Aquatic life uses oxygen that is dissolved in the water and is in much smaller quantities than in the air. Oxygen in water is measured as dissolved oxygen (DO). Through rain, wind, waves, and currents, oxygen is mixed in from the air above. Water that moves faster contains more dissolved oxygen because it has more contact with the air than still water. Underwater plants and algae also contribute oxygen that is given off during photosynthesis. Weather, temperature and salinity affect DO levels. Dissolved Oxygen Percent Saturation is also important to determine because cold, fresh water holds more oxygen than warm or salty water. For example, water at 8°C will be 100% saturated with 8 ppm DO.

Low DO is also caused by fertilizer and manure runoff from streets, lawns and farms. Fertilizers and fecal matter encourage the growth of too much algae, which uses up the oxygen quickly. Plants and animals die and are decayed by bacteria, which also uses up a great deal of oxygen. If more oxygen is consumed than is produced, dissolved oxygen levels decline and some sensitive animals may move away, weaken, or die.

Aquatic animals are most vulnerable to lowered DO levels in the early morning on hot summer days when stream flows are low, water temperatures are high, and aquatic plants have not been producing oxygen since sunset.

1. Record the temperature of the water sample.
2. Submerge the small tube (0125) into the water sample. Carefully remove the tube from the water sample, keeping the tube full to the top.
3. Drop two Dissolved Oxygen TesTabs (3976) into the tube. Water will overflow when tablets are added.
4. Screw the cap on the tube. More water will overflow as the cap is tightened. Make sure no air bubbles are present in the sample.
5. Mix by inverting the tube over and over until the tablets have disintegrated. This will take about 4 minutes.
6. Wait 5 more minutes for the color to develop.
7. Compare the color of the sample to the Dissolved Oxygen color chart.
8. Record the result as ppm Dissolved Oxygen.

• Locate the temperature of the water on the % Saturation chart.
• Locate the Dissolved Oxygen result of the water sample at the top of the chart.
• The % Saturation of the water sample is where the temperature row and the Dissolved Oxygen column intersect. For Example:
  • If the water sample temperature is 16º C and the Dissolved Oxygen result is 4 ppm, then the % Saturation is 41.

Results will support aquatic life well. Waters with consistently high DO levels (greater than 5 ppm) are most likely healthy and stable environments capable of supporting a variety of aquatic organisms. If DO levels are severely low, large quantities of fish may die. DO levels below 2 ppm will likely not support aquatic life for long.