212
Craig R. SMITH and Amanda W.J. DEMOPOULOS
as well. Megafaunal densities and biomasses appear
to be relatively high compared to adjacent abyssal
plains, almost certainly because of enhanced fluxes
of particulate organic carbon in trenches (Belyaev,
1972). A notable characteristic of trench megafaunal
assemblages is pronounced numerical dominance by
one to three very common species (Belyaev, 1972).
The pattern of numerical dominance intensifies with
increasing depth in trenches (Belyaev, 1972) and
is reminiscent of oxygen-stressed communities in
oxygen-minimum zones (Levin et al., 2000). This
numerical dominance, combined with a high proportion
of endemic megafaunal species in trenches (on average
58% of the total species in each trench), and reduced
diversity below depths of 8500 m, suggest that the
high hydrostatic pressure of trenches serves as a
physiological barrier to many megafaunal species
found in abyssal habitats (Belyaev, 1972).
More recently, a few trench sites have been studied
with more modern techniques, in particular, quantitative box-core sampling for macrofauna in the
Aleutian Trench, and baited camera and trap studies
of scavengers in the Mariana, Philippine and Peru–
Chile Trenches (Fig. 6.1). Based on a single 0.25 m
2
box-core sample, Jumars and Hessler (1976) found a
dense, low-diversity macrofaunal assemblage at a depth
of 7298 m on the central axis of the Aleutian Trench.
Macrofaunal abundance (1272 individuals m
−2 ) was
comparable to that in the eutrophic equatorial abyss and
approached the lower limits of macrofaunal abundance
on the continental slope (Table 6.1). As in most deepsea settings, polychaetes dominated the macrofaunal
assemblage (49%), with tanaids and bivalves also
relatively abundant (Jumars and Hessler, 1976). Unlike
most deep-sea sites, however, aplacophorans (10%),
enteropneusts (8%) and echiurids (3%) were also
quite common, suggesting that trench macrofaunal
communities differ at high taxonomic levels from deepocean assemblages in general (c.f., Belyaev, 1972). The
factors causing unusual taxonomic structure in trenches
are not clear, but could include unusually high food
availability, relatively frequent physical disturbance,
and extreme hydrostatic pressure.
The Aleutian Trench macrofauna sampled by Jumars
and Hessler (1976) appeared to be remarkable by deepsea standards in two other ways. (1) The polychaetes
were unusually dominated by mobile surface-deposit
feeders, and (2) species diversity, as measured by
rarefaction, was remarkably low (Jumars and Hessler,
1976). Both characteristics are likely to be responses
to environmental instability – that is, frequent physical
disturbance (Jumars and Hessler, 1976).
Baited camera and trap studies suggest that the
scavenging fauna of the Mariana, Philippine and Peru–
Chile trenches below 6700 m consists exclusively of
crustacea, and overwhelmingly of large lysianassid
amphipods (Hessler et al., 1978). In contrast, at nearby
abyssal sites below the depth of 6000 m, an abundance
of scavenging fishes of several species are attracted to
bait-falls. One large scavenging amphipod, Hirondellea
gigas, appears to be endemic to the Pacific hadal
zone, occurring in the Mariana, Philippine and Kuril–
Kamchatka trenches below 6000 m (Hessler et al.,
1978). The amphipods of Pacific trenches form large
aggregations at bait parcels very rapidly (within hours)
and are particularly voracious; they often consume tens
of kilograms of dead fish within 1–2 days (Hessler
et al., 1978). It has been suggested that a greater
proportion of the food reaching the trench benthos
arrives in the form of larger, more widely scattered
particles than at shallower depths in the ocean; if so,
scavengers are likely to be especially important in the
energy flow to the seafloor of the hadal zone (Hessler
et al., 1978).
CONCLUSIONS AND OUTSTANDING PROBLEMS
Comparisons among deep benthic ecosystems in the
Pacific indicate the overriding importance of several
key environmental parameters. Perhaps the most important parameter is the flux of particulate organic
carbon to the seafloor. Regional variations in many
aspects of community structure, and in numerous
ecological rates, can be directly related to the amount
of organic material sinking to deep-sea sediments from
the surface ocean (Tables 6.1 and 6.3). These include
variations in the abundance, biomass, and community
structure (in terms of taxonomic composition, relative
importance of size classes, and feeding types) of
both the infauna and scavengers, and the rates of
key processes such as sediment-community oxygen
consumption, bioturbation, rates of trace erasure, and
rates of recolonization. Thus, in the deep Pacific Ocean
(and in the deep-sea generally), flux of particulate
organic carbon appears to play a dominant role in
controlling regional variations in biotic structure, much
as temperature and rainfall control ecosystem structure
in terrestrial habitats. In many ways, ecosystems of the
Craig R. SMITH and Amanda W.J. DEMOPOULOS
as well. Megafaunal densities and biomasses appear
to be relatively high compared to adjacent abyssal
plains, almost certainly because of enhanced fluxes
of particulate organic carbon in trenches (Belyaev,
1972). A notable characteristic of trench megafaunal
assemblages is pronounced numerical dominance by
one to three very common species (Belyaev, 1972).
The pattern of numerical dominance intensifies with
increasing depth in trenches (Belyaev, 1972) and
is reminiscent of oxygen-stressed communities in
oxygen-minimum zones (Levin et al., 2000). This
numerical dominance, combined with a high proportion
of endemic megafaunal species in trenches (on average
58% of the total species in each trench), and reduced
diversity below depths of 8500 m, suggest that the
high hydrostatic pressure of trenches serves as a
physiological barrier to many megafaunal species
found in abyssal habitats (Belyaev, 1972).
More recently, a few trench sites have been studied
with more modern techniques, in particular, quantitative box-core sampling for macrofauna in the
Aleutian Trench, and baited camera and trap studies
of scavengers in the Mariana, Philippine and Peru–
Chile Trenches (Fig. 6.1). Based on a single 0.25 m
2
box-core sample, Jumars and Hessler (1976) found a
dense, low-diversity macrofaunal assemblage at a depth
of 7298 m on the central axis of the Aleutian Trench.
Macrofaunal abundance (1272 individuals m
−2 ) was
comparable to that in the eutrophic equatorial abyss and
approached the lower limits of macrofaunal abundance
on the continental slope (Table 6.1). As in most deepsea settings, polychaetes dominated the macrofaunal
assemblage (49%), with tanaids and bivalves also
relatively abundant (Jumars and Hessler, 1976). Unlike
most deep-sea sites, however, aplacophorans (10%),
enteropneusts (8%) and echiurids (3%) were also
quite common, suggesting that trench macrofaunal
communities differ at high taxonomic levels from deepocean assemblages in general (c.f., Belyaev, 1972). The
factors causing unusual taxonomic structure in trenches
are not clear, but could include unusually high food
availability, relatively frequent physical disturbance,
and extreme hydrostatic pressure.
The Aleutian Trench macrofauna sampled by Jumars
and Hessler (1976) appeared to be remarkable by deepsea standards in two other ways. (1) The polychaetes
were unusually dominated by mobile surface-deposit
feeders, and (2) species diversity, as measured by
rarefaction, was remarkably low (Jumars and Hessler,
1976). Both characteristics are likely to be responses
to environmental instability – that is, frequent physical
disturbance (Jumars and Hessler, 1976).
Baited camera and trap studies suggest that the
scavenging fauna of the Mariana, Philippine and Peru–
Chile trenches below 6700 m consists exclusively of
crustacea, and overwhelmingly of large lysianassid
amphipods (Hessler et al., 1978). In contrast, at nearby
abyssal sites below the depth of 6000 m, an abundance
of scavenging fishes of several species are attracted to
bait-falls. One large scavenging amphipod, Hirondellea
gigas, appears to be endemic to the Pacific hadal
zone, occurring in the Mariana, Philippine and Kuril–
Kamchatka trenches below 6000 m (Hessler et al.,
1978). The amphipods of Pacific trenches form large
aggregations at bait parcels very rapidly (within hours)
and are particularly voracious; they often consume tens
of kilograms of dead fish within 1–2 days (Hessler
et al., 1978). It has been suggested that a greater
proportion of the food reaching the trench benthos
arrives in the form of larger, more widely scattered
particles than at shallower depths in the ocean; if so,
scavengers are likely to be especially important in the
energy flow to the seafloor of the hadal zone (Hessler
et al., 1978).
CONCLUSIONS AND OUTSTANDING PROBLEMS
Comparisons among deep benthic ecosystems in the
Pacific indicate the overriding importance of several
key environmental parameters. Perhaps the most important parameter is the flux of particulate organic
carbon to the seafloor. Regional variations in many
aspects of community structure, and in numerous
ecological rates, can be directly related to the amount
of organic material sinking to deep-sea sediments from
the surface ocean (Tables 6.1 and 6.3). These include
variations in the abundance, biomass, and community
structure (in terms of taxonomic composition, relative
importance of size classes, and feeding types) of
both the infauna and scavengers, and the rates of
key processes such as sediment-community oxygen
consumption, bioturbation, rates of trace erasure, and
rates of recolonization. Thus, in the deep Pacific Ocean
(and in the deep-sea generally), flux of particulate
organic carbon appears to play a dominant role in
controlling regional variations in biotic structure, much
as temperature and rainfall control ecosystem structure
in terrestrial habitats. In many ways, ecosystems of the