Dissolved oxygen (DO) is necessary for nearly all aquatic life to survive. Certain processes add oxygen to a waterbody, while others remove or consume oxygen. Oxygen is added to a stream or lake from the atmosphere through mixing in turbulent areas. Plants also contribute oxygen through photosynthesis. Oxygen is removed in surface water by decomposition or organic material, respiration, and chemical processes. DO in waterbodies can be affected by:* Water Temperature: Cold water holds more oxygen than warm water.* Season: DO levels are higher in winter than in summer.* Time of Day: On a sunny day, DO levels rise from morning through the afternoon as aresult of photosynthesis, reach a maximum in late afternoon, and steadily fall during thenight, reaching their lowest point before dawn.* Stream Flow: DO will vary with the volume and velocity of water in a stream; fastermoving water mixes readily with atmospheric oxygen, thus increasing DO.* Aquatic Plants: Plant and algae growth in a stream will affect the oxygen contributed byphotosynthesis during the day and depleted by plant respiration at night.* Dissolved or Suspended Solids: Oxygen dissolves more readily in water that does notcontain high amounts of salts, minerals, or other solids.* Human Impacts: Lower DO levels may result from human impacts including organicenrichment, urban stormwater runoff, riparian corridor removal, stream channelization, and dams.Dissolved oxygen is measured in milligrams per liter of water (mg/L). Iowa standards, which are set to protect aquatic life, call for a minimum of 5 mg/L of DO in warm water streams and 7 mg/L in coldwater streams.