Home > Encyclopedia of Public Health > Dissolved Oxygen

Dissolved oxygen is the amount of oxygen present in fresh water, such as a stream, or freshwater lake. A certain amount of dissolved oxygen is required to sustain fish, animals, and other aquatic life. Any decrease in dissolved oxygen can cause changes, usually negative, in an aquatic system. These include fish kills and loss of aquatic ecosystems. Changing aerobic conditions to anaerobic conditions can cause putrefactive decomposition, which creates sulfides, mercaptanes, and amines. In liquid wastes, the level of dissolved oxygen determines whether the biological changes are brought about by aerobic or by anaerobic organisms. Anaerobic conditions are generally considered undesirable because of the foul odors produced, such as that caused by hydrogen sulfide. Since both aerobic and anaerobic organisms exist in nature, it is important to maintain aerobic conditions in water.

Maintaining aerobic conditions is also important in natural bodies of water that receive potential pollutants. The transformation and decomposition of organic matter in water occur best under aerobic conditions due to the consumption and oxidation of organic substrates by aerobic microbes. The oxygen is replenished through several mechanisms, one being the simple diffusion of the oxygen from the atmosphere into the water. Additionally, aquatic plants and algae produce oxygen through photosynthesis and the natural movement of water.

The level of dissolved oxygen can decrease in several ways, primarily through increased aerobic microbial action. It can also decrease from increases in water temperature.

Dissolved oxygen determination serves many purposes. For example, it serves as the basis of the biological oxygen demand (BOD) test. These tests are used in the evaluation of domestic and industrial waste's capacity to pollute. Determining the residual dissolved-oxygen in a stream at various time intervals aids in the measurement of biochemical oxidation.

MARK G. ROBSON

(SEE ALSO: Ambient Water Quality; Biological Oxygen Demand; Water Quality)