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Oxygen is expressed in a variety of (nearly) interchangeable ways and those most commonly encountered in the literature have changed over the years. A few basics will help.

1. The total pressure of a gas mixture such as atmospheric air equals the sum of pressures exerted by each constituent gas. This is Dalton’s law.

2. Total (standard) atmospheric pressure is expressed as760 mm Hg (also known as “Torr”) or as 101 kilopascals (kPa), which is the SI unit for pressure.

3. The oxygen partial pressure in a standard atmosphere is the product of the mole fraction of oxygen in air × the barometric pressure:

 PO2 = (0.2095) × (760 mm Hg) = 159.2 mm Hg (usually rounded up to 160 mm Hg)

 PO2 = (0.2095) × (101 kPa) = 21.2 kPa (usually rounded down to 21 kPa)

4. Water in equilibrium with a gas mixture will have the same partial pressures as the gas mixture above it. This is Henry’s law. The snag is that the molar concentrations are a function of the partial pressures and an additional factor, the solubility coefficient. Thus: [A] = pA · α_A

where [A] = mol l⁻¹ of A, pA = partial pressure of A (kPa), α _A = solubility coefficient (mol l⁻¹ kPa⁻¹).

5. Solubilities of oxygen in seawater decrease with increasing temperature and salinity. They are normally determined empirically (with exacting care!) at air saturation (e.g. Murray and Riley 1969) and compiled in a table for researchers to use. For Oregon waters (Chapter 11), the oxygen concentration at 12 °C and 33‰ was used. It was reported as 6.14 ml l⁻¹ in Murray and Riley (1969). Knowing that a mole of oxygen is 22.4 l or 32 g, we can convert 6.14 ml l⁻¹ to 8.77 mg l⁻¹ (6.14/22.4) × 32. For future reference, to interchange ml l⁻¹ with mg l⁻¹ simply divide by 0.7. From there, it is a simple matter to convert to millimoles: 8.77/32 = 0.272 millimoles or 272 μmol kg⁻¹, since the mass of a liter is a kg.

6. When comparing P_cs across pelagic systems with widely varying temperatures and oxygen concentrations, it is easiest to use PO₂ because the mole fractions do not change, but relative solubilities do.