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 in liquid media, usually water. In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O2) dissolved in the water. Dissolved oxygen is measured in standard solution units such as milliliters O2 per liter (ml/L), or milligrams O2 per liter (mg/L, also referred to as parts per million or ppm). Dissolved oxygen is used as a water quality indicator. The typical DO of Montana streams and rivers is about 6-10 mg/L. Lower DO values can harm trout populations. DO levels lower than this range could also be an indicator of pollution (for example, sewage effluent, or too many nutrients in a stream that cause algae to grow too fast).
For more, including lesson plans, visit the Science Junction page on dissolved oxygen, URL: http://www.ncsu.edu/sciencejunction/depot/experiments/water/lessons/do/