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EARTH SCIENCE > TERRESTRIAL HYDROSPHERE > WATER QUALITY/WATER CHEMISTRY > GASES > DISSOLVED OXYGEN

Definition:
A colorless, odorless, gaseous element, constituting 21 percent of the volume of the atmsophere, and essential for aerobic forms of life. Gaseous oxygen has a chemical forumla of O2. Despite its abundance in air, the concentration of oxygen in water is usually quite low, at just a few parts per million. Nevertheless, this concentration is a good indicator of water quality. If it is too low, many organisms, including fish, will die.

Reference:
Porteous, A. 1996. Dictionary of Environmental Science and Technology. John Wiley and Sons. p. 392. Websters College Dictionary. 1991. Random House.