FOOD INPUTS, UTILIZATION, CARBON FLOW AND ENERGETICS
359
as rat-tails, Coryphaenoides spp. (family Macrouridae)
has been shown in elegant experiments tracking individual fish which have ingested acoustic tags. These
experiments have given rise to astonishingly large
estimates of population densities (Armstrong et al.,
1992; Bagley et al., 1994; Priede et al., 1990, 1991).
Accurate range data using scanning sonar (Bagley
et al., 1994; Priede et al., 1994a) confirm that the
foraging pattern of scavenging fishes is important in
dispersing organic material from a food-fall laterally
over a wide area (Collins et al., 1998).
DISSOLVED ORGANIC CARBON AT THE DEEP-SEA
BENTHIC BOUNDARY
Quantities and sources
Quantities of dissolved organic material are produced
by phytoplankton in the euphotic zone. However, the
amount produced still remains controversial. A new
method for measuring dissolved organic carbon using
high-temperature catalytic oxidation (HTCO) revealed
far higher levels of dissolved organic carbon in the
upper water column than previously thought (Sugimura
and Suzuki, 1988). Here, bacterioplankton are important in the transformation of dissolved organic matter (DOM) into particulate organic matter (Azam and
Hodson, 1977. Some of this may be transported downwards (Toggweiler, 1989); zooplankton are thought to
contribute towards this dissolved organic matter flux
by diel vertical migrations (Dam et al., 1995). Vertical
profiles in the water column show marked increases in
concentration at the benthic boundary layer of the deep
ocean. Within the benthic boundary layer and sediment,
dissolved organic matter is produced by free-living
bacteria, by excretion of metabolites by metazoans, and
by microbial degradation of dead organisms.
The deep-ocean pool of dissolved organic carbon,
like that of suspended fine organic matter, is now
thought to be very large – far larger than that of sinking
particulate organic carbon (Bauer and Druffel, 1998).
The source of this dissolved organic carbon, however,
is also controversial. The annual global discharge of
dissolved organic carbon from rivers is sufficient to
cover turnover of the entire pool in sea water (Hedges
et al., 1997). What happens to this ultimately landderived carbon remains a mystery. However, although
Bauer and Druffel (1998) suggested from mass-balance
calculations, that input from the continental margins
may be much greater than that from the ocean surface,
most of this is of marine origin. Support for this
comes from recent measurements of high efflux of
dissolved organic carbon from the sediments on the
continental margin off southern Ireland (Otto and
Balzer, 1998). Because this dissolved organic carbon on
the continental margin tends to be old, deep open-ocean
carbon appears to be older than might be expected if its
main source is from new production at the surface.
Dissolved organic matter thus typically consists of
two main fractions. First, there are labile compounds
readily metabolized by heterotrophic bacteria, and thus
maintained at threshold levels. Second, and by far
the largest part, consists of much more refractory
compounds. These are mostly of very high molecular
weight (in the region of 20 000 or more), such as
lignins, humic acids and proteins, which are by no
means as available for utilization by bacteria.
Dissolved organic carbon as a food source
Williams (1975) reviewed the issue of dissolved
organic carbon as a possible food source. The majority
of free-living bacteria subsist partly on the smallest
organic particles, and also on dissolved organic compounds. The interstitial water of deep-sea sediments
contains concentrations of dissolved organic matter
about ten times those in the overlying water column
(Tanone and Handa, 1980), but unlike that in the
water above the dissolved organic matter is much
more analytically identifiable. Free amino acids can
reach concentrations of 5.6 mg °
−1 and free sugars
0.4 mg g
−1 dry weight of sediment (Southward and
Southward, 1982). The importance of dissolved organic
carbon in the nutrition of deep-sea biota is still
speculative. However, it seems likely its utilization
may be widespread, and is certainly very likely to
occur among deep-sea Foraminifera (Gooday et al.,
1992). Free amino acids are taken up by certain freeliving metazoans, particularly those living in reducing
conditions in the sediment in shallow water. However,
the importance of such sources to deep-sea taxa usually
can only be guessed from the absence of obvious
structures for ingestion of particulate food.
For example, a mouthless deep-sea nematode, described from the surface layer of sediment in the
abyssal Northeast Atlantic at 47ºN (Riemann, 1993),
may be able to utilize pore-water dissolved organic carbon. Xenophyophores (large agglutinating protozoans
found only in the deep sea) are also thought to be able
359
as rat-tails, Coryphaenoides spp. (family Macrouridae)
has been shown in elegant experiments tracking individual fish which have ingested acoustic tags. These
experiments have given rise to astonishingly large
estimates of population densities (Armstrong et al.,
1992; Bagley et al., 1994; Priede et al., 1990, 1991).
Accurate range data using scanning sonar (Bagley
et al., 1994; Priede et al., 1994a) confirm that the
foraging pattern of scavenging fishes is important in
dispersing organic material from a food-fall laterally
over a wide area (Collins et al., 1998).
DISSOLVED ORGANIC CARBON AT THE DEEP-SEA
BENTHIC BOUNDARY
Quantities and sources
Quantities of dissolved organic material are produced
by phytoplankton in the euphotic zone. However, the
amount produced still remains controversial. A new
method for measuring dissolved organic carbon using
high-temperature catalytic oxidation (HTCO) revealed
far higher levels of dissolved organic carbon in the
upper water column than previously thought (Sugimura
and Suzuki, 1988). Here, bacterioplankton are important in the transformation of dissolved organic matter (DOM) into particulate organic matter (Azam and
Hodson, 1977. Some of this may be transported downwards (Toggweiler, 1989); zooplankton are thought to
contribute towards this dissolved organic matter flux
by diel vertical migrations (Dam et al., 1995). Vertical
profiles in the water column show marked increases in
concentration at the benthic boundary layer of the deep
ocean. Within the benthic boundary layer and sediment,
dissolved organic matter is produced by free-living
bacteria, by excretion of metabolites by metazoans, and
by microbial degradation of dead organisms.
The deep-ocean pool of dissolved organic carbon,
like that of suspended fine organic matter, is now
thought to be very large – far larger than that of sinking
particulate organic carbon (Bauer and Druffel, 1998).
The source of this dissolved organic carbon, however,
is also controversial. The annual global discharge of
dissolved organic carbon from rivers is sufficient to
cover turnover of the entire pool in sea water (Hedges
et al., 1997). What happens to this ultimately landderived carbon remains a mystery. However, although
Bauer and Druffel (1998) suggested from mass-balance
calculations, that input from the continental margins
may be much greater than that from the ocean surface,
most of this is of marine origin. Support for this
comes from recent measurements of high efflux of
dissolved organic carbon from the sediments on the
continental margin off southern Ireland (Otto and
Balzer, 1998). Because this dissolved organic carbon on
the continental margin tends to be old, deep open-ocean
carbon appears to be older than might be expected if its
main source is from new production at the surface.
Dissolved organic matter thus typically consists of
two main fractions. First, there are labile compounds
readily metabolized by heterotrophic bacteria, and thus
maintained at threshold levels. Second, and by far
the largest part, consists of much more refractory
compounds. These are mostly of very high molecular
weight (in the region of 20 000 or more), such as
lignins, humic acids and proteins, which are by no
means as available for utilization by bacteria.
Dissolved organic carbon as a food source
Williams (1975) reviewed the issue of dissolved
organic carbon as a possible food source. The majority
of free-living bacteria subsist partly on the smallest
organic particles, and also on dissolved organic compounds. The interstitial water of deep-sea sediments
contains concentrations of dissolved organic matter
about ten times those in the overlying water column
(Tanone and Handa, 1980), but unlike that in the
water above the dissolved organic matter is much
more analytically identifiable. Free amino acids can
reach concentrations of 5.6 mg °
−1 and free sugars
0.4 mg g
−1 dry weight of sediment (Southward and
Southward, 1982). The importance of dissolved organic
carbon in the nutrition of deep-sea biota is still
speculative. However, it seems likely its utilization
may be widespread, and is certainly very likely to
occur among deep-sea Foraminifera (Gooday et al.,
1992). Free amino acids are taken up by certain freeliving metazoans, particularly those living in reducing
conditions in the sediment in shallow water. However,
the importance of such sources to deep-sea taxa usually
can only be guessed from the absence of obvious
structures for ingestion of particulate food.
For example, a mouthless deep-sea nematode, described from the surface layer of sediment in the
abyssal Northeast Atlantic at 47ºN (Riemann, 1993),
may be able to utilize pore-water dissolved organic carbon. Xenophyophores (large agglutinating protozoans
found only in the deep sea) are also thought to be able