Dissolved oxygen (DO) is one of the most important indicators of water quality. It is essential for the survival of fish and other aquatic organisms. Oxygen dissolves in surface water due to the aerating action of winds. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis. When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive.
The DO test tells how much oxygen is dissolved in the water. However, it does not tell you how much oxygen the water is capable of dissolving at the temperature at which it was measured. When water dissolves all of the oxygen it is capable of holding at a given temperature it is said to be 100% saturated. The colder the water is, the greater the amount of oxygen the water can hold. As the water becomes warmer, less oxygen can dissolve in the water. Salinity is also an important factor in determining the amount of oxygen a body of water can hold. As the amount of dissolved salts in the water increases, the amount of oxygen the water can hold decreases. Conversely, as the water becomes more fresh (lower salinity), more oxygen can dissolve into the water.
Oxygen levels may be reduced because the water becomes too warm or because there are too many bacteria or algae in the water. After the algae complete their life cycle and die, bacteria consume the dead algae. During this decay process the bacteria also consume the oxygen dissolved in the water. This consumption of oxygen by the bacteria can lead to decreased levels of dissolved oxygen and in some cases completely strip the water of all DO. This process is termed eutrophication. This decrease in dissolved oxygen can cause fish kills and death to other aquatic organisms.
Dissolved oxygen is routinely recorded as part of basic water quality sampling in most surface waters and near-shore coastal systems. There are three common methods for measuring DO. The most practical and consistently accurate method for field measurements employs the polarographic DO sensor. If calibrated correctly, this method provides accurate measurements that can be performed in-situ (i.e., within the water under natural conditions), easily at any depth, and countless times per day. The Winkler Method is still preferred by some investigators, and is used by many agencies for sensor calibration verification and quality assurance/quality control testing. However, since this method requires a relatively high degree of titration skill, the handling of hazardous reagents (chemicals), and great care with sample collection technique and preservation, it is recommended that it be used in the laboratory by skilled analysts.
Dissolved oxygen levels are expressed as milligrams per liter (mg/L) or parts per million.
The accuracy of DO measurement is completely dependent upon proper calibration and maintenance (re. DO sensor method), and strict adherence to analytical methodology (re. Winkler Method).