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
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.
You need the following
equipment for the microcosm study. See the Microcosm Handout
for details on setting up and testing your microcosms.
- LaMotte or Hach Kits or equivalent for analyzing each of the following:
- pH meter
- 6 x 8 oz jars with caps per group (microcosms)
- Approximately 12 x 2 inch minnows per group
- Approximately 12 x 3 sprigs of the common pondweed, Elodea sp.,
- Sodium or calcium bicarbonate
- pH Worksheet
- Microcosm setup and sample collection instructions
You need to make a prediction about dissolved
oxygen, pH, and carbon dioxide in each of the microcosms.
Complete the microcosm data collection
section of the pH Worksheet.
Complete the microcosm
data management and analysis section of the worksheet.
Based on what you
know about pH,
photosynthesis, and respiration - were
the results what would be expected? What might explain unexpected results?
Be prepared to present your
results to the class. Were results consistent among groups? If not,
you complete Part 2, turn your worksheet and graphs in to your teacher.
Part 2 - Effect
of pH in Lakes
Review your knowledge of pH
in lakes. Would pH change by depth in lakes? How? Would changes vary
Why? How does the microcosm study relate to lakes?
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?
Complete the pH portion of the worksheet
for a summer and winter date.
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.
Use the worksheet
to record your answers to questions about your 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.