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Oxygen is introduced into oceanic waters (and all other waters) by diffusion from the atmosphere, aided by wind‐induced turbulence and mixing, and sometimes supplemented to a small degree by photosynthetically produced oxygen. Its solubility in water is an inverse function of salinity and temperature. Once a water mass has left the surface, its dissolved oxygen is consumed through time, mainly by microorganisms but also by larger species such as fishes and crustaceans. As a consequence, oxygen content is an indication of how long the water has been away from the surface. It is a nonconservative property of a water mass that is sometimes used as a tracer. Water masses vary substantially in their time away from the surface. At the extremes, AABW in the Pacific retains its character for 1600 years (Garrison 2002), whereas the residence time for most deep water is 200–300 years. NADW takes about 1000 years to reach the surface after sinking at the northern end of the Great Ocean Conveyor.

Regions of low oxygen concentration are termed oxygen minima, and they vary widely in their severity. If an oceanic region includes a highly productive surface layer with large seasonal algae blooms, the bacterial oxidation of sinking organic matter may remove nearly all the oxygen from the deeper waters. Examples of such areas include the waters off the California coast, the Eastern Tropical Pacific, and the Arabian Sea. Compare the oxygen profiles from the Arabian Sea and the California Current in Figure 1.17 with the profiles for the Antarctic and the Gulf of Mexico. Oxygen concentrations at the surface and at 500 m of depth in the world ocean are shown in Figure 1.18. Most of the world’s severest minima are in regions where upwelling is very strong, and algae blooms are episodically very extensive.

Oxygen minima may also occur because of topography. For example, the Black Sea and the Cariaco Basin off the coast of Venezuela have restricted communications with the rest of the open ocean because of a shallow sill restricting circulation of their deeper waters. As a consequence, their deep waters are isolated and become anoxic.

Because deeper waters have always spent some time away from the surface, an oxygen minimum of some degree is always present. However, in most places it is not severe and not limiting to animal life. In the Gulf of Mexico, for example, the oxygen drops to about 50% of surface values at a depth of 600 m, as it does in the Antarctic. In contrast, the waters off southern California drop to about 5% of surface values, and in the Arabian Sea oxygen levels drop to zero below 200 m. Such low values pose severe challenges to animal life. Moreover, oxygen minima in the Cariaco Basin and the Black Sea include the presence of sulfides, which are metabolic poisons. How animals cope (or not) with such low levels of oxygen will be covered in the next chapter.