Dissolved Oxygen and Water Quality

A high dissolved oxygen (DO) level in a community water supply is good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes. For this reason, industries use water with the least possible amount of dissolved oxygen. Water used in very low-pressure boilers has no more than 2.0 ppm of DO, but most boiler plant operators try to keep oxygen levels to 0.007 ppm or less.

Oxygen comprises approximately 21% of the total gas in the atmosphere; however, it is much less available in the water. Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water and is particularly important in limnology (aquatic ecology). Oxygen enters the water when algae and rooted aquatic plants undergo photosynthesis and oxygen is transferred across the air-water interface. Photosynthesis is the primary process affecting the dissolved-oxygen/temperature relation; water clarity and strength and duration of sunlight, in turn, affect the rate of photosynthesis. The amount of oxygen water can hold depends upon water temperature (more oxygen can be dissolved in colder water), pressure (more oxygen can be dissolved in water at greater pressure), and salinity (freshwater holds more oxygen than saltwater does). Both the partial pressure and the degree of saturation of oxygen will change with altitude. 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 when warm water holds less oxygen.

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Thus, the sun-warmed water will remain at the surface of the water body epilimnion), while the denser, cooler water sinks to the bottom (hypolimnion). The layer of rapid temperature change separating the two layers is called the thermocline. At the beginning of the summer, the hypolimnion of the lake will contain more dissolved oxygen because colder water holds more oxygen than warmer water. However, as time progresses, an increased number of dead organisms from the epilimnion sink to the bottom and are broken down by microorganisms. Continued microbial decomposition eventually results in an oxygen-deficient hypolimnion. If the lake has high concentrations of nutrients, this process may be accelerated. When the growth rate of microorganisms is not limited by a specific nutrient, such as phosphorus, the dissolved oxygen in the lake could be depleted before the summer's end.

Microbes play a key role in the loss of oxygen from surface waters. Microbes use oxygen as energy to break down long-chained organic molecules into simpler, more stable end products such as carbon dioxide, water, phosphate and nitrate. As microbes break down the organic molecules, oxygen is removed from the system and must be replaced by the exchange at the air-water interface. Each step above results in the consumption of dissolved oxygen. If high levels of organic matter are present in the water, microbes may use all available oxygen. This does not mean, however, that the removal of microbes from the ecosystem would solve this problem. Although microbes are responsible for decreasing levels of dissolved oxygen, they play a very important role in the aquatic ecosystem. If the dead matter is not broken down it will "pile up," much as leaves would if they were not broken down each year.

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