40
Martin V. ANGEL
consideration needs to be given to the exceptional
characteristics of water as a medium.
THE NATURE OF WATER
Water is a compound with many unique physicochemical properties. Its boiling and freezing points
are considerably higher than might be predicted from
the characteristics of other oxides of elements of low
molecular weight, which lie close to hydrogen in the
periodic table. Carbon dioxide, for example, is a gas
with a much lower boiling point than that of water.
Moreover its solid phase, or dry ice, is heavier than
its liquid phase. The unusual properties of liquid
water result from the molecules loosely associating into
groups of five. This effectively increases its molecular
weight fivefold, which greatly modifies its physical
characteristics. Ice, the solid phase of water, is equally
unusual, in that it floats; its lower density results from
the “looser” packing of the molecules within the solid
crystal structure.
Water has also an unusually high heat capacity. This
serves to dampen the impact of variations in heating
and cooling, and enables ocean currents to transfer
copious amounts of heat from low latitudes to high
latitudes; hence, they play a major role in modifying
global climate. Its latent heats are the highest known
for any liquid. The latent heat of ice formation is
80 calories per gram, so that when 1 g of ice melts it
lowers the temperature of 80 g of liquid water by 1ºC.
Even more important from an oceanographic point of
view: the latent heat of evaporation is 537 calories
per gram. Thus, when water evaporates from the sea’s
surface, it cools the skin temperature considerably.
Both ice formation and evaporative losses are very
important processes in the determination of “water
mass” characteristics (see below).
Chemically, water is amphoteric (i.e. it can function
either as a base or as an acid). Thus, although
pure water has a neutral pH (i.e. pH 7), when it
dissociates it can equally well act either as a provider
of hydrogen or of hydroxyl ions. Hence it is a
good solvent for many inorganic compounds, and
for those organic compounds with radicals that can
dissociate. Organic compounds without such radicals
tend to be insoluble in water, and these include
compounds that are important structural components
of cells and membranes. Since water is the major
chemical constituent of the internal and external milieu
of organisms at all levels of organization – from
cells to tissues and whole bodies – an understanding
of its physicochemical characteristics (Dorsey, 1940)
is fundamental to understanding the physiology and
ecology of living organisms and communities.
Chemical constituents of ocean water
Nearly all 92 of the naturally occurring elements
have been detected dissolved in sea water, although
the majority of them occur only in trace amounts
(Open University, 1989). “Salinity” is the term used
to quantify the total amounts of dissolved salts in
seawater. In most oceanic water it ranges between 33
and 37, averaging about 35
1 . Most constituents of
seawater are conservative, so that their concentrations
vary in direct proportion to the salinity. This is
particularly true for the major constituents (i.e. those
occurring at concentrations >1 ppm) which together
contribute 99.9% of the salinity value (Table 3.2). Most
Table 3.2
Mean concentrations and estimated total content of the major
constituents of sea-water
Element
Mean conc. g. ° −1
Total in 10 12 tonnes
Chlorine
1.95×10 4
2.57×10 4
Sodium
1.08×10 4
1.42×10 4
Magnesium
1.29×10 3
1.71×10 3
Sulphur
905
1.20×10 3
Calcium
412
545
Potassium
380
502
Bromine
67
88.6
Carbon
28
37
Nitrogen 1
11.5
15
Strontium
8
10.6
Oxygen 1
6
7.93
Boron
4.4
5.82
Silicon
2
2.64
Fluorine
1.3
1.72
1 Elemental oxygen and nitrogen are not normally considered to be
major constituents of sea-water because they are dissolved gases.
are in steady state – that is, their chemical budgets
are in balance, although their turnover times (i.e., the
ratio of the total mass of the substance in the oceans
1 See Glossary, p. 477, for an explanation of the way in which salinity is expressed.
Martin V. ANGEL
consideration needs to be given to the exceptional
characteristics of water as a medium.
THE NATURE OF WATER
Water is a compound with many unique physicochemical properties. Its boiling and freezing points
are considerably higher than might be predicted from
the characteristics of other oxides of elements of low
molecular weight, which lie close to hydrogen in the
periodic table. Carbon dioxide, for example, is a gas
with a much lower boiling point than that of water.
Moreover its solid phase, or dry ice, is heavier than
its liquid phase. The unusual properties of liquid
water result from the molecules loosely associating into
groups of five. This effectively increases its molecular
weight fivefold, which greatly modifies its physical
characteristics. Ice, the solid phase of water, is equally
unusual, in that it floats; its lower density results from
the “looser” packing of the molecules within the solid
crystal structure.
Water has also an unusually high heat capacity. This
serves to dampen the impact of variations in heating
and cooling, and enables ocean currents to transfer
copious amounts of heat from low latitudes to high
latitudes; hence, they play a major role in modifying
global climate. Its latent heats are the highest known
for any liquid. The latent heat of ice formation is
80 calories per gram, so that when 1 g of ice melts it
lowers the temperature of 80 g of liquid water by 1ºC.
Even more important from an oceanographic point of
view: the latent heat of evaporation is 537 calories
per gram. Thus, when water evaporates from the sea’s
surface, it cools the skin temperature considerably.
Both ice formation and evaporative losses are very
important processes in the determination of “water
mass” characteristics (see below).
Chemically, water is amphoteric (i.e. it can function
either as a base or as an acid). Thus, although
pure water has a neutral pH (i.e. pH 7), when it
dissociates it can equally well act either as a provider
of hydrogen or of hydroxyl ions. Hence it is a
good solvent for many inorganic compounds, and
for those organic compounds with radicals that can
dissociate. Organic compounds without such radicals
tend to be insoluble in water, and these include
compounds that are important structural components
of cells and membranes. Since water is the major
chemical constituent of the internal and external milieu
of organisms at all levels of organization – from
cells to tissues and whole bodies – an understanding
of its physicochemical characteristics (Dorsey, 1940)
is fundamental to understanding the physiology and
ecology of living organisms and communities.
Chemical constituents of ocean water
Nearly all 92 of the naturally occurring elements
have been detected dissolved in sea water, although
the majority of them occur only in trace amounts
(Open University, 1989). “Salinity” is the term used
to quantify the total amounts of dissolved salts in
seawater. In most oceanic water it ranges between 33
and 37, averaging about 35
1 . Most constituents of
seawater are conservative, so that their concentrations
vary in direct proportion to the salinity. This is
particularly true for the major constituents (i.e. those
occurring at concentrations >1 ppm) which together
contribute 99.9% of the salinity value (Table 3.2). Most
Table 3.2
Mean concentrations and estimated total content of the major
constituents of sea-water
Element
Mean conc. g. ° −1
Total in 10 12 tonnes
Chlorine
1.95×10 4
2.57×10 4
Sodium
1.08×10 4
1.42×10 4
Magnesium
1.29×10 3
1.71×10 3
Sulphur
905
1.20×10 3
Calcium
412
545
Potassium
380
502
Bromine
67
88.6
Carbon
28
37
Nitrogen 1
11.5
15
Strontium
8
10.6
Oxygen 1
6
7.93
Boron
4.4
5.82
Silicon
2
2.64
Fluorine
1.3
1.72
1 Elemental oxygen and nitrogen are not normally considered to be
major constituents of sea-water because they are dissolved gases.
are in steady state – that is, their chemical budgets
are in balance, although their turnover times (i.e., the
ratio of the total mass of the substance in the oceans
1 See Glossary, p. 477, for an explanation of the way in which salinity is expressed.