Several indices to measure bioavailability have been suggested: Respiration Index, Oxygen Supply Index, and the Metabolic Index. The Respiration Index describes oxygen availability based on the free energy available in the reactants and products of the stoichiometric equation for respiration. However, organisms have ways of altering their oxygen intake and carbon dioxide release, so the strict stoichiometric equation is not necessarily accurate. The Oxygen Supply Index accounts for oxygen solubility and partial pressure, along with the Q of the organism, but does not account for behavioral or physiological changes in organisms to compensate for reduced oxygen availability. The Metabolic Index accounts for the supply of oxygen in terms of solubility, partial pressure, and diffusivity of oxygen in water, and the organism's metabolic rate. The metabolic index is generally viewed as a closer approximation of oxygen bioavailability than the other indices.
There are two thresholds of oxygen required by organisms:
* P (critical partial pressure)- the oxygen level below which an organism cannot support a normal respiration rate
* P (lethal partial pressure)- the oxygen level below which an organism cannot support the minimum respiration rate necessary for survival.
Since bioavailability is specific to each organism and temperature, calculation of these thresholds is done experimentally by measuring activity and respiration rates under different temperature and oxygen conditions, or by collecting data from separate studies.