THE DEEP PACIFIC OCEAN FLOOR
199
Gupta and Machain-Castillo, 1993). On the Peru–
Chile margin, the biomass of benthic invertebrates
and of demersal-fish is relatively high near the upper
and lower boundaries of the oxygen-minimum zone
(i.e., at oxygen concentrations >0.6 ml °
−1 ); at lower
oxygen concentrations, the macrobenthos is dominated
by polychaetes, nematodes, and bivalves (Arntz et al.,
1991). In addition, dense mats of sulfur-oxidizing
bacteria (e.g., Thioploca) may co-occur with the Peru–
Chile macrobenthos at oxygen concentrations below
0.2 ml °
−1 (Arntz et al., 1991). Interestingly, the macrobenthos within persistent oxygen-minimum zones on
continental slopes is more resistant to oxygen stress
than is the fauna of continental shelves exposed to
periodic hypoxia. For example, high standing crops of
macrobenthos, especially polychaetes, occur at oxygen
levels as low as 0.11 ml °
−1 within oxygen-minimum
zones (Levin et al., 1991b), whereas on continental
shelves mass faunal mortality often occurs if the
oxygen concentration of the bottom water drops below
~1.0 ml °
−1 (Diaz and Rosenberg, 1995). The relative
stability of gradients in the oxygen-minimum zone,
combined with a persistent availability of labile organic
material on the seafloor, apparently allows a welladapted opportunistic community to thrive, and perhaps
to have evolved, at the boundaries of oxygen-minimum
zones (Levin et al., 1994; Diaz and Rosenberg, 1995).
Rates of ecologically important processes within
oxygen-minimum zones in the eastern Pacific have not
been well studied. Off Point Sur on the California
margin, total rates of sediment-community respiration
(i.e., organic-carbon mineralization) at the core of
the oxygen-minimum zone do not differ markedly
from those at deeper stations (Fig. 6.9). As expected,
sulfate reduction is quantitatively more important in
the oxygen-minimum zone than deeper on the slope,
but still accounts for less than 25% of total organiccarbon mineralization (Fig. 6.9). Bioturbation rates
and depths within oxygen-minimum zones have not
been well quantified with radio-isotopic measurements
(e.g., excess
210 Pb profiles) in the eastern Pacific;
however, some qualitative bioturbation patterns are
evident. Below oxygen concentrations of 0.1 ml °
−1
in the bottom water, the bioturbating macro- and
megabenthos may be excluded, yielding laminated
(i.e., unmixed) sediments (Savrda and Bottjer, 1991).
At concentrations between 0.1 and 0.5 ml °
−1 , Savrda
and Bottjer (1991) hypothesized that the rates and
depths of bioturbation increase with increasing oxygen concentration, as larger-bodied, deeper-burrowing
species enter the community. The only data to test
this hypothesis come from the oxygen-minimum zone
in the Arabian Sea, which suggest that the depth
of bioturbation increases as oxygen concentrations
rise from 0.1 to 0.3 ml °
−1 or more, but that the
intensity of mixing (as indicated by eddy-diffusion
coefficients) within the bioturbated layer does not
change substantially with oxygen (Smith et al., 2000).
Because these hypotheses are used in reconstructions
of oxygenation patterns in paleo-environments (Savrda
and Bottjer, 1991), it would be very useful to test
the quantitative relationships between oxygen and
bioturbation depths and rates on the California and
Peru–Chile margins.
Oxygen minimum zones may have played an important role in generation of the high species diversity found in bathyal deep-sea habitats (Jumars
and Gallagher, 1982; Grassle and Maciolek, 1992).
Intense oxygen-minimum zones, such as occur in the
eastern tropical Pacific and on the Peru–Chile margin,
impose barriers to gene flow between populations
above and below this zone, potentially facilitating
speciation in otherwise relatively homogeneous deepsea water masses (Rogers, 2000). Over geologic time,
oxygen-minimum zones have expanded and contracted,
periodically isolating populations in slope and basin
habitats on continental margins, and on islands and
seamounts (Kennett, 1982; Rogers, 2000); this too is
likely to have stimulated allopatric speciation. Finally,
the steep gradients in oxygen concentrations and labile
organic matter found at the lower boundaries of some
oxygen-minimum zones (Levin et al., 1991b; Arntz
et al., 1991) undoubtedly yield strong gradients in
selective pressure for particular life histories, optimal
growth rates, and types of species interactions within
the benthos (Levin et al., 1991c, 1994); such selective
gradients are likely to yield enhanced rates of speciation near the lower boundaries of oxygen-minimum
zones (Rogers, 2000).
The abyssal equatorial Pacific
Surface waters in the equatorial Pacific sustain relatively high primary production as a result of upwelling
of nutrients (in particular nitrate and iron) along the
equatorial divergence (Berger, 1989; Murray et al.,
1994; Landry et al., 1997). The enhanced productivity
is most intense in the eastern Pacific, where equatorial
upwelling and eddies combine to increase nutrient flux
over a broad latitutidinal band; for example, primary
199
Gupta and Machain-Castillo, 1993). On the Peru–
Chile margin, the biomass of benthic invertebrates
and of demersal-fish is relatively high near the upper
and lower boundaries of the oxygen-minimum zone
(i.e., at oxygen concentrations >0.6 ml °
−1 ); at lower
oxygen concentrations, the macrobenthos is dominated
by polychaetes, nematodes, and bivalves (Arntz et al.,
1991). In addition, dense mats of sulfur-oxidizing
bacteria (e.g., Thioploca) may co-occur with the Peru–
Chile macrobenthos at oxygen concentrations below
0.2 ml °
−1 (Arntz et al., 1991). Interestingly, the macrobenthos within persistent oxygen-minimum zones on
continental slopes is more resistant to oxygen stress
than is the fauna of continental shelves exposed to
periodic hypoxia. For example, high standing crops of
macrobenthos, especially polychaetes, occur at oxygen
levels as low as 0.11 ml °
−1 within oxygen-minimum
zones (Levin et al., 1991b), whereas on continental
shelves mass faunal mortality often occurs if the
oxygen concentration of the bottom water drops below
~1.0 ml °
−1 (Diaz and Rosenberg, 1995). The relative
stability of gradients in the oxygen-minimum zone,
combined with a persistent availability of labile organic
material on the seafloor, apparently allows a welladapted opportunistic community to thrive, and perhaps
to have evolved, at the boundaries of oxygen-minimum
zones (Levin et al., 1994; Diaz and Rosenberg, 1995).
Rates of ecologically important processes within
oxygen-minimum zones in the eastern Pacific have not
been well studied. Off Point Sur on the California
margin, total rates of sediment-community respiration
(i.e., organic-carbon mineralization) at the core of
the oxygen-minimum zone do not differ markedly
from those at deeper stations (Fig. 6.9). As expected,
sulfate reduction is quantitatively more important in
the oxygen-minimum zone than deeper on the slope,
but still accounts for less than 25% of total organiccarbon mineralization (Fig. 6.9). Bioturbation rates
and depths within oxygen-minimum zones have not
been well quantified with radio-isotopic measurements
(e.g., excess
210 Pb profiles) in the eastern Pacific;
however, some qualitative bioturbation patterns are
evident. Below oxygen concentrations of 0.1 ml °
−1
in the bottom water, the bioturbating macro- and
megabenthos may be excluded, yielding laminated
(i.e., unmixed) sediments (Savrda and Bottjer, 1991).
At concentrations between 0.1 and 0.5 ml °
−1 , Savrda
and Bottjer (1991) hypothesized that the rates and
depths of bioturbation increase with increasing oxygen concentration, as larger-bodied, deeper-burrowing
species enter the community. The only data to test
this hypothesis come from the oxygen-minimum zone
in the Arabian Sea, which suggest that the depth
of bioturbation increases as oxygen concentrations
rise from 0.1 to 0.3 ml °
−1 or more, but that the
intensity of mixing (as indicated by eddy-diffusion
coefficients) within the bioturbated layer does not
change substantially with oxygen (Smith et al., 2000).
Because these hypotheses are used in reconstructions
of oxygenation patterns in paleo-environments (Savrda
and Bottjer, 1991), it would be very useful to test
the quantitative relationships between oxygen and
bioturbation depths and rates on the California and
Peru–Chile margins.
Oxygen minimum zones may have played an important role in generation of the high species diversity found in bathyal deep-sea habitats (Jumars
and Gallagher, 1982; Grassle and Maciolek, 1992).
Intense oxygen-minimum zones, such as occur in the
eastern tropical Pacific and on the Peru–Chile margin,
impose barriers to gene flow between populations
above and below this zone, potentially facilitating
speciation in otherwise relatively homogeneous deepsea water masses (Rogers, 2000). Over geologic time,
oxygen-minimum zones have expanded and contracted,
periodically isolating populations in slope and basin
habitats on continental margins, and on islands and
seamounts (Kennett, 1982; Rogers, 2000); this too is
likely to have stimulated allopatric speciation. Finally,
the steep gradients in oxygen concentrations and labile
organic matter found at the lower boundaries of some
oxygen-minimum zones (Levin et al., 1991b; Arntz
et al., 1991) undoubtedly yield strong gradients in
selective pressure for particular life histories, optimal
growth rates, and types of species interactions within
the benthos (Levin et al., 1991c, 1994); such selective
gradients are likely to yield enhanced rates of speciation near the lower boundaries of oxygen-minimum
zones (Rogers, 2000).
The abyssal equatorial Pacific
Surface waters in the equatorial Pacific sustain relatively high primary production as a result of upwelling
of nutrients (in particular nitrate and iron) along the
equatorial divergence (Berger, 1989; Murray et al.,
1994; Landry et al., 1997). The enhanced productivity
is most intense in the eastern Pacific, where equatorial
upwelling and eddies combine to increase nutrient flux
over a broad latitutidinal band; for example, primary