Dissolved Oxygen
Dissolved oxygen (DO) refers to the amount of oxygen (O₂) dissolved in water. Because fish and other aquatic organisms cannot survive without oxygen, DO is one of the most important water quality parameters. DO is usually expressed as a concentration of oxygen in a volume of water (milligrams of oxygen per liter of water, or mg/L).

In nature, oxygen from the atmosphere can be mixed into (diffused into) a body of water. The mixing is easiest where water is rough (for example, where water is tumbling over rocks or where there are waves). Oxygen is also introduced into water by green aquatic plants and algae during photosynthesis.

Cold water holds more oxygen than warm water. For example, pure water at 4°C (40F) can hold about 13.2 mg/L DO at 100% saturation, while pure water at 25°C (77F) can hold only 8.4 mg/L at 100% saturation. Water with a high concentration of dissolved minerals cannot hold as much DO as pure water.

Dissolved Oxygen in the Red River
Aquatic life is put under stress when the DO concentration falls below 5 mg/L (red band on graph); if the DO concentration falls under 2 mg/L for just a few hours, large fish kills can result. Good fishing waters have a DO concentration around 9 mg/L.

As shown on the graph, DO concentrations in the Red River vary from highs of 15 mg/L in the winter (water temperature from 0.2° to 10°C) to summer lows of 7 mg/L (water temperature from 20° to 26°C). Human activity can also affect DO levels in the Red. Summer increases in the amount of nutrients (phosphorus, nitrogen [N] as ammonia, nitrite, and nitrate) from lawn and farm fertilizers in runoff, runoff from feedlots, storm water, and other discharges. This can result in the increased growth of plants and algae. Bacteria take up oxygen and reduce DO as they decompose this excess organic matter.

Graph of dissolved oxygen (DO; mg/L) for the Red River in the Fargo-Moorhead metro area for the period January 2001 to July 2003 in relation to the level of DO (less than 5 mg/L; red band) stressful to aquatic life.