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Can Nutrients in Water Cause Harm?

Can Nutrients in Water Cause Harm?

Eutrification in the waters of Lake Maracaibo, Venezuela.
Courtesy of Wilfredo R. Rodriguez H.

  • Grades:
  • Length: Variable


Environmental Science and Health

Students create pond water cultures and investigate the impacts of adding chemical or natural nutrients. Student sheets are provided in English and in Spanish.

This activity is from The Science of Water Teacher's Guide. Although it is most appropriate for use with students in grades 3-5, the lessons are easily adaptable for other grade levels. The guide also is available in print format.

Teacher Background

In general, two types of sources contribute to water pollution in the United States. Point sources, such as factories, sewage treatment plants, abandoned mines and oil tankers, introduce pollutants into waterways at single places. This type of pollution is not always significant in terms of volume, but it is the major point of entry for toxic chemicals into water supplies. In most cases, point sources of pollution can be identified and monitored by government agencies.

Non-point source pollution occurs across large areas of land that drain into underground and surface water sources. Pollutants are collected and deposited by water as it travels over land and through layers of soil. Major contributors to non-point source pollution include agricultural activities (which can add chemical fertilizers, pesticides, manure and soil to water), logging and other activities that leave the soil surface bare (allowing soil to be washed into waterways), urban and suburban areas (where lawn chemicals, household chemicals, motor oil and gasoline can enter water supplies), and septic systems (which can contaminate underground water supplies with disease-causing bacteria). Non-point sources of water pollution are difficult to control because they are spread over large areas, and often result from the actions of many individuals.

In the Water unit's storybook, Mystery of the Muddled Marsh, which accompanies this unit, runoff from a new park development introduces soil and fertilizers into a marsh and stream ecosystem. Excess soil and fertilizers lead to murky water and overgrowth of plants, green algae and some microorganisms in the marsh, threatening the marsh animals and their habitat. Riff and Rosie (characters in the story) are able to connect development of the park to changes that they have observed in the marsh.

In this activity, students investigate, on a small scale, the changes that occur when fertilizers are added to pond water cultures.

Objectives and Standards


  • Many different kinds of organisms live in water.

  • Excess nutrients will cause over-abundant growth of some organisms living in water.

  • Non-point source pollution is a major threat to water supplies in the United States.

Science, Health and Math Skills

  • Measuring

  • Predicting

  • Observing

  • Drawing conclusions

Materials and Setup

Teacher Materials (see Setup)

  • 3 clear, 2-liter soft drink bottles,

  • Hay (or dried grass), 2 oz

  • Small container of fish food

  • Small container of liquid fertilizer

  • Spring water, gal (or tap water without chlorine, see Setup)

Materials per Student

  • Copy of “My Marsh Observations” page


  1. You will need to use a hay infusion kit (or pond water) to carry out this activity. Hay infusion kits may be ordered from science education supply companies. Set up your hay infusion culture in a two-liter bottle about one week before beginning the activity. Use one gallon of spring water (or let the same amount of tap water rest uncovered for 24 hours).

  2. As an alternative to creating your own “pond water” by means of a hay infusion kit, you may use water that you or your students have collected from a pond, ditch or stream. In this case, try to find water that has bits of green algae floating in it.

  3. You also will need three clear 2-liter plastic soft drink bottles. Cut the tops off the bottles to make cylindrical containers.

  4. Conduct this activity as a class demonstration. Have each student record his or her own observations on the “My Marsh Observations” sheets.

Procedure and Extensions


Three 30-minute sessions

Session 1: Set up pond water cultures

  1. Begin by asking students if they remember what happened to Marigold Marsh in the story, Mystery of the Muddled Marsh. Allow time for everyone to share his or her ideas. Then, tell students that they will be able to see some of the tiny plants and animals that lived in the muddled marsh while they conduct an investigation of what happens when fertilizer is added to a water ecosystem.

  2. Have one or two students label the three bottles “NF” (no fertilizer, or control), “N” (natural fertilizer), and “C” (chemical fertilizer). In bilingual classrooms, label the containers “SF” (sin fertilizante), “N” (fertilizante natural), and “Q” (fertilizante químico).

  3. Show students the prepared (or pond) water. If possible, put a few drops of the water under a microscope for students to observe. Explain that they will be growing similar living things in the bottles. Add about 250–500 mL of the hay infusion or pond water, along with some hay/dried grass, to each bottle.

  4. Set the soft drink bottles in a bright window or under bright fluorescent lights for 1–2 days to allow the culture to develop. (In conditions with low light, hay infusions will tend to develop mold and/or foul smelling bacteria within 2–3 days.) If using pond water that already has plenty of green algae and other growth, proceed directly to the next step without resting the cultures.

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?


  • Visit a nearby stream, marsh, or ditch with standing water and let students collect small samples of water. Have students observe their water samples in class using hand lenses or low-power microscopes. Students should compare what they see to their observations of the water in the soft drink bottles.

  • Keep one or more cultures of pond water alive in the classroom for longer periods of time by aerating the culture with a simple aquarium pump and plastic tubing inserted into the water.

  • Have student groups set up their own cultures and investigate the effects of one or both kinds of fertilizers on their systems.

Related Content

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  • Mystery of the Muddled Marsh

    Mystery of the Muddled Marsh Reading

    Mr. Slaptail and the cousins, Rosie and Riff, investigate harmful changes occurring in the local creek, pond and marsh.

  • Water

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    Students take a fresh look at water and examine its critical importance to the well-being of all living creatures. (11 activities)


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