Oxygen is important to many different forms of life, but for aquatic organisms there’s a difference; the oxygen they need to survive is dissolved in water.
Water is made up of hydrogen and oxygen, but the oxygen in water molecules (H20) is not what is breathed by aquatic life. Oxygen, as well as other atmospheric gasses, readily diffuses or dissolves into water until an equilibrium is reached. The amount of oxygen that will dissolve into water is dependent upon pressure and temperature; cold water can hold more oxygen than warm water. Dissolved oxygen in water also comes from photosynthetic respiration by aquatic plants and algae.
Dissolved oxygen in the water is consumed by aquatic organisms, bacteria, decay of organic matter and plants (yes, plants also need oxygen when not photosynthesizing.).
In summertime when water temperatures rise, dissolved oxygen levels become critical because the water cannot hold as much oxygen, and the aquatic organisms are still consuming oxygen. Summertime is also the height of plant productivity. Through photosynthesis, aquatic plants and algae cause the levels of dissolved oxygen to peak in late afternoon but then drop at night. Algae also consume oxygen and compete for resources. When algae and other organisms die, the organic matter left behind as well as the bacteria consuming the organic matter consume oxygen. Excessive levels of algae can cause the levels of dissolved oxygen to drop rapidly to the point where fish and other organisms begin to suffer and die.
Missouri's Water Quality Standards for protecting aquatic life is located in Code of State Regulations, 10 CSR 20-7. These standards change for cold water species protection versus warm water species protection. Flowing waters must maintain a minimum of 5.0 mg/L of dissolved oxygen to support warm water aquatic life, and 6.0 mg/L of dissolved oxygen to support cold water aquatic life. At concentrations below 5.0 mg/L sensitive species of fish and other organisms start to show signs of distress such as lethargy and “piping,” swimming to the water surface and gulping or gasping.
Nutrients and organic material are the major pollutants that can cause excess consumption of dissolved oxygen in streams.