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Critter Clues

Objectives:  Students will learn how aquatic organisms can be indicators of water quality.

Grade Level: 3-6

Groupings: Small groups

Materials: Pictures of different bodies of water; Biotic Index of Water Quality; plastic sandwich bags with a selection of macroinvertebrate pictures cut and duplicated from the Biotic Index; water critter costume or mask (optional, see step #2).

Time Allotment:  20 Minutes


1.  Show the students pictures of different bodies of fresh water: lakes, rivers, streams, ponds, puddles. Include some pictures that show pollution. Ask for their impressions of the quality of the water. Would they like to visit there? Would they like to go swimming or boating? Does any of the water look clean enough to drink? What do they think they would find living in these different bodies of water? Record their list of organisms. Would they expect to find the same things living in each of the bodies of water? Why or why not?

teacher with masked student 2.  Explain that good indicators of water quality are the small plants and animals who live there. The organisms scientists look for are benthic macroinvertebrates. You may make a costume or mask depicting one of the critters by enlarging their picture from the Biotic Index of Water Quality onto poster board. Use this critter to introduce the class to the world of macroinvertebrates and to introduce how and why they are important indicators of water quality. (See Listen to Lydia). Use an aquatic field guide to help with the critter's introduction.

3.   Divide the children into small groups. Pass out a Biotic Index of Water Quality to each group. Ask them to find the organism they were just introduced to. In what quality of water are they typically found? Explain that one organism is not enough to classify the water quality of a body of water. Scientists need to look at many organisms and consider the types and amounts of each. Tell the students they will now get a chance to use the Biotic Index to analyze some fictional bodies of water.

4.   Give each group a plastic sandwich bag containing a selection of pictures of benthic macroinvertebrates (see Guidelines for Creating Sample Bags). This water sample bag represents the diversity of organisms that were collected from a body of water. Have the students sort these organisms by type and record the amounts of each. Then have the students analyze this information using the Biotic Index to determine the water quality.

5.  When all groups have rated their body of water, have the groups with similar ratings compare their findings. Have the students discuss variations and similarities. As a class, share the results from all the groups.

Extensions: a.   Have your students use field guides to write descriptions of other benthic macroinvertebrates. Have them create costumes and introduce their critter to the class.

b.   Have the class analyze a local body of water. Visit the area and record general impressions of the water and surrounding area. Sample the water using wire mesh kitchen strainers as dip nets. Look for organisms living in aquatic vegetation and samples of mud. Bring at least two buckets for collecting. Use one for sorting critters from the water and mud, then transfer them to a second bucket filled with clear water. Have the students count and record the number of each type of critter. Using the Biotic Index, determine the quality of the water. How could they maintain or improve the water quality?

Enlarge the pictures of the various benthic macroinvertebrates from the Biotic Index. Make 10- 15 copies of each critter and mount these on index cards and label them. Make up sample bags by adding these cards to plastic sample bags based on the following general guidelines.

Good quality Majority Several A few
Wide Ranging Quality A few Majority A few
Poor Quality None to a few A few Majority

Good quality water has a high diversity of critters living in it. A sample bag would contain several different types of critters, possibly from all three groups with the majority in Group 1. There is usually a low diversity of critters in poor quality water. A sample bag would contain only a few different critters.


Who lives in a body of water depends on the physical features of the water. Several of these features are easy to measure and give important clues to water quality.

Temperature Most aquatic creatures are cold-blooded, so surrounding water temperature determines and limits their growth and metabolism. Colder water has more oxygen in it, and likewise, warmer water has less oxygen in it. Using aquarium thermometers, have students take water temperatures in different areas of a body of water.
pH Most aquatic animals thrive in water whose pH is in the neutral range (6.5-8.5). Acid rain is the main contributor to aquatic pH changes. Using pH paper, have students test a body of water in several locations, collect and test rain water, and compare readings.
Turbidity Turbid water is cloudy water. Loose soil, sediments, and pollutants from erosion, runoff and waste water contribute to turbidity, as well as algal growth caused by excess nutrients such as phosphorus and nitrogen. The effect of turbid water on water quality is complex. Less light can enter, inhibiting plant growth, which in turn limits food supply and reduces oxygen levels. Suspended sediments also absorb heat, causing temperatures to rise, further reducing oxygen levels and diverse life forms. Create a Secchi Disk by painting a black "X" on an 8-inch diameter jar lid. Punch a hole in the center and suspend it from a long rope. Have students lower the disk into standing water until it is no longer visible, then direct them to slowly raise it until they can just barely distinguish the "X" mark. Have them grasp the string at the surface and measure the distance between the water surface and the disk. Compare readings in several places.

Meet Lydia. Her real name is Plathemis lydia. That's Latin for White-tailed Dragonfly. She is one of the many benthic macroinvertebrates who live in water. Benthic means bottom dwelling. Look in the sediment and debris at the bottom of a pond to find Lydia or her close relatives. Macro means she's big enough to be seen without a microscope, and invertebrates describes a whole group of critters without internal backbones unlike you and me. Most benthic macroinvertebrates are insects in one or another stage of their life cycles. Lydia is just a young nymph, but through gradual metamorphosis, she'll become a dragonfly in a few years. Benthic macroinvertebrates are good storytellers, especially when it comes to water quality. Consider Lydia. She's been living in the same neighborhood for a few years, calling the muck at the bottom of Shelbourne Pond home sweet home. She can tell you about the history of the pond during her lifetime because she and other benthic macroinvertebrates are sensitive to both physical and chemical changes in the water. Although fast enough to escape predators, even Lydia, with her own special form of jet propulsion, can't swim away from pollution. She tolerates it, but only to a certain point when she gets sick and dies. Some critters are more tolerant and even flourish as the water quality declines, while others die or disappear with the slightest change. The saying "the more the merrier" applies to benthic macroinvertebrates as well. The greater the variety that lives in any body of water, the healthier it is. So next time you scoop up some water critters, listen to their story.

Biotic Index of Water Quality

* This material has been used with permission from Shelburne Farms, Copyright © 1995.

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