Dissolved oxygen (DO) is the amount of oxygen gas (O₂) that is present in water in a dissolved form and available for use by aquatic organisms. In stormwater management and water quality analysis, dissolved oxygen is a critical indicator of the health and functionality of aquatic systems, as it directly supports the respiration of fish, invertebrates, and aerobic microorganisms.
Dissolved oxygen enters water through two primary mechanisms: atmospheric diffusion and photosynthesis. Oxygen from the atmosphere dissolves into the water at the surface, particularly where there is turbulence, such as in streams, riffles, or areas of wind-driven mixing. Aquatic plants and algae also produce oxygen as a byproduct of photosynthesis during daylight hours, contributing to DO levels.
The concentration of dissolved oxygen is typically expressed in milligrams per liter (mg/L) or as a percentage of saturation. DO levels are influenced by several environmental factors, including temperature, pressure, salinity, and biological activity. Cold water can hold more dissolved oxygen than warm water, and increased biological respiration or decomposition can reduce DO concentrations.
In stormwater systems, dissolved oxygen is closely tied to water quality and ecosystem health. Adequate DO levels are essential for sustaining aerobic life. When DO levels drop too low, a condition known as hypoxia can occur, which stresses or kills aquatic organisms. In extreme cases, anoxia (the absence of oxygen) can develop, leading to severe ecological impacts.
Stormwater runoff can affect dissolved oxygen in several ways. Runoff often carries organic matter, nutrients, and pollutants into receiving waters. The decomposition of organic material by bacteria consumes oxygen, a process measured as biochemical oxygen demand (BOD). Elevated nutrient levels can lead to algal blooms, which may initially increase oxygen through photosynthesis but ultimately decrease DO when the algae die and decompose.
Maintaining healthy dissolved oxygen levels is a key objective in stormwater management. Best Management Practices (BMPs) such as vegetated swales, wetlands, and detention systems can help reduce pollutant loads and moderate temperature, thereby supporting more stable DO conditions in downstream waters.
Dissolved oxygen is a fundamental water quality parameter that reflects the balance between oxygen-producing and oxygen-consuming processes in aquatic environments, making it essential for assessing the impacts of stormwater runoff and the overall health of water bodies.