The unique thing about oxygen is that it can diffuse directly between the air and water and this diffusion is improved by both air and water turbulence. The addition of chemicals to water will usually result in lower dissolved oxygen due to stimulation of biological processes.
Thus, it is biological processes, even though they are impacted by physical and chemical conditions that play the most important role in regulating dissolved oxygen concentrations in surface waters. Plants growing in water release oxygen during photosynthesis and both plants and animals living in water consume this oxygen in respiration. The rate of photosynthesis is affected primarily by temperature, light, nutrient levels, species of plants, abundance of plants, and water turbulence, although several other factors may be involved.
Photosynthesis is carried out in water primarily by microscopic plants called phytoplankton. When these organisms die they settle to the bottom and are decomposed by bacteria. This bacterial decomposition also uses up oxygen. When additional organic matter is added to water other than that generated in the water through photosynthesis, there is a greater biological oxygen demand (BOD) for the aerobic digestion of these carbon compounds. The BOD is defined as the amount of oxygen used over time to degrade organic matter and is the most commonly used parameter in the analysis of oxygen resources in water. 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 some streams in have relatively low salinity values, this factor is typically disregarded for 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.
Dissolved oxygen is affected greatly by physical, chemical and biological processes. A change in either one can easily affect the another. It is the delicate interaction of the three that create a favorable or unfavorable DO rate. But research has shown that it may be largely the biological process that influences DO rates.