THE DEEP ATLANTIC OCEAN
165
of 100 days or 100 years by use of particle reactive
radiotracers such as
234 Th or
210 Pb actually occurs
within hours or days after reactive particles reach the
seabed. Levin et al. (1997), for example, estimated
that mixing by maldanid polychaetes observed on
the Northwest Atlantic slope (at a depth of 850 m)
within 1.5 days after diatom deposition could account
for 25–100% of the mixing below 5 cm estimated
from naturally occurring
234 Th (100-day time scale).
Rapid downward transport of particles within the
sediment column also can result from infilling of
vacant burrows. Trapping of reactive organic matter
within burrows on the Nova Scotian Rise (4800 m) was
shown by Aller and Aller (1986) to cause intensive
decomposition and to generate hot-spots of enhanced
microbial, meiofaunal and macrofaunal activity in
otherwise organic-poor sediment.
Disturbance, colonization and succession
Present understanding of the responses of macrobenthic
communities in the Atlantic to disturbance is derived
from experiments in which defaunated sediments (rendered azoic by freezing and thawing), plant material,
fish or wood were placed in or on the seabed, and
subsequent colonization was observed at varying time
intervals (Smith and Hessler, 1987).
Initial investigations of infaunal succession were
carried out in the Atlantic by deploying deep-sea
sediments in trays lying on the seabed. Grassle’s
(1977) observations of faunal colonization after 2 and
26 months in trays at a depth of 1760 m (Station DOS1) in the Northwest Atlantic were the first to suggest
that recovery rates following disturbance are very slow
for the naturally occurring macrobenthic communities.
Numbers of colonizing individuals were low, and most
belonged to species not previously reported from the
study area. Among the commonest were dorvilleid
and capitellid polychaetes, priapulids, wood-boring
bivalves and snails ectoparasitic on echinoderms. Later
tray recolonization studies in the Northwest Atlantic
conducted by Grassle and Morse-Porteous (1987) at
DOS-1, and at 3600 m (DOS-2), for periods ranging
from 2 to 59 months, supported initial findings of slow
recovery. Even after 59 months, faunal densities did not
attain those in background sediments. Screening of tray
sediments revealed enhanced colonizer densities, and
led to the speculation that predation may control rates
of succession (Grassle and Morse-Porteous, 1987).
Sediment-tray experiments conducted for 6 and
11 months at depths of 2160 m and 4150 m in the
Bay of Biscay also indicated very slow colonization by
macrofauna (Desbruy` eres et al., 1985a). However, in an
earlier experiment Desbruy` eres et al. (1980) reported
much more rapid colonization of defaunated sediments
at 2160 m after 6 months’ exposure, with densities in
organically enriched sediments overshooting those of
control sediments. As in Grassle’s experiments, many
of the recruits in the Bay of Biscay belonged to species
absent from control samples.
These early sediment-tray experiments were fraught
with hydrodynamic artifacts, and deemed unrealistic
because the experimental substrata were separated from
natural sediments (Smith, 1985b; Snelgrove et al.,
1995). However, they were very important in revealing
the availability in the deep Atlantic of opportunistic
colonizers highly adapted to disturbed or enriched
settings. Subsequent deployments of hydrodynamically
unbiased colonization trays were made by Snelgrove
et al. (1992, 1994, 1996) for 23 days and 28 months
at a depth of 900 m near St. Croix, U.S. Virgin Islands
in the equatorial West Atlantic. These experiments
indicated that colonizing macrofauna can attain or
exceed ambient densities in 2+ years, but again the
colonists were species relatively rare in background
sediments (Snelgrove et al., 1996). This trend was
even more evident in organically enriched treatments,
which were colonized by large numbers of capitellid
and hesionid polychaetes, cumaceans and leptostracans
that were not observed in control treatments. The
characteristic response of specialized, opportunistic
species to disturbance in the deep Atlantic has not been
observed in the few comparable colonization studies
carried out for Pacific macrobenthos. In the eastern
Pacific the initial respondents to non-reducing sediment
disturbance appear to be species relatively common in
the background community (Levin and Smith, 1984;
Kukert and Smith, 1992; Levin and DiBacco, 1995).
Opportunists such as Capitella have not been reported
from deep-water experiments in the Pacific, though
Capitella and leptostracans are known to colonize
detritus in shallow submarine canyons (Vetter and
Dayton, 1998). Too few sites and forms of disturbance
have been examined to determine whether the Atlantic
is really distinct from the Pacific in the distribution of
deep-sea opportunists, or to permit speculation about
the causes.
Enrichment of experimental sediments deployed in
recolonization trays have produced colonizer assemblages distinct from the fauna of surrounding sediments
165
of 100 days or 100 years by use of particle reactive
radiotracers such as
234 Th or
210 Pb actually occurs
within hours or days after reactive particles reach the
seabed. Levin et al. (1997), for example, estimated
that mixing by maldanid polychaetes observed on
the Northwest Atlantic slope (at a depth of 850 m)
within 1.5 days after diatom deposition could account
for 25–100% of the mixing below 5 cm estimated
from naturally occurring
234 Th (100-day time scale).
Rapid downward transport of particles within the
sediment column also can result from infilling of
vacant burrows. Trapping of reactive organic matter
within burrows on the Nova Scotian Rise (4800 m) was
shown by Aller and Aller (1986) to cause intensive
decomposition and to generate hot-spots of enhanced
microbial, meiofaunal and macrofaunal activity in
otherwise organic-poor sediment.
Disturbance, colonization and succession
Present understanding of the responses of macrobenthic
communities in the Atlantic to disturbance is derived
from experiments in which defaunated sediments (rendered azoic by freezing and thawing), plant material,
fish or wood were placed in or on the seabed, and
subsequent colonization was observed at varying time
intervals (Smith and Hessler, 1987).
Initial investigations of infaunal succession were
carried out in the Atlantic by deploying deep-sea
sediments in trays lying on the seabed. Grassle’s
(1977) observations of faunal colonization after 2 and
26 months in trays at a depth of 1760 m (Station DOS1) in the Northwest Atlantic were the first to suggest
that recovery rates following disturbance are very slow
for the naturally occurring macrobenthic communities.
Numbers of colonizing individuals were low, and most
belonged to species not previously reported from the
study area. Among the commonest were dorvilleid
and capitellid polychaetes, priapulids, wood-boring
bivalves and snails ectoparasitic on echinoderms. Later
tray recolonization studies in the Northwest Atlantic
conducted by Grassle and Morse-Porteous (1987) at
DOS-1, and at 3600 m (DOS-2), for periods ranging
from 2 to 59 months, supported initial findings of slow
recovery. Even after 59 months, faunal densities did not
attain those in background sediments. Screening of tray
sediments revealed enhanced colonizer densities, and
led to the speculation that predation may control rates
of succession (Grassle and Morse-Porteous, 1987).
Sediment-tray experiments conducted for 6 and
11 months at depths of 2160 m and 4150 m in the
Bay of Biscay also indicated very slow colonization by
macrofauna (Desbruy` eres et al., 1985a). However, in an
earlier experiment Desbruy` eres et al. (1980) reported
much more rapid colonization of defaunated sediments
at 2160 m after 6 months’ exposure, with densities in
organically enriched sediments overshooting those of
control sediments. As in Grassle’s experiments, many
of the recruits in the Bay of Biscay belonged to species
absent from control samples.
These early sediment-tray experiments were fraught
with hydrodynamic artifacts, and deemed unrealistic
because the experimental substrata were separated from
natural sediments (Smith, 1985b; Snelgrove et al.,
1995). However, they were very important in revealing
the availability in the deep Atlantic of opportunistic
colonizers highly adapted to disturbed or enriched
settings. Subsequent deployments of hydrodynamically
unbiased colonization trays were made by Snelgrove
et al. (1992, 1994, 1996) for 23 days and 28 months
at a depth of 900 m near St. Croix, U.S. Virgin Islands
in the equatorial West Atlantic. These experiments
indicated that colonizing macrofauna can attain or
exceed ambient densities in 2+ years, but again the
colonists were species relatively rare in background
sediments (Snelgrove et al., 1996). This trend was
even more evident in organically enriched treatments,
which were colonized by large numbers of capitellid
and hesionid polychaetes, cumaceans and leptostracans
that were not observed in control treatments. The
characteristic response of specialized, opportunistic
species to disturbance in the deep Atlantic has not been
observed in the few comparable colonization studies
carried out for Pacific macrobenthos. In the eastern
Pacific the initial respondents to non-reducing sediment
disturbance appear to be species relatively common in
the background community (Levin and Smith, 1984;
Kukert and Smith, 1992; Levin and DiBacco, 1995).
Opportunists such as Capitella have not been reported
from deep-water experiments in the Pacific, though
Capitella and leptostracans are known to colonize
detritus in shallow submarine canyons (Vetter and
Dayton, 1998). Too few sites and forms of disturbance
have been examined to determine whether the Atlantic
is really distinct from the Pacific in the distribution of
deep-sea opportunists, or to permit speculation about
the causes.
Enrichment of experimental sediments deployed in
recolonization trays have produced colonizer assemblages distinct from the fauna of surrounding sediments