The Effect of pH Level Change on an Aquatic System

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 second 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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Materials:

• 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
• 300 ml beakers or jars to set up miniature water ecosystems
• 1% NaOH in dropper bottles
• 1% HCl in dropper bottles
• Wide range pH paper or pH probe
• Dissecting microscope and hand lenses
• Concave slides
• Pipettes

Objectives:

Students will be able to:

• Determine if changes in pH affect the quality of an aquatic ecosystem.
• Evaluate the optimal pH levels for a macroinvertebrate population.

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.

Procedure:

1. Divide students into working groups and provide each group with a sample of the collected water (including debris) of approximately 500 ml.
2. Have students pour a portion of the sample into a petri dish, then retrieve drops of the sample with a pipette, making sure any noticeable debris is captured.
3. Have them place 1-2 drops on a concave slide and examine it under low power on the microscope. Ask students to record their observations of the water sample.
4. Using the identification charts, have students identify the predominant species of macroinvertebrate found in their water sample.
5. Once an identification has been made, have students divide the 1000 ml sample equally into five separate containers. Each container should have 200 ml of the sample. Have students label each container:
   
   Container 1 – pH 1
   Container 2 – pH 4
   Container 3 – pH 8
   Container 4 – pH 12
   Container 5 – Control
   
   
6. Add 1% HCl drop-wise to Container 1 until the liquid has a pH of 1. Test the pH after the addition of each drop.
7. Add 1% HCl drop-wise to Container 2 until the liquid has a pH of 4. Test the pH after the addition of each drop.
8. Add 1% NaOH drop-wise to Container 3 until the liquid has a pH of 8. Test the pH after the addition of each drop.
9. Add 1% NaOH drop-wise to Container 4 until the liquid has a pH of 12. Test the pH after the addition of each drop.
10. Do NOT add anything to Container 5 (Control).
11. Place the containers in a place where they will not be disturbed for a 24-hour period.
12. Based on the observations of their water sample, ask students to hypothesize about how the contents of each container will change during the 24-hour period.
13. After the 24-hour period, have students observe the contents of each container and record their observations in a data table: