360
John D. GAGE
to utilize dissolved organic carbon, and also act as sites
of microbial enhancement (Levin and Gooday, 1992).
The Pogonophora are a group of worms lacking mouth
and gut whose nutrition is thought at least partly to be
dependent on dissolved organic matter (Southward and
Southward, 1982). Examination of different species by
these workers has shown that dissolved organic carbon
supplies the total energy need of the adult in only
one species, Siboglinum eckmani; however, most other
pogonophores examined had an energy deficit of up to
70%. Southward and Southward (1982) also estimated
that the uptake of dissolved organic carbon by the
bathyal sea star Plutonaster and the polychaete Tharyx
may provide up to 30% of their energy requirements.
George (1981) was able to maintain the solitary coral
Thecopsammia and the polychaete worm Hyalinoecia,
collected from depths of 1000 to 1800 m, for long
periods without food in water containing high levels
of dissolved organic carbon. Organisms potentially
able to benefit from dissolved organic carbon possess
soft bodies with a high surface-to-body ratio, such as
sponges and suspension-feeding cnidarians.
FOOD WEBS, ENERGETICS AND CARBON FLOW
AT THE DEEP-SEA BENTHIC BOUNDARY
Food webs
It has been established above how the benthic sedimentdwelling community responds to the various sorts of
organic flux to the bottom, derived either from sinking
particles or laterally advected particulate material, from
the finest particle to the largest dead body. I have
also touched on some of the early trophic links,
from these first consumers to secondary trophic links
dependent on their faeces or on them as prey. Food
webs provide a graphical conceptualization of these
links as a map of food chains describing which kinds
of organisms in the community eat, or use the ejecta,
of other kinds. These linkages, as prey and predator,
competitors, mutualists or symbionts, make a complex
mesh of interactions (Lawton, 1989). They are not
only interesting in their own right as windows on the
natural complexity and organization of ecosystems, but
important in understanding the functional basis for
biodiversity and, as here, in terms of mapping and
quantifying energy flow within the system.
The benthic ecosystem classically is considered as a
complex web of energy and material flows fuelled by
many different chemical reactions, and with substrates
supplied by a variety of physical processes. In shallow
water, diversity in chemical pathways allows benthic
organisms to extract energy from reduced organic
matter over a range of oxic and anoxic conditions.
In the deep sea the options usually are more limited.
This is because organic inputs are rarely sufficient
for labile particles to escape utilization at the sdiment
surface. Hence, in the absence of reducing conditions
at depth, modelling the deep-sea benthic ecosystem
need only concern itself with oxidant flux. However,
because of the practical difficulties of work in deep
water, and limitations in tools available, only a limited
number of the food-web flows can be sampled. These
either provide a sample of abundance and biomass, or
measure rates of community or (more rarely) individual
metabolism using in situ methods; while, as has been
described, the most usually measured fluxes are those
from, or to, the overlying water. These are the particle
flux usually estimated by sediment traps, or as an
oxidant flux measured from concentration changes
in a benthic chamber, or modelled from a vertical
concentration gradient in pore-water composition.
It is very difficult to measure the energy content
of shallow-water marine organims, let alone those in
the deep sea. Therefore to understand the energetics of
material flows in food webs by this approach presents
a daunting task. In practical terms carbon, being the
essential building block for organic matter, is used as
a proxy for energy. Nitrogen can also be used, this
and carbon having a roughly constant ratio in marine
organisms (the Redfield ratio). Carbon is taken up in
inorganic form during photosynthesis and returned to
sea water as carbon dioxide during respiration, and
thus can be tracked through the system when labelled
using radioactive carbon,
14 C. One important universal
constraint on the length of food chains is the energy
loss at each step. At most only about 50%, and often
much less, of available energy is transferred over to the
next upwards link in ectotherms. Two general features
ought to apply to the deep-sea ecosystem. These are:
a) the rapid rate of attenuation in energy transfer means
that food chains tend to be short, even in the most
productive systems; b) food chains also tend to be
scale-invariant, independent of the number of links in
the web such as the proportions of bottom, intermediate
and top species. Additionally, the density of links in
the food web might be expected to be high because
of the high species richness among deep-sea sedimentdwelling invertebrates (see Chapter 10). The number
John D. GAGE
to utilize dissolved organic carbon, and also act as sites
of microbial enhancement (Levin and Gooday, 1992).
The Pogonophora are a group of worms lacking mouth
and gut whose nutrition is thought at least partly to be
dependent on dissolved organic matter (Southward and
Southward, 1982). Examination of different species by
these workers has shown that dissolved organic carbon
supplies the total energy need of the adult in only
one species, Siboglinum eckmani; however, most other
pogonophores examined had an energy deficit of up to
70%. Southward and Southward (1982) also estimated
that the uptake of dissolved organic carbon by the
bathyal sea star Plutonaster and the polychaete Tharyx
may provide up to 30% of their energy requirements.
George (1981) was able to maintain the solitary coral
Thecopsammia and the polychaete worm Hyalinoecia,
collected from depths of 1000 to 1800 m, for long
periods without food in water containing high levels
of dissolved organic carbon. Organisms potentially
able to benefit from dissolved organic carbon possess
soft bodies with a high surface-to-body ratio, such as
sponges and suspension-feeding cnidarians.
FOOD WEBS, ENERGETICS AND CARBON FLOW
AT THE DEEP-SEA BENTHIC BOUNDARY
Food webs
It has been established above how the benthic sedimentdwelling community responds to the various sorts of
organic flux to the bottom, derived either from sinking
particles or laterally advected particulate material, from
the finest particle to the largest dead body. I have
also touched on some of the early trophic links,
from these first consumers to secondary trophic links
dependent on their faeces or on them as prey. Food
webs provide a graphical conceptualization of these
links as a map of food chains describing which kinds
of organisms in the community eat, or use the ejecta,
of other kinds. These linkages, as prey and predator,
competitors, mutualists or symbionts, make a complex
mesh of interactions (Lawton, 1989). They are not
only interesting in their own right as windows on the
natural complexity and organization of ecosystems, but
important in understanding the functional basis for
biodiversity and, as here, in terms of mapping and
quantifying energy flow within the system.
The benthic ecosystem classically is considered as a
complex web of energy and material flows fuelled by
many different chemical reactions, and with substrates
supplied by a variety of physical processes. In shallow
water, diversity in chemical pathways allows benthic
organisms to extract energy from reduced organic
matter over a range of oxic and anoxic conditions.
In the deep sea the options usually are more limited.
This is because organic inputs are rarely sufficient
for labile particles to escape utilization at the sdiment
surface. Hence, in the absence of reducing conditions
at depth, modelling the deep-sea benthic ecosystem
need only concern itself with oxidant flux. However,
because of the practical difficulties of work in deep
water, and limitations in tools available, only a limited
number of the food-web flows can be sampled. These
either provide a sample of abundance and biomass, or
measure rates of community or (more rarely) individual
metabolism using in situ methods; while, as has been
described, the most usually measured fluxes are those
from, or to, the overlying water. These are the particle
flux usually estimated by sediment traps, or as an
oxidant flux measured from concentration changes
in a benthic chamber, or modelled from a vertical
concentration gradient in pore-water composition.
It is very difficult to measure the energy content
of shallow-water marine organims, let alone those in
the deep sea. Therefore to understand the energetics of
material flows in food webs by this approach presents
a daunting task. In practical terms carbon, being the
essential building block for organic matter, is used as
a proxy for energy. Nitrogen can also be used, this
and carbon having a roughly constant ratio in marine
organisms (the Redfield ratio). Carbon is taken up in
inorganic form during photosynthesis and returned to
sea water as carbon dioxide during respiration, and
thus can be tracked through the system when labelled
using radioactive carbon,
14 C. One important universal
constraint on the length of food chains is the energy
loss at each step. At most only about 50%, and often
much less, of available energy is transferred over to the
next upwards link in ectotherms. Two general features
ought to apply to the deep-sea ecosystem. These are:
a) the rapid rate of attenuation in energy transfer means
that food chains tend to be short, even in the most
productive systems; b) food chains also tend to be
scale-invariant, independent of the number of links in
the web such as the proportions of bottom, intermediate
and top species. Additionally, the density of links in
the food web might be expected to be high because
of the high species richness among deep-sea sedimentdwelling invertebrates (see Chapter 10). The number