Dissolved oxygen

Oxygen saturation (symbol SO₂) is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. The standard unit of oxygen saturation is percent (%).

Oxygen saturation can be measured regionally and noninvasively. Arterial oxygen saturation (SaO₂) is commonly measured using pulse oximetry. Tissue saturation at peripheral scale can be measured using NIRS. This technique can be applied on both muscle and brain.

In medicine, oxygen saturation refers to oxygenation, or when oxygen molecules (O
2) enter the tissues of the body. In this case blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygen saturation ((O
2) sats) measure the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Fish, invertebrates, plants, and aerobic bacteria all require oxygen for respiration.

In aquatic environments, oxygen saturation is a ratio of the concentration of dissolved oxygen (O₂) in the water to the maximum amount of oxygen that will dissolve in the water at that temperature and pressure under stable equilibrium. Well-aerated water (such as a fast-moving stream) without oxygen producers or consumers is 100% saturated.

It is possible for stagnant water to become somewhat supersaturated with oxygen (i.e. reach more than 100% saturation) either because of the presence of photosynthetic aquatic oxygen producers or because of a slow equilibration after a change of atmospheric conditions. Stagnant water in the presence of decaying matter will typically have an oxygen concentration much less than 100%.

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