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.

As a biologist, you have received an Environmental Protection Agency grant to investigate acid deposition in Minnesota lakes. Your first charge is to analyze CO₂ and dissolved oxygen of lake specimens in the lab. Next, you need to analyze WOW data for evidence that a lake may be affected by acid deposition.

Part 1 - pH in a Microcosm

Knowledge Base
Prepare a brief statement that provides a brief overview of acid deposition in lakes. Why does your study require analysis of pH and dissolved oxygen? How is your lab study related to acid deposition in northern lakes?

Experimental Design
Describe how to use the following equipment to analyze your specimens and demonstrate levels of pH that might occur naturally due to photosynthesis and respiration by aquatic plants and animals.

1. LaMotte or Hach Kits or equivalent
2. Meter or Winkler Titration Kit
3. pH meter
4. 6 x 8 oz jars with caps per group (microcosms)
5. Approximately 12 x 2 inch minnows per group
6. Approximately 12 x 3 sprigs of the common pondweed, Elodea sp.
7. Sodium or calcium bicarbonate

What water chemistry measurements do you plan to analyze?

Data Collection
Develop a plan for recording your measurements so they will be useful for your analysis. Collect and record your measurements.

Data Management and Analysis
Produce a series of graphs to show your results. Be sure to label axes and use titles and legends. What do your results show?

Interpretation of Results
How do your results relate to acid deposition? Are your measurements consistent with levels that might occur naturally due to photosynthesis and respiration by aquatic plants and animals? Did your experimental plan work? What suggestions do you have for other researchers completing a similar study?

Reporting Results
You need to use your graphs and analysis of results when you prepare your final presentation (after Part 2). Begin to consider the best way to report your results. Your teacher will specify the format for your presentation: written report, oral presentation, poster, or multi-media presentation.

Part 2 - Effect of pH in Lakes

Knowledge Base
Review your knowledge of pH in lakes. Would pH change by depth? How? Would changes vary by season? Why? How might the microcosm study relate to lakes? What pH levels might occur naturally due to photosynthesis and respiration by aquatic plants and animals?

Experimental Design
Choose a WOW lake where you could demonstrate evidence of acid deposition. Why do you suspect acid deposition could be demonstrated in this lake? What WOW measures should be used to demonstrate acid deposition? Explain your rationale for using these measures?

Data Collection
Develop a plan for recording your measurements so they will be useful for your analysis. Collect and record your measurements.

Data Management and Analysis
Create a graph(s) of your data. Be sure to title your graph(s) and label axes. Do the results show evidence of acid deposition?

Interpretation of Results
What are your suggestions for maintaining or improving the water quality of this lake as it may be subject to increasing acid precipitation? Can your results be applied to other area lakes? Why? What suggestions do you have for other researchers completing a similar study?

Reporting Results
Use the results of the microcosm study and analysis of WOW data to create a poster, oral presentation, written paper, or multi-media presentation about acid deposition. Be sure to include your suggestions for maintaining or improving the water quality of this lake and your suggestions for other researchers.