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Dissolved Oxygen

General Information: Dissolved oxygen (DO) refers to the volume of oxygen that is contained in water. Oxygen enters the water by photosynthesis of aquatic biota and by the transfer of oxygen across the air-water interface. The amount of oxygen that can be held by the water depends on the water temperature, salinity, and pressure. Gas solubility increases with decreasing temperature (colder water holds more oxygen). Gas solubility increases with decreasing salinity (freshwater holds more oxygen than does saltwater). Both the partial pressure and the degree of saturation of oxygen will change with altitude . Finally, gas solubility decreases as pressure decreases. Thus, the amount of oxygen absorbed in water decreases as altitude increases because of the decrease in relative pressure (Smith, 1990).

Once absorbed, oxygen is either incorporated throughout the water body via internal currents or is lost from the system. Flowing water is more likely to have 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 as a result of turbulence, creating a greater potential for exchange of oxygen across the air-water interface. Because stagnant water undergoes less internal mixing, the upper layer of oxygen-rich water tends to stay at the surface, resulting in lower dissolved oxygen levels throughout the water column. Oxygen losses readily occur when water temperatures rise, when plants and animals respire, and when microbes aerobically decompose organic matter.

Dissolved oxygen may play a large role in the survival of biota in temperate lakes and reservoirs during the summer months, due to a phenomenon called stratification. Seasonal stratification occurs as a result of water's temperature-dependent density. As water temperatures increase, the density decreases. Thus, the sun-warmed water will remain at the surface of the water body (forming the epilimnion), while the more dense, cooler water sinks to the bottom (hypolimnion). The layer of rapid temperature change separating the two layers is called the thermocline (Smith, 1990).

At the beginning of the summer, the hypolimnion 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 hypolimnion and are broken down by microorganisms. Continued microbial decomposition eventually results in an oxygen-deficient hypolimnion. If the lake is in a eutrophic state, this process may be accelerated and 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 (Dunne et al., 1978). As the organic molecules are broken down by microbes, oxygen is removed from the system and must be replaced by exchange at the air-water interface.

Each step above results in consumption of dissolved oxygen. If high levels of organic matter are present in a water, microbes may use all available oxygen.

Numerical Categories:

Criteria to maintain designated use:
Designated Use Lowest acceptable DO levels (mg/l)*


Aquatic life
Warm water fish 5.0
Cold water fish 6.0
Spawning season 7.0
Estuarine biota 5.0
Recreation
Primary Contact 3.0
Secondary Contact 3.0
* Summary of state standards
Preferred ranges for designated use:
Designated Use Ranges (mg/l DO)

Industry (Morton, 1986)
Boiler Feed Water
High Pressure 0
Low Pressure 0.1 - 1.4

Environmental Effects: The introduction of excess organic matter may result in a depletion of oxygen from an aquatic system. Prolonged exposure to low dissolved oxygen levels (<5 - 6 mg/l ) may not directly kill an organism, but will increase its susceptibility to other environmental stresses. Exposure to < 30% saturation (<2 mg/l oxygen) for one to four days may kill most of the biota in a system. If oxygen-requiring organisms perish, the remaining organisms will be air-breathing insects and anaerobic (not requiring oxygen) bacteria (Gower, 1980).

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Recreation: If all oxygen is depleted, aerobic (oxygen-consuming) decomposition ceases and further organic breakdown is accomplished anaerobically. Anaerobic microbes obtain energy from oxygen bound to other molecules such as sulfate compounds. Thus, anoxic conditions result in the mobilization of many otherwise insoluble compounds. The breakdown of sulfate compounds will often impart a "rotten-egg" smell to the water, affecting its aesthetic value and preventing recreational use.

Sources: Low dissolved oxygen levels may occur during warm, stagnant conditions that prevent mixing. (For more information, see Temperature section.) In addition, high natural organic levels will often cause a depletion of dissolved oxygen. (For more information, see Organic Matter section.)

Mode of Transport: Not applicable.


Analytical Techniques:

(APHA, 1992)

  1. Iodometric Method: Most reliable method. Requires the addition of divalent manganese solution, followed by a strong alkali, in a stoppered glass bottle. The dissolved oxygen oxidizes to manganous hydroxide precipitate. With the addition of iodide, the oxidized manganese reverts back to divalent state, releasing iodide equivalent to original dissolved oxygen content. Iodide is titrated with thiosulfate.

  2. Azide Method: This method is suitable for samples containing more than 50 ug/l nitrite and not more than 1 mg/l of ferrous iron.

  3. Permanganate Modification: This method is suitable for samples containing ferrous iron.

  4. Alum Flocculation Modification:

  5. Membrane Electrode Method: This method is ideal for field testing. Not applicable for industrial and domestic wastewater. Polarographic or galvanic oxygen-sensitive membrane electrodes are composed of two metal electrodes in contact with a supporting electrolyte that is separated from the test solution by a selective membrane.
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