Dissolved oxygen typically refers to the amount of gaseous oxygen that has dissolved into a body of water. It is a common ecological measurement that indicates the overall health of the aquatic ecosystem.
All living objects in an aquatic ecosystem use oxygen in some way. If the amount of oxygen in the water is too high or too low, it can cause life to die. The amount of dissolved oxygen is affected by many factors, but mainly temperature and dead organic matter. During colder months, the water is colder, which allows it to hold much more oxygen than in summer months. Oxygen can also be consumed by bacteria as they break down dead organic matter.