<p>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.</p>
<p>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.</p>