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. 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. 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 process is termed eutrophication.