Dissolved Oxygen

The oxygen that makes aquatic life possible does not form bubbles, nor is it the oxygen that is part of the H2O water molecule. It is separate O2 molecules that are dissolved in the water and invisible to our eyes. The oxygen concentration of most healthy streams is between 6 and 12 oxygen molecules per one million water molecules (mg/L).

Oxygen dissolves in water by:

  • Atmospheric oxygen mixes into streams and lakes through diffusion and  in areas of turbulence, such as riffles.
  • Aquatic plants release oxygen into the water during photosynthesis.

There are variety of causes for change in dissolved oxygen concentrations:

  • Elevation
  • Temperature
  • Saltiness
  • Turbulence
  • Aquatic Life
  • Vegetation
  • Land Uses
  • Introduction to organic wastes

Dissolved oxygen is important as all aquatic animals need it to breathe underwater. Furthermore, oxygen concentrations affect the behaviors of other chemicals in the water.

The State of Utah has set minimum dissolved oxygen concentrations to protect fish and other aquatic animals. In addition, different species and different activities require different concentrations of oxygen. You can find the water quality standards as part of the Utah Administrative Code R 317-2. For more information on dissolved oxygen read Understanding Your Watershed: Dissolved Oxygen.

When sampling in the field:

 

Steps

Illustration

Rinse the collection cup with stream water three times and then fill to 25 mL with water 15 cm below the surface
Place glass ampoule in cup and break tip under the water. Let ampoule fill with water (30 seconds).
Mix the ampoule by turning it up and down several times. Wait 2 minutes.
With even light shining on the comparator place the test ampoule in front of the color standards. Place on both sides to determine the best color match. Record the concentration that the test ampule most closely matches.