72
Martin V. ANGEL
differing benthic fluxes and with different intra-annual
fluctuations. Over the Porcupine Abyssal Plain, in
some years the sediments become blanketed seasonally
by a cover of phytodetritus within a relatively few
days of the onset of the Spring Bloom. In this area
the benthic communities are dominated by sediment
feeders, which create considerable bioturbation of the
sediment interface. In contrast, only 16º further south
on the sediments of the Madeiran Abyssal Plain
where no seasonal deposition of phytodetritus has been
observed, there is little modification of the sediment
interface by bioturbation and there is a notable absence
of benthic megafauna (Lampitt, 1985; Rice et al.,
1994). Thus, there is a very direct link between
the processes and community structure of deep-sea
benthic environments and the processes occurring in
the overlying water column.
Lampitt and Antia (1997) identified three potential
reasons why fluxes may not increase in line with high
levels of primary production:
(1) Increases in primary production above 200 g C m
−2
y
−1 result from an increasing dependence on regenerated nutrients, and hence the f-ratio decreases
with increasing production.
(2) The degradation of particles proceeds faster within
the upper 2000 m when the levels of flux are
higher – that is: the higher productivity promotes
larger mesopelagic populations. But why are these
populations more efficient in their assimilation?
(3) Horizontal advection prevents the sedimenting
material from reaching depths of 2000 m.
Other possible explanations are based on the methodology: for example, sediment traps may not measure
all components of the fluxes. Significant fluxes may
occur in the form of dissolved organic matter (DOM)
carried into deeper water by downwelling and deepwater formation. Extra large particles that are not intercepted by the traps may make proportionately larger
contributions to the total flux. There are empirical
relationships between primary productivity and the
organic content of sediments used as palaeo-indicators
(Summerhayes et al., 1995b), and such relationships
imply that organic fluxes continue to increase with
increasing productivity. Populations of benthopelagic
scavengers attracted to baited cameras and traps are
known to consume large food packages, and tend to
increase below highly productive regions (Thurston,
1990); since the mean size of pelagic populations
tends to increase between low, temperate and subpolar
latitudes (Angel, 1997), fluxes of large packages may
increase with increasing primary production. This may
also contribute to the discrepancies between the flux estimates produced by Jahnke (1996) based on sediment
oxygen consumption and those of Lampitt and Antia
(1997) based on sediment trap fluxes extrapolated to
total ocean areas. These two studies were in reasonable
agreement for the Atlantic (0.126 and 0.100 Gt C org y
−1 ,
respectively), but quite disparate for the Pacific (0.214
and 0.110 Gt C org y
−1 , respectively) and even more so
for the Indian Ocean (0.142 and 0.012 Gt C org y
−1 ,
respectively; Gt = Gigatonnes), although riverine inputs
may also be contributing to the sediment metabolism.
PHYSIOLOGY AND BEHAVIOUR
All organisms are faced with three basic challenges:
1) how to survive; 2) how to find enough food for
growth and reproduction; and 3) how to reproduce
successfully. The optimal strategies to meet these challenges individually often conflict. Where food abounds,
other animals are attracted in, so that the dangers of
predation (e.g., Lampitt et al., 1983) or being injured
accidentally in feeding-frenzies increase. Most species
occur at very low densities, which means that finding
a mate cannot be left to chance encounters; instead,
species either form breeding swarms or “advertise”
their presence with scent or sound, again increasing
their vulnerability to predation. Observed macroplankton abundances are generally less than 1 per 1000 m
3 ,
but observations from submersibles (e.g., Wolff, 1971;
Childress et al., 1989) have suggested that random net
observations may well miss significant concentrations
of pelagic organisms, which often occur in thin layers
and may be assembled either passively as a result
of currents and eddies, or actively as a result of
behavioural responses to environmental cues.
The need to take risks associated with fulfilling
these basic activities shifts during the life cycle of
a species. Early larval stages are often spent in the
upper water column where food abounds. The higher
risk of predation is presumably outweighed by the
benefit of achieving more rapid growth. Size also
plays a role: the smaller an animal is, the shorter the
range over which it can be visually detected. Thus, as
an animal grows, it either has to adopt an effective
camouflage (such as a high degree of transparency) or
it may move down into deeper water where the light
is dimmer, which diminishes the range at which it can
seen. Hence many pelagic species undergo ontogenetic
Martin V. ANGEL
differing benthic fluxes and with different intra-annual
fluctuations. Over the Porcupine Abyssal Plain, in
some years the sediments become blanketed seasonally
by a cover of phytodetritus within a relatively few
days of the onset of the Spring Bloom. In this area
the benthic communities are dominated by sediment
feeders, which create considerable bioturbation of the
sediment interface. In contrast, only 16º further south
on the sediments of the Madeiran Abyssal Plain
where no seasonal deposition of phytodetritus has been
observed, there is little modification of the sediment
interface by bioturbation and there is a notable absence
of benthic megafauna (Lampitt, 1985; Rice et al.,
1994). Thus, there is a very direct link between
the processes and community structure of deep-sea
benthic environments and the processes occurring in
the overlying water column.
Lampitt and Antia (1997) identified three potential
reasons why fluxes may not increase in line with high
levels of primary production:
(1) Increases in primary production above 200 g C m
−2
y
−1 result from an increasing dependence on regenerated nutrients, and hence the f-ratio decreases
with increasing production.
(2) The degradation of particles proceeds faster within
the upper 2000 m when the levels of flux are
higher – that is: the higher productivity promotes
larger mesopelagic populations. But why are these
populations more efficient in their assimilation?
(3) Horizontal advection prevents the sedimenting
material from reaching depths of 2000 m.
Other possible explanations are based on the methodology: for example, sediment traps may not measure
all components of the fluxes. Significant fluxes may
occur in the form of dissolved organic matter (DOM)
carried into deeper water by downwelling and deepwater formation. Extra large particles that are not intercepted by the traps may make proportionately larger
contributions to the total flux. There are empirical
relationships between primary productivity and the
organic content of sediments used as palaeo-indicators
(Summerhayes et al., 1995b), and such relationships
imply that organic fluxes continue to increase with
increasing productivity. Populations of benthopelagic
scavengers attracted to baited cameras and traps are
known to consume large food packages, and tend to
increase below highly productive regions (Thurston,
1990); since the mean size of pelagic populations
tends to increase between low, temperate and subpolar
latitudes (Angel, 1997), fluxes of large packages may
increase with increasing primary production. This may
also contribute to the discrepancies between the flux estimates produced by Jahnke (1996) based on sediment
oxygen consumption and those of Lampitt and Antia
(1997) based on sediment trap fluxes extrapolated to
total ocean areas. These two studies were in reasonable
agreement for the Atlantic (0.126 and 0.100 Gt C org y
−1 ,
respectively), but quite disparate for the Pacific (0.214
and 0.110 Gt C org y
−1 , respectively) and even more so
for the Indian Ocean (0.142 and 0.012 Gt C org y
−1 ,
respectively; Gt = Gigatonnes), although riverine inputs
may also be contributing to the sediment metabolism.
PHYSIOLOGY AND BEHAVIOUR
All organisms are faced with three basic challenges:
1) how to survive; 2) how to find enough food for
growth and reproduction; and 3) how to reproduce
successfully. The optimal strategies to meet these challenges individually often conflict. Where food abounds,
other animals are attracted in, so that the dangers of
predation (e.g., Lampitt et al., 1983) or being injured
accidentally in feeding-frenzies increase. Most species
occur at very low densities, which means that finding
a mate cannot be left to chance encounters; instead,
species either form breeding swarms or “advertise”
their presence with scent or sound, again increasing
their vulnerability to predation. Observed macroplankton abundances are generally less than 1 per 1000 m
3 ,
but observations from submersibles (e.g., Wolff, 1971;
Childress et al., 1989) have suggested that random net
observations may well miss significant concentrations
of pelagic organisms, which often occur in thin layers
and may be assembled either passively as a result
of currents and eddies, or actively as a result of
behavioural responses to environmental cues.
The need to take risks associated with fulfilling
these basic activities shifts during the life cycle of
a species. Early larval stages are often spent in the
upper water column where food abounds. The higher
risk of predation is presumably outweighed by the
benefit of achieving more rapid growth. Size also
plays a role: the smaller an animal is, the shorter the
range over which it can be visually detected. Thus, as
an animal grows, it either has to adopt an effective
camouflage (such as a high degree of transparency) or
it may move down into deeper water where the light
is dimmer, which diminishes the range at which it can
seen. Hence many pelagic species undergo ontogenetic