Certain factors affect the water temperature. These factors are seasons of the year, time of day, and water depth. 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. 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.