158
Lisa A. LEVIN and Andrew J. GOODAY
Thiel, 1975). Based on a comparison of data for
macrofauna in the Northwest with data for meiofauna
in the Northeast Atlantic, Thiel (1983) reported that
the slope of density decline with water depth is
steeper for macrofauna than for meiofauna by a factor
of 2. A reasonable explanation is that smaller-sized
organisms (the meiofauna) are better able to cope with
declining food inputs at greater depths. However, east–
west differences in patterns of carbon flux with depth
may have influenced the outcome of this analysis.
Several authors have suggested that megafauna are
more sensitive to changes in food supply than are
smaller organisms (Sibuet et al., 1984; Lampitt et al.,
1986). Sibuet et al. (1989) have reported that there is
a constant proportional relationship between metazoan
meiofauna, macrofauna, and megafauna, but that the
relation of particulate organic matter to abundance
of megafauna differs from that of the other groups.
Gal´ eron et al. (2000) found that megafaunal biomass
in the tropical Northeast Atlantic exceeded that of
macrofauna and meiofauna only at the most eutrophic
of the three EUMELI sites. Grassle et al. (1975)
estimated that, in the Northwest Atlantic, megafauna
were three orders of magnitude less abundant than
the macrofauna at 1850 m, whereas biomass for the
two size groups was about equal (Haedrich and Rowe,
1977). Megafauna appear to dominate where food input
is high (e.g., off Cape Hatteras, off Northwest Africa,
or on the Amazon Cone), or where advective transport
enhances particle flux (e.g., the Rockall Trough). They
appear less important in systems where food supply
is lower, although their role in these ecosystems may
not be accurately reflected by counts (Lampitt et al.,
1986).
There have been no systematic comparisons of
the western and eastern Atlantic Ocean in terms of
benthic abundance or biomass in deep waters. Too few
meiofaunal data exist, and methods are too variable to
draw valid comparisons between the two sides of the
Atlantic, although they have been discussed by Thiel
(1983).
Regressions of macrofaunal density against water
depth reveal higher densities in the Northwest than
Northeast Atlantic at bathyal depths (Fig. 5.7A).
The Northwest Atlantic macrofauna show a strong
exponential decline in density with depth (exponential
fit: r
2 = 0.70; P < 0.0001); this relationship is much
weaker in the Northeast Atlantic (exponential fit:
r
2 = 0.16; P = 0.007), where the density relationship
with depth is more linear (r
2 = 0.31; P < 0.0001). The
regression lines for the two areas cross just below
4000 m (Fig. 5.7A), suggesting that, at abyssal depths,
the macrofauna may be more abundant in the Northeast
than the Northwest Atlantic. It is possible that these
differences, driven largely by high margin densities in
the South Atlantic Bight, are due to activities of the
western boundary current (Gulf Stream) in focusing
organic-matter inputs. However, the same trends are not
evident for macrofaunal biomass which, on the basis of
limited data (Fig. 5.7B, Table 5.3), appears to be greater
in the Northeast than in the Northwest Atlantic.
Strong regional variation is evident in the Atlantic.
The Porcupine Seabight yielded invertebrate megafaunal biomass eighteen times and ten times higher,
respectively, than depths of 500 m and 4000 m on the
slope off southern New England, prompting Lampitt
et al. (1986) to suggest that there is higher megafaunal
biomass in the Northeast than in the Northwest Atlantic. Megafaunal biomass on the Demerara Abyssal
Plain (4.5 mg dry wt. m
−2 ) was less than half that
on the Porcupine Seabight (11.7 mg m
−2 ), while the
margin off the Amazon had more than twice as
much biomass as the Porcupine Seabight. Ocean-wide
comparisons of megafaunal biomass, of the sort carried
out above for the macrofauna, are made difficult by
the broad range of methods and variable minimum
body sizes considered in megafaunal investigations
(Thurston et al., 1994) (Table 5.3).
Community respiration
Sediment community respiration, also termed sediment
community oxygen consumption (SCOC), is of interest
because it represents a highly integrated measure of
activity levels of aerobic bacteria, protozoans and
metazoans. This activity is believed to be strongly
controlled by the flux of particulate organic carbon
(i.e., food availability) at the sea floor. Sediment
community oxygen consumption has been measured
using several approaches (Smith and Hinga, 1983).
These include (a) in situ respirometry, which is
based on loss of dissolved oxygen in water overlying
enclosed sediments, with measurements made within
the seabed or in cores suspended just above the bottom;
(b) shipboard core incubations, which also measure
changes in dissolved oxygen; (c) biochemical assay of
the respiratory electron-transport system (ETS) activity
in the laboratory; and (d) calculations of oxygen
consumption derived from oxygen concentration gradients in sediment pore-waters. Several other approaches
Lisa A. LEVIN and Andrew J. GOODAY
Thiel, 1975). Based on a comparison of data for
macrofauna in the Northwest with data for meiofauna
in the Northeast Atlantic, Thiel (1983) reported that
the slope of density decline with water depth is
steeper for macrofauna than for meiofauna by a factor
of 2. A reasonable explanation is that smaller-sized
organisms (the meiofauna) are better able to cope with
declining food inputs at greater depths. However, east–
west differences in patterns of carbon flux with depth
may have influenced the outcome of this analysis.
Several authors have suggested that megafauna are
more sensitive to changes in food supply than are
smaller organisms (Sibuet et al., 1984; Lampitt et al.,
1986). Sibuet et al. (1989) have reported that there is
a constant proportional relationship between metazoan
meiofauna, macrofauna, and megafauna, but that the
relation of particulate organic matter to abundance
of megafauna differs from that of the other groups.
Gal´ eron et al. (2000) found that megafaunal biomass
in the tropical Northeast Atlantic exceeded that of
macrofauna and meiofauna only at the most eutrophic
of the three EUMELI sites. Grassle et al. (1975)
estimated that, in the Northwest Atlantic, megafauna
were three orders of magnitude less abundant than
the macrofauna at 1850 m, whereas biomass for the
two size groups was about equal (Haedrich and Rowe,
1977). Megafauna appear to dominate where food input
is high (e.g., off Cape Hatteras, off Northwest Africa,
or on the Amazon Cone), or where advective transport
enhances particle flux (e.g., the Rockall Trough). They
appear less important in systems where food supply
is lower, although their role in these ecosystems may
not be accurately reflected by counts (Lampitt et al.,
1986).
There have been no systematic comparisons of
the western and eastern Atlantic Ocean in terms of
benthic abundance or biomass in deep waters. Too few
meiofaunal data exist, and methods are too variable to
draw valid comparisons between the two sides of the
Atlantic, although they have been discussed by Thiel
(1983).
Regressions of macrofaunal density against water
depth reveal higher densities in the Northwest than
Northeast Atlantic at bathyal depths (Fig. 5.7A).
The Northwest Atlantic macrofauna show a strong
exponential decline in density with depth (exponential
fit: r
2 = 0.70; P < 0.0001); this relationship is much
weaker in the Northeast Atlantic (exponential fit:
r
2 = 0.16; P = 0.007), where the density relationship
with depth is more linear (r
2 = 0.31; P < 0.0001). The
regression lines for the two areas cross just below
4000 m (Fig. 5.7A), suggesting that, at abyssal depths,
the macrofauna may be more abundant in the Northeast
than the Northwest Atlantic. It is possible that these
differences, driven largely by high margin densities in
the South Atlantic Bight, are due to activities of the
western boundary current (Gulf Stream) in focusing
organic-matter inputs. However, the same trends are not
evident for macrofaunal biomass which, on the basis of
limited data (Fig. 5.7B, Table 5.3), appears to be greater
in the Northeast than in the Northwest Atlantic.
Strong regional variation is evident in the Atlantic.
The Porcupine Seabight yielded invertebrate megafaunal biomass eighteen times and ten times higher,
respectively, than depths of 500 m and 4000 m on the
slope off southern New England, prompting Lampitt
et al. (1986) to suggest that there is higher megafaunal
biomass in the Northeast than in the Northwest Atlantic. Megafaunal biomass on the Demerara Abyssal
Plain (4.5 mg dry wt. m
−2 ) was less than half that
on the Porcupine Seabight (11.7 mg m
−2 ), while the
margin off the Amazon had more than twice as
much biomass as the Porcupine Seabight. Ocean-wide
comparisons of megafaunal biomass, of the sort carried
out above for the macrofauna, are made difficult by
the broad range of methods and variable minimum
body sizes considered in megafaunal investigations
(Thurston et al., 1994) (Table 5.3).
Community respiration
Sediment community respiration, also termed sediment
community oxygen consumption (SCOC), is of interest
because it represents a highly integrated measure of
activity levels of aerobic bacteria, protozoans and
metazoans. This activity is believed to be strongly
controlled by the flux of particulate organic carbon
(i.e., food availability) at the sea floor. Sediment
community oxygen consumption has been measured
using several approaches (Smith and Hinga, 1983).
These include (a) in situ respirometry, which is
based on loss of dissolved oxygen in water overlying
enclosed sediments, with measurements made within
the seabed or in cores suspended just above the bottom;
(b) shipboard core incubations, which also measure
changes in dissolved oxygen; (c) biochemical assay of
the respiratory electron-transport system (ETS) activity
in the laboratory; and (d) calculations of oxygen
consumption derived from oxygen concentration gradients in sediment pore-waters. Several other approaches