318
John D. GAGE
The biological particle cycle in midwater
Because of the dependency of the deep-sea ecosystem
on surface processes, one needs to look briefly at the
euphotic zone, rarely extending deeper than 200 m,
from where most of the organic particles that sustain
deep-sea life originate. Classically, the phytoplankton
are grazed by zooplankton, which package most of their
waste into faecal pellets. These, in turn, are eaten and
decomposed by other organisms, including bacteria,
within this surface layer. Hence, nearly all the organic
matter present is constantly recycled within a surfacelayer loop. Only a very small fraction, perhaps up to
about 10%, escapes to be exported from the euphotic
zone into deeper water by sinking as dead bodies and
faecal material (Fowler and Knauer, 1986; and see
Chapter 3).
Even before it reaches the deep-sea bottom far
below, this organic matter provides food for various
midwater animals, the mesopelagic zooplankton and
nekton. Many of these organisms are the diel migrators contributing to active biological transport. These
animals are able to intercept the particles, so that they
may be repackaged many times before reaching the
seabed. Sediment traps through the water column show
that overall only a tiny proportion, about 1–3%, of
the surface primary production reaches the abyssal
seabed (e.g., Deuser, 1986) as a result of utilization by
midwater organisms. Each particle will also carry its
own little community of microbial decomposers. About
three-quarters of the organic particles sinking from the
euphotic zone may be decomposed and recycled within
the uppermost 500 to 1000 m of the water column –
that is, above the winter mixed layer, or permanent
thermocline, constituting the upper boundary of the
deep open ocean. The proportion of surface production
arriving at the bottom increases at shallower depths, so
that bathyal depths and above will receive values up to
the total export flux from the euphotic zone.
As a result of the midwater consumption, the proportion of skeletal material will increase with increasing
depth in the water column. This is composed of the
calcium carbonate and siliceous structures of the shells
of phytoplankton and chitinous exoskeletal structures
of small crustaceans and other invertebrates, which
are dissolved progressively with increasing depth. This
means that, compared to its inorganic composition, the
organic content decreases markedly and particle size
becomes more variable by the time it reaches bottom
(Honjo, 1982).
Episodic mass deposition from the euphotic zone
Superimposed on the scheme outlined above are
episodic phenomena in the euphotic zone, ranging
from the spring ‘bloom’ to short-lived mini-blooms
with high spatial as well as temporal variability.
The latter may only be easily detectable by optical
sensors. Such short-lived events are not recorded in
sediment traps, which integrate sedimentation over
long enough periods (varying from 3–4 days to
1 month) to collect enough material. The largest events,
by overwhelming the normal recycling processes by
herbivore populations in the euphotic zone, bypass
the tightly coupled processes previously thought to
characterize the biogeochemistry of the euphotic zone.
On the other hand, data from the Northeast Atlantic
suggest that quickly expanding populations of efficient
grazers such as salps may effectively prevent the further
development of the oceanic phytoplankton blooms
(Stienen et al., 1988).
Boyd and Newton (1999) suggested that variability
in size-structure of the phytoplankton community,
rather than primary production itself, is the cause of
mass sinking of ungrazed algal cells. Hence the flux
of particulate material to the deep ocean may depend
on the development of certain algal communities rather
than on total new production. By such means large
quantities of material may escape remineralization during these periods to sink as highly labile, fast-sinking
particles into the ocean’s interior, and eventually fuel
the benthic boundary layer ecosystem.
This idea is supported from Deuser’s (1986) timeseries observations, as the percentage of organic
carbon in traps is highest in winter/spring when
carbon utilization is least efficient during the period
of highest productivity, and lowest in the autumn
when the plankton community is mature and utilization
is maximal. Even if not detectable as peaks in
particle export, faecal pellets enriched with fresh plant
cells may occur, enhancing the quality of sedimented
particles as potential food (Fisher et al., 1996). It is
now thought likely that such transient ‘rain’ events,
rather than the slow background drizzle of refractory
particles, makes up a large part of the export flux
of biologically available organic carbon and easily
remineralized elements, throughout the deep ocean
(Conte et al., 1998).
The question how much of the total sedimentation
of intermediate-size particles arrives at the deep seabed
at rates significantly above the ‘background’ level still
John D. GAGE
The biological particle cycle in midwater
Because of the dependency of the deep-sea ecosystem
on surface processes, one needs to look briefly at the
euphotic zone, rarely extending deeper than 200 m,
from where most of the organic particles that sustain
deep-sea life originate. Classically, the phytoplankton
are grazed by zooplankton, which package most of their
waste into faecal pellets. These, in turn, are eaten and
decomposed by other organisms, including bacteria,
within this surface layer. Hence, nearly all the organic
matter present is constantly recycled within a surfacelayer loop. Only a very small fraction, perhaps up to
about 10%, escapes to be exported from the euphotic
zone into deeper water by sinking as dead bodies and
faecal material (Fowler and Knauer, 1986; and see
Chapter 3).
Even before it reaches the deep-sea bottom far
below, this organic matter provides food for various
midwater animals, the mesopelagic zooplankton and
nekton. Many of these organisms are the diel migrators contributing to active biological transport. These
animals are able to intercept the particles, so that they
may be repackaged many times before reaching the
seabed. Sediment traps through the water column show
that overall only a tiny proportion, about 1–3%, of
the surface primary production reaches the abyssal
seabed (e.g., Deuser, 1986) as a result of utilization by
midwater organisms. Each particle will also carry its
own little community of microbial decomposers. About
three-quarters of the organic particles sinking from the
euphotic zone may be decomposed and recycled within
the uppermost 500 to 1000 m of the water column –
that is, above the winter mixed layer, or permanent
thermocline, constituting the upper boundary of the
deep open ocean. The proportion of surface production
arriving at the bottom increases at shallower depths, so
that bathyal depths and above will receive values up to
the total export flux from the euphotic zone.
As a result of the midwater consumption, the proportion of skeletal material will increase with increasing
depth in the water column. This is composed of the
calcium carbonate and siliceous structures of the shells
of phytoplankton and chitinous exoskeletal structures
of small crustaceans and other invertebrates, which
are dissolved progressively with increasing depth. This
means that, compared to its inorganic composition, the
organic content decreases markedly and particle size
becomes more variable by the time it reaches bottom
(Honjo, 1982).
Episodic mass deposition from the euphotic zone
Superimposed on the scheme outlined above are
episodic phenomena in the euphotic zone, ranging
from the spring ‘bloom’ to short-lived mini-blooms
with high spatial as well as temporal variability.
The latter may only be easily detectable by optical
sensors. Such short-lived events are not recorded in
sediment traps, which integrate sedimentation over
long enough periods (varying from 3–4 days to
1 month) to collect enough material. The largest events,
by overwhelming the normal recycling processes by
herbivore populations in the euphotic zone, bypass
the tightly coupled processes previously thought to
characterize the biogeochemistry of the euphotic zone.
On the other hand, data from the Northeast Atlantic
suggest that quickly expanding populations of efficient
grazers such as salps may effectively prevent the further
development of the oceanic phytoplankton blooms
(Stienen et al., 1988).
Boyd and Newton (1999) suggested that variability
in size-structure of the phytoplankton community,
rather than primary production itself, is the cause of
mass sinking of ungrazed algal cells. Hence the flux
of particulate material to the deep ocean may depend
on the development of certain algal communities rather
than on total new production. By such means large
quantities of material may escape remineralization during these periods to sink as highly labile, fast-sinking
particles into the ocean’s interior, and eventually fuel
the benthic boundary layer ecosystem.
This idea is supported from Deuser’s (1986) timeseries observations, as the percentage of organic
carbon in traps is highest in winter/spring when
carbon utilization is least efficient during the period
of highest productivity, and lowest in the autumn
when the plankton community is mature and utilization
is maximal. Even if not detectable as peaks in
particle export, faecal pellets enriched with fresh plant
cells may occur, enhancing the quality of sedimented
particles as potential food (Fisher et al., 1996). It is
now thought likely that such transient ‘rain’ events,
rather than the slow background drizzle of refractory
particles, makes up a large part of the export flux
of biologically available organic carbon and easily
remineralized elements, throughout the deep ocean
(Conte et al., 1998).
The question how much of the total sedimentation
of intermediate-size particles arrives at the deep seabed
at rates significantly above the ‘background’ level still