What Is Oxygen Sag?

Occasionally in bodies of water, large amounts of dead fish may turn up, having died all at once in a catastrophic environmental disaster known as "oxygen sag." Knowing exactly what oxygen sag is, how it occurs, and what it means for the environment is an important component of biology, ecological studies, business and water safety policies.

Identification

Oxygen sag is the dip in dissolved oxygen present in water that is the result of the introduction of waste material. Typically bacteria present in the water congregate near the food source (waste) and consume oxygen. If waste is concentrated in a certain area, the oxygen also will be depleted, causing oxygen sag. In some cases, however, some waste compounds consume oxygen on their own and are not the result of bacteria.

Effects

The effect of oxygen sag is the death of oxygen-breathing animals such as fish, invertebrates and bacteria. This may be a small, isolated area such as a shallow point of a pond, or may affect entire streams, depending upon the severity of the pollution. As oxygen-consuming animals die off, carbon dioxide-consuming plants will soon follow as they run out of dissolved carbon dioxide. The result is a "dead" body of water.

Misconceptions

Reduction of a stream or other body of water's flow may also cause oxygen sag independent of pollution. Damming or diversion for irrigation, for example, may reduce the flow of water, thus reducing its ability to churn and mix with air (called "turbidity"), resulting in lower oxygen levels, or sag.

Expert Insight

The rate at which oxygen sag can occur depends of several factors such as the volume of water, the pH level, the turbidity and the temperature of the water in addition to the amount of pollutants introduced in the waterways. These and some other factors affect the time frame in which the oxygen is used up in a body of water.

Sources of Oxygen Sag

The sources of these pollutants are typically human habitation; sewage is either dumped or leaks into waterways and then is carried to oceans. In some cases raw waste water is dumped directly into the oceans. Sometimes heavy rains or runoff from melting snow can overwhelm sewage treatment facilities and cause accidental spills into the bodies of water and oceans.
Proper treatment of waste material can reduce the impact of human habitation on waterways and coasts and minimize the risks of oxygen sag.

Resources

• California State University, Sacramento; Oxygen Sag Curve Calculations
• McGraw-Hill Higher Education Glossary of Biological Terminology; Oxygen Sag