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
