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Author(s): Nancy Moreno, PhD, and Barbara Tharp, MS.

Can Nutrients in Water Cause Harm? (cont.)

Eutrophication is when large amounts of nutrients are present in lakes, streams or the ocean. Algae and other microscopic organisms may grow so abundantly that they choke out other water life. The algal growth blocks sunlight. This causes underwater plants, which provide food and shelter for many animals, to die. In addition, when the algae begin to die and decompose, dissolved oxygen needed by fish and other animals is used up. This process occurs naturally over hundreds or thousands of years in some aquatic ecosystems. However, human activities accelerate eutrophication by increasing the rate at which nutrients enter bodies of water. In the image shown on the slide, eutrofication has occurred in the waters of Lake Maracaibo, Venezuela.

Session 2: Beginning the experiments

  1. Allow time for groups of students to observe the three bottles. Each student should record his or her own observations. Ask, Do you notice any differences among the bottles? Why or why not? Have students observe the water using a hand lens or microscope.

  2. Explain to the students that they will investigate what happens when nutrients, in the form of fertilizer, are added to aquatic ecosystems. Most students will be familiar with the word “fertilizer” from the story, Mystery of the Muddled Marsh. Make sure that they understand that fertilizer has good applications and that it can be very important for food production. 

  3. Show the chemical fertilizer and fish food to the class. Help the students understand that both substances will add nutrients to the water in the bottles. 

  4. Ask one student to add three drops of liquid fertilizer to the bottle labeled “C,” and another student to add a large pinch of fish food to the bottle labeled “N.” Have students predict what will happen in each bottle over the course of the next week. The bottles should be kept in a bright window or under bright fluorescent lights. 

Session 3: Looking at results

  1. Have students observe the bottles every day and write or draw their observations on their student sheets. 

  2. After about a week, have students discuss their results within small groups. Have them compare the appearance of the three bottles. Ask, Which bottle has the cloudiest water? Which bottle has the clearest water? Students also may be able to observe differences in water color and/or the amount of organisms in their bottles. Older students may want to compare the amount of organisms in a drop of water from each bottle. In general, expect the bottles with chemical and natural fertilizers to grow more algae and other microorganisms. Given enough time, these cultures may turn brown and develop a foul smell. 

  3. Discuss the results with the class. Ask, What happened when we added more nutrients to the water in the bottle? What do you think will happen if we continue to add more nutrients to the bottles? Help the students make extensions to other situations by asking, What can we do to reduce the amount of fertilizer that washes into lakes and streams? What would happen if no one used fertilizers at all? Can you think of ways we can use the fertilizer we need to grow food without polluting our waterways?

Funded by the following grant(s)

National Institute of Environmental Health Sciences, NIH

National Institute of Environmental Health Sciences, NIH

My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932

Houston Endowment Inc.

Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education