How does organic material break down and effect the oxygen levels of an aquatic ecosytem?

Objectives:

Students will be able to:


  • Evaluate the relationship between oxygen levels and the amount of organic matter in an aquatic ecosystem.
  • Explain the relationship between oxygen levels, bacteria and the breakdown of organic matter using an indicator solution.

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

    Materials:

    3 different water samples (tap water, rain water, pond water, etc.)
    3 test tubes
    graduated cylinder
    test tube rack
    methylene blue in a dropper bottle

    Note: Bacteria act on methylene blue indicator solution and cause it to change from blue to a colorless liquid. The more bacteria a water sample contains, the faster the color of the methylene blue breaks down.

    Discussing the procedure with your partner, then write a hypothesis about the relationship
    between the amount of organic matter, the amount of bacteria and the oxygen levels in
    a water sample.

    Hypothesis:

    Procedure:


    Working in pairs:


    1. Label three test tubes 1, 2 and 3. Put 5 ml of a different water sample in each tube.
    2. Add 20 drops of methylene blue to each test tube. Put the test tubes in a test tube rack
    and observe them at 10 minute intervals for 1 hour. Record your observations in the data
    table.

    Data Table:

    Test Tube Type of Sample After 10 mins After 20 mins After 30 mins After 40 mins After 50 mins After 60 mins
    1              
    2              
    3              


    After the observations are complete, now answer the following questions.

    Conclusions:


    1. Does the data collected support your hypothesis? Why or why not?



    2. In which test tube did the blue color disappear first?

    3. Which water sample contains the most organic matter? Which contains the least organic matter?

    4. Which water sample contains the most oxygen? Which water sample contains the least oxygen?

    5. Least 3 possible sources of organic matter in each positive sample.


    6. Write a short paragraph explaining the relationship between oxygen levels, bacteria and the breakdown of organic matter. What can you conclude about the dissolved oxygen levels and the amount of bacteria found in a water sample?