Dissolved oxygen (DO) is the concentration of molecular oxygen (O2) dissolved in water, usually expressed in milligrams per liter (mg/L), parts per million, or percent of saturation. The DO level represents one of the most important measurements of water quality and is a critical indicator of a water body’s ability to support healthy ecosystems. Levels above 5 mg/L are considered optimal, and most fish cannot survive for prolonged periods at levels below 3 mg/L. Microbial communities in water use oxygen to breakdown organic materials, such as manure, sewage and decomposing algae. Low levels of dissolved oxygen can be a sign that too much organic material is in a water body.