THE DEEP PACIFIC OCEAN FLOOR
203
with a macrofauna:microbiota biomass ratio of 6:1 in
the bathyal Santa Catalina Basin on the California
margin, suggesting that microbes may be relatively
much more important in the energetics of abyssal
equatorial communities.
Carbon sources and trophic types
The most important sources of organic matter in both
eutrophic and mesotrophic equatorial Pacific habitats
are likely to be: (1) small sinking particles, whose
flux has been evaluated with moored sediment traps;
(2) phytodetrital aggregates, which may be too large
and rare to be reliably captured in traps; and (3) the
sinking carcasses of nekton (crustaceans, fish, whales,
etc.). In the eutrophic equatorial abyss, the flux of
particulate organic carbon at greater depths shows substantial short-term variability, fluxes into deep sediment
traps varying as much as two-fold between 17-day
sampling periods (Honjo et al., 1995). Substantial
interannual variability in the flux of particulate organic
carbon also occurs in eutrophic and mesotrophic
equatorial settings; for example, Dymond and Collier
(1988) found that during the 1982–83 El Ni˜ no, the flux
of particulate organic carbon at a eutrophic equatorial
station (1ºN, 139ºW) was roughly half that in a nonEl Ni˜ no year, whereas flux of particulate organic
carbon at a mesotrophic station (11ºN, 140ºW) roughly
doubled. Despite this variability, in both eutrophic
and mesotrophic sites the remineralization rates of
organic carbon, as evaluated by oxygen uptake in
in situ respirometers, roughly matched the rate of
rain of particulate organic carbon into deep-moored
sediment traps (Hammond et al., 1996; Berelson et al.,
1997; Smith et al., 1997). Thus, the prime source
of organic carbon to infaunal benthos in equatorial
sediments appears to be the flux of small sinking
particles. However, these respirometry measurements
are relatively few in number and cover only small
areas (<0.2 m
2 each); they thus do not include the
deposit-feeding and scavenging megafauna, and may
miss important seafloor hot-spots of metabolic activity.
Xenophyophore tests and echiuran feeding pits in
particular may serve as important traps of food-rich
sedimenting particles, adding heterogeneity to seafloor
mineralization processes (Levin and Gooday, 1992;
Smith et al., 1996). Thus, it is quite possible that other
sources of organic matter, such as phytodetrital aggregates or large sinking carcasses, contribute significant
food energy to equatorial abyssal sediments.
There is some evidence that freshly settled phytodetritus may be an important source of labile organic matter to eutrophic equatorial sediments. Recently, Smith
et al. (1996) found concentrations of fresh, greenish,
phytoplankton detritus on the seafloor from 5ºS to 5ºN
along the 140ºW meridian in Nov.–Dec. 1992. Gardner
et al. (1984) also observed phytodetrital aggregates in
this region in 1977. Phytodetritus collected by Smith
et al. (1996) sustained high rates of microbial activity
and was rich in excess
234 Th activity, suggesting it
had settled from the water column in the previous
100 days. This material appeared to be selectively
grazed by holothurians and echiurans, and was cached
in burrows as deep as 27 cm in the sediment. Smith
et al. estimated that the standing stock of phytodetritus in November and December 1992 constituted
about 3% of the annual flux of particulate organic
carbon to the eutrophic equatorial seafloor. In addition,
modeling of organic-matter reactions indicated that,
during November and December 1992, organic-carbon
degradation in eutrophic sediments along the EqPac
transect was dominated by a very labile component
with a mean degradation half-life of ~20 days; this
labile organic carbon appeared to be derived from
the phytodetritus (Hammond et al., 1996) and was
similar in lability to the dominant material degrading
in sediments from depths of 4000 m on the California
margin (Sayles et al., 1994). Phytodetrital flux in the
equatorial Pacific seems to be related to the formation
of intense convergence zones in the euphotic zone
during the passage of tropical instability waves, which
are most common between August and December
(Smith et al., 1996). Thus, phytodetritus may frequently
settle to the eutrophic equatorial seafloor during the
boreal autumn, and could supply a significant fraction
of the energy requirements of the abyssal benthos. In
the mesotrophic equatorial abyss phytodetritus has not
been observed, suggesting that the flux of particulate
organic carbon may be lower in quality, as well as
quantity, in the mesotrophic regions than in eutrophic
equatorial settings.
As on the California slope, there are faunal components adapted to utilize all the prominent sources
of organic carbon to equatorial Pacific sediments.
Although poorly studied, the megafauna include a well
adapted suite of very mobile, swimming scavengers,
including lyssianisid amphipods, macrourid fish, and
natantian decapods, which rapidly consume fish and
cephalopod bait placed on the seafloor (R. Hessler,
personal communication). Unlike that of the California
203
with a macrofauna:microbiota biomass ratio of 6:1 in
the bathyal Santa Catalina Basin on the California
margin, suggesting that microbes may be relatively
much more important in the energetics of abyssal
equatorial communities.
Carbon sources and trophic types
The most important sources of organic matter in both
eutrophic and mesotrophic equatorial Pacific habitats
are likely to be: (1) small sinking particles, whose
flux has been evaluated with moored sediment traps;
(2) phytodetrital aggregates, which may be too large
and rare to be reliably captured in traps; and (3) the
sinking carcasses of nekton (crustaceans, fish, whales,
etc.). In the eutrophic equatorial abyss, the flux of
particulate organic carbon at greater depths shows substantial short-term variability, fluxes into deep sediment
traps varying as much as two-fold between 17-day
sampling periods (Honjo et al., 1995). Substantial
interannual variability in the flux of particulate organic
carbon also occurs in eutrophic and mesotrophic
equatorial settings; for example, Dymond and Collier
(1988) found that during the 1982–83 El Ni˜ no, the flux
of particulate organic carbon at a eutrophic equatorial
station (1ºN, 139ºW) was roughly half that in a nonEl Ni˜ no year, whereas flux of particulate organic
carbon at a mesotrophic station (11ºN, 140ºW) roughly
doubled. Despite this variability, in both eutrophic
and mesotrophic sites the remineralization rates of
organic carbon, as evaluated by oxygen uptake in
in situ respirometers, roughly matched the rate of
rain of particulate organic carbon into deep-moored
sediment traps (Hammond et al., 1996; Berelson et al.,
1997; Smith et al., 1997). Thus, the prime source
of organic carbon to infaunal benthos in equatorial
sediments appears to be the flux of small sinking
particles. However, these respirometry measurements
are relatively few in number and cover only small
areas (<0.2 m
2 each); they thus do not include the
deposit-feeding and scavenging megafauna, and may
miss important seafloor hot-spots of metabolic activity.
Xenophyophore tests and echiuran feeding pits in
particular may serve as important traps of food-rich
sedimenting particles, adding heterogeneity to seafloor
mineralization processes (Levin and Gooday, 1992;
Smith et al., 1996). Thus, it is quite possible that other
sources of organic matter, such as phytodetrital aggregates or large sinking carcasses, contribute significant
food energy to equatorial abyssal sediments.
There is some evidence that freshly settled phytodetritus may be an important source of labile organic matter to eutrophic equatorial sediments. Recently, Smith
et al. (1996) found concentrations of fresh, greenish,
phytoplankton detritus on the seafloor from 5ºS to 5ºN
along the 140ºW meridian in Nov.–Dec. 1992. Gardner
et al. (1984) also observed phytodetrital aggregates in
this region in 1977. Phytodetritus collected by Smith
et al. (1996) sustained high rates of microbial activity
and was rich in excess
234 Th activity, suggesting it
had settled from the water column in the previous
100 days. This material appeared to be selectively
grazed by holothurians and echiurans, and was cached
in burrows as deep as 27 cm in the sediment. Smith
et al. estimated that the standing stock of phytodetritus in November and December 1992 constituted
about 3% of the annual flux of particulate organic
carbon to the eutrophic equatorial seafloor. In addition,
modeling of organic-matter reactions indicated that,
during November and December 1992, organic-carbon
degradation in eutrophic sediments along the EqPac
transect was dominated by a very labile component
with a mean degradation half-life of ~20 days; this
labile organic carbon appeared to be derived from
the phytodetritus (Hammond et al., 1996) and was
similar in lability to the dominant material degrading
in sediments from depths of 4000 m on the California
margin (Sayles et al., 1994). Phytodetrital flux in the
equatorial Pacific seems to be related to the formation
of intense convergence zones in the euphotic zone
during the passage of tropical instability waves, which
are most common between August and December
(Smith et al., 1996). Thus, phytodetritus may frequently
settle to the eutrophic equatorial seafloor during the
boreal autumn, and could supply a significant fraction
of the energy requirements of the abyssal benthos. In
the mesotrophic equatorial abyss phytodetritus has not
been observed, suggesting that the flux of particulate
organic carbon may be lower in quality, as well as
quantity, in the mesotrophic regions than in eutrophic
equatorial settings.
As on the California slope, there are faunal components adapted to utilize all the prominent sources
of organic carbon to equatorial Pacific sediments.
Although poorly studied, the megafauna include a well
adapted suite of very mobile, swimming scavengers,
including lyssianisid amphipods, macrourid fish, and
natantian decapods, which rapidly consume fish and
cephalopod bait placed on the seafloor (R. Hessler,
personal communication). Unlike that of the California