FOOD INPUTS, UTILIZATION, CARBON FLOW AND ENERGETICS
363
approaches of this kind it is necessary to identify
the different faunal compartments, in terms of size
and feeding type, to determine the biomass of these
compartments and the magnitude of the fluxes of
organic material between them. This is accomplished
in terms of known trophic relations and mass-specific
metabolic rates (Mahaut et al., 1995), in order to
estimate relationships of organic-carbon demand to
biomass (K.L. Smith, 1992). Such work is extremely
difficult and tedious, involving input from many
different investigations. Categorization of trophic type
is assembled using the sort of information summarized
above on page 362. Careful study of body size and
trophic behaviour, as well as stomach contents, is
required in order to avoid mis-categorization. The
question may arise, for example, whether large motile
megafauna or fish are top predators in a benthic food
web, or whether they rely on nekton falls from the
surface, thereby short-circuiting the benthic food web
(Mahaut et al., 1990). Furthermore, possible spatial and
temporal variability in rates of processes mean that
“snapshot” measurements incorporated, such as those
from measurements of sediment community oxygen
demand (see above), carry high uncertainty.
Quantification of trophic energy-flow models:
caveats
The work so far discussed shows that it is now possible
to put some figures for carbon and energy in and
between the boxes corresponding to the compartments
in trophodynamic models of energy flow. But even at
the most comprehensively studied site – that studied
by K.L. Smith in the central North Pacific – not
all compartments and flows could be estimated from
real data, and the budget fails to balance inputs and
outputs. One suggestion, that the ‘missing’ carbon in
these studies might be supplied by dissolved organics
(K.L. Smith, 1989, 1992), was prompted by the inverse
relation found by Sugimura and Suzuki (1988) between
apparent oxygen utilization and dissolved organic carbon measured in the water column. This suggested that
a more utilizable, labile fraction present is being turned
over much more rapidly than that the old and highly
refractory dissolved organic carbon previously thought
to constitute this pool. This labile fraction, then, has
a previously unrecognized importance, presumably by
diffusion and turbulent mixing down to the benthic
boundary layer, in carbon cycling in the deep ocean
(Burdige et al., 1992). There are also tantalizing
data indicating losses of dissolved organic carbon
from the sediment larger than the requirements for
sediment community oxygen consumption (Lampitt
et al., 1995).
Other possible overlooked sources of organic carbon
include net advection from other areas, although this
seems much less likely in the central North Pacific
than for sites nearer, and on, the continental margin.
Also, the possible importance of large food-falls as
another source of organic carbon in the central North
Pacific, as is the case virtually everywhere else, cannot
yet be quantified accurately. Finally, active transport
into the benthic boundary layer by migrating fauna
(which bypass sediment traps) could, both as active
flux mediated by predation on diel migrators, and as
passive flux from faecal ejecta at depth (see pp. 316–
317), account for the total estimated consumption of the
benthic boundary layer community in the central North
Pacific. But this assumes that the migrators are feeding
above the benthic boundary layer, and that their faeces
represent a net gain (that animals leave the benthic
boundary layer with empty guts). Neither assumption
presently can be validated (K.L. Smith, 1992).
Finally, the problem of dealing with seasonal variation involves many uncertainties. Although intense
seasonality may be seen in surface productivity, reflected in the measurements of exported particle flux,
a corresponding variability in sediment community
oxygen consumption, as an expression of seabed
organic remineralization, has not always been seen
(e.g., K.L. Smith, 1989; Lampitt et al., 1995). This
is not to suggest that such coupling might not exist.
Studies elsewhere have demonstrated rapid responses
to pulsed sedimentation from the surface (see above).
Improvements in the range, quality and duration of
data, reflecting both the full size-range of inputs and
the amounts utilized, will be necessary before a full
understanding of the energetic links and dynamic
responses can be said to exist for even one site in
the deep sea. K.L. Smith et al. (1992) concluded that
the inclusion of previously undetected episodic particle
flux to the benthic boundary layer might substantially
reconcile the imbalance between supply as particulate
organic carbon flux and demand measured as sediment
community oxygen consumption. Their studies at an
abyssal station (Station ‘M’) off California utilizing
a long time series of particle-flux measurements
(2.3 years) succeeded in achieving agreement within
15%, compared to shortfalls of up to 97% in previous
short-term measurements (K.L. Smith, 1987, 1989).
363
approaches of this kind it is necessary to identify
the different faunal compartments, in terms of size
and feeding type, to determine the biomass of these
compartments and the magnitude of the fluxes of
organic material between them. This is accomplished
in terms of known trophic relations and mass-specific
metabolic rates (Mahaut et al., 1995), in order to
estimate relationships of organic-carbon demand to
biomass (K.L. Smith, 1992). Such work is extremely
difficult and tedious, involving input from many
different investigations. Categorization of trophic type
is assembled using the sort of information summarized
above on page 362. Careful study of body size and
trophic behaviour, as well as stomach contents, is
required in order to avoid mis-categorization. The
question may arise, for example, whether large motile
megafauna or fish are top predators in a benthic food
web, or whether they rely on nekton falls from the
surface, thereby short-circuiting the benthic food web
(Mahaut et al., 1990). Furthermore, possible spatial and
temporal variability in rates of processes mean that
“snapshot” measurements incorporated, such as those
from measurements of sediment community oxygen
demand (see above), carry high uncertainty.
Quantification of trophic energy-flow models:
caveats
The work so far discussed shows that it is now possible
to put some figures for carbon and energy in and
between the boxes corresponding to the compartments
in trophodynamic models of energy flow. But even at
the most comprehensively studied site – that studied
by K.L. Smith in the central North Pacific – not
all compartments and flows could be estimated from
real data, and the budget fails to balance inputs and
outputs. One suggestion, that the ‘missing’ carbon in
these studies might be supplied by dissolved organics
(K.L. Smith, 1989, 1992), was prompted by the inverse
relation found by Sugimura and Suzuki (1988) between
apparent oxygen utilization and dissolved organic carbon measured in the water column. This suggested that
a more utilizable, labile fraction present is being turned
over much more rapidly than that the old and highly
refractory dissolved organic carbon previously thought
to constitute this pool. This labile fraction, then, has
a previously unrecognized importance, presumably by
diffusion and turbulent mixing down to the benthic
boundary layer, in carbon cycling in the deep ocean
(Burdige et al., 1992). There are also tantalizing
data indicating losses of dissolved organic carbon
from the sediment larger than the requirements for
sediment community oxygen consumption (Lampitt
et al., 1995).
Other possible overlooked sources of organic carbon
include net advection from other areas, although this
seems much less likely in the central North Pacific
than for sites nearer, and on, the continental margin.
Also, the possible importance of large food-falls as
another source of organic carbon in the central North
Pacific, as is the case virtually everywhere else, cannot
yet be quantified accurately. Finally, active transport
into the benthic boundary layer by migrating fauna
(which bypass sediment traps) could, both as active
flux mediated by predation on diel migrators, and as
passive flux from faecal ejecta at depth (see pp. 316–
317), account for the total estimated consumption of the
benthic boundary layer community in the central North
Pacific. But this assumes that the migrators are feeding
above the benthic boundary layer, and that their faeces
represent a net gain (that animals leave the benthic
boundary layer with empty guts). Neither assumption
presently can be validated (K.L. Smith, 1992).
Finally, the problem of dealing with seasonal variation involves many uncertainties. Although intense
seasonality may be seen in surface productivity, reflected in the measurements of exported particle flux,
a corresponding variability in sediment community
oxygen consumption, as an expression of seabed
organic remineralization, has not always been seen
(e.g., K.L. Smith, 1989; Lampitt et al., 1995). This
is not to suggest that such coupling might not exist.
Studies elsewhere have demonstrated rapid responses
to pulsed sedimentation from the surface (see above).
Improvements in the range, quality and duration of
data, reflecting both the full size-range of inputs and
the amounts utilized, will be necessary before a full
understanding of the energetic links and dynamic
responses can be said to exist for even one site in
the deep sea. K.L. Smith et al. (1992) concluded that
the inclusion of previously undetected episodic particle
flux to the benthic boundary layer might substantially
reconcile the imbalance between supply as particulate
organic carbon flux and demand measured as sediment
community oxygen consumption. Their studies at an
abyssal station (Station ‘M’) off California utilizing
a long time series of particle-flux measurements
(2.3 years) succeeded in achieving agreement within
15%, compared to shortfalls of up to 97% in previous
short-term measurements (K.L. Smith, 1987, 1989).