THE DEEP PACIFIC OCEAN FLOOR
197
time scales of years, rather than the weeks to months
typical of shallow communities (e.g., VanBlaricom,
1982; Smith and Brumsickle, 1989; Vetter, 1996).
Submarine canyons
The shelves and slopes of the Pacific basin are dissected
by submarine canyons; in fact, the Pacific contains
49 of the 96 submarine canyons mapped worldwide
by Shepard and Dill (1966). These features typically
begin at depths of 15 to 100 m and form steep,
narrow-walled channels that terminate near the floors
of basins or at the base of the continental slope,
often producing depositional sediment fans (Shepard
and Dill, 1966). All canyons serve both as channels
for energetic currents and turbidity flows, and as
conduits for the transport of detritus (e.g., detrital
kelp and sand) and particle-bound pollutants from
the continental shelf into the deep sea (Vetter, 1994).
Substratum types include rocky outcrops, sediments
ranging from coarse sand to mud, and in some cases,
large parcels of organic debris (Vetter, 1994; Vetter
and Dayton, 1998). Consumers feeding in canyons,
including commercially exploited species, potentially
can experience increased food supply through at least
three mechanisms. Suspension feeders may benefit
from accelerated currents (Rowe, 1971), demersal
planktivores can exploit dense layers of zooplankton
which become concentrated in canyons during vertical
migrations (Greene et al., 1988), and detritivores
may benefit from elevated sedimentation rates and
accumulations of macrophytic debris (Vetter, 1994;
Vetter and Dayton, 1998; Harrold et al., 1998). Because
of high physical energy, rocky outcrops, and enhanced
food availability in canyons, faunal communities differ
markedly from those on the surrounding sedimentcovered slopes.
The Pacific canyons which have been best studied
biologically are the Scripps and La Jolla Canyons off
San Diego, California. Vetter and Dayton (1998) found
evidence of organic enrichment from macrophytic detritus (kelp and seagrass) to depths of 550 m, and coarse
sediments suggestive of strong currents to depths of
700 m within both canyons. Infaunal assemblages in
canyons were distinct from those at similar depths
on the nearby slope, with macrofaunal densities and
biomasses typically 2-fold to 15-fold higher in canyons;
in fact, canyon macrofaunal densities were among the
highest ever measured at slope depths. The most abundant species in canyons generally were detritivores, but
included the bivalve Thyasira flexuosa, which contains
endosymbiotic, sulfur-oxidizing bacteria presumably
utilizing sulfides derived from anaerobic decay of
buried detritus, or from porewater seepage along
the canyon axis (Vetter and Dayton, 1998). Species
composition within canyons also differed from that on
surrounding slopes. Canyon assemblages generally had
lower diversity owing to dominance by a few species
(e.g., the polychaete Capitella sp.); nonetheless, 168
out of a total of 435 species collected by Vetter and
Dayton (1998) occurred only inside the canyons. It is
clear that canyons contribute substantially to habitat
diversity on the continental slope.
On the northeast Pacific slope, the enhanced secondary production of canyons may also figure significantly in the life-history of demersal fishes. Food-rich
patches often are critical for the recruitment success of
many fish stocks, allowing larval and juvenile stages to
pass through “energetic bottlenecks”. In fact, Vetter and
Dayton (1999) found very high densities of juvenile
hake (Merluccius productus) within the Scripps and
La Jolla Canyons, suggesting that the canyons were
acting as nursery grounds. These authors also found
enhanced abundance of turbot (Pleuronichthys sp.)
and zoarcids within canyons. Perhaps not surprisingly,
submarine canyons along the California coast are regularly targeted by commercial and recreational fishermen
exploiting rockfish, rattails and other bottom fishes
(C.R. Smith and E.W. Vetter, personal observations).
Oxygen-minimum zones
As discussed above, the eastern margin of the Pacific
Ocean is intersected by an oxygen-minimum zone
(oxygen-minimum zone), where bottom-water oxygen
concentrations drop below 0.5 ml °
−1 (Fig. 6.10). In
the equatorial zone, the oxygen-minimum zone is
particularly well developed extending from a depth of
50 m to 1300 m, with oxygen concentrations falling
below 0.1 ml °
−1 over most of this range (Wishner et al.,
1991). On the California slope, the oxygen-minimum
zone is not as well developed, but still extends over
depths roughly from 500 m to 1000 m, with minimum
oxygen concentrations below 0.3 ml °
−1 (Emery, 1960;
Reimers et al., 1992; Fig. 6.9). In enclosed basins
(e.g., Santa Monica and Santa Barbara Basins) whose
sill depths intersect the oxygen-minimum zone, lowoxygen conditions may extend to basin floors, which
can be much deeper than 1000 m (Emery, 1960).
197
time scales of years, rather than the weeks to months
typical of shallow communities (e.g., VanBlaricom,
1982; Smith and Brumsickle, 1989; Vetter, 1996).
Submarine canyons
The shelves and slopes of the Pacific basin are dissected
by submarine canyons; in fact, the Pacific contains
49 of the 96 submarine canyons mapped worldwide
by Shepard and Dill (1966). These features typically
begin at depths of 15 to 100 m and form steep,
narrow-walled channels that terminate near the floors
of basins or at the base of the continental slope,
often producing depositional sediment fans (Shepard
and Dill, 1966). All canyons serve both as channels
for energetic currents and turbidity flows, and as
conduits for the transport of detritus (e.g., detrital
kelp and sand) and particle-bound pollutants from
the continental shelf into the deep sea (Vetter, 1994).
Substratum types include rocky outcrops, sediments
ranging from coarse sand to mud, and in some cases,
large parcels of organic debris (Vetter, 1994; Vetter
and Dayton, 1998). Consumers feeding in canyons,
including commercially exploited species, potentially
can experience increased food supply through at least
three mechanisms. Suspension feeders may benefit
from accelerated currents (Rowe, 1971), demersal
planktivores can exploit dense layers of zooplankton
which become concentrated in canyons during vertical
migrations (Greene et al., 1988), and detritivores
may benefit from elevated sedimentation rates and
accumulations of macrophytic debris (Vetter, 1994;
Vetter and Dayton, 1998; Harrold et al., 1998). Because
of high physical energy, rocky outcrops, and enhanced
food availability in canyons, faunal communities differ
markedly from those on the surrounding sedimentcovered slopes.
The Pacific canyons which have been best studied
biologically are the Scripps and La Jolla Canyons off
San Diego, California. Vetter and Dayton (1998) found
evidence of organic enrichment from macrophytic detritus (kelp and seagrass) to depths of 550 m, and coarse
sediments suggestive of strong currents to depths of
700 m within both canyons. Infaunal assemblages in
canyons were distinct from those at similar depths
on the nearby slope, with macrofaunal densities and
biomasses typically 2-fold to 15-fold higher in canyons;
in fact, canyon macrofaunal densities were among the
highest ever measured at slope depths. The most abundant species in canyons generally were detritivores, but
included the bivalve Thyasira flexuosa, which contains
endosymbiotic, sulfur-oxidizing bacteria presumably
utilizing sulfides derived from anaerobic decay of
buried detritus, or from porewater seepage along
the canyon axis (Vetter and Dayton, 1998). Species
composition within canyons also differed from that on
surrounding slopes. Canyon assemblages generally had
lower diversity owing to dominance by a few species
(e.g., the polychaete Capitella sp.); nonetheless, 168
out of a total of 435 species collected by Vetter and
Dayton (1998) occurred only inside the canyons. It is
clear that canyons contribute substantially to habitat
diversity on the continental slope.
On the northeast Pacific slope, the enhanced secondary production of canyons may also figure significantly in the life-history of demersal fishes. Food-rich
patches often are critical for the recruitment success of
many fish stocks, allowing larval and juvenile stages to
pass through “energetic bottlenecks”. In fact, Vetter and
Dayton (1999) found very high densities of juvenile
hake (Merluccius productus) within the Scripps and
La Jolla Canyons, suggesting that the canyons were
acting as nursery grounds. These authors also found
enhanced abundance of turbot (Pleuronichthys sp.)
and zoarcids within canyons. Perhaps not surprisingly,
submarine canyons along the California coast are regularly targeted by commercial and recreational fishermen
exploiting rockfish, rattails and other bottom fishes
(C.R. Smith and E.W. Vetter, personal observations).
Oxygen-minimum zones
As discussed above, the eastern margin of the Pacific
Ocean is intersected by an oxygen-minimum zone
(oxygen-minimum zone), where bottom-water oxygen
concentrations drop below 0.5 ml °
−1 (Fig. 6.10). In
the equatorial zone, the oxygen-minimum zone is
particularly well developed extending from a depth of
50 m to 1300 m, with oxygen concentrations falling
below 0.1 ml °
−1 over most of this range (Wishner et al.,
1991). On the California slope, the oxygen-minimum
zone is not as well developed, but still extends over
depths roughly from 500 m to 1000 m, with minimum
oxygen concentrations below 0.3 ml °
−1 (Emery, 1960;
Reimers et al., 1992; Fig. 6.9). In enclosed basins
(e.g., Santa Monica and Santa Barbara Basins) whose
sill depths intersect the oxygen-minimum zone, lowoxygen conditions may extend to basin floors, which
can be much deeper than 1000 m (Emery, 1960).