Definition: Microscopic oxygen (O2) molecules that are mixed within water...dissolved oxygen is found in thespaces between water (H2O) molecules.

Background:

• Aquatic animals and aerobic bacteria need O2 for respiration...without dissolved oxygen, fish would drown!

• Presence of dissolved oxygen is a positive sign, while its absence is a signal of severe pollution.

• Temperature - dissolved O2 is normally greatest during the winter because cold water can hold more O2...(as temperatures drop, water molecules are spaced farther apart)
  

• Wet weather or melting snow increases flow, which results in greater mixing of atmospheric oxygen.

 Dissolved oxygen added to water through aeration and photosynthesis

Aquatic Life Influences

• Algae and aquatic plants deliver O2 to water through photosynthesis.
• Respiration/decomposition removes dissolved O2.
• During growing seasons, dissolved O2 is highest in early afternoon when aquatic photosynthesis is maximal.

• Temperature changes - any actions that change the temperature of the stream affect dissolved oxygen.

• Nutrient additions - from fertilizers encourage excessive plant growth (algal blooms), which eventually die and need to be decomposed by aerobic (oxygen using) bacteria. DO levels drop.  This is eutrophication.
  
  

• Organic waste additions (anything once part of a living plant or animal) enters waterways through death of aquatic plants, sewage, urban & agricultural runoff and discharge of food processing plants. Aerobic bacteria also consume organic waste, depleting oxygen levels. This use of oxygen is called biological oxygen demand.

• Turbulent water released from a dam can have such a high DO level that it can be toxic to organisms.

• The U.S. Environmental Protection Agency considers healthy water to have 5 mg/L dissolved oxygen, below 4 mg/L water quality is considered poor.