Dissolved Oxygen (DO)

The concentration of free molecular oxygen that is dissolved in water, usually expressed in milligrams per liter (mg/L), parts per million (ppm), or percent of saturation.  DO allows fish and other life to live in water.  Levels of 5 mg O2/L are optimal for sustaining life; most fish cannot survive prolonged periods at low levels of dissolved oxygen. (U.S. EPA, 2002a).