Hutchins and Glenn Merrick
developed this lesson.
introduces students to pH,
and to the qualities of lakes that make them sensitive
to acid deposition.
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
a lakes pH and its buffering capacity or resistance to change
in pH. This exercise uses microcosms and WOW data to explore the primary
and abiotic factors
that determine a lakes pH.
can meet the goals for this lesson by completing either a directed study
or student inquiry lesson.
In the directed
study, students investigate dissolved
dioxide in microcosms. They analyze pH and dissolved oxygen using
WOW data and complete a pH worksheet. Students need to print the worksheet
and microcosm setup and sample collection directions.
study lesson is found in the student section of WOW under the title "Studying the Effect of pH
on Aquatic Organisms."
inquiry lesson places students in the role of an EPA biologist investigating
the possibility of acid deposition in Minnesota lakes. Students use
WOW data to determine pH and acid deposition. Students need to print
the lesson. The final presentation is a poster, oral presentation, written
paper, or multi-media presentation.
inquiry lesson is found in the student section of WOW under the title "Investigating the Effect
of pH on Aquatic Organisms."
sources of acidity in lakes.
the relationship of photosynthesis and respiration with the bicarbonate
buffering equilibrium equation.
why pH changes with depth in some stratified
why lakes with the same pH may not be equally vulnerable to the effects
of acid deposition.
predict if a given lake is vulnerable to acidification.
how the "biology" of a lake, photosynthesis,
can control pH.
of chemical equilibrium equations, photosynthesis, and cellular respiration
helps students complete the lesson successfully.
or Hach Kits or equivalent for analyzing each of the following per
- 6 x 8
oz jars with caps per group (microcosms)
12 x 2 inch minnows per group
12 x 3 sprigs of the common pondweed, Elodea sp.per group
or calcium bicarbonate
see Microcosm Setup and Sample
Worksheet for students completing the directed study lesson
pH in a
Microcosm - 2 hours
pH in Lakes - 1 hour
Chemistry - chemical equilibrium, buffering, titration
Biology - photosynthesis and primary productivity, respiration,
Thermal Stratification, Aquatic
1 - pH in a Microcosm
acid deposition as a nearly invisible problem to the casual observer.
Acidification of lakes in Scandinavia was largely unnoticed until it
was too late. What do acidified lakes look like? Would students swim
in one? Why or why not? Are the local or regional lakes vulnerable
acidification? Why or why not?
play the role of an EPA biologist investigating acid deposition in
Minnesota lakes. Ask students to prepare a brief statement that provides
overview of acid deposition in lakes. Why does their study require
analysis of pH and dissolved oxygen?
into groups of 3-4. Students divide the tasks of microcosm setup, oxygen
testing, pH testing, CO2
testing, and data recording. Refer students to the Microcosm
Setup and Sample Collection Instructions that are attached to the
worksheet. Ask students to make predictions about pH, dissolved oxygen,
and carbon dioxide in each of the microcosms.
or Hach Kits or equivalent
Winkler titration kit
6 x 8 oz
jars with caps per group (microcosms)
12 x 2 inch minnows per group
12 x 3 sprigs of the common pondweed, Elodea sp.
or calcium bicarbonate
to describe how to use the following equipment to analyze their specimens
and demonstrate the levels of pH that might occur naturally due to
and respiration by aquatic plants and animals.
chemistry measurements do students plan to analyze?
to the data collection section of the worksheet.
to develop a plan for recording their measurements. They should collect
and record their measurements.
Management and Analysis
to worksheet questions 1-3.
should produce a series of graphs to show their results. Remind them
to label axes and use titles and legends. What do their results show?
to reflect on their knowledge of pH, buffering, photosynthesis, and
respiration. Were their results what would be expected? (Below are
results for each of the microcosms.)
water/Microcosm A: no changes in O2, CO2, and pH
water/Microcosm B: no changes in O2, CO2, and
only/ Microcosm C: -O2, +CO2, -pH
in buffered water/ Microcosm D: -2, +CO2, less change in pH
only/ Microcosm E: +2, -CO2, +pH
in buffered water/ Microcosm F: +O2, -CO2, less change in pH
relationship of bicarbonate buffering equilibrium with the general equations
for respiration and photosynthesis.
How do students'
results relate to acid deposition? Are their measurements consistent
with levels that might occur naturally due to photosynthesis and respiration
by aquatic plants and animals? Did their experimental design plan work?
What suggestion do they have for researchers completing a similar study?
present their results to the class. Were results consistent among all
groups? If not, why?
should begin to consider organization for their final presentation.
The final presentation is completed after Part 2.
2 - Effect of pH in Lakes
Profiler from the WOW data visualization
tools can help illustrate changes in pH during an extended period
of sampling (see Figure 1). Changes in pH
and DO could also be demonstrated by using the Profile
Plotter, using the Color
Mapper (see Figure 2) or by creating a graph in Excel. You may want
to display these for the students. This could be done either during
your initial discussions for this lesson, or as part of the discussion
and closure for the lesson.
1. pH Changes in Ice Lake (missing data has been interpolated)
students knowledge of pH in lakes. Would pH change by depth?
How? Would changes vary by season? Why? How do the results of the microcosm
study relate to pH in lakes?
Review students 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?
student group a WOW lake. Ask them for predictions about changes in
their lakes pH and dissolved oxygen by depth.
should choose a WOW lake where they could demonstrate evidence of acid
deposition. Why do they suspect acid deposition could be demonstrated
in this lake?
measures should be used to demonstrate acid deposition?
to the questions on the microcosm pH
Worksheet. They should collect pH data for a summer and winter
date. Students should wait to collect dissolved oxygen data.
should develop a plan for recording measurements. Ask them to collect
and record their measurements.
Management and Analysis
their experience with the microcosms, have student groups hypothesize
what each lakes oxygen profile may look like. Can students determine
if the lake is thermally stratified?
should finish by adding oxygen data to the table and graph.
should create a graph(s) of the data. Remind them to title their graph(s)
and label axes. Do the results show evidence of acid deposition?
how the oxygen data may reflect the relative CO2 concentrations
at different depths (epilimnion and
hypolimnion) in the lake. Refer
students to the pH Worksheet.
students suggestions for maintaining or improving the water quality
of this lake as it may be subject to increasing acid precipitation?
Can their results be applied to other area lakes? Why? Do they have
suggestion for researchers completing a similar study?
data recorder from each group reproduce their lakes profile on
the blackboard. Compare and discuss the results of the groups.
should use the results of the microcosm study and their analysis of
WOW data to create a poster, oral presentation, written paper, or multi-media
presentation about acid deposition. Remind them to include their suggestions
for maintaining or improving the water quality of this lake and suggestions
for other researchers.
As a measure
of water quality, pH estimates the activity (concentration) of
dissolved in water. pH is measured on 0-14 scale. Numerically,
pH equals the negative log of hydrogen ion concentration (pH = -log
[H+]), and so a change of 1 pH unit is equivalent to a ten-fold change
in hydrogen ion concentration. Since it is a negative logarithm, an
increase in pH is equivalent to a decrease in hydrogen ion activity
(approximately equal to concentration in typical lake waters). Conceptually,
this means that a decrease in pH represents an increase in acidity,
and an increase in pH represents a decrease in acidity.
pH in lakes
can be expected to range 4.5 in poorly buffered bog lakes to greater
than 9.0 in hard water lakes. It is estimated that 2,200 lakes in Minnesota
are sensitive to acid deposition, with the majority of these found in
the northeastern counties (St. Louis, Itasca, Lake, and Cook); (Twaroski
et al, 1989) (see acid rain websites).
Because aquatic organisms are physiologically sensitive to acidification
fish and other organisms
have been extirpated from some lakes in the northeastern U.S. (link
to Finger Lakes site), eastern Canada (Sudbury restoration link), and
Scandinavian countries. The susceptibility of a lake to acid deposition
depends on: the amount of acid deposition falling within a lakes watershed and
the buffering capacity (acid
neutralizing capacity or ANC) of the watersheds soils.
of acid deposition
falling within a lakes watershed is a function
of the lakes proximity to industrial sources of sulphur dioxide
and oxides of nitrogen and prevailing weather patterns. In Minnesota,
high pressure and associated cold fronts tend to bring in less acidic
rainfall from the north and west, while the counterclockwise circulation
of low pressure cells and associated warm fronts can deliver more highly
acidified precipitation from the south and east.
buffering capacity (also known as alkalinity or
acid neutralizing capacity [ANC]) is a measure of the water to resist
changes in pH by neutralizing
acid (or base) inputs. Compounds that contribute most commonly to freshwater
lakes alkalinity include carbonates, bicarbonates, and hydroxides.
Inputs of strong bases such as hydroxides do not typically occur naturally
- if significant, these sources are probably due to industrial waste
discharges. The greater the alkalinity, the greater the ability to neutralize
acidic inputs. Lakes with low alkalinity may have pHs near 7,
but not the buffering capacity to neutralize acidic deposition.
lakes bicarbonate buffering systems operate to maintain pH between 6
systems respiration is the major source of carbon dioxide. Other sources
include microbial methane fermentation,
nitrification of ammonia, and
sulfide oxidation. All sources of carbon dioxide serve to drive the
bicarbonate equilibrium equation to the right, contributing free hydrogen
ions, and therefore acidifying the water. Carbon dioxide is consumed
by photosynthetic organisms. Utilization of carbon dioxide by plants
and algae will, therefore, drive the bicarbonate equilibrium equation
to the left, reducing the availability of hydrogen ions and raising
the pH of the lake.
C. et al. 1989
- Try some of the experiments (http://www.epa.gov/acidrain/student/exprmt.html)
or activities (http://www.epa.gov/acidrain/student/activs.html)
found on the Environmental Protection Agency's Acid Rain Program website.
- Find additional background on the environmental effects of acid
rain on the Environmental Protection Agency's Acid Rain Program website
- Compare the amount of sulfur dioxide falling in northeastern Minnesota
with the amount falling in Pennsylvania or New York during 1996 in
kg/hectare (2.5 acres) and calculate the total amount of sulphur dioxide
falling on a lake near your location. The National Atmospheric Deposition
Program's website. Does this data explain why many lakes in Minnesota,
while poorly buffered, have not been impacted, at least noticeably,
by acid deposition?
- Try some of the laboratory exercises related to acid rain found
on the Woodrow Wilson Leadership Program in Chemistry.
- Find out what the Minnesota Pollution Control Agency does to monitor,
regulate, and educate Minnesotans about acid rain.
- Use the MARIS Lake Characteristics query for Wisconsin to determine
what morphometry and watershed characteristics lakes with low alkalinity
and possibly low pH typically have. Selecting Trout Lake in Vilas
County gives a variety of lakes, some of which are theoretically vulnerable
to acid rain.