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.
Criteria to maintain designated use:
Designated Use Lowest acceptable DO levels (mg/l)*
|Warm water fish||5.0|
|Cold water fish||6.0|
|* Summary of state standards|
|Preferred ranges for designated use:|
|Designated Use||Ranges (mg/l DO)|
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).
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.