Dissolved oxygen (DO) is a measure of the amount of oxygen (O2) dissolved in water. DO is a key indicator of aquatic water quality and sufficient concentrations of DO are needed for aquatic organisms to survive.
DO concentrations of 6 mg/l and above are best for aquatic organisms. Values between 2.5 and 6 mg/l represent stressed conditions and values below 2.5 mg/l are extremely poor, with values less than 1 mg/l representing anoxic (absence of oxygen) conditions.