly constant
along the equator from 103ºW to 140ºW, with rates
of 0.6 to 0.8 mmol m
−2 d
−1 (equivalent to roughly
2 g C m
−2 y
−1 ) (Hammond et al., 1996). Seafloor respiration rate declined roughly symmetrically with
distance from the equator along the 140ºW meridian