THE DEEP ATLANTIC OCEAN
129
the South Atlantic Bight with comparable data from
the North and Mid-Atlantic Bights, indicate that both
the highest and lowest macrofaunal diversities can be
found in the South Atlantic Bight. Generally it is midslope depths (1220 to 1500 m) that exhibit highest
species richness and evenness, but in the South Atlantic
Bight some stations at 800 m and 3000 m were diverse.
The low-diversity assemblages found off Cape Hatteras
(Blake and Grassle, 1994) are discussed below.
Analyses of macrofaunal species composition for
146 quantitative boxcore samples collected from depths
of 600–3500 m between Cape Hatteras and Charleston
Bump yielded 1300 species, of which over 30% were
new to science (Hilbig, 1994). Half of these were
polychaetes, 22% were crustaceans and 16% were
molluscs. This study found that the zoogeographic
barrier reported to occur between Capes Lookout
and Hatteras (Cutler, 1975) was non-existent for
bivalves, but was confirmed for most cumaceans, some
aplacophorans and many polychaetes (Hilbig, 1994).
Cape Hatteras: The slope off Cape Hatteras appears
to be atypical with respect to the rest of the Western
Atlantic slope in terms of sedimentation and benthic
standing stocks (Table 5.2). It has been suggested
that the sedimentary/nutrient regime is more typical of
estuarine or shelf environments (Rhoads and Hecker,
1994). The Cape Hatteras region receives high input of
organic matter comparable to that occurring in coastal
estuaries (DeMaster et al., 1994). The high inputs
are associated with high sedimentation rates resulting
from outwelling from nearshore embayments and
topographic funneling of nutrients from the shelf out
to the slope (Blake and Diaz, 1994). The Gulf Stream
and Virginia currents converge in this region. This
convergence, combined with the topographic position
of the Cape on the outer edge of the shelf, leads
to a funneling of water masses and their constituents
offshore (Rhoads and Hecker, 1994). Thus, much of
the organic input in this region is terrigenous, coming
from Chesapeake and Delaware Bays to the north,
North Carolina sounds, and the shelf. Evidence for
terrigenous inputs comes from lighter d
13 C values for
sediments (−21.2) than is observed at sites further
south (−18.7 to −19.6) (Blair et al., 1994). The
composition of fatty acids and sterols are typical
of refractory shelf and estuarine sediments (Harvey,
1994). The concentrations of chlorophyll a in sediments
at depths from 530 m to 2003 m averaged 19.9 mg m
−2 ,
a value much higher than observed elsewhere on the
eastern continental slope of the United States. Viable
diatoms present in cores up to 14 cm below the surface
suggest high rates of bioturbation (Cahoon et al.,
1994). Observations of rapid subduction of diatoms
by maldanid polychaetes at this site support this idea
(Levin et al., 1997, 1999), as do other experimental
studies of particle mixing (DeMaster et al., 1994; Blair
et al., 1996; Fornes et al., 1999). Analyses of fatty acids
and sterols suggest that diatoms and dinoflagellates
are the principal source of labile organic matter to
the sediments, with a minor input of vascular-plant
material (Harvey, 1994).
The Cape Hatteras margin supports extraordinarily
large numbers of megafauna (Hecker, 1994) and
dense infaunal assemblages with unusually low species
diversity (Schaff et al., 1992; Blake and Grassle, 1994;
Blake and Hilbig, 1994). The abundant megafauna
includes large populations of brittle stars and asteroids, the foraminiferan Bathysiphon filiformis, three
demersal fish, two eelpouts and a large anemone
(Fig. 5.5) (Hecker, 1994). These taxa attain much
higher population densities off Cape Hatteras than at
any other site on the eastern margin of the United States
(Hecker, 1994).
Macrofaunal densities at depths between 530 and
850 m off Cape Hatteras are also extraordinarily high
(46 000–89 000 individuals m
−2 ), about 2–9 times
higher than at comparable depths elsewhere on the
eastern United States slope (Blake and Hilbig, 1994;
Blake and Grassle, 1994; Schaff et al., 1992). These
densities are typical of those found in shallow water,
and some of the species are characteristic of shelf
depths (Schaff et al., 1992; Blake and Grassle, 1994).
Bioturbation activity in this area is much higher than in
other regions of the Northwest Atlantic margin (Schaff
et al., 1992; Diaz et al., 1994; DeMaster et al., 1994;
Levin et al., 1997; Fornes et al., 1999).
The diversity and evenness of benthic foraminiferal
assemblages (the fraction >63 mm – i.e., within the
meiofaunal size range) at depths between 600 m and
2000 m off Cape Hatteras are reported by Cutter
et al. (1994) to be high (H
= 4.02–4.42, log e ). About
9% of the species present are those characteristic of
shelf environments (Cutter et al., 1994), comparable
to values from the metazoan macrobenthos. These
data, however, refer to ‘total’ assemblages – that
is, there was no discrimination between ‘dead’ and
‘live’ individuals (the ‘live’ being those stained with
rose Bengal). Gooday et al. (2001) found that large
Foraminifera (the fraction >300 mm – i.e., within the
129
the South Atlantic Bight with comparable data from
the North and Mid-Atlantic Bights, indicate that both
the highest and lowest macrofaunal diversities can be
found in the South Atlantic Bight. Generally it is midslope depths (1220 to 1500 m) that exhibit highest
species richness and evenness, but in the South Atlantic
Bight some stations at 800 m and 3000 m were diverse.
The low-diversity assemblages found off Cape Hatteras
(Blake and Grassle, 1994) are discussed below.
Analyses of macrofaunal species composition for
146 quantitative boxcore samples collected from depths
of 600–3500 m between Cape Hatteras and Charleston
Bump yielded 1300 species, of which over 30% were
new to science (Hilbig, 1994). Half of these were
polychaetes, 22% were crustaceans and 16% were
molluscs. This study found that the zoogeographic
barrier reported to occur between Capes Lookout
and Hatteras (Cutler, 1975) was non-existent for
bivalves, but was confirmed for most cumaceans, some
aplacophorans and many polychaetes (Hilbig, 1994).
Cape Hatteras: The slope off Cape Hatteras appears
to be atypical with respect to the rest of the Western
Atlantic slope in terms of sedimentation and benthic
standing stocks (Table 5.2). It has been suggested
that the sedimentary/nutrient regime is more typical of
estuarine or shelf environments (Rhoads and Hecker,
1994). The Cape Hatteras region receives high input of
organic matter comparable to that occurring in coastal
estuaries (DeMaster et al., 1994). The high inputs
are associated with high sedimentation rates resulting
from outwelling from nearshore embayments and
topographic funneling of nutrients from the shelf out
to the slope (Blake and Diaz, 1994). The Gulf Stream
and Virginia currents converge in this region. This
convergence, combined with the topographic position
of the Cape on the outer edge of the shelf, leads
to a funneling of water masses and their constituents
offshore (Rhoads and Hecker, 1994). Thus, much of
the organic input in this region is terrigenous, coming
from Chesapeake and Delaware Bays to the north,
North Carolina sounds, and the shelf. Evidence for
terrigenous inputs comes from lighter d
13 C values for
sediments (−21.2) than is observed at sites further
south (−18.7 to −19.6) (Blair et al., 1994). The
composition of fatty acids and sterols are typical
of refractory shelf and estuarine sediments (Harvey,
1994). The concentrations of chlorophyll a in sediments
at depths from 530 m to 2003 m averaged 19.9 mg m
−2 ,
a value much higher than observed elsewhere on the
eastern continental slope of the United States. Viable
diatoms present in cores up to 14 cm below the surface
suggest high rates of bioturbation (Cahoon et al.,
1994). Observations of rapid subduction of diatoms
by maldanid polychaetes at this site support this idea
(Levin et al., 1997, 1999), as do other experimental
studies of particle mixing (DeMaster et al., 1994; Blair
et al., 1996; Fornes et al., 1999). Analyses of fatty acids
and sterols suggest that diatoms and dinoflagellates
are the principal source of labile organic matter to
the sediments, with a minor input of vascular-plant
material (Harvey, 1994).
The Cape Hatteras margin supports extraordinarily
large numbers of megafauna (Hecker, 1994) and
dense infaunal assemblages with unusually low species
diversity (Schaff et al., 1992; Blake and Grassle, 1994;
Blake and Hilbig, 1994). The abundant megafauna
includes large populations of brittle stars and asteroids, the foraminiferan Bathysiphon filiformis, three
demersal fish, two eelpouts and a large anemone
(Fig. 5.5) (Hecker, 1994). These taxa attain much
higher population densities off Cape Hatteras than at
any other site on the eastern margin of the United States
(Hecker, 1994).
Macrofaunal densities at depths between 530 and
850 m off Cape Hatteras are also extraordinarily high
(46 000–89 000 individuals m
−2 ), about 2–9 times
higher than at comparable depths elsewhere on the
eastern United States slope (Blake and Hilbig, 1994;
Blake and Grassle, 1994; Schaff et al., 1992). These
densities are typical of those found in shallow water,
and some of the species are characteristic of shelf
depths (Schaff et al., 1992; Blake and Grassle, 1994).
Bioturbation activity in this area is much higher than in
other regions of the Northwest Atlantic margin (Schaff
et al., 1992; Diaz et al., 1994; DeMaster et al., 1994;
Levin et al., 1997; Fornes et al., 1999).
The diversity and evenness of benthic foraminiferal
assemblages (the fraction >63 mm – i.e., within the
meiofaunal size range) at depths between 600 m and
2000 m off Cape Hatteras are reported by Cutter
et al. (1994) to be high (H
= 4.02–4.42, log e ). About
9% of the species present are those characteristic of
shelf environments (Cutter et al., 1994), comparable
to values from the metazoan macrobenthos. These
data, however, refer to ‘total’ assemblages – that
is, there was no discrimination between ‘dead’ and
‘live’ individuals (the ‘live’ being those stained with
rose Bengal). Gooday et al. (2001) found that large
Foraminifera (the fraction >300 mm – i.e., within the