202
Craig R. SMITH and Amanda W.J. DEMOPOULOS
sediments is concentrated in the top 5 cm of sediment,
where there is access to labile organic matter depositing
on the sediment–water interface. Macrofaunal species
diversity has not been fully evaluated in equatorial
Pacific sediments, but the local diversity of the
dominant taxon, the polychaetes, appears to be high.
At three equatorial sites in the northeastern Pacific,
Paterson et al. (1998) found between 11 and 14 species
among 20 individuals, and, for pooled box cores,
about 40 species among 100 individuals. This rivals
or exceeds the extremely high diversity previously
described for continental-slope habitats (Fig. 6.8).
In the mesotrophic abyss, for instance, at 9ºN along
the EqPac transect, macrofaunal abundance (290 m
−2 )
and biomass (0.12 mg m
−2 ) are roughly 25% of those
in eutrophic abyssal sediments (Table 6.1). However,
mean macrofaunal body size (~0.4 mg) remains similar
to that between 0º and 5ºN (Table 6.1).
The dominant meiofaunal taxon in the equatorial
Pacific, the Nematoda, has received substantial study
along the EqPac transect (Brown, 1998; Brown et al.,
2002). In eutrophic sediments (e.g., from 0º–5ºN),
the nematodes attain mean densities of 130 000 to
140 000 individuals m
−2 , and biomasses of 0.03 to
0.06 g wet weight m
−2 in the top 5 cm of sediment
(Brown, 1998; Brown et al., 2002). These nematode
assemblages attain very high local diversity, with
over 32 species among 50 individuals collected in
a single 80 cm
2 sample (Lambshead et al., 2002).
In mesotrophic sediments at 9ºN, the abundance and
biomass of nematodes has dropped somewhat to 90 000
individuals and 0.02 g cm
−2 , respectively, while local
species diversity changes only slightly (Lambshead
et al., 2002). The abundance and biomass of nematodes
in the equatorial Pacific abyss falls at the low end of
the ranges of nematode abundance and biomass in the
abyssal northeast Atlantic near the continental margin
(e.g., the Porcupine Abyssal Plain) (Brown, 1998),
whereas the local species diversity of equatorial nematode fauna is relatively high (Lambshead et al., 2002).
Microbial biomass in eutrophic sediments along the
EqPac transect is surprisingly high, ranging from 0.2
to 0.3 g C m
−2 in the top 0.5 cm of sediment (Smith
et al., 1997). Assuming that wet-weight biomass is
10% organic carbon, this yields a microbial wet weight
between 2 and 3 g m
−2 – roughly five-fold greater than
that of the macrofauna (Table 6.1) and 100-fold higher
than that of the nematodes. In mesotrophic equatorial
sediments, microbial biomass declines somewhat in
absolute terms (to 1.4 g C m
−2 : Smith et al., 1997),
but the ratio to other size classes increases, microbial
biomass being about ten-fold larger than that of the
macrofauna. Although much of the bacterial biomass in
sediments may consist of cells sinking out of the water
column (Novitsky, 1987), the high microbial biomass
relative to other size classes suggests that the microbes
may account for a large proportion of the respiration of
the sediment community in eutrophic and mesotrophic
sediments of the equatorial abyss.
Manganese nodules occur in the mesotrophic abyss,
and occasionally in the eutrophic abyss (Fig. 6.6), and
provide solid substrata for communities fundamentally
different from those in surrounding soft sediments.
These polymetallic accretions often attain densities
between 100 and 300 m
−2 , covering 20 to 50% of the
plan area of the seafloor (e.g., Heezen and Hollister,
1971; Mullineaux, 1987). At 5ºN, 125ºW, roughly 10%
of exposed nodule surfaces are covered by sessile,
eukaryotic organisms, with Foraminifera accounting
for over 98% of community abundance and areal
cover (Mullineaux, 1987). Metazoans found attached to
nodules include small sponges, molluscs, polychaetes
and bryozoans; according to Mullineaux, the vast
majority of the nodule species are not found in
surrounding sediments. Mullineaux found that the areal
density of animals >63 mm in diameter attached to
nodules was roughly 10% of that of the sedimentdwelling meiofauna. Local species diversity on nodules
is roughly comparable to that of the sediment-dwelling
nematodes, with ~25 species among 50 individuals
(Mullineaux, 1987).
In addition to manganese nodules, xenophyophores
are likely to provide substantial habitat heterogeneity
on the seafloor in the equatorial abyss. Although the
ecology of xenophyophores in the equatorial abyss
has not been explicitly studied, in other areas (e.g.,
seamounts) the tests of these organisms provide shelter
and/or food resources for a specialized community of
macrofaunal invertebrates (Levin and Gooday, 1992).
Because of their abundance (2 to 6 m
−2 ), xenophyophores are very likely to contribute fundamentally
to macrofaunal community structure in the equatorial
abyss.
Nowhere in the equatorial Pacific have the biomasses
for all size classes of benthos (i.e., the megafauna,
macrofauna, meiofauna and microbiota) been tabulated. The best biomass data come from the EqPac
transect, where macrofauna and microbiota occur in
biomass ratios of roughly 1:5 in eutrophic sediments,
as against 1:10 in mesotrophic settings. This contrasts
Craig R. SMITH and Amanda W.J. DEMOPOULOS
sediments is concentrated in the top 5 cm of sediment,
where there is access to labile organic matter depositing
on the sediment–water interface. Macrofaunal species
diversity has not been fully evaluated in equatorial
Pacific sediments, but the local diversity of the
dominant taxon, the polychaetes, appears to be high.
At three equatorial sites in the northeastern Pacific,
Paterson et al. (1998) found between 11 and 14 species
among 20 individuals, and, for pooled box cores,
about 40 species among 100 individuals. This rivals
or exceeds the extremely high diversity previously
described for continental-slope habitats (Fig. 6.8).
In the mesotrophic abyss, for instance, at 9ºN along
the EqPac transect, macrofaunal abundance (290 m
−2 )
and biomass (0.12 mg m
−2 ) are roughly 25% of those
in eutrophic abyssal sediments (Table 6.1). However,
mean macrofaunal body size (~0.4 mg) remains similar
to that between 0º and 5ºN (Table 6.1).
The dominant meiofaunal taxon in the equatorial
Pacific, the Nematoda, has received substantial study
along the EqPac transect (Brown, 1998; Brown et al.,
2002). In eutrophic sediments (e.g., from 0º–5ºN),
the nematodes attain mean densities of 130 000 to
140 000 individuals m
−2 , and biomasses of 0.03 to
0.06 g wet weight m
−2 in the top 5 cm of sediment
(Brown, 1998; Brown et al., 2002). These nematode
assemblages attain very high local diversity, with
over 32 species among 50 individuals collected in
a single 80 cm
2 sample (Lambshead et al., 2002).
In mesotrophic sediments at 9ºN, the abundance and
biomass of nematodes has dropped somewhat to 90 000
individuals and 0.02 g cm
−2 , respectively, while local
species diversity changes only slightly (Lambshead
et al., 2002). The abundance and biomass of nematodes
in the equatorial Pacific abyss falls at the low end of
the ranges of nematode abundance and biomass in the
abyssal northeast Atlantic near the continental margin
(e.g., the Porcupine Abyssal Plain) (Brown, 1998),
whereas the local species diversity of equatorial nematode fauna is relatively high (Lambshead et al., 2002).
Microbial biomass in eutrophic sediments along the
EqPac transect is surprisingly high, ranging from 0.2
to 0.3 g C m
−2 in the top 0.5 cm of sediment (Smith
et al., 1997). Assuming that wet-weight biomass is
10% organic carbon, this yields a microbial wet weight
between 2 and 3 g m
−2 – roughly five-fold greater than
that of the macrofauna (Table 6.1) and 100-fold higher
than that of the nematodes. In mesotrophic equatorial
sediments, microbial biomass declines somewhat in
absolute terms (to 1.4 g C m
−2 : Smith et al., 1997),
but the ratio to other size classes increases, microbial
biomass being about ten-fold larger than that of the
macrofauna. Although much of the bacterial biomass in
sediments may consist of cells sinking out of the water
column (Novitsky, 1987), the high microbial biomass
relative to other size classes suggests that the microbes
may account for a large proportion of the respiration of
the sediment community in eutrophic and mesotrophic
sediments of the equatorial abyss.
Manganese nodules occur in the mesotrophic abyss,
and occasionally in the eutrophic abyss (Fig. 6.6), and
provide solid substrata for communities fundamentally
different from those in surrounding soft sediments.
These polymetallic accretions often attain densities
between 100 and 300 m
−2 , covering 20 to 50% of the
plan area of the seafloor (e.g., Heezen and Hollister,
1971; Mullineaux, 1987). At 5ºN, 125ºW, roughly 10%
of exposed nodule surfaces are covered by sessile,
eukaryotic organisms, with Foraminifera accounting
for over 98% of community abundance and areal
cover (Mullineaux, 1987). Metazoans found attached to
nodules include small sponges, molluscs, polychaetes
and bryozoans; according to Mullineaux, the vast
majority of the nodule species are not found in
surrounding sediments. Mullineaux found that the areal
density of animals >63 mm in diameter attached to
nodules was roughly 10% of that of the sedimentdwelling meiofauna. Local species diversity on nodules
is roughly comparable to that of the sediment-dwelling
nematodes, with ~25 species among 50 individuals
(Mullineaux, 1987).
In addition to manganese nodules, xenophyophores
are likely to provide substantial habitat heterogeneity
on the seafloor in the equatorial abyss. Although the
ecology of xenophyophores in the equatorial abyss
has not been explicitly studied, in other areas (e.g.,
seamounts) the tests of these organisms provide shelter
and/or food resources for a specialized community of
macrofaunal invertebrates (Levin and Gooday, 1992).
Because of their abundance (2 to 6 m
−2 ), xenophyophores are very likely to contribute fundamentally
to macrofaunal community structure in the equatorial
abyss.
Nowhere in the equatorial Pacific have the biomasses
for all size classes of benthos (i.e., the megafauna,
macrofauna, meiofauna and microbiota) been tabulated. The best biomass data come from the EqPac
transect, where macrofauna and microbiota occur in
biomass ratios of roughly 1:5 in eutrophic sediments,
as against 1:10 in mesotrophic settings. This contrasts