The oxygen content of seawater varies between 0 and 8.5 ml/€, mainly within the range 1-6 ml/€. High values occur at the surface, where dissolved oxygen tends to equilibrate with atmospheric oxygen. Rapid photosynthesis may sometimes produce supersaturation. Because oxygen is more soluble in cold water than in warm, the oxygen content of surface water is usually greater at high latitudes than nearer the Equator, and the sinking of cold surface water in polar seas carries oxygen-rich water to the bottom of the deep ocean basins.

Although the deep layers of water are mostly well oxygenated, oxygen is by no means uniformly distributed with depth, and in some areas there is an oxygen-minimum layer at a depth somewhere between 100 and 1000 m. This is most evident in low latitudes, where the water at 100-500 m has sometimes been found to be almost completely lacking in oxygen. The reasons for this are uncertain, but the oxygen-minimum zone often appears to be well populated, and one cause of the deficiency of oxygen may be depletion by a large amount of animal and bacterial respiration in water where relatively little circulation is taking place.

The exceptional conditions in the Black Sea were mentioned earlier. Cut off from the Mediterranean by the shallow water of the Bosphorus, there is little mixing between the low density surface water (see page 111) and the denser, more saline deep water. The deep levels are virtually stagnant and have become completely depleted of oxygen. Animal life is impossible below some 150-200 m, but anaerobic bacteria flourish in the deep layers, mainly sulphur bacteria which metabolize sulphate to sulphide and produce large quantities of H2S, giving the deep water a very objectionable smell. Comparable conditions sometimes arise in other land-locked areas of deficient circulation, such as deep lochs, fiords and lagoons.

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