Science JunctionLesson 1

 Lesson 1-What is dissolved oxygen and how is it important to an aquatic ecosystem?

Students will be able to: Introduction:
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 5-6 parts per million to support a diverse population. 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.

Note: Safety goggles and aprons should be worn at all times during this lab activity.

Fresh water sample from a local stream or pond
1000 milliliter beaker or large container
3 500 milliliter beakers or jars to set up miniature water ecosystems
Dissecting microscope and hand lenses
Graduated cylinder
Dissolved Oxygen test kit (Chemical)
Petri dishes
Fish tank pump and tubing with a t-connection so the tubing can provide air to 2 beakers
Air regulator for beaker 2 or a paper clip

Working with a partner:
1. Take an approximate 1000 ml sample of the water sample from a local stream or pond.
2. Pour a portion of the sample into a petri dish and examine it under low power on the dissecting microscope or with a hand lens.
3. Provide a detailed description of the water sample. What types of organisms are found?
Are the organisms living?

4. Test the dissolved oxygen levels of the water sample.
Initial dissolved oxygen reading:

5. Separate the water sample into the three beakers or jars and label as follows:

6. Place beaker 1, the control, in an area where it will not be disturbed for a 24 hour period.
7. Place beaker 2, slight aeration, and beaker 3, increased aeration, side by side.
8. Plug in the fish tank pump and place the tubing in beakers 2 and 3.
9. To reduce the amount of air generated to beaker 2, the tubing may be crimped slightly with a paper clip or a regulator may be used.

Note: Beaker 2 should be receiving less air than beaker 3.

10. Place beakers 2 and 3 in an area where they will be undisturbed for a 24 hour period and the air pump can remain plugged in and running.
11. Based on the initial levels of dissolved oxygen, hypothesize how the biodiversity of each water sample will change over the 24 hour period. Will the organisms increase or decrease? Make sure your hypothesis includes a statement about the dissolved oxygen levels.

12. After the 24 hour period, place a portion of each of the water samples from beakers 1, 2 and 3 in a petri dish and examine it under low and high power on a dissecting microscope. Provide a detailed description of each sample. Record your description in the appropriate column in the data table.
13. Test the dissolved oxygen level of each sample and record the number in the appropriate column in the data table.

Data Table:

After the observations are complete, answer the following questions.
1. Does the data collected support your hypothesis? Why or why not?

2. Write a short paragraph explaining how each water sample changed based on the dissolved oxygen levels. What can you conclude about the dissolved oxygen levels and biodiversity?

3. Research the effects of decreased levels of dissolved oxygen on aquatic plants and animals. Describe the causes of decreased oxygen levels and determine how these causes can be reduced or eliminated.


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©1998 April J. Cleveland for Science Junction, NC State University. All rights reserved.
page design by Lisa Leonor Grable and April J. Cleveland 
Last Modified: 6/8/98
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