Science JunctionLesson 2

 Does an increase in nitrate and phosphate levels effect the growth of algae?

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

3 test tubes with screw caps
distilled water
graduated cylinder
10% nitrate/phosphate solution
20% nitrate/phosphate  solution
Chlorella culture
light source

After reading through the activity, develop a hypothesis which states which of the solutions, distilled water, 10% nitrate/phosphate solution or 20% nitrate/phosphate solution, will contain the largest amount of Chlorella after a five day period.


Working with a partner:
1. Write your name and your lab partner's name on each test tube.
2. Label the first test tube distilled water and fill in three quarters full with distilled water. This is the control.
3. Label the second test tube 10% nitrate/phosphate solution and the third test tube 20% nitrate/phosphate solution. Fill both test tubes three quarters full with the appropriate solution.
4. Gently mix the Chlorella culture. Using the dropper, place 10 drops of the Chlorella culture into each tube. Shake each tube gently to mix the contents.
5. Place the caps on the test tubes tightly and lay them on their sides in a well lit area where they will be undisturbed for the five day period.

Note: A light source may be used to provide a well lit area.

6. Check the test tubes every day for the next five days. Record the observations on the data table. Growth of  Chlorella can be determined by an increase in the density of the algae as well as an increase in the darkness of the green color in the tubes. Record your results on the data table each day.

Data Table:

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

2. Were there any changes in the distilled water tube? Explain.

3. At what level of nitrate, 10% or 20%, did the most growth occur? Why?

4. Based on your observations, what would you conclude about the effects of excessive
nitrates and phosphates on the growth of algae?

5. How could the increased use of nitrates and phosphates (fertilizers), adversely effect an
aquatic ecosystem?


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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: 6/9/98
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