In freshwater or marine systems apparent oxygen utilisation (AOU) is the difference between the measured dissolved oxygen concentration and its equilibriumsaturation concentration in water with the same physical and chemical properties[1]. Such differences typically occur when biological activity acts to change the ambient concentration of oxygen. For example, primary production liberates oxygen and increases its concentration, while respiration consumes it and decreases its concentration.
Consequently, the AOU of a water sample represents the sum of the biological activity that the sample has experienced since it was last in equilibrium with the atmosphere. In shallow water systems (e.g. lakes), the full water column is generally in close contact with the atmosphere, so oxygen concentrations are typically close to saturation. In deep water systems (e.g. oceans), water can be out of contact with the atmosphere for extremely long periods of time (years, decades, centuries) and large AOU values are possible.
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