Dissolved oxygen refers to the amount of oxygen that is present in a liquid, such as water or wastewater. This measurement is important because it affects the ability of aquatic organisms to survive and thrive. Oxygen can dissolve in water through various physical and biological processes, such as diffusion from the atmosphere, photosynthesis by aquatic plants, and aeration from waterfalls or rapids. Dissolved oxygen is commonly measured in milligrams per liter (mg/L) or as a percentage of the maximum amount of oxygen that can be dissolved in the water, which is known as percent saturation. Low levels of dissolved oxygen can lead to hypoxia or anoxia, which can have harmful effects on aquatic organisms, including fish kills and altered food webs. Therefore, monitoring and maintaining dissolved oxygen levels in bodies of water is essential for ecosystem health and sustainability.