The specific gravity of seawater varies with temperature and salinity and very slightly with pressure. At 20oC and atmospheric pressure, seawater of salinity 35%o has a specific gravity of 1.026.
At salinities above 24.7%o the temperature of maximum density lies below the freezing point. Because of this, cooling of the sea's surface to freezing point does not produce a surface layer of light water, as it does in fresh water where the temperature of maximum density is 40C. In contrast to fresh water, surface cooling of the sea below 4oC therefore causes the density of the surface water to increase, and convectional mixing to continue, right up to the point at which ice crystals begin to separate. Consequently, there is no formation of a winter thermocline in the sea as occurs in freshwater ponds and lakes.
At sea level the atmosphere presses down with a force of approximately 1 kg for every square centimetre of the Earth's surface (14.7 pounds per square inch). In simple terms this is 1 atmosphere or approximately 1 bar (1 atm = 1.013 bar). A more accurate measure is the pascal and 100 00 pascals (Pa) is equal to 1 bar. Hydrostatic pressure increases by approximately 1 atm per 10 m increase in depth. In the deepest ocean trenches, pressures exceed 1100 atm.
Although water is only very slightly compressible, such enormous pressures are
sufficient to produce a slight adiabatic compression of the deep water, resulting in a detectable increase in temperature. In some areas, temperature readings taken from the 4000 m level down to 10 000 m show a rise of about 1°C (Figure 4.8). The water column is not unstable because this rise of temperature at the lower levels is the result of compression and there is no decrease of density. Deep water temperatures are often expressed as potential temperature, 9, i.e. the temperature to which the water would come if brought to atmospheric pressure without heat change. Increase of pressure also reduces the pH of seawater through its effect on the dissociation of bicarbonate (see page 115).
Although marine creatures are found at all depths, each species is restricted in the range of levels it inhabits. It is usually difficult to know to what extent this limitation is due to pressure because the associated gradients of temperature and illumination are probably in many cases the dominant factors regulating distribution in depth. However, the pressure gradient must certainly play some part. Organisms which live in the surface layers of the sea can be killed by subjecting them to the very high pressure of abyssal depths. Alternatively, organisms brought up from the deep-sea bottom require a high pressure, comparable to that of their usual environment, for normal activity. Evidently marine creatures are adapted to suit particular pressure ranges.
The physiological effects of pressure are not well understood but various responses to experimental changes in pressure have been observed (Knight-Jones and Morgan, 1966; Newell, 1976; Rice, 1964). It has a direct influence on several aspects of cell chemistry including weak acid and base dissociations, ionic interactions, protein structures and it affects the rates of enzymatic catalysis in deep-sea organisms (Somero et al., 1983). Pressure changes influence metabolic rates and oxygen consumption, and are observed to produce alterations of behaviour in certain marine organisms. Experiments on small pelagic animals indicate that pressure increase commonly causes an increase of swimming activity, and movement upwards towards light. In natural conditions the pressure gradient may therefore be one factor which helps to prevent surface forms from descending too deep. Some shallow water species are remarkably sensitive to pressure variations, in some cases, e.g. the amphipod Corophium, responding to changes as small as 200-500 Pa. It is uncertain how organisms without gas cavities can detect pressure change. Possibly the compressibility of their tissues differs appreciably from that of seawater.
Many species are fairly limited in vertical distribution, but some are eurybathic forms found over a great range of depths. Examples from the British fauna which extend from the continental shelf to below 2000 m, listed by Ekman (1953), include the polychaetes Lumbriconereis impatiens, Notomastus latericeus and Hydroides norvegica, to at least 3000 m; the keel worm Pomatoceros triqueter, 5-3000 m; the polychaete Amphicteis gunneri, 20-5000 m; the cumaceans Diastylis laevis 9-2820 m; and Eudorella truncatula, 9-2820 m; the starfish Henricia sanguinolenta, 0-2450 m; the brittlestar Ophiopholis aculeata, 02040 m. An exceptional example of extensive depth distribution is reported for the pogonophore Siboglinum caulleryi, ranging from 22 m to over 8000 m.
Most of the organisms which are found near the surface of the sea seem to have a more restricted depth range than those which inhabit deep levels. The pressure gradient may be partly accountable because the greatest relative changes of pressure with depth occur close to the surface. Between the surface and 10 m the pressure doubles, and on descending a further 10 m the pressure increases by a further 50 per cent. An organism which changes its depth between the surface and 20 m experiences the same relative pressure change as one which moves between 2000 and 6000 m.
Earlier we mentioned the dangers of breathing air beneath the sea's surface (see page 81). Air-breathing aquatic vertebrates such as seals and whales face some risk of gas embolism when surfacing from deep dives, but the danger is far less than for human divers because the animals are not continually breathing air under pressure while diving. When they dive they take down relatively little air, the lungs being only partially inflated. Water pressure tends to collapse the lungs, driving the air into the trachea from which little absorption takes place. There is consequently not much gas dissolved under pressure in the blood. These animals contain large quantities of myoglobin from which they draw oxygen during submergence. They can accumulate a considerable oxygen debt and can tolerate a high level of CO2 in the blood, the threshold sensitivity of the respiratory centre in the brain being higher than in terrestrial mammals. In seals the heart rate is generally reduced during diving, the peripheral circulation is shut down and the blood flow mainly restricted to supplying the brain (Hempleman and Lockwood, 1978). Little is known of the movements of seals and whales under water. It is thought that seals do not often dive very deeply, probably seldom much over 30 m, although a depth of 600 m has been recorded for a Weddell seal. Some whales go even deeper than this, with sperm whales known to descend to over 1000 m and occasionally even to over 2000 m (Clarke, 1978).
A distinctive feature of certain species living on the deepest parts of the sea floor in the ocean trenches is the exceptionally large size they attain compared with closely related forms in shallow water. This phenomenon of 'gigantism' is specially marked in some groups of small crustacea, notably amphipods and isopods, which seldom exceed lengths of 2-3 cm in shallow and middle depths but grow to 8-10 cm or more in the hadal zones. The explanation of these unusually large sizes is uncertain. The water temperature in ocean trenches is not much lower than on shallower parts of the abyssal floor, so it is possible that gigantism is an effect of pressure on metabolism, perhaps associated with a longer growing period and delayed maturation. Also, because deep-sea sediments are more radioactive than near-shore deposits, the mutation rate may be greater at hadal depths resulting in a higher rate of speciation.
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