Dissolved Oxygen

What is dissolved oxygen?

Oxygen is a natural element needed by many forms of life, including aquatic organisms. Most aquatic animals use oxygen dissolved in water for respiration. Oxygen primarily enters water through diffusion from the surrounding air, which can be enhanced by turbulence, and from photosynthesis by aquatic plants. 

Dissolved oxygen is measured in units of milligrams/liter (mg/L) or as a percent of saturation (%).

Oxygen bubbles under water.
Photo Credit: Pixabay

Turbulence at the water’s surface can increase the amount of oxygen that is dissolved into the water from the atmosphere. 

Why do we measure dissolved oxygen?

Oxygen is necessary for living things and for many of the chemical processes that take place in water. The amount of dissolved oxygen needed by an aquatic organism depends on a variety of factors, including the species, the organism’s metabolic rate and overall health, and water temperature. Organisms typically have an optimum range in which they do best.

Graphic showing the range of tolerance for dissolved oxygen in fish. 3 mg/L is the lowest tolerated. Between 3-5 mg/L there are few fish. Reproduction is possible at 6 mg/L and Growth and Activities is optimized at 7 mg/L. There are Abundant fish at 10 mg/L.
Source: EPA; Graphic Credit: Illinois-Indiana Sea Grant

What affects dissolved oxygen measurements?

The temperature and salinity of water influence how much oxygen it can hold. Warm water holds less dissolved oxygen than cold water because the molecules are moving faster than in cold water, thereby allowing oxygen to escape from the water. Freshwater can hold more dissolved oxygen than saltwater because saltwater has less space for oxygen molecules due to the sodium and chloride ions it contains. Therefore, the warmer and saltier the water, the less dissolved oxygen it will contain.

As mentioned previously, oxygen is added to water at the surface where gases in the atmosphere come into contact with it. Therefore, the movement of water from wind and waves also can help oxygenate water. In addition, deeper water gets oxygen from the upper layers when mixing occurs.

Dissolved oxygen can also be influenced by humans. For instance, additional nutrients can enter a waterbody in runoff from lawns or farm fields, and cause a large increase in aquatic plant growth. While initially this may raise oxygen levels through photosynthesis, excess algal growth will consume oxygen on cloudy days and at night via respiration. Also, as all the plants die off, oxygen will be consumed by bacteria in the decomposition process.

Dissolved Oxygen Factsheet

Dissolved Oxygen