186
Craig R. SMITH and Amanda W.J. DEMOPOULOS
column relatively shallow, particulate organic carbon
rains to the seafloor at high annual rates – for instance,
~10 g C m
−2 y
−1 at 1000 m on the California slope
(Smith and Hinga, 1983). At abyssal depths (4000–
6000 m) beneath productive waters (e.g., the California
Current or the equatorial upwelling zone), the annual
flux of particulate organic carbon declines to roughly
1–3 g C m
−2 y
−1 (K.L. Smith et al., 1992; C.R. Smith
et al., 1997). Beneath the vast oligotrophic gyres (see
Chapter 2, Fig. 2.13) where the water column is deep
(>5000 m) and annual primary production very low, the
annual flux of particulate organic carbon may be as
little as 0.3 g C m
−2 y
−1 (K.L. Smith, 1992).
REPRESENTATIVE DEEP PACIFIC HABITATS
Continental slopes and marginal basins
Continental-slope and marginal-basin habitats surround
the Pacific at water depths from 200 m to 4000 m.
Habitat conditions vary dramatically over spatial scales
of tens to thousands of kilometers along these slopes,
yielding a broad array of communities. For example,
the sinking flux of particulate organic carbon typically
decreases more than three fold as depth increases
from 500 m to 4000 m (Martin et al., 1987; Berelson
et al., 1996). Substratum and current velocities differ
dramatically from depositional fans, where sediments
often are muddy and currents sluggish, to submarine
canyons, where rocky outcrops and erosive currents
abound. In the eastern Pacific, the oxygen minimum
is superimposed on this topographically induced complexity, yielding a layer of oxygen-stressed habitats
between ocean depths of 100 m and 1000 m. Below we
discuss several habitat types found in slope regions:
depositional slopes and basins, canyons, and oxygenminimum zones.
Depositional slopes and basins on the California
margin
The best studied slopes and basins occur along the
margin of the American state of California, where deep
benthic ecosystems have been intensively investigated
since the late 1950s (Emery, 1960). The general
patterns here are almost certainly representative of
Pacific slopes in general, although specific details (e.g.,
species identities, absolute flux rates of particulate
organic carbon, intensity of the oxygen-minimum zone)
may vary with geographic location.
Habitat and community description: Along the
open California slope, in areas of relatively low current
velocity, sediments generally grade from sandy on the
upper slope (~200 m to 600 m) to soft muds at greater
depths (Emery, 1960; Reimers et al., 1992; Vetter and
Dayton, 1998). The borderland basins off southern
California, formed by a series of ridges and troughs
parallel to the coastline, are floored predominantly by
fine muds at depths from 1000 m to 2000 m. Muddy
surface sediments in these slope habitats are heavily
modified by biological activity, which forms a patina of
animal tracks, trails, mounds, tubes and fecal casts on
the seafloor (Fig. 6.6a; see also Jumars, 1975; Thistle,
1979b; Smith and Hamilton, 1983). Many of these
biogenic structures are surprisingly dynamic, being
formed and destroyed by faunal activity rather than by
water flow. In the 1240 m-deep Santa Catalina Basin,
the fecal mounds of echiurans (5–10 cm high and
30 cm across) can grow several centimeters in height
in 100 hours (Smith et al., 1986); when abandoned, the
mounds disappear within 11 months as a consequence
of sediment reworking by brittle stars and other benthos
(Kukert and Smith, 1992). Smaller structures, such
as gastropod trails or fecal casts of holothuroids, are
erased from the basin floor by brittle stars within a
few weeks (Wheatcroft et al., 1989). Thus, much of
the biogenic structure of the sediment–water interface
appears to change many times during the life spans
(years to decades) of macro- and megabenthos on the
California margin.
The California slopes and basins harbor richer
benthic assemblages than more oligotrophic settings,
such as the North Pacific central gyre. Epibenthic
megafauna (animals greater than 2 cm in smallest
dimension) often are abundant, attaining densities from
0.3 to 17 individuals m
−2 (Table 6.1; see also Smith
and Hamilton, 1983; Bennett et al., 1994; Lauerman
et al., 1996). Echinoderms are particularly common,
with brittle stars (e.g., Ophiomusium lymani, Ophiophthalmus normani) and holothuroids (e.g., the “sea
pig” Scotoplanes globosa) dominating the megafauna
(Barham et al., 1967; C.R. Smith and Hamilton, 1983;
Lauerman et al., 1996), and at times attaining high
biomasses (e.g., a mean of 67±30 g wet weight m
−2 in
the Santa Catalina Basin). In addition to the dominant
echinoderms, many other taxa are represented in the
megafauna, including gastropods (e.g., neptunids and
trochids), hexactinellid sponges, fishes (macrourids,
zoarcids, and hagfish), decapods and galatheids (Smith
and Hamilton, 1983; Wakefield, 1990; Lauerman et al.
Craig R. SMITH and Amanda W.J. DEMOPOULOS
column relatively shallow, particulate organic carbon
rains to the seafloor at high annual rates – for instance,
~10 g C m
−2 y
−1 at 1000 m on the California slope
(Smith and Hinga, 1983). At abyssal depths (4000–
6000 m) beneath productive waters (e.g., the California
Current or the equatorial upwelling zone), the annual
flux of particulate organic carbon declines to roughly
1–3 g C m
−2 y
−1 (K.L. Smith et al., 1992; C.R. Smith
et al., 1997). Beneath the vast oligotrophic gyres (see
Chapter 2, Fig. 2.13) where the water column is deep
(>5000 m) and annual primary production very low, the
annual flux of particulate organic carbon may be as
little as 0.3 g C m
−2 y
−1 (K.L. Smith, 1992).
REPRESENTATIVE DEEP PACIFIC HABITATS
Continental slopes and marginal basins
Continental-slope and marginal-basin habitats surround
the Pacific at water depths from 200 m to 4000 m.
Habitat conditions vary dramatically over spatial scales
of tens to thousands of kilometers along these slopes,
yielding a broad array of communities. For example,
the sinking flux of particulate organic carbon typically
decreases more than three fold as depth increases
from 500 m to 4000 m (Martin et al., 1987; Berelson
et al., 1996). Substratum and current velocities differ
dramatically from depositional fans, where sediments
often are muddy and currents sluggish, to submarine
canyons, where rocky outcrops and erosive currents
abound. In the eastern Pacific, the oxygen minimum
is superimposed on this topographically induced complexity, yielding a layer of oxygen-stressed habitats
between ocean depths of 100 m and 1000 m. Below we
discuss several habitat types found in slope regions:
depositional slopes and basins, canyons, and oxygenminimum zones.
Depositional slopes and basins on the California
margin
The best studied slopes and basins occur along the
margin of the American state of California, where deep
benthic ecosystems have been intensively investigated
since the late 1950s (Emery, 1960). The general
patterns here are almost certainly representative of
Pacific slopes in general, although specific details (e.g.,
species identities, absolute flux rates of particulate
organic carbon, intensity of the oxygen-minimum zone)
may vary with geographic location.
Habitat and community description: Along the
open California slope, in areas of relatively low current
velocity, sediments generally grade from sandy on the
upper slope (~200 m to 600 m) to soft muds at greater
depths (Emery, 1960; Reimers et al., 1992; Vetter and
Dayton, 1998). The borderland basins off southern
California, formed by a series of ridges and troughs
parallel to the coastline, are floored predominantly by
fine muds at depths from 1000 m to 2000 m. Muddy
surface sediments in these slope habitats are heavily
modified by biological activity, which forms a patina of
animal tracks, trails, mounds, tubes and fecal casts on
the seafloor (Fig. 6.6a; see also Jumars, 1975; Thistle,
1979b; Smith and Hamilton, 1983). Many of these
biogenic structures are surprisingly dynamic, being
formed and destroyed by faunal activity rather than by
water flow. In the 1240 m-deep Santa Catalina Basin,
the fecal mounds of echiurans (5–10 cm high and
30 cm across) can grow several centimeters in height
in 100 hours (Smith et al., 1986); when abandoned, the
mounds disappear within 11 months as a consequence
of sediment reworking by brittle stars and other benthos
(Kukert and Smith, 1992). Smaller structures, such
as gastropod trails or fecal casts of holothuroids, are
erased from the basin floor by brittle stars within a
few weeks (Wheatcroft et al., 1989). Thus, much of
the biogenic structure of the sediment–water interface
appears to change many times during the life spans
(years to decades) of macro- and megabenthos on the
California margin.
The California slopes and basins harbor richer
benthic assemblages than more oligotrophic settings,
such as the North Pacific central gyre. Epibenthic
megafauna (animals greater than 2 cm in smallest
dimension) often are abundant, attaining densities from
0.3 to 17 individuals m
−2 (Table 6.1; see also Smith
and Hamilton, 1983; Bennett et al., 1994; Lauerman
et al., 1996). Echinoderms are particularly common,
with brittle stars (e.g., Ophiomusium lymani, Ophiophthalmus normani) and holothuroids (e.g., the “sea
pig” Scotoplanes globosa) dominating the megafauna
(Barham et al., 1967; C.R. Smith and Hamilton, 1983;
Lauerman et al., 1996), and at times attaining high
biomasses (e.g., a mean of 67±30 g wet weight m
−2 in
the Santa Catalina Basin). In addition to the dominant
echinoderms, many other taxa are represented in the
megafauna, including gastropods (e.g., neptunids and
trochids), hexactinellid sponges, fishes (macrourids,
zoarcids, and hagfish), decapods and galatheids (Smith
and Hamilton, 1983; Wakefield, 1990; Lauerman et al.