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The easiest physical characteristic of the ocean to understand is pressure. Pressure increases by 1 atmosphere (atm) (the barometric pressure at sea level) or 14.7 pounds per square inch (psi) with each increase of 10 m in depth. The metric unit of pressure is the pascal (Pa). One atmosphere is equivalent to 101.3 kPa. Pressure in the deepest point in the ocean, the Challenger Deep (depth: 10 916 m), is 16 046 psi or 1.11 × 10⁵ kPa. In contrast, pressure at the average depth of the ocean is 5586 psi or 3.85 × 10⁴ kPa. Pressure can be important in shaping the characteristics of species living in the deep sea and will be discussed in the next chapter.

Figure 1.17 Oxygen and temperature profiles from four oceanic regions. (a) Antarctic (Southern Ocean); (b) Arabian Sea; (c) Gulf of Mexico; (d) California Current.

Source: Torres et al. (2012), figure 1 (p. 1909). Reproduced with the permission of The Company of Biologists.

Figure 1.18 Typical oceanic oxygen concentrations. (a) Surface; (b) 500 m.