Oxygen is necessary for higher life on this planet and terrestrial plants and animals obtain oxygen as a gas from the air. Despite the fact that all of us are taking oxygen into our bodies and converting it to carbon dioxide as part of a process to extract energy from our food, the amount of oxygen in the atmosphere remains remarkably stable at about 20.95% of the air. We credit plants with contributing oxygen back into the atmosphere in the process of photosynthesis : utilizing sunlight to produce organic molecules from water and carbon dioxide. Oxygen is a by-product of this process. The energy needs of plants are modest per unit body weight, most of the production of organic material contributing to increasing biomass. The result is an excess of oxygen production over oxygen use. In aquatic environments, the uptake of carbon dioxide during photosynthesis can influence the pH of the water.

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 water is then said to be supersaturated. How oxygen solubility relates to field measurements is discussed below.

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