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Dissolved oxygen infers to the volume of oxygen that is contained in water. Oxygen enters water through photosynthesis and across the water interface. The amount of oxygen that can be held by the water depends on the water pressure, salinity, and temperature. Gas solubility increases with decreasing temperature; thus colder water holds more oxygen. A reason why freshwater holds more oxygen than does saltwater is because gas solubility increases with decreasing salinity. When there is a change in altitude, Both the partial pressure and the saturation degree of oxygen will change. In addition, gas solubility decreases as pressure decreases. Therefore, the amount of oxygen absorbed in water decreases as altitude increases because of the decrease in relative pressure (Smith, 1990).

Once oxygen is absorbed, it is inaugurated throughout the water body through internal currents or is lost in the system. Flowing water has high dissolved oxygen levels than is stagnant water because of the water movement at the air-water interface. In flowing water, oxygen-rich water at the surface is constantly being replaced by water containing less oxygen because of turbulence. This creates a greater potential for exchange of oxygen across the air-water interface. The upper layer of oxygen-rich water tends to stay at the surface because stagnant water undergoes less internal mixing, resulting in lower dissolved oxygen levels throughout the water column. Oxygen losses occur when water temperatures rise, when plants and animals respire, and when microbes aerobically decompose organic matter (Smith 1990).

Dissolved oxygen may play a large role in the survival of biota in temperate lakes and reservoirs during the summer months. This is because of stratification. Seasonal stratification occurs as a result of water’s temperature-dependent density. As water temperatures increase, the density decreases. Therefore, the sun-warmed water will remain at the surface of the water body while the more dense, cooler water sinks to the bottom. The layer of rapid temperature change separating the two layers is called the thermocline. At the beginning of the summer, the cooler water will contain more dissolved oxygen because colder water holds more oxygen than warmer water. However, as time passes on, an increased number of dead organisms from the warmer water sink to the cooler water layer and are broken down by microorganisms. Continued microbial decomposition eventually results in an oxygen-deficient cooler water (Corel 1990).

Microbes have a crucial role in the oxygen loss. Microbes use oxygen as energy to break down long-chained organic molecules into simpler end products such as carbon dioxide, water, phosphate and nitrate. As organic molecules are broken down by microbes, oxygen is removed from the system and must be replaced by exchange at the air-water interface. If high levels of organic matter are present in water, microbes may use all available oxygen.

If dissolved gas concentrations in water exceed 110 percent, they could be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from “gas bubble disease.? In addition, aquatic invertebrates are affected by gas bubble disease but at levels higher than those that are deadly to fish (Oran 1992).

Adequate dissolved oxygen is necessary for good water quality. Oxygen is a necessary element to all forms of life. Natural stream purification processes require adequate oxygen levels in order to provide for aerobic life forms. As dissolved oxygen levels in water drop below 5.0 mg/l, aquatic life is put under anxiety. The lower the concentration, the greater the anxiety. Oxygen levels that remain below 1-2 mg/l for a few hours can result in large fish deaths (Oran 1992).

Corel, Kent. ?Dissolved Oxygen,? KY Water Watch, 1994. 5 June 2000.


Smith, Stephanie. ?Dissolved Oxygen (DO),? Water Shedss, 1990. 5 June 2000.


Oran, Mike, ?Aquatic Systems,? Scientific American, May 1992


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