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Oxygen saturation or dissolved oxygen (DO) is a measure of amount of oxygen dissolved in a given medium.
In medicine, oxygen saturation (SaO2) measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. An SaO2 value below 90% is termed hypoxia.
In aquatic ecology oxygen saturation indicates the status of the oxygen (O2) regime. Dissolved oxygen is usually measured in standard solution units such as millimoles O2 per liter (mmol/L), milligrams O2 per liter (mg/L) , milliliter O2 per liter (ml/L), or parts per thousand (ppt). Measurements in these units will change with temperature and salinty. As in the medical sense, here oxygen saturation is measured as a percentage of the theoretical maximum concentration given the current temperature, pressure and salinity. Well aerated water (in free interchange with air) will be 100% saturated. Regimes of low concentrations in the range between 0 and 30% are often called hypoxic. The state of 0% saturation is called anoxia. Most fish can not live below 30% saturation. Healthy ocean water is usually 100 - 110% saturated, the slight oversaturation caused by phytoplankton. Oversaturation can sometimes be harmful for organisms and cause gas bubble disease.
Tables of DO in milliliters per liter (ml/L) are often based on an equation by Weiss (1970, Deep-Sea Res. 17:721-735):
-
where Al = - 173.4292, A2 = 249.6339, A3 = 143.3483, A4 = - 21.8492, Bl = - 0.033096, B2 = 0.014259, B3 = - 0.001700, T = temperature in degrees K, and S = salinity in g/kg.
Solubility tables and corrections for changes in salinity and pressure can be found at the USGS web site (http://water.usgs.gov/owq/FieldManual/Chapter6/6.2.4.pdf). In general, the colder the water the more O2 it can dissolve and the more saline water the less O2 it can dissolve.
Some examples:
- 0 °C, normal pressure, freshwater: 14.6 mg/L = 100 %
- 10 °C, normal pressure, freshwater: 11.3 mg/L = 100 %
- 20 °C, normal pressure, freshwater: 9.1 mg/L = 100 %
See also
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