What is Dissolved Oxygen (DO)?
Dissolved oxygen is essential for animals living in the waterways of the Minnesota River Basin. The concentration of molecular oxygen (O2) dissolved in water, usually expressed in milligrams per liter (mg/L), parts per million, or percent of saturation.

Why is DO important?
The dissolved oxygen 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 milligrams per liter (mg/L) are considered optimal. 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.

What is the standard for DO?
In Minnesota, a standard dissolved-oxygen level of 5 milligrams per liter (mg/L) has been set to protect aquatic life.

What is the status of DO in the Minnesota River Basin?
In the past, dissolved oxygen levels in the Minnesota River below Shakopee have fallen beneath the 5 mg/L standard during the warm summer months. Dissolved oxygen is essential to aquatic life but many factors reduce it in the river, including higher temperatures, inadequate re-aeration, high respiration, chemical-oxidation, and excessive oxygen demand from sediment. Low dissolved oxygen occurs more often in the summer because warm water holds less oxygen than cold water and the presence of more decaying algae and other organisms consumes the oxygen available (Basin Information Document, 1997).

References:
State of the Minnesota River: 2002 Surface Water Quality Monitoring.
Minnesota River: Basin Information Document. Minnesota Pollution Control Agency. November, 1997.
MPCA Glossary - http://www.pca.state.mn.us/gloss/index.shtml