If samples are collected the day before, aeration is necessary as some of the aquatic organisms are extremely sensitive to environmental changes. As the dissolved oxygen levels decrease, the organisms will rapidly die off. The dissolved oxygen levels begin to decrease almost immediately after the sample is taken from the water. Fish tank pumps, tubing and regulators may be purchased from a pet supply store or a department store and are fairly inexpensively. 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.
Bacteria use oxygen as part of the decomposition process.
Methylene blue indicator solution changes from blue to colorless in response to low oxygen levels and is an indirect indication of how much organic material is present in a water sample. The faster the color changes, the less oxygen is present, and the more carbon dioxide and bacteria. If it is impractical to complete this lesson in a one hour time frame, the test tubes may be prepared and examined during the next class period. Lesson 3-Design an experiment to test the effects of increases or decreases in dissolved oxygen levels on plants and animals. This will familiarize them with the dissolved oxygen test kits and also give them a basic understanding of how dissolved oxygen effects aquatic organisms.