THE DEEP ATLANTIC OCEAN
125
period, exhibited increased speeds (15–23 cm sec
−1 )
during periods termed benthic storms. These occurred
approximately every 21 days, and lasted on average
for 7 days. Every 10 months there was a storm
with average velocity >23 cm sec
−1 (Aller, 1989).
The storms caused the following sequence of events:
(a) erosion, in which surface sediment, surficial organic
matter, bacteria, larvae and juveniles were swept away;
(b) intermediate current velocity, during which there
was deposition of fresh organic matter, removal of
metabolites, and mechanical stimulation of microbes;
and (c) decelerating currents, involving deposition of
several centimeters of sediment, burial of organisms
and infilling of burrows. Maximal foraminiferal and
metazoan abundances were observed during this period
(Aller, 1989).
At the HEBBLE site the sediments support unusually high densities of bacteria, polychaetes, bivalves,
isopods and tanaids relative to other locations at comparable depths (Thistle et al., 1985, 1991). Thistle et al.
(1991) suggested that this indicated a fairly high flux
of food to the seafloor. Community structure is distinct,
in that macrofaunal assemblages are dominated by
2 species of ampharetid polychaetes, which comprise
over 60% of the individuals collected. A predominance
in the macrofauna of juveniles, rather than adults (as
is more typical of abyssal sites), suggests that the
fauna is continually responding to disturbance (Thistle
et al., 1985). Comparable ‘opportunistic’ characteristics
were observed in deep-sea recolonization experiments
carried out at a depth of around 2000 m by Grassle
(1977) and Desbruy` eres et al. (1980). The macrofaunal
taxa at the HEBBLE site are mainly deposit feeders
which can take advantage of organic matter deposited
on the sediment surface (Thistle et al., 1985). They
appear to mix sediments actively (DeMaster et al.,
1991).
Densities of nematodes and harpacticoid copepods
at the HEBBLE site, and diversity of copepods, did
not differ from those reported in other abyssal areas
(Thistle et al., 1985, 1991). Thistle and Sherman (1985)
suggested that some nematodes use long retractable
tails to avoid resuspension, but in general the nematode
fauna exhibited few of the adaptations normally
found in nematodes from high-energy, coarse-grained
environments.
Taxa residing in surficial sediment (isopods, nematodes and harpacticoid copepods) exhibited variation in
abundance over time that was attributed to erosion by
storms (Thistle, 1988; Thistle et al., 1991). Standing
stocks of polychaetes, bivalves, and tanaids, many of
which burrow, remained relatively constant during the
study (Thistle et al., 1991). Thistle and Wilson (1987,
1996) observed that surface-dwelling isopods, presumably exposed to erosion, were much less common at the
HEBBLE site than in other, more quiescent, deep-sea
regions.
North-Atlantic (NAB) and Mid-Atlantic
Bights (MAB)
Continental slope and rise: Hecker (1990a) examined
variation in the megafauna with depth and geographic
location on the continental margin south of New
England, on the eastern and western edges of Georges
Bank, and along the SEEP I transect (34 m to 2394 m)
(Fig. 5.2). In this region, 80% of the continental
slope consists of gully and ridge morphology (Scanlon,
1984). Hecker (1990a) described four megafaunal
zones with fairly abrupt boundaries. The upper slope
was dominated by solitary scleractinians and quill
worms (Hyalinoecia artifex), and the lower slope by the
brittlestar Ophiomusium lymani, cerianthid anemones,
sea pens and the urchin Echinus affinis. These two
regions exhibited highest densities. The upper midslope was occupied by lower numbers of red crabs
and fishes, and the transition zone by cerianthids,
sea pens and ophiuroids. Animal distributions were
controlled by effects of local topography on currents,
and accompanying effects on food availability. The
mid-slope, where densities were lowest, is a broad
depositional band; higher currents are present on the
upper and lower slopes where megafaunal densities
were greatest.
Comparable studies of megafaunal zonation in the
Mid-Atlantic Bight were carried out by Hecker et al.
(1983) on the continental margin (100 m to 2300 m)
east of New Jersey. Five major zones were observed,
with faunal breaks at 400 m, 750 m, 1450 m and
1600 m. The megafauna between 200 and 400 m
comprised mainly crabs (Cancer spp., Munida iris),
sea pens (Stylatula elegans), and anemones (Cerianthus
borealis). Between 400 m and 750 m dominants were
the red crab (Geryon quinquedens), the anemone
Bolocera tuediae, quill worms (Hyalinoecia artifex),
rattails (Nezumia spp.) and hake (Urophycis chesteri).
Between 700 m and 1400 m the eel Synaphobranchus
spp. became dominant. From 1400 m to 2300 m
Ophiomusium lymani and Echinus affinis, cerianthid
anemones and the sea pen Distichoptilum gracile were
dominant. As in the North Atlantic Bight, megafaunal
abundances were highest in the shallower (<600 m) and
125
period, exhibited increased speeds (15–23 cm sec
−1 )
during periods termed benthic storms. These occurred
approximately every 21 days, and lasted on average
for 7 days. Every 10 months there was a storm
with average velocity >23 cm sec
−1 (Aller, 1989).
The storms caused the following sequence of events:
(a) erosion, in which surface sediment, surficial organic
matter, bacteria, larvae and juveniles were swept away;
(b) intermediate current velocity, during which there
was deposition of fresh organic matter, removal of
metabolites, and mechanical stimulation of microbes;
and (c) decelerating currents, involving deposition of
several centimeters of sediment, burial of organisms
and infilling of burrows. Maximal foraminiferal and
metazoan abundances were observed during this period
(Aller, 1989).
At the HEBBLE site the sediments support unusually high densities of bacteria, polychaetes, bivalves,
isopods and tanaids relative to other locations at comparable depths (Thistle et al., 1985, 1991). Thistle et al.
(1991) suggested that this indicated a fairly high flux
of food to the seafloor. Community structure is distinct,
in that macrofaunal assemblages are dominated by
2 species of ampharetid polychaetes, which comprise
over 60% of the individuals collected. A predominance
in the macrofauna of juveniles, rather than adults (as
is more typical of abyssal sites), suggests that the
fauna is continually responding to disturbance (Thistle
et al., 1985). Comparable ‘opportunistic’ characteristics
were observed in deep-sea recolonization experiments
carried out at a depth of around 2000 m by Grassle
(1977) and Desbruy` eres et al. (1980). The macrofaunal
taxa at the HEBBLE site are mainly deposit feeders
which can take advantage of organic matter deposited
on the sediment surface (Thistle et al., 1985). They
appear to mix sediments actively (DeMaster et al.,
1991).
Densities of nematodes and harpacticoid copepods
at the HEBBLE site, and diversity of copepods, did
not differ from those reported in other abyssal areas
(Thistle et al., 1985, 1991). Thistle and Sherman (1985)
suggested that some nematodes use long retractable
tails to avoid resuspension, but in general the nematode
fauna exhibited few of the adaptations normally
found in nematodes from high-energy, coarse-grained
environments.
Taxa residing in surficial sediment (isopods, nematodes and harpacticoid copepods) exhibited variation in
abundance over time that was attributed to erosion by
storms (Thistle, 1988; Thistle et al., 1991). Standing
stocks of polychaetes, bivalves, and tanaids, many of
which burrow, remained relatively constant during the
study (Thistle et al., 1991). Thistle and Wilson (1987,
1996) observed that surface-dwelling isopods, presumably exposed to erosion, were much less common at the
HEBBLE site than in other, more quiescent, deep-sea
regions.
North-Atlantic (NAB) and Mid-Atlantic
Bights (MAB)
Continental slope and rise: Hecker (1990a) examined
variation in the megafauna with depth and geographic
location on the continental margin south of New
England, on the eastern and western edges of Georges
Bank, and along the SEEP I transect (34 m to 2394 m)
(Fig. 5.2). In this region, 80% of the continental
slope consists of gully and ridge morphology (Scanlon,
1984). Hecker (1990a) described four megafaunal
zones with fairly abrupt boundaries. The upper slope
was dominated by solitary scleractinians and quill
worms (Hyalinoecia artifex), and the lower slope by the
brittlestar Ophiomusium lymani, cerianthid anemones,
sea pens and the urchin Echinus affinis. These two
regions exhibited highest densities. The upper midslope was occupied by lower numbers of red crabs
and fishes, and the transition zone by cerianthids,
sea pens and ophiuroids. Animal distributions were
controlled by effects of local topography on currents,
and accompanying effects on food availability. The
mid-slope, where densities were lowest, is a broad
depositional band; higher currents are present on the
upper and lower slopes where megafaunal densities
were greatest.
Comparable studies of megafaunal zonation in the
Mid-Atlantic Bight were carried out by Hecker et al.
(1983) on the continental margin (100 m to 2300 m)
east of New Jersey. Five major zones were observed,
with faunal breaks at 400 m, 750 m, 1450 m and
1600 m. The megafauna between 200 and 400 m
comprised mainly crabs (Cancer spp., Munida iris),
sea pens (Stylatula elegans), and anemones (Cerianthus
borealis). Between 400 m and 750 m dominants were
the red crab (Geryon quinquedens), the anemone
Bolocera tuediae, quill worms (Hyalinoecia artifex),
rattails (Nezumia spp.) and hake (Urophycis chesteri).
Between 700 m and 1400 m the eel Synaphobranchus
spp. became dominant. From 1400 m to 2300 m
Ophiomusium lymani and Echinus affinis, cerianthid
anemones and the sea pen Distichoptilum gracile were
dominant. As in the North Atlantic Bight, megafaunal
abundances were highest in the shallower (<600 m) and