THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
41
to the rate at which it is being supplied or removed)
can vary considerably. The mean ocean mixing time
is of the order of 500 years (Stuiver et al., 1983),
so any substance whose turn-over takes 10
5 years or
more is uniformly distributed, except very close to
major point sources or sinks. However, there are many
substances whose concentrations fluctuate widely, notably those that play a role in biological systems –
oxygen, carbon, and plant nutrients. Many higher
organisms regulate their internal ionic concentrations;
for example, potassium and calcium are concentrated
in the blood of fish, whereas sodium is excreted.
Some of the rarer elements are essential in trace
concentrations; for example, vanadium is a constituent
of the oxygen-carrying compound in the blood of
salps, and copper is a constituent of haemocyanin,
the blood pigment in crustaceans. However, elements
that are essential in trace concentrations become toxic
if their concentrations rise too much in the ambient
seawater. Emissions from hydrothermal vents (Parson
et al., 1995) result in the occurrence of high local
concentrations of heavy metals such as manganese,
cobalt and mercury. To survive, the organisms that
inhabit the vicinity of vents must either avoid the
discharge plumes, or be capable of tolerating the
presence of the heavy metals in some way. Some
detoxify them and store them; others control the
concentrations in their blood and tissues by ionic
regulation and excretion. Hydrothermal vents may have
played and still be playing a significant evolutionary
role by creating chemical variability in the otherwise
monotonous pelagic environment of the deep ocean.
The density of sea water
The density of sea water plays a key, but indirect,
role in ecological processes, through its control of the
stability of the upper water column and its contribution to ocean circulation. Three factors, temperature,
salinity and hydrostatic pressure, determine its density.
Generally water density increases with depth. However,
wherever and whenever the density is uniform with
depth, the water readily mixes vertically. However, only
quite small increases in density can inhibit vertical
mixing and the water becomes stratified. Typically in
the upper few tens of metres and also close to the
sea floor, the density of the water is almost always
uniform. Near the surface it is the action of the
winds in the atmosphere that keeps the upper part
of the “water column” well mixed and uniform. This
layer of uniform water is called the wind-mixed layer.
The other uniform layer immediately overlying the
interface between the water and the seabed, which
is again created by frictional forces but this time
between the water currents and the seabed, is termed
the “benthic boundary layer”. This layer is important
to those pelagic animal species that live close to the
seabed in deep water, but it is even more important
to the ecology of the benthic organisms that inhabit
the sea floor. Between these two extremes, quite
small differences in density between contiguous layers
of water inhibit vertical mixing and the water is
horizontally layered. This layering is so stable that
even quite large perturbations (caused by tides or the
seamounts) may not disrupt it, but instead generate
internal waves. Internal waves are analogous to the
waves at the surface, but they can have amplitudes
of tens or even hundreds of metres, and wave lengths
(i.e., the distance from crest to crest) that are very
long. Under the right conditions they can break,
just as surface waves approaching a shoreline break.
Generally horizontal gradients in oceanic waters are
extremely weak, whereas vertical gradients are often
steep; an animal migrating 50 m vertically across the
thermocline may experience a greater change in water
temperature than if it swims 1000 km horizontally.
Thus, patterns of distribution (of both physicochemical
and biological characteristics) within the water column
are often clearly defined vertically, but weakly defined
horizontally.
At the base of the wind-mixed layer there is often a
sharp increase in density, which is usually associated
with a rapid decline in water temperature. This zone of
transition is described as the “seasonal thermocline”,
and is a very important ecological feature (see p. 49).
In the tropical and sub-tropical zones the seasonal
thermocline is present throughout the year except
in regions where upwelling occurs (see pp. 67–68).
In temperate to tropical latitudes there is another
deeper temperature discontinuity, which is described as
the permanent thermocline. This marks the transition
between the warm waters and the cool deep waters,
and this transition outcrops at the surface along the
Polar Fronts at high latitudes. Thus the oceans can be
regarded as comprising a “cold-water sphere” which
extends from the polar seas and across low latitudes
at depths of c. 800 to 1000 m, and a “warm-water
sphere” that fills the shallower depths at low and middle
latitudes.
Hydrostatic pressure is almost entirely a function
41
to the rate at which it is being supplied or removed)
can vary considerably. The mean ocean mixing time
is of the order of 500 years (Stuiver et al., 1983),
so any substance whose turn-over takes 10
5 years or
more is uniformly distributed, except very close to
major point sources or sinks. However, there are many
substances whose concentrations fluctuate widely, notably those that play a role in biological systems –
oxygen, carbon, and plant nutrients. Many higher
organisms regulate their internal ionic concentrations;
for example, potassium and calcium are concentrated
in the blood of fish, whereas sodium is excreted.
Some of the rarer elements are essential in trace
concentrations; for example, vanadium is a constituent
of the oxygen-carrying compound in the blood of
salps, and copper is a constituent of haemocyanin,
the blood pigment in crustaceans. However, elements
that are essential in trace concentrations become toxic
if their concentrations rise too much in the ambient
seawater. Emissions from hydrothermal vents (Parson
et al., 1995) result in the occurrence of high local
concentrations of heavy metals such as manganese,
cobalt and mercury. To survive, the organisms that
inhabit the vicinity of vents must either avoid the
discharge plumes, or be capable of tolerating the
presence of the heavy metals in some way. Some
detoxify them and store them; others control the
concentrations in their blood and tissues by ionic
regulation and excretion. Hydrothermal vents may have
played and still be playing a significant evolutionary
role by creating chemical variability in the otherwise
monotonous pelagic environment of the deep ocean.
The density of sea water
The density of sea water plays a key, but indirect,
role in ecological processes, through its control of the
stability of the upper water column and its contribution to ocean circulation. Three factors, temperature,
salinity and hydrostatic pressure, determine its density.
Generally water density increases with depth. However,
wherever and whenever the density is uniform with
depth, the water readily mixes vertically. However, only
quite small increases in density can inhibit vertical
mixing and the water becomes stratified. Typically in
the upper few tens of metres and also close to the
sea floor, the density of the water is almost always
uniform. Near the surface it is the action of the
winds in the atmosphere that keeps the upper part
of the “water column” well mixed and uniform. This
layer of uniform water is called the wind-mixed layer.
The other uniform layer immediately overlying the
interface between the water and the seabed, which
is again created by frictional forces but this time
between the water currents and the seabed, is termed
the “benthic boundary layer”. This layer is important
to those pelagic animal species that live close to the
seabed in deep water, but it is even more important
to the ecology of the benthic organisms that inhabit
the sea floor. Between these two extremes, quite
small differences in density between contiguous layers
of water inhibit vertical mixing and the water is
horizontally layered. This layering is so stable that
even quite large perturbations (caused by tides or the
seamounts) may not disrupt it, but instead generate
internal waves. Internal waves are analogous to the
waves at the surface, but they can have amplitudes
of tens or even hundreds of metres, and wave lengths
(i.e., the distance from crest to crest) that are very
long. Under the right conditions they can break,
just as surface waves approaching a shoreline break.
Generally horizontal gradients in oceanic waters are
extremely weak, whereas vertical gradients are often
steep; an animal migrating 50 m vertically across the
thermocline may experience a greater change in water
temperature than if it swims 1000 km horizontally.
Thus, patterns of distribution (of both physicochemical
and biological characteristics) within the water column
are often clearly defined vertically, but weakly defined
horizontally.
At the base of the wind-mixed layer there is often a
sharp increase in density, which is usually associated
with a rapid decline in water temperature. This zone of
transition is described as the “seasonal thermocline”,
and is a very important ecological feature (see p. 49).
In the tropical and sub-tropical zones the seasonal
thermocline is present throughout the year except
in regions where upwelling occurs (see pp. 67–68).
In temperate to tropical latitudes there is another
deeper temperature discontinuity, which is described as
the permanent thermocline. This marks the transition
between the warm waters and the cool deep waters,
and this transition outcrops at the surface along the
Polar Fronts at high latitudes. Thus the oceans can be
regarded as comprising a “cold-water sphere” which
extends from the polar seas and across low latitudes
at depths of c. 800 to 1000 m, and a “warm-water
sphere” that fills the shallower depths at low and middle
latitudes.
Hydrostatic pressure is almost entirely a function