366
John D. GAGE
sediment community, measured as interfacial fluxes
such as oxygen consumption, indicate enhanced rates
of benthic carbon mineralization at the base of the
slope. This led Jahnke et al. (1990) to suggest that
about half of the total organic-carbon input to the
deep-sea floor occurs within 500 km of the continental
slope. The American SEEP programs in the MidAtlantic Bight in the Northwest Atlantic showed that,
although rates characteriztically decline precipitously
down slope, values at depths of about 1000 m may
be almost as high as those on the continental shelf.
This result supported the idea of an organic-carbon
‘depocentre’ and a corresponding peak in sediment
community oxygen consumption (Rowe et al., 1994).
The SEEP programs tested the idea that large amounts
of fine particles including shelf phytoplankton production may be swept over the shelf edge and deposited on
the adjacent slope; this phenomenon was periodically
observed after storm-driven resuspension from shelf
sediments (Biscaye et al., 1994).
Large-scale comparisons and projections
In the abyssal realm, comparisons between different
latitudes and levels of primary production at the
surface may show differences in the size, structure
and composition of the community. These reflect
differences in benthic response to amount and seasonal
pattern of organic flux to the bottom (e.g., Sibuet et al.,
1993; Rice et al., 1994). French work at contrasting
sites at roughly the same latitude in the tropical
Northeast Atlantic showed relatively high abundance of
carnivorous necrophages (e.g., scavenging lysianassid
amphipods) at the most oligotrophic station, despite the
very low particle flux to the bottom. This was also consistent with the pattern in proportions of size classes,
the largest organisms (megafauna) showing the least
diminution in biomass from eutrophic to oligotrophic
sites compared to macro- and meiofauna (Sibuet et al.,
1993). K.L. Smith (1992) also found that the largest
size groups formed a much larger component of the
consumer community in the oligotrophic central North
Pacific than at a more eutrophic, abyssal site near the
California margin.
Such differences cannot easily be explained by
differences in seasonality, as the stations were located
at about the same latitude. Instead, possible body
differences in foraging strategies related to body size
may be implicated. Life-styles reliant on local particle
rain become unrewarding at very low particle flux,
with increasing importance of motile necrophages able
to respond and scavenge large, albeit sparse, falls
from the death of large surface swimmers (some
perhaps migratory) which collect food over a wide
area. Something similar may possibly explain the
differing composition of abyssal necrophages at two
sites further north in the Northeast Atlantic referred
to earlier. One is dominated by crustaceans such as
scavenging amphipods, and the other by macrourid fish;
this difference possibly is a response to differences
in relative proportions of mid-water zooplankton and
nekton (Thurston et al., 1995). Sediment bacterial
activity shows a better relationship to the particle
rain intercepted by sediment traps, bacterial numbers
being 3 to 4 times higher at the mesotrophic site
than at the oligotrophic one (Sibuet et al., 1993).
Otherwise, latitudinal comparisons in the Atlantic
Ocean, from 30ºS up to about 80ºN, show a first-order
relationship of meio- and macrofaunal abundance to
particle rain, as integrated by the organic-carbon signal
buried in the sediment during the Holocene (Sibuet
et al., 1989).
Physical variables, such as hydrodynamics, are
also important. The most extreme conditions are in
areas experiencing frequent sediment-eroding benthic
storms. Up to a point, strong flow may support a larger
benthic standing crop than would be expected. But at
higher energy it may have a negative impact on larger
size classes such as motile megafauna, but lead to much
enhanced abundances of certain smaller taxa, especially
bacteria (Thistle et al., 1985, 1991). On the other
hand, a large amount of microbial decomposition that
would have occurred in the sediment has already been
mediated by bacteria attached to suspended particles.
Also, the finding that the macrofauna may consist
largely of subadults indicates a constraint imposed on
population size structure by such disturbances (Thistle
et al., 1985).
Hydrodynamics also affects the efficiency of sediment traps as measures of passive particle rain. It
has been shown above how vertical particle flux on
the continental slope may be augmented by downslope
processes, and by lateral advection of resuspended
particles. Even in the abyssal basins, traps set within
the benthic boundary layer may provide higher measurements than those a few hundred metres above,
owing to resuspension. On the other hand workers have
felt it necessary to measure particle flux as near the
bed as possible in order to include near-bed advective
John D. GAGE
sediment community, measured as interfacial fluxes
such as oxygen consumption, indicate enhanced rates
of benthic carbon mineralization at the base of the
slope. This led Jahnke et al. (1990) to suggest that
about half of the total organic-carbon input to the
deep-sea floor occurs within 500 km of the continental
slope. The American SEEP programs in the MidAtlantic Bight in the Northwest Atlantic showed that,
although rates characteriztically decline precipitously
down slope, values at depths of about 1000 m may
be almost as high as those on the continental shelf.
This result supported the idea of an organic-carbon
‘depocentre’ and a corresponding peak in sediment
community oxygen consumption (Rowe et al., 1994).
The SEEP programs tested the idea that large amounts
of fine particles including shelf phytoplankton production may be swept over the shelf edge and deposited on
the adjacent slope; this phenomenon was periodically
observed after storm-driven resuspension from shelf
sediments (Biscaye et al., 1994).
Large-scale comparisons and projections
In the abyssal realm, comparisons between different
latitudes and levels of primary production at the
surface may show differences in the size, structure
and composition of the community. These reflect
differences in benthic response to amount and seasonal
pattern of organic flux to the bottom (e.g., Sibuet et al.,
1993; Rice et al., 1994). French work at contrasting
sites at roughly the same latitude in the tropical
Northeast Atlantic showed relatively high abundance of
carnivorous necrophages (e.g., scavenging lysianassid
amphipods) at the most oligotrophic station, despite the
very low particle flux to the bottom. This was also consistent with the pattern in proportions of size classes,
the largest organisms (megafauna) showing the least
diminution in biomass from eutrophic to oligotrophic
sites compared to macro- and meiofauna (Sibuet et al.,
1993). K.L. Smith (1992) also found that the largest
size groups formed a much larger component of the
consumer community in the oligotrophic central North
Pacific than at a more eutrophic, abyssal site near the
California margin.
Such differences cannot easily be explained by
differences in seasonality, as the stations were located
at about the same latitude. Instead, possible body
differences in foraging strategies related to body size
may be implicated. Life-styles reliant on local particle
rain become unrewarding at very low particle flux,
with increasing importance of motile necrophages able
to respond and scavenge large, albeit sparse, falls
from the death of large surface swimmers (some
perhaps migratory) which collect food over a wide
area. Something similar may possibly explain the
differing composition of abyssal necrophages at two
sites further north in the Northeast Atlantic referred
to earlier. One is dominated by crustaceans such as
scavenging amphipods, and the other by macrourid fish;
this difference possibly is a response to differences
in relative proportions of mid-water zooplankton and
nekton (Thurston et al., 1995). Sediment bacterial
activity shows a better relationship to the particle
rain intercepted by sediment traps, bacterial numbers
being 3 to 4 times higher at the mesotrophic site
than at the oligotrophic one (Sibuet et al., 1993).
Otherwise, latitudinal comparisons in the Atlantic
Ocean, from 30ºS up to about 80ºN, show a first-order
relationship of meio- and macrofaunal abundance to
particle rain, as integrated by the organic-carbon signal
buried in the sediment during the Holocene (Sibuet
et al., 1989).
Physical variables, such as hydrodynamics, are
also important. The most extreme conditions are in
areas experiencing frequent sediment-eroding benthic
storms. Up to a point, strong flow may support a larger
benthic standing crop than would be expected. But at
higher energy it may have a negative impact on larger
size classes such as motile megafauna, but lead to much
enhanced abundances of certain smaller taxa, especially
bacteria (Thistle et al., 1985, 1991). On the other
hand, a large amount of microbial decomposition that
would have occurred in the sediment has already been
mediated by bacteria attached to suspended particles.
Also, the finding that the macrofauna may consist
largely of subadults indicates a constraint imposed on
population size structure by such disturbances (Thistle
et al., 1985).
Hydrodynamics also affects the efficiency of sediment traps as measures of passive particle rain. It
has been shown above how vertical particle flux on
the continental slope may be augmented by downslope
processes, and by lateral advection of resuspended
particles. Even in the abyssal basins, traps set within
the benthic boundary layer may provide higher measurements than those a few hundred metres above,
owing to resuspension. On the other hand workers have
felt it necessary to measure particle flux as near the
bed as possible in order to include near-bed advective