What is dissolved oxygen?

Dissolved oxygen refers specifically to the free molecular oxygen that is present in water, which is essential for the survival of aquatic organisms like fish, invertebrates, and some microorganisms. This oxygen is not chemically combined with other elements; instead, it exists in a free state, allowing it to be readily available for respiration by aquatic life. The concentration of dissolved oxygen can vary depending on several factors, including temperature, turbulence, and the presence of organic materials in the water.

Knowing that dissolved oxygen is critical for maintaining healthy ecosystems, it is important for water quality assessments. The other choices define concepts that relate to water quality but do not accurately describe dissolved oxygen. For instance, chemically combined oxygen refers to oxygen in compounds, salts pertain to dissolved minerals, and salinity is a measure of concentration of all salts in water, none of which pertain to the free molecular state of oxygen necessary for aquatic life.