THE DEEP ATLANTIC OCEAN
153
below 4000 m. Desbruy` eres et al. (1985b) noted a
correlation between maximum current velocities and
the highest abundances of macrourids at their 4700 m
station. They also observed that an attack on the bait
by a single fish provoked an immediate concentration
of congeneric individuals around the bait.
Unique information about the biology of abyssal
scavenging fish in the Atlantic and Pacific Oceans has
been obtained using AUDOS, a free vehicle which
attracts fish to bait in which small transponders are
embedded (e.g., Priede et al., 1990, 1991, 1994a,b;
Armstrong et al., 1992; reviewed by Merrett and
Haedrich, 1997). The fish readily ingest these transponders along with the bait. The transponders report
back acoustically to a sonar on the central AUDOS
vehicle, allowing the movements of the fish on the
ocean floor to be tracked over a range of up to
500 m. The AUDOS experiments have demonstrated
that benthopelagic species living in the water near
the seafloor do not sit and wait for carcasses, as
previously believed, but actively search for them. The
first fish to arrive at bait in the Northeast Atlantic
were Coryphaenoides armatus and Histiobranchus
bathybius. Having ingested the bait (and transponder),
the fish generally wandered slowly away from the food
source, usually disappearing from the range of the
AUDOS sonar within a period of twelve hours. Recent
observations made on the Porcupine Abyssal Plain
suggest that there were no significant differences in the
arrival times or swimming speeds of fish before and
after the arrival of phytodetritus at this site (Smith et al.,
1997a). These results contrast with the observation that
macrourids arrived more slowly at bait in the North
Pacific Ocean before the spring bloom than they did
later in the year following a pulsed input of organic
matter (Priede et al., 1994a).
Equatorial and South Atlantic
Compared with the North Atlantic, the deep-sea
benthos of the Equatorial Atlantic, and particularly
the South Atlantic, is rather poorly known. Most
studies have concerned the abundance, diversity and
distribution of particular groups, in some cases as part
of an ocean-wide study of distribution and diversity
patterns. Examples include ascidians (Monniot and
Monniot, 1978; Monniot, 1979), protobranch bivalves
(Allen and Sanders, 1996) and echinoderms (Sibuet,
1979, 1985). Russian expeditions have also contributed
much to knowledge of the taxonomy and distribution of
animals in the deep South Atlantic (e.g., volumes edited
by Vinogradova, 1990, 1993; see also Vinogradova,
1997). A synthesis of meiofaunal, macrofaunal and
megafaunal densities in the Angola and Cape Basins
(Southeast Atlantic), the Demerara Abyssal Plain and
the Vema Fracture Zone (Equatorial Atlantic) as well
as in North Atlantic basins has been presented by
Sibuet et al. (1989). More recently, Rex et al. (1993)
have analyzed latitudinal trends in diversity among
macrofaunal bivalves, gastropods and isopods from the
Greenland–Norwegian Sea in the north to the Argentine
Basin in the south. Southern Hemisphere sites include
the Brazil, Angola and Cape Basins in addition to the
Argentine Basin.
Relatively few studies, however, have addressed
smaller-size fractions of the benthic fauna at particular
sites in the South Atlantic. One of the few is that of
Sanders (1969), who presented data for macrofauna
(>420 mm) from a transect through the oxygen minimum zone (OMZ) off Walvis Bay, Namibia. At depths
of 100 m, where the oxygen content of the bottom
water was <2% saturation, faunal density was low
(125 individuals m
−2 ). Species richness was low as
well, comparable to that found at 5000 m under the
oligotrophic Sargasso Sea. At 200 m (11% oxygen saturation), densities were exceptionally high (30 000 individuals m
−2 ), but diversity was only slightly increased.
At 300 m (15% saturation) diversity remained low, but
densities dropped to 8000 individuals m
−2 . Densities
at 450 m, 630 m, 975 m and 2140 m were 2300, 5400,
4750, and 4140 individuals m
−2 , respectively. Diversity,
estimated by rarefaction, increased with depth and
increasing oxygen saturation through the 975 m station,
but declined again at 2140 m. Sanders (1969) noted that
macrofaunal patterns along the Walvis Bay transect,
a gradient of oxygen stress, resemble those observed
along gradients of organic or chemical pollution.
Maximal densities combined with low diversity are
observed at the edge of the stressed conditions.
In a recent study, Soltwedel (1997) described meiofauna on the shelf, slope and abyssal plains (27–
4601 m) off the tropical West African coast between
Guinea (10ºN) and Angola (17ºS). This part of the
eastern equatorial Atlantic is subject to coastal upwelling, which varies seasonally and geographically but
is never sufficiently intense to result in the formation
of an oxygen minimum zone. Meiofaunal abundance
and biomass generally decrease fairly regularly with
increasing water depth. However, the rate of increase
varies in different parts of the margin, and is usually
153
below 4000 m. Desbruy` eres et al. (1985b) noted a
correlation between maximum current velocities and
the highest abundances of macrourids at their 4700 m
station. They also observed that an attack on the bait
by a single fish provoked an immediate concentration
of congeneric individuals around the bait.
Unique information about the biology of abyssal
scavenging fish in the Atlantic and Pacific Oceans has
been obtained using AUDOS, a free vehicle which
attracts fish to bait in which small transponders are
embedded (e.g., Priede et al., 1990, 1991, 1994a,b;
Armstrong et al., 1992; reviewed by Merrett and
Haedrich, 1997). The fish readily ingest these transponders along with the bait. The transponders report
back acoustically to a sonar on the central AUDOS
vehicle, allowing the movements of the fish on the
ocean floor to be tracked over a range of up to
500 m. The AUDOS experiments have demonstrated
that benthopelagic species living in the water near
the seafloor do not sit and wait for carcasses, as
previously believed, but actively search for them. The
first fish to arrive at bait in the Northeast Atlantic
were Coryphaenoides armatus and Histiobranchus
bathybius. Having ingested the bait (and transponder),
the fish generally wandered slowly away from the food
source, usually disappearing from the range of the
AUDOS sonar within a period of twelve hours. Recent
observations made on the Porcupine Abyssal Plain
suggest that there were no significant differences in the
arrival times or swimming speeds of fish before and
after the arrival of phytodetritus at this site (Smith et al.,
1997a). These results contrast with the observation that
macrourids arrived more slowly at bait in the North
Pacific Ocean before the spring bloom than they did
later in the year following a pulsed input of organic
matter (Priede et al., 1994a).
Equatorial and South Atlantic
Compared with the North Atlantic, the deep-sea
benthos of the Equatorial Atlantic, and particularly
the South Atlantic, is rather poorly known. Most
studies have concerned the abundance, diversity and
distribution of particular groups, in some cases as part
of an ocean-wide study of distribution and diversity
patterns. Examples include ascidians (Monniot and
Monniot, 1978; Monniot, 1979), protobranch bivalves
(Allen and Sanders, 1996) and echinoderms (Sibuet,
1979, 1985). Russian expeditions have also contributed
much to knowledge of the taxonomy and distribution of
animals in the deep South Atlantic (e.g., volumes edited
by Vinogradova, 1990, 1993; see also Vinogradova,
1997). A synthesis of meiofaunal, macrofaunal and
megafaunal densities in the Angola and Cape Basins
(Southeast Atlantic), the Demerara Abyssal Plain and
the Vema Fracture Zone (Equatorial Atlantic) as well
as in North Atlantic basins has been presented by
Sibuet et al. (1989). More recently, Rex et al. (1993)
have analyzed latitudinal trends in diversity among
macrofaunal bivalves, gastropods and isopods from the
Greenland–Norwegian Sea in the north to the Argentine
Basin in the south. Southern Hemisphere sites include
the Brazil, Angola and Cape Basins in addition to the
Argentine Basin.
Relatively few studies, however, have addressed
smaller-size fractions of the benthic fauna at particular
sites in the South Atlantic. One of the few is that of
Sanders (1969), who presented data for macrofauna
(>420 mm) from a transect through the oxygen minimum zone (OMZ) off Walvis Bay, Namibia. At depths
of 100 m, where the oxygen content of the bottom
water was <2% saturation, faunal density was low
(125 individuals m
−2 ). Species richness was low as
well, comparable to that found at 5000 m under the
oligotrophic Sargasso Sea. At 200 m (11% oxygen saturation), densities were exceptionally high (30 000 individuals m
−2 ), but diversity was only slightly increased.
At 300 m (15% saturation) diversity remained low, but
densities dropped to 8000 individuals m
−2 . Densities
at 450 m, 630 m, 975 m and 2140 m were 2300, 5400,
4750, and 4140 individuals m
−2 , respectively. Diversity,
estimated by rarefaction, increased with depth and
increasing oxygen saturation through the 975 m station,
but declined again at 2140 m. Sanders (1969) noted that
macrofaunal patterns along the Walvis Bay transect,
a gradient of oxygen stress, resemble those observed
along gradients of organic or chemical pollution.
Maximal densities combined with low diversity are
observed at the edge of the stressed conditions.
In a recent study, Soltwedel (1997) described meiofauna on the shelf, slope and abyssal plains (27–
4601 m) off the tropical West African coast between
Guinea (10ºN) and Angola (17ºS). This part of the
eastern equatorial Atlantic is subject to coastal upwelling, which varies seasonally and geographically but
is never sufficiently intense to result in the formation
of an oxygen minimum zone. Meiofaunal abundance
and biomass generally decrease fairly regularly with
increasing water depth. However, the rate of increase
varies in different parts of the margin, and is usually