Macro-invertebrates

Background Information

The organisms that live in a stream are affected by the quality of the water in the stream. The poorer the water, the fewer the organisms. Some aquatic organisms, such as fish and frogs, can leave a polluted stretch of a stream; other organisms cannot. By identifying the organisms present in a water source, the water quality can be predicted.

Benthic macro-invertebrates are good indicator organisms and can be used to determine the health of a stream. Benthic means bottom dwelling, macro means large enough to be seen, and invertebrate means an animal without a backbone. Most benthic macro-invertebrates are aquatic insects or the aquatic stages of insects. Examples include stonefly nymphs, mayflly nymphs, caddisfly larvae, dragonfly nymphs, and midge larvae. Also included are such things as mollusks, crustaceans and worms.

Benthic macro-invertebrates are good indicator organisms because:

1. They are sensitive to physical and chemical changes in their habitat.
2. They cannot easily escape pollution.
3. They are easily collected and classified.

Test Procedure

1. Work in small groups, 2-4 students per group recommended.
2. Review pictures of pond water microorganisms that you have been given.
3. Prepare wet mount slides and use your microscope to search and find at least 5 organisms.
4. If you find some you cannot identify immediately, prepare a sketch for later use in identification.
5. For each microorganism you find, you must supply the following information:
1. Name
2. Classification (kingdom, phylum, etc.)
3. Sketch
4. Description
5. Description of movement (if any)
6. Approximate population count (devise a method) and graph this data
7. Concluding statement

What to Expect

Examples of macro-invertebrates very sensitive to pollution:

• Stonefly nymph
• Mayfly nymph
• Caddisfly nymph
• Riffle Beetle
• Water Penny
• Planarian
• Gilled Snail
• Dobsonfly

Examples of macro-invertebrates somewhat sensitive to pollution

• Crayfish
• Sowbug
• Scud
• Alderfly
• Crane Fly
• Fishfly
• Aquatic Beetle
• Dragonfly
• Clam or Mussel
• Damselfly
• Watersnipe Fly
• Whirligig

Examples of macro-invertebrate not sensitive to pollution

• Aquatic Worm
• Midge Fly
• Leech
• Lunged Snail
• Black Fly

Click here for help in identifying some of these organisms.

References