Dissolved Oxygen in Streams Why Is It Important? Like terrestrial animals, fish and other aquatic organisms need oxygen to live. As water moves past their gills (or other breathing apparatus), microscopic bubbles of oxygen gas in the water, called dissolved oxygen (DO), are transferred from the water to their blood. Like any other gas diffusion process, the transfer is efficient only above certain concentrations. So, a certain minimum amount of oxygen must be present in water for aquatic life to survive. In other words, oxygen can be present in the water, but at too low a concentration to sustain aquatic life. In addition to being required by aquatic organisms for respiration, oxygen also is used for decomposition of organic matter and other biological and chemical processes. Reasons for Natural Variation Oxygen is produced during photosynthesis and consumed during respiration and decomposition. Because it requires light, photosynthesis occurs only during daylight hours. Respiration and decomposition, on the other hand, occur 24 hours a day. This difference alone can account for large daily variations in DO concentrations. During the night, when photosynthesis cannot counterbalance the loss of oxygen through respiration and decomposition, DO concentrations steadily decline. They are lowest just before dawn, when photosynthesis resumes. Dissolved oxygen concentrations increase wherever the water flow becomes turbulent, such as in a riffle area, waterfall, or a dam. Oxygen concentrations are much higher in air, which is about 21 percent oxygen, than in water, which is a tiny fraction of 1 percent oxygen. Where the air and water meet, this tremendous difference in concentration causes oxygen molecules in the air to dissolve into the water until saturation is reached. More oxygen dissolves into water when turbulence caused by rocky bottoms or steep gradients brings more water into contact with the surface. A similar process happens when you add sugar to a cup of coffee. The sugar dissolves, but it will dissolve more quickly if you stir the coffee. Another physical process that impacts DO concentrations has to do with the temperature of the water and gas saturation. Cold water can hold more gas – that is DO – than warm water. So, during the summer months when stream water is warmer, oxygen can be limited by the ability of the water to "soak up" more oxygen gas. A table comparing oxygen saturation at different water temperatures can be found in the lake temperature section. There are other reasons for seasonal variation. During late summer, streamflows can get very low in the Puget Sound area. Many of the tributaries that provide oxygenated water to the main stream dry up, and as water moves slowly over what may previously have been riffles or rapids, there is less opportunity for aeration and oxygenation. Warmer summer temperatures also cause increased biological activity (growth, productivity, respiration, and decomposition), and therefore greater daily variability in DO. Expected Impact of Pollution Pollution tends to cause a decrease in stream oxygen concentrations. This change can be caused by addition of effluent or runoff water with a low concentration of DO or chemical or biological constituents that have a high oxygen demand – that is they require large amounts of oxygen before they can be thoroughly decomposed. The latter is often the more typical and more serious case. The demand for oxygen doesn’t occur directly where the effluent or runoff water is discharged but instead somewhere downstream where decomposition finally occurs. This can make it difficult to show a direct relationship between addition of an oxygen demanding pollution source and decrease in oxygen concentrations. There is a way to determine the amount of oxygen required for decomposition of a pollutant source by measuring whet is called the biochemical oxygen demand (BOD). Since this measurement is not typically included in citizen monitoring efforts it is not covered in this text. Standard Methods for the Examination of Water and Wastewater is a commonly used reference manual for water quality studies and provides information on BOD analysis. The full reference is included in the reference section. Stormwater runoff also delivers oxygen-demanding substances to streams. When a watershed becomes developed, greater quantities of pollutants are released and the total volume of runoff increases. Most conventional pollutants (sediments, nutrients, organic matter) require oxygen for decomposition or for chemical reactions. Consequently, DO concentrations often decrease in streams located in a developed or developing watershed.
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