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.