What does the term dissolved oxygen (DO) refer to?

Dissolved oxygen (DO) specifically refers to the amount of oxygen that is actually dissolved in water. This is a critical parameter in aquatic environments because it is the form of oxygen that aquatic organisms, such as fish and invertebrates, can utilize for respiration. High levels of dissolved oxygen are essential for maintaining healthy aquatic ecosystems, as they support the metabolic processes of living organisms.

Other options describe different aspects of oxygen but do not accurately define dissolved oxygen. For example, the total amount of oxygen in a water body encompasses all forms of oxygen, including that which might be bound to other substances or present as gases. The percentage of oxygen in the atmosphere pertains to the concentration of oxygen in the air, not in the water. The oxygen utilized by aquatic life refers to the consumption of DO by organisms rather than its presence in dissolved form. Therefore, focusing specifically on the oxygen dissolved in water captures the essential meaning of dissolved oxygen as it relates to aquatic life.