Thus, unambiguous sources and site observations can be used to eliminate DO as a candidate cause. Biological evidence should not be used to exclude DO since several stressors alone or combined may cause similar symptoms of low or high DO.
Further investigation will be needed. This type of initial screening saves time only when unnecessary listing of candidate causes is avoided.
Early screenings should be conservative because the premature elimination of an actual cause will increase the time and cost of stressor identification.
Sources : Low concentrations of DO are physically precluded by consistent aeration from turbulence. Spillways, waterfalls, and turbulent flows in streams and rivers naturally aerate water. However, if flow changes during part of the year, DO will be affected and this should be considered.
Strong wave action in marine coastal areas may ensure aeration, whereas gentle wave action and riffles may or may not be sufficient, depending on the depth of the water and rigor of mixing. Screening in these situations should be complemented with measures of DO concentrations see Ways to Measure tab. When not listing low DO as a candidate cause due to turbulence, consider listing altered hydrologic flow or insufficient sediment retention or supply. Both are known to occur below spillways and waterfalls due to retention of sediment behind the dam, and the power of water turbulence below the dam that can remove sediment and dislodge organisms.
Site observations : When continuous measures of DO are available that document diurnal patterns over a long period of time, and they show DO concentrations consistent with those found at unimpaired sites, you may choose not to list low DO lack of spatial co-occurrence. However, we strongly caution against using benchmarks of effects for excluding DO from your initial list of candidate causes, because different species have different oxygen requirements e.
EPA DO concentrations and percent saturation are related, but not equivalent. Saturation level varies naturally, as water can contain more DO at lower temperatures, higher pressures, and lower salinities. The Winkler titration procedure was the first recognized method for determining DO concentrations in natural waters Winkler , cited in Mitchell More recently, this method was found prone to over-reporting DO under hypoxic conditions and under-reporting DO under nearly anoxic conditions.
Fairly simple and reliable DO measurements now can be obtained with DO meters or field test kits. Percent saturation is calculated by dividing the measured DO concentration by the saturation level and multiplying by Saturation levels can be obtained from U.
Geological Survey solubility tables based on water temperature and corrected for different salinities and pressures. Equations for calculating percent saturation are available from Water on the Web.
Biochemical oxygen demand BOD and chemical oxygen demand COD are measures of the potential consumption of oxygen by microbial respiration and the oxidation of chemicals in the water, respectively. The actual rate of oxygen consumption in a stream is affected by a number of variables including temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water. The lowest concentrations of DO are usually measured before photosynthesis begins for the day i.
Documentation of DO concentrations over a hour period may be useful for identifying diurnal patterns and may reveal information about DO depletion. Conceptual diagrams are used to describe hypothesized relationships among sources, stressors and biotic responses within aquatic systems.
Human activities can significantly affect DO concentrations in streams, most notably by decreasing oxygenation and by increasing chemical or biochemical oxygen demand. Agricultural practices, forestry practices, and other activities may involve channel alteration e. Impoundments upstream of a location may discharge low oxygen water downstream, but releases also may increase turbulence and oxygenate water.
These land use practices also may directly introduce nutrients e. The resulting chemical reactions and increased respiration of microbes and plants can increase oxygen demand in streams, leading to decreases in DO.
These sources also may affect DO via interactions with other stressors. For example, DO saturation occurs at lower concentrations in warm versus cold water, so factors contributing to increased water temperatures e.
Similar relationships are seen with increasing ionic strength and sediment. Although most impairments associated with DO result from insufficient oxygen levels, in rare cases DO concentrations may be too high e. Even if elevated DO levels do not cause direct impairment, they may contribute to stressful DO fluctuations when followed by significant drops in DO at night. This conceptual diagram Figure 11 illustrates linkages between DO-related stressors middle of diagram , the human activities and sources that result in those stressors top of diagram , and the biological responses that can result bottom of diagram.
In some cases, additional steps leading from sources to stressors, modes of action leading from stressors to responses, and other modifying factors also are shown. This narrative generally follows the diagram top to bottom, left to right. Certain human activities, such as agricultural, residential, and industrial practices, can contribute to DO depletion or, less frequently, DO supersaturation and subsequent biological impairment. These practices may directly introduce chemical contaminants, organic loading, and nutrients to streams, via point and non-point sources e.
Increases in these substances can increase chemical and biochemical oxygen demand, most notably due to increased respiration of plants and microbes. Physical alteration of the stream channel, through impoundments or channel alterations, can contribute to low dissolved oxygen concentrations in several ways. For example, an impoundment downstream of a location will slow water velocities and increase water depths, which will tend to reduce turbulence and lower incorporation of oxygen into the water column via aeration, as well as reduce diffusion of oxygen from the atmosphere.
Channel incision also reduces oxygen diffusion due to decreases in surface-to-volume ratio with increasing stream depth. An impoundment upstream of a location upper far right of diagram may reduce DO levels if downstream water releases come from deeper, oxygen-depleted waters of the reservoir i.
Land cover alterations also may reduce stream DO levels by altering in-stream physical characteristics. For example, decreases in riparian vegetation often associated with these activities can reduce large woody debris inputs to the channel, reducing turbulence and aeration; homogenization of stream substrates can have similar effects.
In addition these alterations may increase delivery of chemical contaminants, organic material, and nutrients to streams with surface runoff. DO concentrations are also closely linked to several other stressors.
Interactions between nutrient concentrations and DO were mentioned earlier—basically, nutrient enrichment stimulates oxygen-generating photosynthesis and oxygen-depleting respiration processes. In addition, DO levels are affected by water temperature, ionic strength, and dissolved solids: oxygen solubility decreases as these parameters increase, reducing the amount of DO in the water. Dissolved oxygen DO is a relative measure of the amount of oxygen O 2 dissolved in water. Oxygen gets into the water by diffusion from the atmosphere, aeration of the water as it tumbles over rocks and waterfalls, and as a product of photosynthesis.
The oxygen content of water will decrease when there is an increase in nutrients and organic materials from industrial wastewater, sewage discharges, and runoff from the land. Intensive land uses such as farming produce more nutrients in runoff than native forest.
Excessive plant and algae growth and decay in response to increasing nutrients in waterways can significantly affect the amount of dissolved oxygen available. A wastewater indicator such as biochemical oxygen demand BOD is a laboratory test that measures the relative oxygen-depletion effect of a waste contaminant when the contaminant reacts through biochemical reactions with nutrients and bacteria.
The negative effect wastewater has on mahinga kai and aquatic plant life, by reducing the amount of available oxygen, is indicated by an elevated BOD reading. Potential impacts of low dissolved oxygen DO on water quality and mahinga kai. Information provided may be out of date, and you are advised to check for newer sources in this section. The benefits of avoiding stratification are increased dissolved oxygen levels throughout a resource and increased usable habitat for fish.
Perhaps the most important benefit to a properly built and designed aeration system is the constant optimization of dissolved oxygen levels. In a highly oxygenated environment, nutrients that cause algae blooms bind to free molecules such as Iron and precipitate out of the water column. Another strategy to combat algae growth is the application of beneficial bacteria. Beneficial bacteria out-compete algae for available nutrients thus eliminating the aggressive blooms that can cause dissolved oxygen sags.
By eliminating stratification and reducing the chances of dense algae and phytoplankton blooms, a pond owner can keep dissolved oxygen levels high and eliminate the stressors to the fish populations. Contact Aqua Sierra today if you feel your pond experiences dissolved oxygen issues and your fish are suffering! We can solve your problems!
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