350
John D. GAGE
(Mikkelsen et al., 1982). It appears that, as in shallow
water, a broad range in growth strategies will be found
corresponding to differing lifestyles in the fauna (Gage,
1991).
Rates of secondary production
Estimates of secondary production from such data
have not yet been attempted by direct methods (see
Gage, 1992, for summary). However, demographic
models fitted to size frequencies and skeletal growth
markers offer the means to calculate production using
a fitted size/mass relationship. Using models fitted
to four bivalve community dominants, the ratio of
annual production to biomass (P/B ratio) ranged from
0.49 to 1.65 (Gage, 1992), although their combined
production (114.9 mg wet weight m
−2 yr
−1 ) is less than
half that predicted (363 mg wet weight m
−2 yr
−1 ) from
empirical relationships established from coastal studies
(Brey, 1990). Total non-foraminiferal macrobenthic
community production equivalent to 122 mg organic
carbon based on a mean annual P/B of 0.98, was
estimated by Gage (1992) for a depth of 2900 m in
the Rockall Trough. This value for somatic production
represents between 3 and 12% of total respiratory
carbon uptake for the sediment community at this
depth, suggesting that secondary production by the
macrobenthos alone may be a small, but non-trivial,
component of carbon consumption and turnover at the
deep-sea floor.
So far the contribution of anabolic metabolism (as
reproductive and somatic production) in the overall assimilation of larger deposit-feeding megafauna,
which may consume a large proportion of available
particulate organic carbon flux to the bottom has not
been taken into account. Although it has been assumed
that such rates in these animals will be very slow, recent
observations of dramatic population changes among
megafauna in the Northeast Atlantic (Billett et al. 2001)
indicate that much faster rates may occur.
Implications of episodic food input to the benthic
boundary layer community
Episodic particulate flux is of great importance because
it appears to bypass a large part of the processes of
recycling and biogenic repackaging of the export flux
from the surface. It has been estimated that >50% of
annual biogenic particle flux in the Northeast Atlantic
is supplied to the bottom during the spring particle
bloom (Honjo and Manganini, 1993). If this is true for
large areas of the deep sea, then most of the utilizable
food for deep-sea benthic organisms, including bacteria
and other micro-organisms, through small meiofaunal
metazoans, to macrofauna and megafauna, both motile
and buried, will arrive as pulses of labile material. The
tightness in the coupling between supply and demand,
however, may depend on supply not outstripping demand. Smith et al. (1994) suggested that, although the
North Atlantic and North Pacific both show variability
in particle flux, this tends to occur as small increments
in the Pacific rather than as mass deposition; in
consequence, trophic coupling is more efficient in the
Pacific than in the Atlantic. In the Atlantic detrital
carpets accumulate over large areas, presumably as a
result of supply vastly exceeding demand. However, for
the rest of the time the reverse is the case, making
labile particulate food for deposit feeders, and perhaps
also for suspension feeders, almost vanishingly scarce.
This may mean that overall efficiency in utilization
(defined as the ratio of net production entering the
benthic boundary layer to its assimilation) might be
higher in the Pacific than the Atlantic Ocean.
Clearly these events, by imposing temporal variability on the nutrient flux to the seabed, should have a
profound effect on the deep-ocean carbon dynamics.
As yet, such intermittency has not been fully taken
into account in models of organic-carbon diagenesis in
the oceanic biogeochemical carbon cycle. Seasonally
pulsed input imparts dramatic temporal variability
to the solubilizing of complex organic molecules to
inorganic constituents, contributing to nutrient efflux
from the sediment, as well as increased rates in
carbon burial (C.R. Smith, 1992; Pfannkuche, 1993).
Furthermore, in view of their high quality as food, the
seasonal input pulses may represent the single most
important nutritional input to the community of the
deep-sea bed.
Simple box models of the energetic flows in the
sediment at the German BIOTRANS site before and
after deposition are shown in Fig. 11.17, and a
similar representation for flows in the phytodetrital
layer in Fig. 11.18. Considerable heterogeneity may
be imparted to the sediment because of the complexity and different size scales involved. For example,
analysis of lipid content in the abyssal sediment has
uncovered considerable small-scale spatial variability,
very possibly related, via the feeding activity of benthic
organisms, to patchiness in phytodetrital accumulation
in topographic lows in the sediment (Santos et al.,
1994). On the continental slope several data sets,
John D. GAGE
(Mikkelsen et al., 1982). It appears that, as in shallow
water, a broad range in growth strategies will be found
corresponding to differing lifestyles in the fauna (Gage,
1991).
Rates of secondary production
Estimates of secondary production from such data
have not yet been attempted by direct methods (see
Gage, 1992, for summary). However, demographic
models fitted to size frequencies and skeletal growth
markers offer the means to calculate production using
a fitted size/mass relationship. Using models fitted
to four bivalve community dominants, the ratio of
annual production to biomass (P/B ratio) ranged from
0.49 to 1.65 (Gage, 1992), although their combined
production (114.9 mg wet weight m
−2 yr
−1 ) is less than
half that predicted (363 mg wet weight m
−2 yr
−1 ) from
empirical relationships established from coastal studies
(Brey, 1990). Total non-foraminiferal macrobenthic
community production equivalent to 122 mg organic
carbon based on a mean annual P/B of 0.98, was
estimated by Gage (1992) for a depth of 2900 m in
the Rockall Trough. This value for somatic production
represents between 3 and 12% of total respiratory
carbon uptake for the sediment community at this
depth, suggesting that secondary production by the
macrobenthos alone may be a small, but non-trivial,
component of carbon consumption and turnover at the
deep-sea floor.
So far the contribution of anabolic metabolism (as
reproductive and somatic production) in the overall assimilation of larger deposit-feeding megafauna,
which may consume a large proportion of available
particulate organic carbon flux to the bottom has not
been taken into account. Although it has been assumed
that such rates in these animals will be very slow, recent
observations of dramatic population changes among
megafauna in the Northeast Atlantic (Billett et al. 2001)
indicate that much faster rates may occur.
Implications of episodic food input to the benthic
boundary layer community
Episodic particulate flux is of great importance because
it appears to bypass a large part of the processes of
recycling and biogenic repackaging of the export flux
from the surface. It has been estimated that >50% of
annual biogenic particle flux in the Northeast Atlantic
is supplied to the bottom during the spring particle
bloom (Honjo and Manganini, 1993). If this is true for
large areas of the deep sea, then most of the utilizable
food for deep-sea benthic organisms, including bacteria
and other micro-organisms, through small meiofaunal
metazoans, to macrofauna and megafauna, both motile
and buried, will arrive as pulses of labile material. The
tightness in the coupling between supply and demand,
however, may depend on supply not outstripping demand. Smith et al. (1994) suggested that, although the
North Atlantic and North Pacific both show variability
in particle flux, this tends to occur as small increments
in the Pacific rather than as mass deposition; in
consequence, trophic coupling is more efficient in the
Pacific than in the Atlantic. In the Atlantic detrital
carpets accumulate over large areas, presumably as a
result of supply vastly exceeding demand. However, for
the rest of the time the reverse is the case, making
labile particulate food for deposit feeders, and perhaps
also for suspension feeders, almost vanishingly scarce.
This may mean that overall efficiency in utilization
(defined as the ratio of net production entering the
benthic boundary layer to its assimilation) might be
higher in the Pacific than the Atlantic Ocean.
Clearly these events, by imposing temporal variability on the nutrient flux to the seabed, should have a
profound effect on the deep-ocean carbon dynamics.
As yet, such intermittency has not been fully taken
into account in models of organic-carbon diagenesis in
the oceanic biogeochemical carbon cycle. Seasonally
pulsed input imparts dramatic temporal variability
to the solubilizing of complex organic molecules to
inorganic constituents, contributing to nutrient efflux
from the sediment, as well as increased rates in
carbon burial (C.R. Smith, 1992; Pfannkuche, 1993).
Furthermore, in view of their high quality as food, the
seasonal input pulses may represent the single most
important nutritional input to the community of the
deep-sea bed.
Simple box models of the energetic flows in the
sediment at the German BIOTRANS site before and
after deposition are shown in Fig. 11.17, and a
similar representation for flows in the phytodetrital
layer in Fig. 11.18. Considerable heterogeneity may
be imparted to the sediment because of the complexity and different size scales involved. For example,
analysis of lipid content in the abyssal sediment has
uncovered considerable small-scale spatial variability,
very possibly related, via the feeding activity of benthic
organisms, to patchiness in phytodetrital accumulation
in topographic lows in the sediment (Santos et al.,
1994). On the continental slope several data sets,