8
David THISTLE
gen concentrations of ~0.45 ml °
−1 (Levin and Gage,
1998).
In terms of abundance, standing stocks at some
low-oxygen sites are very low (Sanders, 1969; Levin
et al., 1991), whereas at others they are remarkably
high (Levin et al., 2000). Sites of high abundance
seem to occur where oxygen concentration exceeds
~0.16 ml °
−1 (Levin et al., 2000) and the flux of organic
carbon is high (Sanders, 1969). Where abundances
are high, the number of species that constitute the
fauna tends to be low relative to that at comparable,
high-oxygen sites, suggesting that only a few species
have solved the physiological problems presented by
the low oxygen concentration and that the ecological
reward for those that have is substantial. Interestingly,
the identity of the successful species varies from
site to site, suggesting that adaptation to low oxygen
concentrations has occurred many times. In general,
tolerance of reduced oxygen increases from crustaceans
to molluscs to polychaetes, but some exceptions are
known (Levin et al., 2000).
Meiofauna
1 are also sensitive to reduced oxygen.
In oxygen-minimum zones, the diversity of benthic
foraminiferan faunas tends to be reduced, and most
individuals belong to a small number of species
(Sen Gupta and Machain-Castillo, 1993). Experimental
evidence from shallow water reveals that tolerance
to oxygen stress generally decreases from benthic
copepods to nematodes and soft-shelled foraminifers to
hard-shell foraminifers (Moodley et al., 1997). These
taxon-specific differences in tolerance imply that as
oxygen-stress increases the meiofauna will change in
composition.
Oxygen concentration also varies with depth in
the sediment. Oxygen enters the pore water of deepsea sediments by diffusion and by the activities of
organisms that pump or mix water into the sediment.
Oxygen is consumed by animal and microbial respiration and by chemical reactions in the sediment.
Where the deposition rate of labile organic matter is
relatively high and the oxygen concentration in the
bottom water is low, as in the basins of the California
Continental Borderland, free oxygen disappears within
the first centimeter (Reimers, 1987). Where organicmatter deposition rates are low and the bottom water
is well oxygenated, as beneath the oligotrophic waters
of the central North Pacific, abundant free oxygen is
present several centimeters into the seabed (Reimers,
1987). The depth of oxygen penetration into the
sediment limits the vertical distribution of organisms
that require it, such as most metazoans.
Light
Light intensity decreases exponentially with depth
in the water column because incident photons are
absorbed or scattered. Particles suspended in the water
(sediment particles, phytoplankton cells) increase both
absorption and scattering, but even in the clearest ocean
water no photosynthetically useful light reaches the
sea floor below about 250 m (Fig. 2.5). Therefore,
the deep-sea floor (except the shallowest 50 m) differs
from more familiar ecosystems in that plant primary
production does not occur. Except for hydrothermalvent and cold-seep communities, the food of deep-seafloor organisms must be imported (see Chapter 11).
The paucity of food reaching the deep-sea floor has
profound consequences for the ecology of organisms
living there.
The decrease of light intensity with increasing depth
has other consequences for deep-sea species. For
example, in shallow water most isopods have eyes.
As depth increases, the proportion of isopod species
without eyes increases until, at abyssal depths, eyes
are absent (Hessler and Thistle, 1975; see Thurston
and Bett, 1993, for amphipods). The implication of
this pattern is that vision is of decreasing importance
for some animal groups as depth increases. Its role in
the ecology of these species (in prey location, in mate
location, in movement) must be taken over by other
senses such as chemoreception and mechanoreception.
Also, the blindness suggests that they do not use
bioluminescence, which is important to many animals
of the deep water column (Chapter 3). Demersal fishes
(e.g., Macrouridae) show a parallel pattern. They can
have eyes, even at great depth, but eyes are smaller in
deeper-living species (Marshall, 1979).
Near-bottom flow
In much of the deep sea, the near-bottom water
moves slowly compared to that in shallow-water
environments. Speeds in the bathyal zone tend to be
less than 10 cm s
−1 at 1 m above the bottom, those in
the abyssal zone less than 4 cm s
−1 . Speeds in both
environments vary little from day to day at a location
(Eckman and Thistle, 1991). Because the horizontal
flow speed must decrease to zero at a solid boundary
(Vogel, 1981), the horizonal speeds just above the
seabed will be much less than those 1 m above. These
flows are benign in that they are too slow to erode
sediment or benthic organisms. The flow does move
David THISTLE
gen concentrations of ~0.45 ml °
−1 (Levin and Gage,
1998).
In terms of abundance, standing stocks at some
low-oxygen sites are very low (Sanders, 1969; Levin
et al., 1991), whereas at others they are remarkably
high (Levin et al., 2000). Sites of high abundance
seem to occur where oxygen concentration exceeds
~0.16 ml °
−1 (Levin et al., 2000) and the flux of organic
carbon is high (Sanders, 1969). Where abundances
are high, the number of species that constitute the
fauna tends to be low relative to that at comparable,
high-oxygen sites, suggesting that only a few species
have solved the physiological problems presented by
the low oxygen concentration and that the ecological
reward for those that have is substantial. Interestingly,
the identity of the successful species varies from
site to site, suggesting that adaptation to low oxygen
concentrations has occurred many times. In general,
tolerance of reduced oxygen increases from crustaceans
to molluscs to polychaetes, but some exceptions are
known (Levin et al., 2000).
Meiofauna
1 are also sensitive to reduced oxygen.
In oxygen-minimum zones, the diversity of benthic
foraminiferan faunas tends to be reduced, and most
individuals belong to a small number of species
(Sen Gupta and Machain-Castillo, 1993). Experimental
evidence from shallow water reveals that tolerance
to oxygen stress generally decreases from benthic
copepods to nematodes and soft-shelled foraminifers to
hard-shell foraminifers (Moodley et al., 1997). These
taxon-specific differences in tolerance imply that as
oxygen-stress increases the meiofauna will change in
composition.
Oxygen concentration also varies with depth in
the sediment. Oxygen enters the pore water of deepsea sediments by diffusion and by the activities of
organisms that pump or mix water into the sediment.
Oxygen is consumed by animal and microbial respiration and by chemical reactions in the sediment.
Where the deposition rate of labile organic matter is
relatively high and the oxygen concentration in the
bottom water is low, as in the basins of the California
Continental Borderland, free oxygen disappears within
the first centimeter (Reimers, 1987). Where organicmatter deposition rates are low and the bottom water
is well oxygenated, as beneath the oligotrophic waters
of the central North Pacific, abundant free oxygen is
present several centimeters into the seabed (Reimers,
1987). The depth of oxygen penetration into the
sediment limits the vertical distribution of organisms
that require it, such as most metazoans.
Light
Light intensity decreases exponentially with depth
in the water column because incident photons are
absorbed or scattered. Particles suspended in the water
(sediment particles, phytoplankton cells) increase both
absorption and scattering, but even in the clearest ocean
water no photosynthetically useful light reaches the
sea floor below about 250 m (Fig. 2.5). Therefore,
the deep-sea floor (except the shallowest 50 m) differs
from more familiar ecosystems in that plant primary
production does not occur. Except for hydrothermalvent and cold-seep communities, the food of deep-seafloor organisms must be imported (see Chapter 11).
The paucity of food reaching the deep-sea floor has
profound consequences for the ecology of organisms
living there.
The decrease of light intensity with increasing depth
has other consequences for deep-sea species. For
example, in shallow water most isopods have eyes.
As depth increases, the proportion of isopod species
without eyes increases until, at abyssal depths, eyes
are absent (Hessler and Thistle, 1975; see Thurston
and Bett, 1993, for amphipods). The implication of
this pattern is that vision is of decreasing importance
for some animal groups as depth increases. Its role in
the ecology of these species (in prey location, in mate
location, in movement) must be taken over by other
senses such as chemoreception and mechanoreception.
Also, the blindness suggests that they do not use
bioluminescence, which is important to many animals
of the deep water column (Chapter 3). Demersal fishes
(e.g., Macrouridae) show a parallel pattern. They can
have eyes, even at great depth, but eyes are smaller in
deeper-living species (Marshall, 1979).
Near-bottom flow
In much of the deep sea, the near-bottom water
moves slowly compared to that in shallow-water
environments. Speeds in the bathyal zone tend to be
less than 10 cm s
−1 at 1 m above the bottom, those in
the abyssal zone less than 4 cm s
−1 . Speeds in both
environments vary little from day to day at a location
(Eckman and Thistle, 1991). Because the horizontal
flow speed must decrease to zero at a solid boundary
(Vogel, 1981), the horizonal speeds just above the
seabed will be much less than those 1 m above. These
flows are benign in that they are too slow to erode
sediment or benthic organisms. The flow does move