The amount of oxygen that dissolves is limited by physical condition, such as water temperature and atmospheric pressure. Assuming a constant atmospheric pressure, low water temperatures produce a higher potential dissolved oxygen level than do high water temperatures. Photo: Courtesy of Dr. Ben Stout

One measure of dissolved oxygen in water is parts per million (ppm), which is the number of oxygen molecules (O₂) per million total molecules in a sample. Calculating the percent saturation is another way to analyze dissolved oxygen levels. Percent saturation is the measured dissolved oxygen level divided by the greatest amount of oxygen that the water can hold under various temperature and atmospheric pressure conditions multiplied by 100. Left: This graph shows the maximum amount of oxygen that can be dissolved in water at different temperatures. Graph data: Courtesy of Dr. Ben Stout

Dissolved Oxygen / Greatest Amount of Oxygen x 100 = % Saturation

Oxygen is essential for fish, invertebrate, plant, and aerobic bacteria respiration. Dissolved oxygen levels below 3 ppm are stressful to most aquatic organisms. Levels below 2 or 1 ppm will not support fish. Fish growth and activity usually require 5--6 ppm of dissolved oxygen.

Low dissolved oxygen levels indicate an excessive demand on the oxygen of the system. The build up of organic material from human activities is one source of oxygen depletion. Microorganisms in the stream consume oxygen as they decompose sewage, urban and agricultural runoff, and discharge from food-processing plants, meat-packaging plants, and dairies.

Natural organic materials, such as leaves, accumulate in the stream and create an oxygen demand as they decompose.

Some pollutants, such as acid mine drainage, produce direct chemical demands on oxygen in the water. Dissolved oxygen is consumed in the oxidation-reduction reactions of introduced chemical compounds, such as nitrate (NO₃^1-) and ammonia (NH₄¹⁺), sulfate (SO₄^2-) and sulfite (SO₃^2-), and ferrous (Fe²⁺) and ferric (Fe³⁺) ions.

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