Acidity varies widely among natural lakes. At first this variation appears as simple differences in pH values among lakes. A closer look reveals that the relative amounts of materials that produce acidity in lakes determine both a lake's pH and its buffering capacity or resistance to change in pH. In this lesson you use microcosms and WOW data to explore factors that determine a lake's pH.

Part 1 - pH in a Microcosm

Knowledge Base
Review your knowledge of acidity in lakes. Is it a local problem? Do local or regional lakes have the potential to become acidified? Read the sections on pH and acid deposition in the WOW primer.

Experimental Design
You need the following equipment for the microcosm study. See the Microcosm Handout for details on setting up and testing your microcosms.

1. LaMotte or Hach Kits or equivalent for analyzing each of the following:
   • CO₂ (by titration)
   • dissolved oxygen (meter or Winkler Titration Kit)
2. pH meter
3. 6 x 8 oz jars with caps per group (microcosms)
4. Approximately 12 x 2 inch minnows per group
5. Approximately 12 x 3 sprigs of the common pondweed, Elodea sp., per group
6. Sodium or calcium bicarbonate
7. pH Worksheet
8. Microcosm setup and sample collection instructions

You need to make a prediction about dissolved oxygen, pH, and carbon dioxide in each of the microcosms.

Data Collection
Complete the microcosm data collection section of the pH Worksheet.

Data Management and Analysis
Complete the microcosm data management and analysis section of the worksheet.

Interpretation of Results
Based on what you know about pH, buffering, photosynthesis, and respiration - were the results what would be expected? What might explain unexpected results?

Reporting Results
Be prepared to present your results to the class. Were results consistent among groups? If not, why? After you complete Part 2, turn your worksheet and graphs in to your teacher.

Part 2 - Effect of pH in Lakes

Knowledge Base
Review your knowledge of pH in lakes. Would pH change by depth in lakes? How? Would changes vary by season? Why? How does the microcosm study relate to lakes?

Experimental Design
Your teacher has asked you to analyze Ice Lake. How might pH change by depth in your lake? Why? How might dissolved oxygen change by depth in your lake? Why?

Data Collection
Complete the pH portion of the worksheet for a summer and winter date.

Data Management and Analysis
Based on your experience with the microcosms hypothesize what the lake's oxygen profile may look like. Can you determine if the lake is thermally stratified? Finish the pH-DO table by adding oxygen data.

Interpretation of Results
Use the worksheet to record your answers to questions about your results.

Reporting Results
The data recorder from your group needs to reproduce the lake's profile on the blackboard. Be prepared to compare and discuss the results of different groups in your class. Turn your worksheet and graphs in to your teacher.