Unlike terrestrial environments, oxygen is typically a limiting factor in aquatic ecosystems. Dissolved oxygen (DO) concentrations are expressed as milligrams of oxygen per liter of water (mg/L). The amount of DO affects what types of aquatic life are present in a stream, because many species of fish and macroinvertebrates are sensitive to low DO levels. DO also regulates the availability of certain nutrients in the water. Many physical and biological factors affect the amount of dissolved oxygen in a stream.
The physical factors that influence DO are temperature, altitude, salinity, and stream structure. Temperature inversely controls the solubility of oxygen in water; as temperature increases, oxygen is less soluble. In contrast, there is a direct relationship between atmospheric pressure and DO; as the pressure increases due to weather or elevation changes, oxygen solubility increases. Salinity also reduces the solubility of oxygen in water. However, because streams in Northeast Wisconsin have relatively low salinity values, this factor is typically disregarded for our calculations. Stream structure also influences DO concentrations. Atmospheric oxygen becomes mixed into a stream at turbulent, shallow riffles, resulting in increased DO levels. Because there is less surface interaction between water and air in slow-moving water and deep sections of a stream, DO concentrations often decrease between surface and bottom measurements.
The biological processes of photosynthesis and respiration also affect dissolved oxygen concentrations in streams. As aquatic plants photosynthesize, they give off large amounts of DO during daylight hours. However, respiration from aquatic vegetation, microorganisms, and algae consume oxygen at all hours of the day and night. A stream experiencing an algal bloom exhibits large daily fluctuations in DO as extreme oxygen production during the day contrasts with the bacterial decomposition of algal detritus at night. Thus, the lowest concentrations of DO in the summer are typically observed just before dawn.
Biochemical oxygen demand (BOD) is another important factor that effects DO concentrations in streams. BOD is the amount of oxygen consumed by microbial decomposition of organic waste, and is measured by the change in DO in a sealed water sample over a five-day incubation period. High levels of organic pollution, such as that from sewage treatment plants, agricultural runoff, or industrial wastes, can significantly increase the BOD in a stream. Relatively healthy streams will have a 5-day BOD reading of less than 2 mg/L, whereas polluted streams may approach 10 mg/L.
Streams in the Fox River Basin typically have dissolved oxygen values between 2 and 14 mg/L. The State of Wisconsin has set a minimum water quality standard of 5 mg/L DO as necessary for a stream to support fish and aquatic life. Trout streams may not have a DO level of less than 6.0 mg/L at any time, and may not have less than 7.0 mg/L DO during the spawning season.