Why Monitor for Dissolved Oxygen

Let’s talk about why people monitor for dissolved oxygen.

Most aquatic organisms require dissolved oxygen in order to survive, but what exactly is it? Dissolved oxygen, or DO, is just like the oxygen humans use to breathe, but has been dissolved in water. Unlike the bonded molecules of H20, DO refers to the free oxygen that is present in water or other liquids.

Oxygen can enter the water in several ways, such as from the atmosphere, aeration, or photosynthesis. Once dissolved in water, oxygen is available for use by living organisms and can play a large role in many chemical processes in the environment.

In general, a body of water will trend toward 100% air saturation – achieving equilibrium with the atmosphere. This is key to a healthy system, however different events can greatly affect levels of dissolved oxygen.

Supersaturation happens when DO increases above 100%. This type of imbalance is common during peak growth phases of algal blooms where more oxygen is being generated than can be consumed. Turbulent conditions can also lead to supersaturation if more air gets mixed into the water.

Hypoxia occurs when DO decreases to a level that can no longer support aquatic life. As algal blooms fade and die, more oxygen can be consumed and DO levels will drop. Rapid increases in temperature, such as thermal pollution, can also cause hypoxia. As temperature increases, the level of DO in water decreases. Salty runoff can also result in a fish kill DO levels will decrease as salinity increases.

So why do we monitor for dissolved oxygen? A scientist interested in surface water would view it as a direct indicator of a water body’s ability to support aquatic life. A scientist studying ground water could use DO to help determine when stable conditions are met during purging or to regulate the analysis of metals or volatile compounds. A treatment plant operator would be concerned with DO levels for the microbes used to transform wastewater into harmless end products. And of course the public will be concerned about the health of fish and other aquatic species swimming in their local waters.

You can learn more at YSI.com/how-sensors-work