332
John D. GAGE
RESPONSE AND UTILIZATION OF ORGANIC
SMALL-PARTICLE FLUX BY THE DEEP-SEA
BENTHIC BOUNDARY LAYER COMMUNITY
The benthic boundary layer community as a whole
includes the bacterioplankton and zooplankton residing
in the immediately overlying water, the near-bottom
or benthopelagic fauna, and the community more
intimately associated with the sediment and hard
substrata, collectively known as the benthos. The
total benthopelagic community would also include
the larger, swimming organisms such as scavenging
crustaceans and fish. This scavenging community will
be addressed later (pp. 354–358) in dealing with large
packages of passively sinking organic material. Apart
from the small percentage living as predators or opportunist scavengers, such as some benthic crustaceans,
brittle stars and sea stars, the remaining parts of the
community are consumers of the detrital small-particle
flux, whether sinking or laterally advected, and hence
important in remineralization of particulate organic
carbon. However, in view of the emerging view that
much of the flux of new production from the surface
arrives as episodic detrital pulses, the extent to which
the community depends on such intermittent (possibly
very infrequent), but probably highly nutritious, detrital
food rather than on low-level ‘background’ fall-out is
an important question. The subject has stimulated a
relatively large research effort which will be referred
to in the following pages, but a considerable amount
of work is as yet unpublished.
Scaling of response
The manner in which detrital particles are dealt with
by the benthic boundary layer community as food, and
the consequences of benthic utilization on sediment
mixing and particle transportation within the sediment,
as well as on benthic population processes, is closely
scaled to body size. For bacteria and micro-organisms
the individual particles themselves provide habitat,
and the population’s response may cycle entirely on
a single flake of marine snow, or faecal pellet. Such
a population-level response will also apply to the
smallest meiofaunal metazoans whose individual life
spans occupy only a few weeks at most. Here increases
in population densities will be the main response
to seasonally pulsed flux in particles. Moreover, the
strategy of utilizing particles for such small animals
will be markedly different to that employed by larger
metazoans. This results from the ability of meiofauna
(and the young postlarval stages of macrofauna) to
perceive detritus as individual particles so that they are
able to select those of the narrow size range associated
with most bacteria and organic particles (Self and
Jumars, 1988). They will hence be macrophages
specializing probably on the most nutritious particles,
and their activity is restricted to the superficial layer of
sediment.
In contrast, larger organisms perceive sediment as
a bulk substratum that will, because of its usually
low food value, require a range of specialized adaptations in order to maximize the return from foraging
effort. Some of these mechanisms have important
consequences in rapid relocation of fresh organic
material deep within the sediment. Furthermore such
larger metazoans have individual growth rates that may
encompass several annual cycles in organic flux to
the bottom. Hence, populations tend, as far is known,
to be interannually stable, although with seasonally
varying age structure reflecting any periodicity in
reproduction and recruitment (Gage, 1994). This is not
to say that densities of megafauna will necessarily be
spatially homogeneous on the bottom. Indeed optimal
foraging of motile species may involve mass shifts of
populations, like the constantly moving herds of buffalo
of the prairie. There may even be temporal shifts in
community composition, as has been detected in the
Northeast Atlantic on the Porcupine Abyssal Plain.
These different activities will be considered below; but
first one needs to consider data on the response of
micro-organisms, which is, because of their tiny size,
detected and measured in terms of its effects, such as
solute flux or degradation of material substrates.
Response and utilization by micro-organisms,
protists and smaller metazoan size classes
Microbial degradation
Data on rates of microbial degradation of organic
inputs at the deep-sea bed date from the 1970s from
experiments with labile material, such as bread or
flesh, held just above the abyssal seabed. While some
experiments indicated rapid rates of degradation, with
some material being consumed or decayed within
10 weeks (Seki et al., 1974), others showing markedly
reduced rates (Wirsen and Jannasch, 1976). Subsequent
in situ studies have clarified this discrepancy. These
observations confirm that barotolerant and barophilic
bacteria are found right at the sediment–water interface
John D. GAGE
RESPONSE AND UTILIZATION OF ORGANIC
SMALL-PARTICLE FLUX BY THE DEEP-SEA
BENTHIC BOUNDARY LAYER COMMUNITY
The benthic boundary layer community as a whole
includes the bacterioplankton and zooplankton residing
in the immediately overlying water, the near-bottom
or benthopelagic fauna, and the community more
intimately associated with the sediment and hard
substrata, collectively known as the benthos. The
total benthopelagic community would also include
the larger, swimming organisms such as scavenging
crustaceans and fish. This scavenging community will
be addressed later (pp. 354–358) in dealing with large
packages of passively sinking organic material. Apart
from the small percentage living as predators or opportunist scavengers, such as some benthic crustaceans,
brittle stars and sea stars, the remaining parts of the
community are consumers of the detrital small-particle
flux, whether sinking or laterally advected, and hence
important in remineralization of particulate organic
carbon. However, in view of the emerging view that
much of the flux of new production from the surface
arrives as episodic detrital pulses, the extent to which
the community depends on such intermittent (possibly
very infrequent), but probably highly nutritious, detrital
food rather than on low-level ‘background’ fall-out is
an important question. The subject has stimulated a
relatively large research effort which will be referred
to in the following pages, but a considerable amount
of work is as yet unpublished.
Scaling of response
The manner in which detrital particles are dealt with
by the benthic boundary layer community as food, and
the consequences of benthic utilization on sediment
mixing and particle transportation within the sediment,
as well as on benthic population processes, is closely
scaled to body size. For bacteria and micro-organisms
the individual particles themselves provide habitat,
and the population’s response may cycle entirely on
a single flake of marine snow, or faecal pellet. Such
a population-level response will also apply to the
smallest meiofaunal metazoans whose individual life
spans occupy only a few weeks at most. Here increases
in population densities will be the main response
to seasonally pulsed flux in particles. Moreover, the
strategy of utilizing particles for such small animals
will be markedly different to that employed by larger
metazoans. This results from the ability of meiofauna
(and the young postlarval stages of macrofauna) to
perceive detritus as individual particles so that they are
able to select those of the narrow size range associated
with most bacteria and organic particles (Self and
Jumars, 1988). They will hence be macrophages
specializing probably on the most nutritious particles,
and their activity is restricted to the superficial layer of
sediment.
In contrast, larger organisms perceive sediment as
a bulk substratum that will, because of its usually
low food value, require a range of specialized adaptations in order to maximize the return from foraging
effort. Some of these mechanisms have important
consequences in rapid relocation of fresh organic
material deep within the sediment. Furthermore such
larger metazoans have individual growth rates that may
encompass several annual cycles in organic flux to
the bottom. Hence, populations tend, as far is known,
to be interannually stable, although with seasonally
varying age structure reflecting any periodicity in
reproduction and recruitment (Gage, 1994). This is not
to say that densities of megafauna will necessarily be
spatially homogeneous on the bottom. Indeed optimal
foraging of motile species may involve mass shifts of
populations, like the constantly moving herds of buffalo
of the prairie. There may even be temporal shifts in
community composition, as has been detected in the
Northeast Atlantic on the Porcupine Abyssal Plain.
These different activities will be considered below; but
first one needs to consider data on the response of
micro-organisms, which is, because of their tiny size,
detected and measured in terms of its effects, such as
solute flux or degradation of material substrates.
Response and utilization by micro-organisms,
protists and smaller metazoan size classes
Microbial degradation
Data on rates of microbial degradation of organic
inputs at the deep-sea bed date from the 1970s from
experiments with labile material, such as bread or
flesh, held just above the abyssal seabed. While some
experiments indicated rapid rates of degradation, with
some material being consumed or decayed within
10 weeks (Seki et al., 1974), others showing markedly
reduced rates (Wirsen and Jannasch, 1976). Subsequent
in situ studies have clarified this discrepancy. These
observations confirm that barotolerant and barophilic
bacteria are found right at the sediment–water interface