Some aquatic plants and animals have special ways of getting
oxygen directly from the air, but most remove oxygen dissolved
in the water. Because oxygen dissolved in water is far less
abundant than oxygen mixed in the air, the actual amount of
oxygen present is an important water quality parameter. As a
general rule, the more oxygen dissolved in water the better the
water quality. A consistently high oxygen content allows a body
of water to support more numbers and variety of aquatic
organisms. Survival under low oxygen conditions is a
specialization of a limited number of species. There is a limit
to how much oxygen water can hold before it is saturated. This
amount, called the oxygen solubility or saturation value, is not
fixed, but depends upon oxygen pressure in the air, water
temperature, and dissolved salts present. Solubility is greater
for fresh water than salt water, and greater for cold water than
warm water. Under some circumstances, oxygen concentration can
exceed the saturation value, and the wat