Water School

What is dissolved oxygen?

Dissolved oxygen, or DO, is the amount of oxygen in surface water available for aquatic life.

Aquatic plants and algae contribute to the presence of dissolved oxygen. Dissolved oxygen is necessary for aquatic species, such as fish and insects, that rely on water for their oxygen intake, absorbing it through their gills or bodies.

Dissolved oxygen is an important parameter used in water quality monitoring. The concentration of DO in water is dependent on the temperature of the water. Warm water holds less pure oxygen gas than cold water because water molecules expand as they heat up, leaving less room for oxygen molecules to dissolve.

Dissolved oxygen in water follows daily and seasonal patterns based on biological processes and water temperature. DO is highest when the sun is at its peak in the afternoon as aquatic plants are photosynthesizing, producing food by taking up carbon dioxide and releasing pure oxygen gas into the water. The rate is lowest in the morning after bacteria have been respiring or breathing, which is when they take up pure oxygen gas from the water and release carbon dioxide during the night. 

Biological functions, such as photosynthesis and respiration, are major factors in the daily cycle of DO, whereas ambient air and water temperatures factor more into the seasonal cycle. In the winter months when the water is colder, more pure oxygen gas can dissolve into the water. As the water heats during the summer, sometimes very quickly when the water is shallow or pooled, less oxygen dissolves into the water. The reduced oxygen holding capability of the water and high temperatures can result in the mortality or death of aquatic animals when experienced for a prolonged period.

To learn more about how dissolved oxygen impacts the Brazos River and its reservoirs, click here.

Return to Water School to learn more about water!

Indicators: Dissolved Oxygen | US EPA 
11 Amazing facts about Dissolved Oxygen in the Water - RLT Solutions




The information provided on this site is intended as background on water within the Brazos River basin. There should be no expectation that this information is all encompassing, complete or in any way examines every aspect of this very complex natural resource.

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