THE PELAGIC ENVIRONMENT OF THE OPEN OCEAN
43
not react with water generally have low solubilities,
whereas those that do, such as sulphur dioxide, nitrogen
dioxide and ammonia, are highly soluble. Carbon
dioxide reacts relatively slowly with water to form
carbonic acid, and is usually considered to have a low
solubility. However, its solubility is complicated by the
chemical equilibria governing the dynamic balances
between dissolved carbon dioxide and bicarbonate and
carbonate ions. These interactions play an important
ecological role in buffering sea water against changes in
pH, which in sea water generally ranges from 7.7 to 8.2
(Brewer et al., 1995). The higher values occur where
photosynthetic rates are high enough to result in a
significant draw-down in dissolved carbon dioxide (i.e.,
the partial pressure of CO 2 in the surface waters falls
well below that of the atmosphere). These interactions
are also influenced by pressure, because like all gases
carbon dioxide becomes more soluble with increasing
pressure, and so carbonate also becomes more soluble
with increasing depth. Thus, as depth increases, those
animals which use calcium carbonate for their skeletons
have to work progressively harder to secrete and
maintain them.
Exchanges of gases across the sea–surface interface
are rapid, especially in windy conditions when breaking
waves inject suspended bubbles to depths of 10 m or
more. Thus at the surface the partial pressures of the
gases dissolved in the seawater rapidly equilibrate with
those in the atmosphere (e.g., Thorpe, 1995), so that
either the oceans are taking up gases or they are venting
excess dissolved gases into the atmosphere. Since the
partial pressures of gases increase as temperatures rise,
their solubilities decrease – this is why water in a kettle
bubbles long before it starts to boil (one may note
that this is counter to the effect of temperature on the
solubility of salts, which increases with increases in
temperature). Thus, more oxygen and carbon dioxide
can dissolve in the cold surface waters of polar seas
than in warm surface waters of tropical seas. In
consequence, the formation of bottom and deep water
transfers surface waters with high concentrations of
dissolved oxygen and carbon dioxide into the interior of
the ocean. This enables animals and microbes to respire
aerobically almost everywhere in the deep ocean (the
exceptions will be discussed below), and has also been
reducing the rate at which concentrations of carbon
dioxide have been building up in the atmosphere as
a result of the burning of fossil fuels. Even so, it
has been estimated that anthropogenic emissions of
carbon dioxide to the atmosphere have already resulted
in a reduction of pH in the global ocean by 0.1
(Sarmiento et al., 1992), and further reductions are to
be expected. These may begin to influence the depth
of the lysocline (the depth at which calcium carbonate
begins to dissolve) (see Chapter 13). When rates of
primary production are high, as during seasonal bloom
conditions at temperate latitudes, the partial pressure of
carbon dioxide in the surface waters may be lowered
well below that of the atmosphere, so that the ocean
rapidly absorbs more from the atmosphere. Conversely,
when cold deep water, enriched with dissolved carbon
dioxide as a result of the breakdown of organic carbon
by respiration, upwells to the surface, the excess carbon
dioxide is vented back into the atmosphere, even
though primary production may be stimulated by the
high inputs of nutrients (see p. 50).
In the North Atlantic, the water is sinking along the
Polar Front, which is similar in density to the water at
the bottom – Antarctic Bottom Water. This water mass
originates in the Weddell Sea and spreads northwards
from the Southern Ocean along the bottom of the
Atlantic. Being similar in density, these two types of
water can mix freely, and they together form a new and
highly characteristic water mass called North Atlantic
Deep Water (NADW). North Atlantic Deep Water is
important because it is rich in dissolved oxygen and
pervades the deep waters of all the major oceans via
the “Great Conveyor” – the term coined by Broecker
(1992) to describe the general pathway of circulation
throughout the global ocean. Its concentration of
dissolved oxygen decreases as it ages (i.e., as the time
since it was last at the surface increases). In the Atlantic
the deep waters are well flushed with young waters,
which contain high concentrations of dissolved oxygen
(>5.5 ml °
−1 ), but in the several hundred years required
for NADW to reach the Pacific and Indian Oceans its
oxygen content has fallen to <3 ml °
−1 .
Another sub-surface source of oxygen is photosynthesis. As plants combine carbon dioxide and water
in the synthesis of carbohydrates using energy from
the sun, a byproduct of the reaction is oxygen. Thus,
when the rate at which photosynthesis is producing
oxygen exceeds the rate at which it is being used up
by the respiration of the pelagic organisms, oxygen
concentrations build up. In some tropical seas the
partial pressure of the oxygen dissolved in the water
at depths of about 100 m exceeds its partial pressure
in the atmosphere; the water is then described as being
supersaturated with oxygen. However, vertical profiles
of oxygen concentrations generally show a steady
43
not react with water generally have low solubilities,
whereas those that do, such as sulphur dioxide, nitrogen
dioxide and ammonia, are highly soluble. Carbon
dioxide reacts relatively slowly with water to form
carbonic acid, and is usually considered to have a low
solubility. However, its solubility is complicated by the
chemical equilibria governing the dynamic balances
between dissolved carbon dioxide and bicarbonate and
carbonate ions. These interactions play an important
ecological role in buffering sea water against changes in
pH, which in sea water generally ranges from 7.7 to 8.2
(Brewer et al., 1995). The higher values occur where
photosynthetic rates are high enough to result in a
significant draw-down in dissolved carbon dioxide (i.e.,
the partial pressure of CO 2 in the surface waters falls
well below that of the atmosphere). These interactions
are also influenced by pressure, because like all gases
carbon dioxide becomes more soluble with increasing
pressure, and so carbonate also becomes more soluble
with increasing depth. Thus, as depth increases, those
animals which use calcium carbonate for their skeletons
have to work progressively harder to secrete and
maintain them.
Exchanges of gases across the sea–surface interface
are rapid, especially in windy conditions when breaking
waves inject suspended bubbles to depths of 10 m or
more. Thus at the surface the partial pressures of the
gases dissolved in the seawater rapidly equilibrate with
those in the atmosphere (e.g., Thorpe, 1995), so that
either the oceans are taking up gases or they are venting
excess dissolved gases into the atmosphere. Since the
partial pressures of gases increase as temperatures rise,
their solubilities decrease – this is why water in a kettle
bubbles long before it starts to boil (one may note
that this is counter to the effect of temperature on the
solubility of salts, which increases with increases in
temperature). Thus, more oxygen and carbon dioxide
can dissolve in the cold surface waters of polar seas
than in warm surface waters of tropical seas. In
consequence, the formation of bottom and deep water
transfers surface waters with high concentrations of
dissolved oxygen and carbon dioxide into the interior of
the ocean. This enables animals and microbes to respire
aerobically almost everywhere in the deep ocean (the
exceptions will be discussed below), and has also been
reducing the rate at which concentrations of carbon
dioxide have been building up in the atmosphere as
a result of the burning of fossil fuels. Even so, it
has been estimated that anthropogenic emissions of
carbon dioxide to the atmosphere have already resulted
in a reduction of pH in the global ocean by 0.1
(Sarmiento et al., 1992), and further reductions are to
be expected. These may begin to influence the depth
of the lysocline (the depth at which calcium carbonate
begins to dissolve) (see Chapter 13). When rates of
primary production are high, as during seasonal bloom
conditions at temperate latitudes, the partial pressure of
carbon dioxide in the surface waters may be lowered
well below that of the atmosphere, so that the ocean
rapidly absorbs more from the atmosphere. Conversely,
when cold deep water, enriched with dissolved carbon
dioxide as a result of the breakdown of organic carbon
by respiration, upwells to the surface, the excess carbon
dioxide is vented back into the atmosphere, even
though primary production may be stimulated by the
high inputs of nutrients (see p. 50).
In the North Atlantic, the water is sinking along the
Polar Front, which is similar in density to the water at
the bottom – Antarctic Bottom Water. This water mass
originates in the Weddell Sea and spreads northwards
from the Southern Ocean along the bottom of the
Atlantic. Being similar in density, these two types of
water can mix freely, and they together form a new and
highly characteristic water mass called North Atlantic
Deep Water (NADW). North Atlantic Deep Water is
important because it is rich in dissolved oxygen and
pervades the deep waters of all the major oceans via
the “Great Conveyor” – the term coined by Broecker
(1992) to describe the general pathway of circulation
throughout the global ocean. Its concentration of
dissolved oxygen decreases as it ages (i.e., as the time
since it was last at the surface increases). In the Atlantic
the deep waters are well flushed with young waters,
which contain high concentrations of dissolved oxygen
(>5.5 ml °
−1 ), but in the several hundred years required
for NADW to reach the Pacific and Indian Oceans its
oxygen content has fallen to <3 ml °
−1 .
Another sub-surface source of oxygen is photosynthesis. As plants combine carbon dioxide and water
in the synthesis of carbohydrates using energy from
the sun, a byproduct of the reaction is oxygen. Thus,
when the rate at which photosynthesis is producing
oxygen exceeds the rate at which it is being used up
by the respiration of the pelagic organisms, oxygen
concentrations build up. In some tropical seas the
partial pressure of the oxygen dissolved in the water
at depths of about 100 m exceeds its partial pressure
in the atmosphere; the water is then described as being
supersaturated with oxygen. However, vertical profiles
of oxygen concentrations generally show a steady