THE DEEP ATLANTIC OCEAN
135
to those of near-shore species. The echinoid Echinus
affinis, however, appears to live longer (up to 28 years)
and grow more slowly than related shallow-water
species, although some other echinoderm species may
grow faster (Gage and Tyler, 1985).
Porcupine Seabight (PSB): This embayment of the
continental slope, lying southwest of Ireland and
south of the Rockall Trough, is bounded by the
Porcupine Bank to the west, the Irish shelf to the
east and the Goban Spur to the south (Fig. 5.1). It
is connected to the Porcupine Abyssal Plain by a
narrow southwestern opening. An excellent general
account of the topography, sedimentology, geology and
hydrography of the Porcupine Seabight has been given
by Rice et al. (1991). To the north and west the slopes
are fairly gentle but the eastern side is cut by canyons;
this makes conventional sampling gear difficult to use.
An important feature is the Gollum Channel System,
which runs through the axis of the Seabight and out
onto the Porcupine Abyssal Plain. Recent submersible
observations suggest that the upper part of the channel
system (down to at least 940 m depth) is active, but
that at 3000 m depth bottom currents are too weak
to resuspend fine-grained sediments (Tudhope and
Scoffin, 1995).
Between 1977 and 1986, the Porcupine Seabight
was subject to an extensive sampling program by the
Institute of Oceanographic Sciences (U.K.), summarized by Rice et al. (1991). The main focus was
on the megafauna, but the meiofauna (including the
Foraminifera) was also studied. This program yielded
some important results, the most notable being the
discovery of phytodetritus deposition (Billett et al.,
1983), described elsewhere in this chapter (p. 121).
The invertebrate megafauna was sampled with a
semi-balloon otter trawl and an epibenthic sledge,
and photographed in situ using a camera attached
to the sledge. Rice et al. (1982) used an epibenthic
sledge fitted with an odometer wheel, and simultaneously obtained seafloor photographs, to quantify
the megafauna in the Porcupine Seabight. Despite
various shortcomings, a combination of sampling and
photography provided reasonably accurate estimates
of densities for some non-sessile taxa such as the
larger echinoderms. For example, the holothurian
Benthogone rosea had a mean density of 0.098
and 0.114 individuals m
−2 (photographic and catch
data respectively) in a haul taken at 1400 m. Dense
aggregations of the small holothurian Kolga hyalina
are occasionally observed in the Porcupine Seabight.
Billett and Hansen (1982) counted 50 individuals m
−2
in photographs taken around a depth of 3700 m, and
34 individuals m
−2 around 4000 m depth, these high
densities being associated with the Gollum Channel
System. Specimens caught at particular stations were
small and had a narrow size distribution, suggesting
periodic synchronous reproduction and other opportunistic life-history characteristics. Large numbers of
possibly opportunistic holothurians have also been
observed in some Northwest Atlantic canyons (Rowe,
1971b).
Megafaunal biomass in the Porcupine Seabight is
between two and five times greater than in the Bay
of Biscay (Billett, 1991). Despite considerable scatter,
total biomass values (whether measured as wet weight,
dry weight, or ash-free dry weight [AFDW]) show a
clear logarithmic decrease between 500 and 4100 m
depth, with echinoderm biomass decreasing less steeply
with depth than the biomass of crustaceans and
‘other phyla’ (Lampitt et al., 1986). On the upper
slope (200–1000 m), the main megafaunal animals are
crustaceans (particularly the crab Geryon tridens and
other decapods), sponges and cnidarians. Echinoderms
predominate at greater depths, constituting >75%
and sometimes >90% of the invertebrate megafaunal
AFDW at 1500 m, 2000 m, 2500 m, 3500 m and 4000 m
depth (Billett, 1991). Holothurians are always the most
important taxon, except around 2500 m depth in the
center of the Seabight where the megafauna is dominated by asteroids. Billett (1991) has summarized the
bathymetric distribution of holothurian species in the
Porcupine Seabight based on epibenthic sledge catches
(Fig. 5.6). He recognized three zones: the upper and
middle slope where species occupy relatively narrow
depth bands related to changing hydrographic conditions, the middle and lower slope, where holothurians
are uncommon for poorly understood reasons, and
areas beyond the base of the slope where bathymetric
distributions are relatively broad.
Lampitt et al. (1986) repeatedly sampled a station
at a depth of 1300 m using an epibenthic sledge.
Differences in megafaunal biomass between samples
collected at this site on a single cruise spanned an
order of magnitude, from <0.1 g m
−2 to >1 g m
−2 ashfree dry weight. These disparities were due entirely to
variations in the abundance of Pheronema carpenteri.
This hexactinellid sponge is abundant along parts of the
Northwest European margin and forms dense aggregations between 1000 m and 1300 m in the Porcupine
135
to those of near-shore species. The echinoid Echinus
affinis, however, appears to live longer (up to 28 years)
and grow more slowly than related shallow-water
species, although some other echinoderm species may
grow faster (Gage and Tyler, 1985).
Porcupine Seabight (PSB): This embayment of the
continental slope, lying southwest of Ireland and
south of the Rockall Trough, is bounded by the
Porcupine Bank to the west, the Irish shelf to the
east and the Goban Spur to the south (Fig. 5.1). It
is connected to the Porcupine Abyssal Plain by a
narrow southwestern opening. An excellent general
account of the topography, sedimentology, geology and
hydrography of the Porcupine Seabight has been given
by Rice et al. (1991). To the north and west the slopes
are fairly gentle but the eastern side is cut by canyons;
this makes conventional sampling gear difficult to use.
An important feature is the Gollum Channel System,
which runs through the axis of the Seabight and out
onto the Porcupine Abyssal Plain. Recent submersible
observations suggest that the upper part of the channel
system (down to at least 940 m depth) is active, but
that at 3000 m depth bottom currents are too weak
to resuspend fine-grained sediments (Tudhope and
Scoffin, 1995).
Between 1977 and 1986, the Porcupine Seabight
was subject to an extensive sampling program by the
Institute of Oceanographic Sciences (U.K.), summarized by Rice et al. (1991). The main focus was
on the megafauna, but the meiofauna (including the
Foraminifera) was also studied. This program yielded
some important results, the most notable being the
discovery of phytodetritus deposition (Billett et al.,
1983), described elsewhere in this chapter (p. 121).
The invertebrate megafauna was sampled with a
semi-balloon otter trawl and an epibenthic sledge,
and photographed in situ using a camera attached
to the sledge. Rice et al. (1982) used an epibenthic
sledge fitted with an odometer wheel, and simultaneously obtained seafloor photographs, to quantify
the megafauna in the Porcupine Seabight. Despite
various shortcomings, a combination of sampling and
photography provided reasonably accurate estimates
of densities for some non-sessile taxa such as the
larger echinoderms. For example, the holothurian
Benthogone rosea had a mean density of 0.098
and 0.114 individuals m
−2 (photographic and catch
data respectively) in a haul taken at 1400 m. Dense
aggregations of the small holothurian Kolga hyalina
are occasionally observed in the Porcupine Seabight.
Billett and Hansen (1982) counted 50 individuals m
−2
in photographs taken around a depth of 3700 m, and
34 individuals m
−2 around 4000 m depth, these high
densities being associated with the Gollum Channel
System. Specimens caught at particular stations were
small and had a narrow size distribution, suggesting
periodic synchronous reproduction and other opportunistic life-history characteristics. Large numbers of
possibly opportunistic holothurians have also been
observed in some Northwest Atlantic canyons (Rowe,
1971b).
Megafaunal biomass in the Porcupine Seabight is
between two and five times greater than in the Bay
of Biscay (Billett, 1991). Despite considerable scatter,
total biomass values (whether measured as wet weight,
dry weight, or ash-free dry weight [AFDW]) show a
clear logarithmic decrease between 500 and 4100 m
depth, with echinoderm biomass decreasing less steeply
with depth than the biomass of crustaceans and
‘other phyla’ (Lampitt et al., 1986). On the upper
slope (200–1000 m), the main megafaunal animals are
crustaceans (particularly the crab Geryon tridens and
other decapods), sponges and cnidarians. Echinoderms
predominate at greater depths, constituting >75%
and sometimes >90% of the invertebrate megafaunal
AFDW at 1500 m, 2000 m, 2500 m, 3500 m and 4000 m
depth (Billett, 1991). Holothurians are always the most
important taxon, except around 2500 m depth in the
center of the Seabight where the megafauna is dominated by asteroids. Billett (1991) has summarized the
bathymetric distribution of holothurian species in the
Porcupine Seabight based on epibenthic sledge catches
(Fig. 5.6). He recognized three zones: the upper and
middle slope where species occupy relatively narrow
depth bands related to changing hydrographic conditions, the middle and lower slope, where holothurians
are uncommon for poorly understood reasons, and
areas beyond the base of the slope where bathymetric
distributions are relatively broad.
Lampitt et al. (1986) repeatedly sampled a station
at a depth of 1300 m using an epibenthic sledge.
Differences in megafaunal biomass between samples
collected at this site on a single cruise spanned an
order of magnitude, from <0.1 g m
−2 to >1 g m
−2 ashfree dry weight. These disparities were due entirely to
variations in the abundance of Pheronema carpenteri.
This hexactinellid sponge is abundant along parts of the
Northwest European margin and forms dense aggregations between 1000 m and 1300 m in the Porcupine