Determining Overall Water Quality Using a Macroinvertebrate Survey
To develop a foundation and complete understanding of the role that macroinvertebrates play in aquatic ecosystems, we must be aware of their presence and activity within them. This is the first of two class activities to help you and your students gain knowledge of the organisms found in your own source, which can be translated into true stewardship of your environment. Note: Safety goggles and aprons should be worn at all times during this lab activity.
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- Water sample from a local stream or pond (sample must be fresh and contain debris from the bottom and edges of the water)
- Macroinvertebrate Identification charts
- Assorted trays and petri dishes for separation of sample
- Dissecting microscope and hand lenses
- Concave slides
Students will be able to:
- Identify various types of macroinvertebrates found in water ecosystems
- Perform a quantitative macroinvertebrate survey to determine the overall water quality of a water ecosystem.
For best results, observations should be made within 24 hours after collection of the samples. If samples are not observed immediately, aeration of the samples is needed during the next 24-hour holding period. This is necessary because of the susceptibility of some of the organisms to environmental changes. Organisms which are most susceptible are also ones which are intolerant to pollution. As the dissolved oxygen levels decrease, these organisms will rapidly die off. Dissolved oxygen levels begin to decrease almost immediately after the sample is taken.
Divide students into working groups and provide each group with a sample of the collected water (including debris) of approximately 500 ml.Have students pour a portion of the sample into a petri dish, then retrieve drops of the sample using a pipette, making sure any noticeable debris is captured. Place 1-2 drops on a concave slide and examine under low power on a microscope.Using identification charts, count and identify 100 macroinvertebrates. Explain that students should not count only the large ones, or their sample will be biased. If 100 macroinvertebrates cannot be found, provide an additional 500 ml sample. Continue this procedure until students have counted and identified 100 macroinvertebrates.
Provide students with a chart to keep count of the number of each type of organism listed.
Using the Chart:
Multiply the number of each type of organism by its biotic value. Add all the numbers and divide by 10 to determine the Biotic Index Value for their sample. Check the Biotic Value Chart to determine the quality of their water sample. For example if you find: