Dissolved oxygen (DO) is the amount of oxygen gas dissolved in water, essential for the survival of aquatic organisms. It is one of the most important indicators of water quality and ecosystem health.
Dissolved oxygen refers to the concentration of molecular oxygen (O2) present in water, originating primarily from atmospheric diffusion and photosynthesis by aquatic plants and algae. DO levels are influenced by water temperature, atmospheric pressure, salinity, and the rate of biological and chemical oxygen consumption. Cold water holds more dissolved oxygen than warm water; at 0 degrees Celsius, saturated fresh water contains about 14.6 mg/L of DO, while at 30 degrees Celsius it holds only about 7.5 mg/L. Most fish species require DO levels above 5 mg/L for healthy survival, and levels below 2 mg/L are considered hypoxic, creating dead zones where most aquatic life cannot survive. DO concentrations typically follow a diurnal cycle in productive waters, peaking in the afternoon when photosynthesis is greatest and declining overnight when respiration dominates. Measurement is performed using electrochemical sensors (Clark electrode or optical luminescence) or chemical titration (Winkler method). DO monitoring is fundamental to assessing river health, evaluating wastewater treatment effectiveness, and managing aquatic habitats.