208
Craig R. SMITH and Amanda W.J. DEMOPOULOS
For this sediment nanobiota, Snider et al. estimated
numerical density of 6.6×10
7 m
−2 and biomass of
0.13 g wet weight m
−2 .
Manganese nodules are common in the oligotrophic
abyss, typically covering roughly 30% of the seafloor
(Mullineaux, 1987). As in the mesotrophic equatorial Pacific, Mullineaux (1987) found the eukaryotic
nodule fauna of the MPG-I site to be dominated
(>99%) in numbers and biomass by Foraminifera
and related rhizopod protozoans, which covered approximately 10% of exposed nodule surfaces. Interestingly, 92% of the nodule taxa found at MPG-I
were also found on nodules 4000 kilometers away
in the equatorial Pacific, whereas virtually none were
found in surrounding sediments (Mullineaux, 1987).
With densities of very roughly 4000 m
−2 of total
seafloor, these hard-substratum “meiobenthos” were
roughly two orders of magnitude less abundant than
their meiofaunal counterparts dwelling in surrounding
MPG-I sediments (Mullineaux, 1987). The abundance
and species diversity of the MPG-I nodule fauna was
approximately half that on nodules in the mesotrophic
equatorial Pacific, presumably reflecting lower inputs of
particulate organic carbon (Mullineaux, 1987).
The biomass distribution of the total benthic community has perhaps been better studied at MPG-I
than at any other site in the deep Pacific Ocean.
K.L. Smith (1992) compiled biomass data from
the vicinity of MPG-I to examine carbon cycling
through the oligotrophic abyssal benthos. The ratios of
biomass between megafauna, macrofauna, meiofauna
and microbiota at this 5800-m site were roughly as
0.5:0.03:0.6:1.0 (K.L. Smith, 1992). Thus, the microbes
(which here includes bacteria >10 mm in diameter)
dominate community biomass, with megafauna and
meiofauna also being relatively important. In the
oligotrophic abyss, relatively little metabolically active
biomass appears to be concentrated in the macrofauna,
suggesting that other size classes, especially the
microbes and meiofauna, dominate metabolism (K.L.
Smith, 1992). This situation contrasts with biomass
distributions on the California slope and in shallow
water, where megafauna and macrofauna typically
dominate the biomass distribution (Gray, 1981; Snider
et al., 1984; Gerlach et al., 1985). Thus, under
extremely oligotrophic conditions, the smallest size
classes of benthos appear to assume much greater
importance in the recycling of organic matter on the
deep-sea floor.
Carbon sources and trophic types
The primary sources of organic matter for the
oligotrophic Pacific abyss are likely to be (1) the flux
of small sinking particles measured in sediment traps
and (2) the sinking carcasses of nekton (particularly
crustaceans, fishes and whales). Other sources of
organic matter found in more eutrophic settings (e.g.,
phytodetrital aggregates, macroalgal debris) have not
been observed in the oligotrophic abyssal Pacific.
The flux of fine particulate organic carbon to the
oligotrophic seafloor, as measured in sediment traps in
the MPG-I area, is roughly 0.3 g C m
−2 y
−1 (K.L. Smith,
1992); this is equivalent to one-sixth to one-third of
the flux measured in the eutrophic equatorial abyss,
and only about one-thirtieth of the flux measured on
the California slope (Table 6.1). When compared to
the organic-carbon demand of the sediment community
measured by seafloor respirometry, this sinking flux
of particulate organic carbon appears inadequate by
up to 50% in meeting the metabolic requirements of
the oligotrophic benthos (K.L. Smith, 1992). This may
imply that other sources of organic matter, such as
large food falls, provide substantial carbon flux to the
oligotrophic seafloor. Given the normal sparseness of
scavengers and the low biomass of scavenging rattails
and amphipods (Priede et al., 1990, 1994; K.L. Smith,
1992), it seems unlikely that nekton falls constitute a
large proportion of the organic-carbon flux. A more
likely explanation for the inadequacy of the flux of
small particulate organic carbon is that all deployments
of sediment traps in the MPG-I region have been for
very short periods of time (7 to 14 days); thus, as
on the continental margin (K.L. Smith et al., 1992),
they are likely to have missed important pulses of
particulate organic carbon flux. Karl et al. (1996)
showed that, even in oligotrophic waters, a substantial
proportion of the flux of particulate organic carbon
may occur as brief pulses following occasional, intense
bursts of primary production. Clearly, sediment-trap
deployments for longer time scales (a year or more)
must be combined with synchronous measurements of
seafloor respiration to determine whether the flux of
small particulate organic carbon is sufficient to feed the
oligotrophic benthos.
The flux of large organic falls to the oligotrophic
seafloor has not been evaluated, but the consumption of
large carrion parcels within hours (Dayton and Hessler,
1972; Hessler, 1974) and the extreme adaptations of
scavengers (Dahl, 1979; Barnard and Ingram, 1986)
indicate that this flux is evolutionarily and ecologically
Craig R. SMITH and Amanda W.J. DEMOPOULOS
For this sediment nanobiota, Snider et al. estimated
numerical density of 6.6×10
7 m
−2 and biomass of
0.13 g wet weight m
−2 .
Manganese nodules are common in the oligotrophic
abyss, typically covering roughly 30% of the seafloor
(Mullineaux, 1987). As in the mesotrophic equatorial Pacific, Mullineaux (1987) found the eukaryotic
nodule fauna of the MPG-I site to be dominated
(>99%) in numbers and biomass by Foraminifera
and related rhizopod protozoans, which covered approximately 10% of exposed nodule surfaces. Interestingly, 92% of the nodule taxa found at MPG-I
were also found on nodules 4000 kilometers away
in the equatorial Pacific, whereas virtually none were
found in surrounding sediments (Mullineaux, 1987).
With densities of very roughly 4000 m
−2 of total
seafloor, these hard-substratum “meiobenthos” were
roughly two orders of magnitude less abundant than
their meiofaunal counterparts dwelling in surrounding
MPG-I sediments (Mullineaux, 1987). The abundance
and species diversity of the MPG-I nodule fauna was
approximately half that on nodules in the mesotrophic
equatorial Pacific, presumably reflecting lower inputs of
particulate organic carbon (Mullineaux, 1987).
The biomass distribution of the total benthic community has perhaps been better studied at MPG-I
than at any other site in the deep Pacific Ocean.
K.L. Smith (1992) compiled biomass data from
the vicinity of MPG-I to examine carbon cycling
through the oligotrophic abyssal benthos. The ratios of
biomass between megafauna, macrofauna, meiofauna
and microbiota at this 5800-m site were roughly as
0.5:0.03:0.6:1.0 (K.L. Smith, 1992). Thus, the microbes
(which here includes bacteria >10 mm in diameter)
dominate community biomass, with megafauna and
meiofauna also being relatively important. In the
oligotrophic abyss, relatively little metabolically active
biomass appears to be concentrated in the macrofauna,
suggesting that other size classes, especially the
microbes and meiofauna, dominate metabolism (K.L.
Smith, 1992). This situation contrasts with biomass
distributions on the California slope and in shallow
water, where megafauna and macrofauna typically
dominate the biomass distribution (Gray, 1981; Snider
et al., 1984; Gerlach et al., 1985). Thus, under
extremely oligotrophic conditions, the smallest size
classes of benthos appear to assume much greater
importance in the recycling of organic matter on the
deep-sea floor.
Carbon sources and trophic types
The primary sources of organic matter for the
oligotrophic Pacific abyss are likely to be (1) the flux
of small sinking particles measured in sediment traps
and (2) the sinking carcasses of nekton (particularly
crustaceans, fishes and whales). Other sources of
organic matter found in more eutrophic settings (e.g.,
phytodetrital aggregates, macroalgal debris) have not
been observed in the oligotrophic abyssal Pacific.
The flux of fine particulate organic carbon to the
oligotrophic seafloor, as measured in sediment traps in
the MPG-I area, is roughly 0.3 g C m
−2 y
−1 (K.L. Smith,
1992); this is equivalent to one-sixth to one-third of
the flux measured in the eutrophic equatorial abyss,
and only about one-thirtieth of the flux measured on
the California slope (Table 6.1). When compared to
the organic-carbon demand of the sediment community
measured by seafloor respirometry, this sinking flux
of particulate organic carbon appears inadequate by
up to 50% in meeting the metabolic requirements of
the oligotrophic benthos (K.L. Smith, 1992). This may
imply that other sources of organic matter, such as
large food falls, provide substantial carbon flux to the
oligotrophic seafloor. Given the normal sparseness of
scavengers and the low biomass of scavenging rattails
and amphipods (Priede et al., 1990, 1994; K.L. Smith,
1992), it seems unlikely that nekton falls constitute a
large proportion of the organic-carbon flux. A more
likely explanation for the inadequacy of the flux of
small particulate organic carbon is that all deployments
of sediment traps in the MPG-I region have been for
very short periods of time (7 to 14 days); thus, as
on the continental margin (K.L. Smith et al., 1992),
they are likely to have missed important pulses of
particulate organic carbon flux. Karl et al. (1996)
showed that, even in oligotrophic waters, a substantial
proportion of the flux of particulate organic carbon
may occur as brief pulses following occasional, intense
bursts of primary production. Clearly, sediment-trap
deployments for longer time scales (a year or more)
must be combined with synchronous measurements of
seafloor respiration to determine whether the flux of
small particulate organic carbon is sufficient to feed the
oligotrophic benthos.
The flux of large organic falls to the oligotrophic
seafloor has not been evaluated, but the consumption of
large carrion parcels within hours (Dayton and Hessler,
1972; Hessler, 1974) and the extreme adaptations of
scavengers (Dahl, 1979; Barnard and Ingram, 1986)
indicate that this flux is evolutionarily and ecologically