44
Martin V. ANGEL
decline to a minimum at the permanent thermocline,
and then a slight increase in the deeper water of
the “cold-water sphere”. In upwelling regions (see
pp. 67–68), such as the Arabian Sea and the Eastern
Tropical Pacific, surface productivity is exceptionally
high, so that an unusually high supply of organic carbon
sediments down to subthermocline depths. These high
organic inputs stimulate microbial respiration to such
an extent that virtually all the dissolved oxygen in
the water is used up. In the Eastern Tropical Pacific
there is a permanent zone of oxygen depletion at
depths of 200 to 1000 m (Wishner et al., 1995); in the
Arabian Sea an oxygen-depleted zone develops during
the upwelling season while the south-west monsoon is
blowing. Within these oxygen minimum zones, both
the organic flux regime and the water chemistry are
drastically altered. Most pelagic species depend on
aerobic respiration, they are excluded from the depleted
zones, but a few that can tolerate the low oxygen
conditions flourish. In a few localities, such as the
Black Sea and the Cariaco Trench (off Venezuela),
there is never any oxygen available in the deep water
below 100 to 200 m.
As the hydrostatic pressure increases, the partial
pressures of the dissolved gases decrease and so
their solubilities increase. Thus the energy required to
extract gases from the water – oxygen for respiration,
carbon dioxide for incorporation into skeletal calcium
carbonate, and other gases to inflate swim-bladders –
increases substantially with depth. Moreover, because
gases are so much more compressible than water,
their effectiveness in regulating buoyancy declines with
depth, because their densities increase more rapidly
than that of the surrounding medium. However, the
acoustic characteristics of gases are maintained, so,
even at abyssal depths there are examples of animals
retaining gas bubbles for the detection of sound.
Transmission of light through water
Light is one of the major ecological factors influencing
distributions of organisms in the upper 1000 m. At
depths greater than this its influence wanes, despite the
almost universal use of bioluminescence by pelagic organisms for inter- and intra-specific signalling (Herring
et al., 1990). In the euphotic zone light is one of the key
factors, together with nutrient availability and vertical
stability, that regulate rates of primary production and
hence the supply of organic material to the deepliving communities. Water is translucent rather than
transparent. It selectively absorbs and scatters light of
different wavelengths (Sathyendranath and Platt, 1990).
The red wavelengths (l = 650–700 nm) are most rapidly
absorbed (except when turbidity is very high), and
even in the clearest oceanic waters all red light has
been absorbed once a depth of 30 m has been reached.
Blue-green light (l = 475–485 nm) penetrates to the
greatest depths and is detectable at depths of 1 km
beneath the clearest oligotrophic water that underlies
the subtropical gyres. Pure water scatters light, but this
scattering is greatly enhanced by suspended particles.
Hence, as productivity increases and phytoplankton
becomes more abundant, the more rapidly is light
attenuated with depth. Profiles of light intensity and
the proportional changes in its colour composition
play an important role in determining the zonation of
the communities in the upper kilometre of the ocean,
through determining the optimal strategies for detecting
prey and countering visual predation.
Below 250 m, the brightest light comes from directly
overhead and, regardless of the elevation of the
sun, there is a symmetrical fall-off in intensity with
increasing angle to the vertical, the dimmest light being
backscattered from the depths (Denton, 1970). Many of
the fish species living at depths of 250–600 m during
the day have elaborate arrangements of photophores
(light-producing organs) along their undersides. Their
function appears to be to break up the silhouette of a
fish when it is viewed from directly underneath. Below
depths of 600–700 m, the downwelling light becomes
so dim that the silhouette ceases to be a problem;
then the ventral photophores no longer confer any
advantage, and they become less elaborate or are totally
lost. As the value of visual cues for detecting prey
dwindles with depth, so chemoreception plays an increasingly important role in both intra- and interspecific
communication. Chemical plumes spread more readily
along isopycnals (i.e., surfaces of constant density)
than vertically across them. Many deep-sea organisms
have highly elaborate chemical receptors [see Marshall
(1971) and Bone et al. (1995) for examples in fish],
and probably have specialized behavioural adaptations
linked to the detection of these chemical cues. These
adaptations are also reflected in the morphology of the
brains of many deep-living fish, those regions involved
in chemosensory perception being well developed, but
those involved in vision are reduced.
Animals that produce light (bioluminescence) may
rely on their own luciferin/luciferinase system, or
“culture” luminescent bacteria within their light organs
Martin V. ANGEL
decline to a minimum at the permanent thermocline,
and then a slight increase in the deeper water of
the “cold-water sphere”. In upwelling regions (see
pp. 67–68), such as the Arabian Sea and the Eastern
Tropical Pacific, surface productivity is exceptionally
high, so that an unusually high supply of organic carbon
sediments down to subthermocline depths. These high
organic inputs stimulate microbial respiration to such
an extent that virtually all the dissolved oxygen in
the water is used up. In the Eastern Tropical Pacific
there is a permanent zone of oxygen depletion at
depths of 200 to 1000 m (Wishner et al., 1995); in the
Arabian Sea an oxygen-depleted zone develops during
the upwelling season while the south-west monsoon is
blowing. Within these oxygen minimum zones, both
the organic flux regime and the water chemistry are
drastically altered. Most pelagic species depend on
aerobic respiration, they are excluded from the depleted
zones, but a few that can tolerate the low oxygen
conditions flourish. In a few localities, such as the
Black Sea and the Cariaco Trench (off Venezuela),
there is never any oxygen available in the deep water
below 100 to 200 m.
As the hydrostatic pressure increases, the partial
pressures of the dissolved gases decrease and so
their solubilities increase. Thus the energy required to
extract gases from the water – oxygen for respiration,
carbon dioxide for incorporation into skeletal calcium
carbonate, and other gases to inflate swim-bladders –
increases substantially with depth. Moreover, because
gases are so much more compressible than water,
their effectiveness in regulating buoyancy declines with
depth, because their densities increase more rapidly
than that of the surrounding medium. However, the
acoustic characteristics of gases are maintained, so,
even at abyssal depths there are examples of animals
retaining gas bubbles for the detection of sound.
Transmission of light through water
Light is one of the major ecological factors influencing
distributions of organisms in the upper 1000 m. At
depths greater than this its influence wanes, despite the
almost universal use of bioluminescence by pelagic organisms for inter- and intra-specific signalling (Herring
et al., 1990). In the euphotic zone light is one of the key
factors, together with nutrient availability and vertical
stability, that regulate rates of primary production and
hence the supply of organic material to the deepliving communities. Water is translucent rather than
transparent. It selectively absorbs and scatters light of
different wavelengths (Sathyendranath and Platt, 1990).
The red wavelengths (l = 650–700 nm) are most rapidly
absorbed (except when turbidity is very high), and
even in the clearest oceanic waters all red light has
been absorbed once a depth of 30 m has been reached.
Blue-green light (l = 475–485 nm) penetrates to the
greatest depths and is detectable at depths of 1 km
beneath the clearest oligotrophic water that underlies
the subtropical gyres. Pure water scatters light, but this
scattering is greatly enhanced by suspended particles.
Hence, as productivity increases and phytoplankton
becomes more abundant, the more rapidly is light
attenuated with depth. Profiles of light intensity and
the proportional changes in its colour composition
play an important role in determining the zonation of
the communities in the upper kilometre of the ocean,
through determining the optimal strategies for detecting
prey and countering visual predation.
Below 250 m, the brightest light comes from directly
overhead and, regardless of the elevation of the
sun, there is a symmetrical fall-off in intensity with
increasing angle to the vertical, the dimmest light being
backscattered from the depths (Denton, 1970). Many of
the fish species living at depths of 250–600 m during
the day have elaborate arrangements of photophores
(light-producing organs) along their undersides. Their
function appears to be to break up the silhouette of a
fish when it is viewed from directly underneath. Below
depths of 600–700 m, the downwelling light becomes
so dim that the silhouette ceases to be a problem;
then the ventral photophores no longer confer any
advantage, and they become less elaborate or are totally
lost. As the value of visual cues for detecting prey
dwindles with depth, so chemoreception plays an increasingly important role in both intra- and interspecific
communication. Chemical plumes spread more readily
along isopycnals (i.e., surfaces of constant density)
than vertically across them. Many deep-sea organisms
have highly elaborate chemical receptors [see Marshall
(1971) and Bone et al. (1995) for examples in fish],
and probably have specialized behavioural adaptations
linked to the detection of these chemical cues. These
adaptations are also reflected in the morphology of the
brains of many deep-living fish, those regions involved
in chemosensory perception being well developed, but
those involved in vision are reduced.
Animals that produce light (bioluminescence) may
rely on their own luciferin/luciferinase system, or
“culture” luminescent bacteria within their light organs