2.7. COOL CHANGE
(Level: 10 acad :: Water Quality)

Purpose: To study the effect of temperature on the rate at which organisms use oxygen.

What You Need: 4 1-litre/quart jars with screw tops :: 12 15-cm strands of an aquatic plant (e.g. Elodea) :: distilled water :: 2 paper bags :: masking tape :: marker :: dissolved oxygen kit* :: refrigerator or cold storage

(*see Unit 12. Resources for sources)

What You Do:

Prior to the experiment, if distilled water is not available, regular tap water can be used provided it has been left standing for 48 hours. The sample plant should be housed under a bright light and have a temperature of 25 deg. C, which has been maintained over several weeks.

1. Label the jars consecutively 1, 2, 3, 4 and fill all the jars to the top with distilled water.
2. Measure the dissolved oxygen content of Jar 1. Be very careful not to agitate the water when taking the sample. Record the results. We can assume that there will be the same amount of oxygen in each of the 4 jars. Slowly refill Jar 1 with distilled water and screw the lid on tightly.
3. Add six 15cm strands of the aquatic plant to each of Jars 2 and 4. Measure carefully - you must have the same amount of plant material in each jar. Screw the lids on tightly.
4. Screw the lid on tightly of Jar 3.
5. Label Jars 1 and 3 as the "Control".
6. Place Jars 1 and 2 in a paper bag and set in cold storage (refrigerate, but do not freeze).
7. Place Jars 3 and 4 in a paper bag and set out in a constant room temperature.
8. Allow both bags and contents to sit overnight.
9. The next day, record the temperature in each of the 4 jars. Test each jar for the dissolved oxygen content. Determine the difference in oxygen between each pair of plant jars and control jars for each temperature. Record all results on the chart provided.

Questions:

1. What temperatures showed the greatest use of oxygen?
2. What is the purpose of the controls?
3. How do different water temperatures affect the rate at which cold-blooded (i.e. variable body temperature) organisms, such as amphibians, use oxygen?
4. If a small pond supports a large number of aquatic organisms like frog tadpoles, how would a hot day affect the amphibians present? At what time of day would the amphibians feel the greatest stress?
5. Do amphibians need more or less oxygen in the winter? Why?

Click here to see the answers to these questions.
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