Download Dissolved Oxygen Test

Dissolved Oxygen Test
Dissolved oxygen (DO) is critical for the animals that live in the water. Just as land-based
creatures need oxygen to live, so do aquatic animals. The more oxygen dissolved in water is
usually better for aquatic life. You typically have the greatest diversity of life in waters with high
levels of dissolved oxygen.
Oxygen comes into water in two ways. The first is photosynthesis. Plants and algae in the water
produce oxygen during the daytime. Those same plants consume oxygen during the night. If
there are many plants in the water, oxygen levels may increase as the day goes on and plants are
photosynthesizing more.
Oxygen also enters the water directly from the air. Moving water and wave action mixes and
dissolves oxygen into it. Waterfalls and rapids tend to increase the amount of oxygen in water.
As water heats up, gases are driven out of the water. For example, a can of soda pop has carbon
dioxide gas dissolved in it, which we call carbonation. As the pop heats up, the carbon dioxide is
driven out and the pop goes flat. Warmer water will have less oxygen in it than colder water.
The Test
Where you get your water and how you collect it for dissolved oxygen is important. Make sure
you fill and cap your sample bottle under the water. After it is capped, if there are air bubbles in
the sample, you need to collect it again.
Try to collect water in the middle of the stream – not too close to the top, bottom or edges. Try to
get the sample in a portion of the stream that “looks” like the rest of the stream, with the same
speed and amount of rocks.
After you conduct the chemical testing, you will get a dissolved oxygen reading in ppm or parts
per million. Because the amount of oxygen water can hold changes on temperature, you need to
use the chart below to determine the percent of saturation of oxygen in the water.
If your dissolved oxygen reading is under 90 percent saturation, you might want to figure out
why. Here are some common reasons:
1. Lack of turbulence in the water – Are the rocks and gravel that are normally at the bottom
of the stream covered by sand or silt? If so, erosion control measures might help.
2. Temperature change – If the water is very warm, it can drive out dissolved oxygen.
3. “Muck” – What does the bottom of the stream look like? If there is a lot of decaying plant
and animal matter, which looks like black muck, that can use up much of the oxygen in
the water
http://msue.anr.msu.edu/news/youth_water_quality_tests_part_2_dissolved_oxygen