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 )

Bibliography

Morgan, M. (1997). Environmental Health, 2nd edition. Englewood, CO: Morton Publishing Company.

Nadakavukaren, A. (2000). Our Global Environment, 5th edition. Prospect Heights, IL: Waveland Press.

Dissolved oxygen

Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water and is particularly important in limnology (aquatic ecology ). Oxygen comprises approximately 21% of the total gas in the atmosphere ; however, it is much less available in water. The amount of oxygen water can hold depends upon temperature (more oxygen can be dissolved in colder water), pressure (more oxygen can be dissolved in water at greater pressure), and salinity (more oxygen can be dissolved in water of lower salinity). Many lakes and ponds have anoxic (oxygen deficient) bottom layers in the summer because of decomposition processes depleting the oxygen. The amount of dissolved oxygen often determines the number and types of organisms living in that body of water. For example, fish like trout are sensitive to low DO levels (less than eight parts per million ) and cannot survive in warm, slow-moving streams or rivers. Decay of organic material in water caused by either chemical processes or microbial action on untreated sewage or dead vegetation can severely reduce dissolved oxygen concentration. This is the most common cause of fish kills , especially in summer months when warm water holds less oxygen anyway.