Significance: Dissolved oxygen is essential for rivers, streams, lakes, and aquatic life.
What is Dissolved Oxygen?
Oxygen gas that is dissolved into water is dissolved oxygen, or DO. DO is essential for rivers, streams, lakes, and aquatic life. It is a positive sign to have DO in water. If there is an absence of oxygen in water, it can be a sign of severe pollution. Water ranges from very high levels of DO to very low levels of DO. Sometimes the DO is so low, there is hardly any aquatic life.
Where Does Dissolved Oxygen Come From?
Most DO in water comes from the atmosphere or plants. The oxygen comes from tumbling water on fast moving rivers that mix atmospheric oxygen with water. Photosynthesis is another way of oxygen entering water. DO levels are often highest in the afternoon because of high levels of sunlight that cause plants and algae to photosynthesize rapidly. Because photosynthesis doesn't occur at night, DO levels exhibit diurnal and nocturnal cycles.
What is "Normal" for Dissolved Oxygen?
DO levels in rivers that are at a 90% saturation level or higher consistently are considered healthy, unless the water has become supersaturated because of cultural eutrophication. If the DO values in rivers are below 90%, there may be large amounts of oxygen demanding materials. Many factors contribute to how high or low a DO level may be. Water temperature is one of the causes. Oxygen and other gases dissolve more easily in cooler water than in warmer water. Certain factors affect the water temperature. These factors are seasons of the year, time of day, and water depth. By affecting temperature, factors affect DO. Levels of DO fluctuate from morning to night. DO is highest just before dark because the plants have photosynthesized all day. At night, levels drop because the plants aren't photosynthesizing and are conducting respiration.
Eutrophication is an increase in nutrients that can lead to an overgrowth of algae. The overgrowth doesn't allow the plants to photosynthesize. If the plants don't photosynthesize, the levels of oxygen decrease possibly causing oxygen sensitive organisms to die.
What Causes Low DO?
If BOD (Biochemical Oxygen Demand) is too high, the DO content of the water becomes depleted, and the DO content of the water becomes too low to support all the life in the water. If a body of water has large amounts of decaying vegetation, like leaves or aquatic microorganisms, then the amount of DO is reduced. Sometimes people dump raw sewage, garbage, grass, and other decay able substances in the water. As these things decay, the amount of DO falls. When bacteria decompose food, leaves, and feces, they require a great amount of oxygen, lowering the DO level. Urban and agricultural runoff, such as fertilizers, makes algae grow. The algae block the sunlight from the other plants, and the plants die so bacteria again decompose this organic matter.
How Does DO Affect Water Supplies?
High DO levels in a community water supply are good because it makes drinking water taste better. On the other hand, high DO levels speed up corrosion in water pipes. Industries use water with the least amount of DO. In boiler waters it is important to remove ALL oxygen. Slightly acid water containing both carbon dioxide (CO2) and oxygen (O2) will be corrosive. The acid water condition and CO2 initiate corrosion, but O2 is necessary for it to continue. Salt water holds less O2 than fresh water.
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