Dissolved oxygen (DO) refers to the amount of oxygen that is present in water. It is an important measure of water quality as it indicates the amount of oxygen available to living aquatic organisms. DO is measured in milligrams per liter (mg/L) or parts per million (ppm) .

Aquatic animals, plants, and bacteria all require oxygen to survive. DO levels can fluctuate periodically, seasonally, and even as part of the natural daily ecology of the aquatic resource. Low levels of oxygen (hypoxia) or no oxygen levels (anoxia) can occur when excess organic materials, such as large algal blooms, are decomposed by microorganisms, and DO in the water is consumed. Low oxygen levels often occur in the bottom of the water column and affect organisms that live in the sediments.

DO levels can tell us a lot about the condition of water. For example, running water, such as that of a fast-moving stream, dissolves more oxygen than the still water of a pond or lake. The optimal levels in an estuary for dissolved oxygen is higher than 6 ppm. Accurate data for the concentration of dissolved oxygen in surface and ground waters are essential for documenting changes in environmental water resources.