What does saturated mean in the context of dissolved oxygen in water?

• A. Water has zero DO
• B. Water is holding all the DO it can at a given temperature
• C. Water cannot dissolve more oxygen
• D. Water is at sea level pressure

Answer: (B)

Saturation means the water is holding all the dissolved oxygen it can at that temperature. The amount of oxygen that can stay dissolved in water depends on temperature (and also pressure and salinity). At lower temperatures, water can keep more oxygen; at higher temperatures, it can hold less. When the DO concentration reaches that temperature- and pressure-specific maximum, the water is saturated. If more oxygen enters under those conditions, it won’t stay dissolved and will tend to escape or be consumed until the concentration again matches the maximum for the current conditions.

Zero DO isn’t saturation; that would be a deficient state. Saying the water cannot dissolve more oxygen is related but incomplete, because saturation is defined for the current conditions. Pressure alone (like sea level pressure) doesn’t define saturation; it’s the combination of temperature and pressure that sets the limit. In practice, DO is often expressed as a percent of the maximum possible DO at that temperature, i.e., 100% saturation.