FOOD INPUTS, UTILIZATION, CARBON FLOW AND ENERGETICS
325
Pfannkuche and Lochte (1993) used Cyanobacteria
as a convenient tracer for tracking, in the faeces of
salps, the rapid transport to the abyssal seabed of
surface-derived phytoplankton during a mass swarming
of salps in the Northeast Atlantic. On the seabed of
the European continental rise, adjacent to the Porcupine
Abyssal Plain off southern Ireland, gelatinous detritus
was deposited in August. This material had a high
chloropigment and nucleic acid content which could
be identified as likely to be mainly from coccolithophorids, blooms of which are common features of
peak surface productivity at the continental margin off
northern Europe.
Beaulieu and K.L. Smith (1998) found the phytodetrital material in the abyssal Northeast Pacific to
change seasonally. Chain-forming diatoms dominate
just before the maximum in sinking flux, and are
followed a month later by dominance by phaeodarians,
an important group of sarcodine protozoans formerly
included in the “Radiolaria”. Both contain evidence of
mucous material from zooplankton, with composition
similar to that in sediment traps set above in the
water column. These data indicate that phytodetritus
can change dramatically in composition over a short
period. Furthermore, K.L. Smith et al. (1998) found the
chemical composition of detrital aggregates sampled
from the bottom at this site in the abyssal Northeast
Pacific to be similar to that collected in sediment traps
60 metres above the bottom.
Perhaps the characteristic of most importance, however, is the reactive state of the phytodetritus. The
content of chloropigments provides a convenient indicator of this quality. Kinetic studies on detritus from
the seafloor of the central equatorial Pacific Ocean
show that reactivity is strongly age-dependent. The
most reactive component includes chlorophyll-a, which
is strongly correlated with phytodetritus (C.R. Smith
et al., 1996). This component accounts for the vast bulk
of on-going degradation, but only a small minority of
the total inventory at any one time (Stephens et al.,
1997). This suggests that other labile material escapes
rapid degradation near the sediment–water interface, to
degrade much more slowly in deeper layers. However,
the role of benthic organisms and oxygen gradients in
this remains unclear.
Causes of mass deposition on the deep-sea bed
A range of oceanographic/climatic phenomena may
be responsible for pulsed particle flux (e.g., Thunell
and Pilskaln, 1994). A phytodetrital floc occurring
off California at a depth of 4100 m consisted of
individual aggregates disappearing within hours to
weeks (K.L. Smith et al., 1994), and with composition
differing over short periods (Beaulieu and K.L. Smith,
1998).
Where they occur, mass accumulations of phytodetritus clearly are the result of blooms of phytoplankton,
especially diatoms (Kemp and Baldauf, 1993), which
deplete available nutrients so that they die and sink,
and, because of their high concentration and sticky
mucus production, become aggregated as rapidly sinking floc. It is still unclear to what extent the strong
seasonality in particle flux detected in sediment traps
(Wefer, 1989; Nair et al., 1989; K.L. Smith et al., 1992)
results in mass accumulation on the bottom. Clearly, for
such material to appear in quantity it has overwhelmed
the usual ability of the benthic biota to consume and
recycle it.
In conclusion, although the timing of their first
appearance can be linked to major flux events resulting
from surface phytoplankton blooms, the occurrence
of mass accumulation does not necessarily mirror
the time-integrated bulk peaks measured in sediment
traps. Clearly a sharper temporal, and possibly spatial,
resolution of bloom events in the euphotic zone, and
the quality as well as quantity of export particles, will
be necessary to understand bentho-pelagic coupling
as manifested in mass accumulation on the deepocean floor. Furthermore, where mass accumulations
occur one still needs to understand better the rate
at which particles sink to the bottom, the associated
biodegradatory processses and the nutritive value and
subsequent utilization of particles by the benthos.
Interannual and mesoscale spatial variability in
particle flux
I have already referred earlier (p. 323) to apparent
interannual differences in mass flux of fresh detritus
trapped just above the seabed in the Northeast Atlantic
(Witbaard et al., 2000). Several other data sets have
now identified year-to-year differences in this seasonal
pattern. In the open ocean, these may result in
qualitative and quantitative differences in delivery of
fresh phytoplankton and heterotroph grazers, such
as planktonic foraminifers, to the bottom. This may
result from surface-water temperature and weatherdriven variability in timing of the spring bloom,
depth of the oceanic mixed layer, and the extent of
surface stratification (Deuser, 1986; Thunell and Honjo,
1987; Vangriesheim and Khripounoff, 1990; Brock and
McClain, 1992; Michaels et al., 1994; Townsend et al.,
325
Pfannkuche and Lochte (1993) used Cyanobacteria
as a convenient tracer for tracking, in the faeces of
salps, the rapid transport to the abyssal seabed of
surface-derived phytoplankton during a mass swarming
of salps in the Northeast Atlantic. On the seabed of
the European continental rise, adjacent to the Porcupine
Abyssal Plain off southern Ireland, gelatinous detritus
was deposited in August. This material had a high
chloropigment and nucleic acid content which could
be identified as likely to be mainly from coccolithophorids, blooms of which are common features of
peak surface productivity at the continental margin off
northern Europe.
Beaulieu and K.L. Smith (1998) found the phytodetrital material in the abyssal Northeast Pacific to
change seasonally. Chain-forming diatoms dominate
just before the maximum in sinking flux, and are
followed a month later by dominance by phaeodarians,
an important group of sarcodine protozoans formerly
included in the “Radiolaria”. Both contain evidence of
mucous material from zooplankton, with composition
similar to that in sediment traps set above in the
water column. These data indicate that phytodetritus
can change dramatically in composition over a short
period. Furthermore, K.L. Smith et al. (1998) found the
chemical composition of detrital aggregates sampled
from the bottom at this site in the abyssal Northeast
Pacific to be similar to that collected in sediment traps
60 metres above the bottom.
Perhaps the characteristic of most importance, however, is the reactive state of the phytodetritus. The
content of chloropigments provides a convenient indicator of this quality. Kinetic studies on detritus from
the seafloor of the central equatorial Pacific Ocean
show that reactivity is strongly age-dependent. The
most reactive component includes chlorophyll-a, which
is strongly correlated with phytodetritus (C.R. Smith
et al., 1996). This component accounts for the vast bulk
of on-going degradation, but only a small minority of
the total inventory at any one time (Stephens et al.,
1997). This suggests that other labile material escapes
rapid degradation near the sediment–water interface, to
degrade much more slowly in deeper layers. However,
the role of benthic organisms and oxygen gradients in
this remains unclear.
Causes of mass deposition on the deep-sea bed
A range of oceanographic/climatic phenomena may
be responsible for pulsed particle flux (e.g., Thunell
and Pilskaln, 1994). A phytodetrital floc occurring
off California at a depth of 4100 m consisted of
individual aggregates disappearing within hours to
weeks (K.L. Smith et al., 1994), and with composition
differing over short periods (Beaulieu and K.L. Smith,
1998).
Where they occur, mass accumulations of phytodetritus clearly are the result of blooms of phytoplankton,
especially diatoms (Kemp and Baldauf, 1993), which
deplete available nutrients so that they die and sink,
and, because of their high concentration and sticky
mucus production, become aggregated as rapidly sinking floc. It is still unclear to what extent the strong
seasonality in particle flux detected in sediment traps
(Wefer, 1989; Nair et al., 1989; K.L. Smith et al., 1992)
results in mass accumulation on the bottom. Clearly, for
such material to appear in quantity it has overwhelmed
the usual ability of the benthic biota to consume and
recycle it.
In conclusion, although the timing of their first
appearance can be linked to major flux events resulting
from surface phytoplankton blooms, the occurrence
of mass accumulation does not necessarily mirror
the time-integrated bulk peaks measured in sediment
traps. Clearly a sharper temporal, and possibly spatial,
resolution of bloom events in the euphotic zone, and
the quality as well as quantity of export particles, will
be necessary to understand bentho-pelagic coupling
as manifested in mass accumulation on the deepocean floor. Furthermore, where mass accumulations
occur one still needs to understand better the rate
at which particles sink to the bottom, the associated
biodegradatory processses and the nutritive value and
subsequent utilization of particles by the benthos.
Interannual and mesoscale spatial variability in
particle flux
I have already referred earlier (p. 323) to apparent
interannual differences in mass flux of fresh detritus
trapped just above the seabed in the Northeast Atlantic
(Witbaard et al., 2000). Several other data sets have
now identified year-to-year differences in this seasonal
pattern. In the open ocean, these may result in
qualitative and quantitative differences in delivery of
fresh phytoplankton and heterotroph grazers, such
as planktonic foraminifers, to the bottom. This may
result from surface-water temperature and weatherdriven variability in timing of the spring bloom,
depth of the oceanic mixed layer, and the extent of
surface stratification (Deuser, 1986; Thunell and Honjo,
1987; Vangriesheim and Khripounoff, 1990; Brock and
McClain, 1992; Michaels et al., 1994; Townsend et al.,