Effect of Nitrate and Phosphate Levels on the Growth of Algae


GRADES: 6-12


TIME NEEDED: 15 minutes (day 1); 5 minutes (days 2-4); ½hour (day 5)




Students will be able to:

1. Determine if an increase in nitrate and phosphate levels will effect the growth of

Chlorella, an algae.

2. Evaluate the nitrate and phosphate levels at which an increase in algal growth occurs in a closed system.




Three 15 ml test tubes with screw caps

distilled water

graduated cylinder

10% nitrate/phosphate fertilizer solution (use liquid fertilizer with a high nitrate/phosphate content)

20% nitrate/phosphate fertilizer solution

Chlorella algae culture (Ward's)

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.






  1. Label the first test tube "distilled water" and fill it¾full with distilled water. This is the control tube.
  2. Label the second test tube "10% nitrate/phosphate solution" and the third test tube "20% nitrate/phosphate solution". Fill both test tubes ¾ full with the appropriate solution.
  3. 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.
  4. 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.
  5. Check the test tubes every day for the next five days. Record the observations on a 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 a data table each day. 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?


Adapted from: April J. Cleveland, "Science Junction", North Carolina State University, 1998.