THE DEEP ATLANTIC OCEAN
157
BASIN-WIDE BIOLOGICAL PATTERNS AND
PROCESSES
Though many studies of deep-sea benthos are regional
in nature, a few have tried to draw global or basinwide comparisons in an attempt to evaluate the relative
importance of factors controlling biological pattern in
the deep sea. In the sections below we review the
conclusions of such studies, and, because we found
large gaps in treatment of these issues, have attempted
to create our own syntheses. We focus on patterns of
density and biomass, community respiration, bioturbation and succession, in the belief that much, though
certainly not all, of the accumulated knowledge about
the deep Atlantic fauna addresses these issues.
Density and biomass
Most early investigations of density and biomass
patterns of Atlantic benthos examined the effects of
water depth. Investigators usually treat meiofauna,
macrofauna, or megafauna, but rarely all three (Table 5.3; but see Gal´ eron et al., 2000). In most places
monotonic, sometimes exponential, declines in density
and biomass were observed with increasing water depth
for each group (Thiel, 1983; figs. 6, 7 therein; Rowe,
1983; fig. 3 therein; Vincx et al., 1994; table 3 therein).
Lampitt et al. (1986) reported a logarithmic decline
in megafaunal biomass along a depth gradient in the
Porcupine Seabight, with a reduction by a factor of 30
between 500 and 4100 m. They observed no significant
change in average body weight with depth. Declines in
macrofaunal density with depth are also evident along
most depth transects in the Northwest Atlantic (Hessler
and Sanders, 1967; Rowe et al., 1974; Smith, 1978;
Blake and Grassle, 1994; Blake and Hilbig, 1994).
Less regular patterns have been reported beneath upwelling regions, where oxygen minima at intermediate
depths depress density (Sanders, 1969; Thiel, 1978;
Pfannkuche et al., 1983), where fluxes of organic matter
to deep water are very high, as in the Norwegian–
Greenland Sea (Thiel, 1983: fig. 9 therein), and in
regions subject to hydrodynamic disturbance (Hecker,
1990b). Where hydrodynamic reworking is intense,
as in the Rockall Trough and Goban Spur, maximal
densities and biomass sometimes occur at mid-slope
depths (Gage, 1978; Flach and Heip, 1996b). These
patterns may be modified by seasonal and interannual
variation (Flach and Heip, 1996b).
A more recent emphasis has been on the comparison
of abundance and biomass at sites differing in organicmatter flux (Sibuet et al., 1989, 1993; Rowe et al.,
1991; Thurston et al., 1994; Paterson et al., 1994a,b;
Vanreusel et al., 1995a; Gooday, 1996b; Cosson
et al., 1997; Gal´ eron et al., 2000). These studies
have invariably found that flux of particulate organic
carbon (POC) is the first-order parameter controlling
macrofaunal and meiofaunal abundances in the deep
Atlantic (Thiel, 1983; Sibuet et al., 1989; Rowe
et al., 1991). Regions of the Atlantic with highest
organic flux support the greatest infaunal densities and
biomass. Particularly notable hot-spots in this regard
are the Norwegian–Greenland Sea (Romero-Wetzel and
Gerlach, 1991), the continental margin off Cap Blanc,
Northwest Africa (Cosson et al., 1997; Gal´ eron et al.,
2000), and off Cape Hatteras, North Carolina. In the
latter area, macrofaunal densities at 850 m depth exceed
40 000 individuals m
−2 (Schaff et al., 1992; Blake and
Hilbig, 1994).
A similar correlation is often apparent along bathymetric transects. Work by Pfannkuche et al. (1983) off
Morocco (35ºN) and Pfannkuche (1985) in the Porcupine Seabight indicates that meiofaunal abundances
are strongly linked to surface productivity regimes and
sediment chloroplastic pigments, but do not necessarily
decrease exponentially downslope. For example, on the
Moroccan transect, meiofaunal maxima were observed
between 400 and 1200 m. Thiel (1978) reported meiofaunal densities to be fairly constant with depth at
sites beneath upwelling areas off Northwest Africa,
with densities sometimes exceeding 10
6 m
−2 . Vanreusel
et al. (1995a) documented lower nematode density
and biomass in oligotrophic waters off Mauritania
(EUMELI 3) than at the Porcupine Abyssal Plain site,
which experiences seasonal phytodetrital pulses. Larger
nematode body size was observed on the Porcupine
Abyssal Plain. In some cases, the macrofaunal and
meiofaunal trends appear linked to patterns of bacterial
biomass and density (Rowe et al., 1991; Vanreusel
et al., 1995b), which are in turn dependent on the
primary productivity of overlying waters (Morita, 1979;
Tan and Ruger, 1989). Rowe et al. (1991) suggested
that the importance of bacteria in the pool of living
carbon (relative to macro and meiofauna) increases
with depth.
Patterns of density and biomass generally track one
another; regions of highest density often exhibit high
biomass. However, for macrofauna, biomass declines
more rapidly with depth than does abundance because
body size declines with depth (Rowe et al., 1974;
157
BASIN-WIDE BIOLOGICAL PATTERNS AND
PROCESSES
Though many studies of deep-sea benthos are regional
in nature, a few have tried to draw global or basinwide comparisons in an attempt to evaluate the relative
importance of factors controlling biological pattern in
the deep sea. In the sections below we review the
conclusions of such studies, and, because we found
large gaps in treatment of these issues, have attempted
to create our own syntheses. We focus on patterns of
density and biomass, community respiration, bioturbation and succession, in the belief that much, though
certainly not all, of the accumulated knowledge about
the deep Atlantic fauna addresses these issues.
Density and biomass
Most early investigations of density and biomass
patterns of Atlantic benthos examined the effects of
water depth. Investigators usually treat meiofauna,
macrofauna, or megafauna, but rarely all three (Table 5.3; but see Gal´ eron et al., 2000). In most places
monotonic, sometimes exponential, declines in density
and biomass were observed with increasing water depth
for each group (Thiel, 1983; figs. 6, 7 therein; Rowe,
1983; fig. 3 therein; Vincx et al., 1994; table 3 therein).
Lampitt et al. (1986) reported a logarithmic decline
in megafaunal biomass along a depth gradient in the
Porcupine Seabight, with a reduction by a factor of 30
between 500 and 4100 m. They observed no significant
change in average body weight with depth. Declines in
macrofaunal density with depth are also evident along
most depth transects in the Northwest Atlantic (Hessler
and Sanders, 1967; Rowe et al., 1974; Smith, 1978;
Blake and Grassle, 1994; Blake and Hilbig, 1994).
Less regular patterns have been reported beneath upwelling regions, where oxygen minima at intermediate
depths depress density (Sanders, 1969; Thiel, 1978;
Pfannkuche et al., 1983), where fluxes of organic matter
to deep water are very high, as in the Norwegian–
Greenland Sea (Thiel, 1983: fig. 9 therein), and in
regions subject to hydrodynamic disturbance (Hecker,
1990b). Where hydrodynamic reworking is intense,
as in the Rockall Trough and Goban Spur, maximal
densities and biomass sometimes occur at mid-slope
depths (Gage, 1978; Flach and Heip, 1996b). These
patterns may be modified by seasonal and interannual
variation (Flach and Heip, 1996b).
A more recent emphasis has been on the comparison
of abundance and biomass at sites differing in organicmatter flux (Sibuet et al., 1989, 1993; Rowe et al.,
1991; Thurston et al., 1994; Paterson et al., 1994a,b;
Vanreusel et al., 1995a; Gooday, 1996b; Cosson
et al., 1997; Gal´ eron et al., 2000). These studies
have invariably found that flux of particulate organic
carbon (POC) is the first-order parameter controlling
macrofaunal and meiofaunal abundances in the deep
Atlantic (Thiel, 1983; Sibuet et al., 1989; Rowe
et al., 1991). Regions of the Atlantic with highest
organic flux support the greatest infaunal densities and
biomass. Particularly notable hot-spots in this regard
are the Norwegian–Greenland Sea (Romero-Wetzel and
Gerlach, 1991), the continental margin off Cap Blanc,
Northwest Africa (Cosson et al., 1997; Gal´ eron et al.,
2000), and off Cape Hatteras, North Carolina. In the
latter area, macrofaunal densities at 850 m depth exceed
40 000 individuals m
−2 (Schaff et al., 1992; Blake and
Hilbig, 1994).
A similar correlation is often apparent along bathymetric transects. Work by Pfannkuche et al. (1983) off
Morocco (35ºN) and Pfannkuche (1985) in the Porcupine Seabight indicates that meiofaunal abundances
are strongly linked to surface productivity regimes and
sediment chloroplastic pigments, but do not necessarily
decrease exponentially downslope. For example, on the
Moroccan transect, meiofaunal maxima were observed
between 400 and 1200 m. Thiel (1978) reported meiofaunal densities to be fairly constant with depth at
sites beneath upwelling areas off Northwest Africa,
with densities sometimes exceeding 10
6 m
−2 . Vanreusel
et al. (1995a) documented lower nematode density
and biomass in oligotrophic waters off Mauritania
(EUMELI 3) than at the Porcupine Abyssal Plain site,
which experiences seasonal phytodetrital pulses. Larger
nematode body size was observed on the Porcupine
Abyssal Plain. In some cases, the macrofaunal and
meiofaunal trends appear linked to patterns of bacterial
biomass and density (Rowe et al., 1991; Vanreusel
et al., 1995b), which are in turn dependent on the
primary productivity of overlying waters (Morita, 1979;
Tan and Ruger, 1989). Rowe et al. (1991) suggested
that the importance of bacteria in the pool of living
carbon (relative to macro and meiofauna) increases
with depth.
Patterns of density and biomass generally track one
another; regions of highest density often exhibit high
biomass. However, for macrofauna, biomass declines
more rapidly with depth than does abundance because
body size declines with depth (Rowe et al., 1974;