THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
59
Ecology of the abyssopelagic zone
A further subtle change occurs in the pelagic assemblages at depths of about 2500–2700 m. At 42ºN 17ºW
in the Northeast Atlantic, Angel (1983) recorded a
sudden decline in abundance of fish at 2700 m, so
that they ceased to be the dominant component of the
micronekton biomass. Initially they were replaced by
decapod crustaceans, which in turn, at even greater
depths, were replaced by mysids. This depth of 2700 m
may be critical for physiological reasons. In some
rather crude preliminary field trials, Menzies and Wilson (1961) compared the survival of benthic specimens
brought up to the surface, and of littoral species
lowered to depth. For both sets of species there was
a critical boundary for survival at 2500 to 2700 m; all
the deep-living animals retrieved from greater depths
succumbed to the change in hydrostatic pressure,
whereas the shallow-living species all survived being
lowered to 2400 m, but succumbed if lowered deeper.
The abyssopelagic zone extends down either to hadal
depths (i.e., depths >6000 m) or to the benthopelagic
zone within c. 100 m of the sea floor.
Ecology of the benthopelagic zone
The benthopelagic zone usually coincides with the
benthic boundary layer (BBL) – the layer of isothermal
and isohaline water contiguous to the sea floor.
However, in regions where there is high mesoscale
eddy activity (e.g., in regions of western boundary
currents such as the Gulf Stream), benthic storms are
frequent and create isohaline and isothermal conditions
extending as much as 1000 m above the bottom
(Weatherly and Kelley, 1985). Wishner (1980) first
noted that the standing crop of plankton increases very
close to the bottom, and may more than double within
the benthopelagic zone. In addition, the rather special
conditions result in many of the species found there
being endemic to the zone (Angel, 1990).
The general reversal of the gradient of pelagic
biomass close to the seabed implies that the declining
gradient in food availability seen higher in the water
column is reversed in close proximity to the seabed.
Above the BBL, the organisms in the water have only
the flux of sedimentary particles as the basic source
of food, and this flux is very patchy and intermittent
in time and space. However, once these particles are
deposited on the seabed, they remain available until
they are either consumed or microbially degraded.
Whenever the current at the bottom exceeds a critical
velocity the particles are resuspended, once again
becoming available to organisms in the water. Any organism that scavenges on the bottom reduces the scale
of the problem of finding food from three-dimensional
in the body of the water to two-dimensional on the
surface of the sediment.
This reversal of the gradient in standing stock also
tends to reverse the gradient of predation pressure.
A pelagic organism becomes safer from the risks of
predation the higher above the bottom it is swimming,
so that the water column above the BBL may be
used as a refuge. Conversely species that feed on the
bottom can find a more dynamic hydrographic regime
by swimming up into the water column, and become
better able to pick up scent plumes from food packages
such as whale carcasses (Smith et al., 1998), or
even potential mates. Baited traps deployed at various
heights above the seabed catch large numbers of these
amphipod scavengers close to the bottom, but their
abundance dwindles almost to zero >50 m above the
bottom (Thurston, 1979; Wickens, 1983; Christiansen,
1996). Occasionally these amphipods have been caught
in trawls several hundreds of metres above the bottom;
the reasons for these extensive migrations are not
clear.
The benthopelagic zone is also used by the pelagic
larvae and post-larvae of benthic species. Several
holothurian species, normally considered to be megabenthos, have the ability to float up off the seabed
and have been caught several hundreds of metres
above the bottom (Billett et al., 1985; Billett, 1991).
Several deep-sea benthic gastropod species have been
found to have fully pelagic larvae, which undergo
extensive development near the surface and then
descend into deep water on maturation (Bouchet and
War´ en, 1985). The dispersion of such larvae within the
deep-water column may result in the establishment of
pseudopopulations of adults of species in areas which
have environmental characteristics unsuitable for their
reproduction (Bouchet and Taviani, 1992).
Vertical migrations
There are three basic types of vertical migrations:
diurnal (or diel), ontogenetic, and seasonal. In terms
of ecological processes, diel vertical migrations, the
behaviour of many pelagic species in swimming up
towards the surface at dusk and returning back down
again at dawn, is by far the most significant. There is
59
Ecology of the abyssopelagic zone
A further subtle change occurs in the pelagic assemblages at depths of about 2500–2700 m. At 42ºN 17ºW
in the Northeast Atlantic, Angel (1983) recorded a
sudden decline in abundance of fish at 2700 m, so
that they ceased to be the dominant component of the
micronekton biomass. Initially they were replaced by
decapod crustaceans, which in turn, at even greater
depths, were replaced by mysids. This depth of 2700 m
may be critical for physiological reasons. In some
rather crude preliminary field trials, Menzies and Wilson (1961) compared the survival of benthic specimens
brought up to the surface, and of littoral species
lowered to depth. For both sets of species there was
a critical boundary for survival at 2500 to 2700 m; all
the deep-living animals retrieved from greater depths
succumbed to the change in hydrostatic pressure,
whereas the shallow-living species all survived being
lowered to 2400 m, but succumbed if lowered deeper.
The abyssopelagic zone extends down either to hadal
depths (i.e., depths >6000 m) or to the benthopelagic
zone within c. 100 m of the sea floor.
Ecology of the benthopelagic zone
The benthopelagic zone usually coincides with the
benthic boundary layer (BBL) – the layer of isothermal
and isohaline water contiguous to the sea floor.
However, in regions where there is high mesoscale
eddy activity (e.g., in regions of western boundary
currents such as the Gulf Stream), benthic storms are
frequent and create isohaline and isothermal conditions
extending as much as 1000 m above the bottom
(Weatherly and Kelley, 1985). Wishner (1980) first
noted that the standing crop of plankton increases very
close to the bottom, and may more than double within
the benthopelagic zone. In addition, the rather special
conditions result in many of the species found there
being endemic to the zone (Angel, 1990).
The general reversal of the gradient of pelagic
biomass close to the seabed implies that the declining
gradient in food availability seen higher in the water
column is reversed in close proximity to the seabed.
Above the BBL, the organisms in the water have only
the flux of sedimentary particles as the basic source
of food, and this flux is very patchy and intermittent
in time and space. However, once these particles are
deposited on the seabed, they remain available until
they are either consumed or microbially degraded.
Whenever the current at the bottom exceeds a critical
velocity the particles are resuspended, once again
becoming available to organisms in the water. Any organism that scavenges on the bottom reduces the scale
of the problem of finding food from three-dimensional
in the body of the water to two-dimensional on the
surface of the sediment.
This reversal of the gradient in standing stock also
tends to reverse the gradient of predation pressure.
A pelagic organism becomes safer from the risks of
predation the higher above the bottom it is swimming,
so that the water column above the BBL may be
used as a refuge. Conversely species that feed on the
bottom can find a more dynamic hydrographic regime
by swimming up into the water column, and become
better able to pick up scent plumes from food packages
such as whale carcasses (Smith et al., 1998), or
even potential mates. Baited traps deployed at various
heights above the seabed catch large numbers of these
amphipod scavengers close to the bottom, but their
abundance dwindles almost to zero >50 m above the
bottom (Thurston, 1979; Wickens, 1983; Christiansen,
1996). Occasionally these amphipods have been caught
in trawls several hundreds of metres above the bottom;
the reasons for these extensive migrations are not
clear.
The benthopelagic zone is also used by the pelagic
larvae and post-larvae of benthic species. Several
holothurian species, normally considered to be megabenthos, have the ability to float up off the seabed
and have been caught several hundreds of metres
above the bottom (Billett et al., 1985; Billett, 1991).
Several deep-sea benthic gastropod species have been
found to have fully pelagic larvae, which undergo
extensive development near the surface and then
descend into deep water on maturation (Bouchet and
War´ en, 1985). The dispersion of such larvae within the
deep-water column may result in the establishment of
pseudopopulations of adults of species in areas which
have environmental characteristics unsuitable for their
reproduction (Bouchet and Taviani, 1992).
Vertical migrations
There are three basic types of vertical migrations:
diurnal (or diel), ontogenetic, and seasonal. In terms
of ecological processes, diel vertical migrations, the
behaviour of many pelagic species in swimming up
towards the surface at dusk and returning back down
again at dawn, is by far the most significant. There is