Water What-ifs
Dissolved Oxygen Mini-workshop

D dissolved oxygen is one of the best indicators of the health of a water ecosystem. Dissolved oxygen can range from 0-18 parts per million (ppm), but most natural water systems require a range of at least 5-6 parts per million to support a diverse population.

Oxygen enters the water by direct absorption from the atmosphere or by plant photosynthesis. The oxygen is used by plants and animals for respiration and by the aerobic bacteria which consume oxygen during the process of decomposition. When organic matter such as animal waste or improperly treated wastewater enters a body of water, algae growth increases and the dissolved oxygen levels decrease as the plant material dies off and is decomposed through the action of the aerobic bacteria.

Decreases in the dissolved oxygen levels can cause changes in the types and numbers of aquatic macroinvertebrates which live in a water ecosystem. Species which cannot tolerate decreases in dissolved oxygen levels include mayfly nymphs, stonefly nymphs, caddisfly larvae and beetle larvae. As the dissolved oxygen levels decrease, these pollution-intolerant organisms are replaced by the pollution-tolerant worms and fly larvae.

Dissolved oxygen levels change and vary according to the time of day, the weather and the temperature. If yearly comparisons are made on dissolved oxygen levels, they should be done at the same time of day, during the same season and on a day with a temperature variation of only 10 degrees Celsius from the previous reading.

A decrease in the dissolved oxygen levels is usually an indication of an influx of some type of organic pollutant, such as animal waste or improperly treated wastewater.

After completing the tutorial you should be able to

• explain the purpose of measuring dissolved oxygen levels and describe how it is determined and recorded.
• Interpret dissolved oxygen levels and analyze their meaning using LaMotte Dissolved Oxygen kits.

Goggles and gloves should be worn while performing the dissolved oxygen tests.

Sampling Method

1. Rinse the sampling bottle (0688-DO) with the sample to be tested.
2. Replace the top on the empty sampling bottle and submerge it in water sample to be tested.
3. After submerging the sampling bottle, remove the top under the water and fill the bottle while tapping the sides to release the air bubbles.
4. Replace top while the bottle is submerged.
5. Remove the full, capped bottle from the water and remove the top.

1. Add 8 drops of Manganous Sulfate Solution (4167) to the bottle of sample water.
2. Add 8 drops of Alkaline Potassium Iodide Azide (7166) to the bottle of sample water. Put the top on the bottle and invert 2 or 3 times. A precipitate will form and should be allowed to settle below the shoulder of the bottle before continuing with the next step.
3. Use the 1.0 gram measuring spoon (0697) provided with the kit to add 1 gram of Sulfamic Acid Powder (6286).
4. Put the top on the bottle and invert to mix until the precipitate is dissolved. A clear yellow to orange brown color will develop depending on the oxygen content of the sample.
5. The sample is now "fixed" and ready to be tested.

Testing the Water Sample

If the color of the fixed sample is already a faint yellow, skip steps #3 and #4 and continue with the second part of step #5 which is the addition of Starch Indicator Solution.

1. Fill the titration tube (0299) to the 20 ml mark with the fixed sample.
2. Fill the Direct Reading titrator (0377) with Sodium Thiosulfate (4169).
3. Place the titrator in the top of the titration tube and add one drop of the Sodium Thiosulfate and swirl.
4. Continue to add Sodium Thiosulfate dropwise until the sample is a faint yellow.
5. Remove the top of the titration tube and the titrator. Add 8 drops of Starch Indicator Solution (4170WT). This will cause the sample to turn blue.
6. Replace the top of the titration tube and the titrator. Continue to add Sodium Thiosulfate dropwise until the blue color disappears.
7. Read the titrator scale in parts per million (ppm).

To support a diverse population of organisms, what should the dissolved oxygen levels be?

If a mayfly nymph is found in a body of water, would the dissolved oxygen level be low or high?

Once you have completed the mini-workshops, test your knowledge with the Water Quality Post-test. Once your completed post-test is submitted, you will receive a password which will give you and your students complete access to all areas of the Water What-ifs web site. Thanks for taking part in this research project on water quality.