How does warmer water affect dissolved oxygen?

• A. Warmer water holds more dissolved oxygen.
• B. Temperature has no effect on dissolved oxygen.
• C. Warmer water holds less dissolved oxygen.
• D. Warmer water increases oxygen absorption from the air.

Answer: (C)

Warmer water holds less dissolved oxygen because the solubility of gases in water decreases as temperature rises. This means cooler water can trap more oxygen, while as the water warms, it can’t keep as much dissolved oxygen even before considering how fast organisms are using it. So, even if oxygen is entering from the air or produced by photosynthesis, the overall dissolved oxygen level tends to fall with higher temperatures. The idea that warmer water would hold more oxygen or that temperature has no effect conflicts with the basic solubility behavior of gases. And while surface exchange can increase with agitation, it doesn’t compensate for the reduced capacity to dissolve oxygen at higher temperatures.