Credits
Sue Hutchins and Glenn Merrick developed this lesson.

Goals
This activity introduces students to pH, acidity, and buffering, and to the qualities of lakes that make them sensitive to acid deposition.

Introduction
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 a lake’s pH and its buffering capacity or resistance to change in pH. This exercise uses microcosms and WOW data to explore the primary biotic and abiotic factors that determine a lake’s pH.

Students can meet the goals for this lesson by completing either a directed study or student inquiry lesson.

In the directed study, students investigate dissolved oxygen, pH, and carbon 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.

The directed study lesson is found in the student section of WOW under the title "Studying the Effect of pH on Aquatic Organisms."

The student 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.

The student inquiry lesson is found in the student section of WOW under the title "Investigating the Effect of pH on Aquatic Organisms."

Outcomes
Students will:

1. Identify sources of acidity in lakes.
2. Diagram the relationship of photosynthesis and respiration with the bicarbonate buffering equilibrium equation.
3. Explain why pH changes with depth in some stratified lakes.
4. Explain why lakes with the same pH may not be equally vulnerable to the effects of acid deposition.
5. Reasonably predict if a given lake is vulnerable to acidification.
6. Describe how the "biology" of a lake, photosynthesis, and respiration, can control pH.

Keywords
Acidity, alkalinity, base, bicarbonate, buffering, pH, hydrogen, ion

Prerequisites
Basic knowledge of chemical equilibrium equations, photosynthesis, and cellular respiration helps students complete the lesson successfully.

Materials/Resources/Software

1. LaMotte or Hach Kits or equivalent for analyzing each of the following per group:
   • 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
   see Microcosm Setup and Sample Collection Instructions
7. pH Worksheet for students completing the directed study lesson

Time Required
pH in a Microcosm - 2 hours
Effect of pH in Lakes - 1 hour

Curriculum Connections
Chemistry - chemical equilibrium, buffering, titration

Biology - photosynthesis and primary productivity, respiration, acid rain issues

WOW Curriculum Links
Conductivity, Thermal Stratification, Aquatic Respiration

Procedure

Part 1 - pH in a Microcosm

Knowledge Base

Directed Study
Discuss 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 to acidification? Why or why not?

Student Inquiry
Students play the role of an EPA biologist investigating acid deposition in Minnesota lakes. Ask students to prepare a brief statement that provides a brief overview of acid deposition in lakes. Why does their study require analysis of pH and dissolved oxygen?

Experimental Design

Directed Study
Divide students 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.

Student Inquiry
Ask students to describe how to use the following equipment to analyze their specimens and demonstrate the levels of pH that might occur naturally due to photosynthesis and respiration by aquatic plants and animals.

What water chemistry measurements do students plan to analyze?

Data Collection

Directed Study
Refer students to the data collection section of the worksheet.

Student Inquiry
Ask students to develop a plan for recording their measurements. They should collect and record their measurements.

Data Management and Analysis

Directed Study
Refer students to worksheet questions 1-3.

Student Inquiry
Students 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?

Interpretation of Results

Directed Study
Ask students to reflect on their knowledge of pH, buffering, photosynthesis, and respiration. Were their results what would be expected? (Below are typical results for each of the microcosms.)

Aged water/Microcosm A: no changes in O₂, CO₂, and pH

Auffered water/Microcosm B: no changes in O₂, CO₂, and pH

Minnows only/ Microcosm C: -O₂, +CO₂, -pH

Minnows in buffered water/ Microcosm D: -₂, +CO₂, less change in pH

Pondweed only/ Microcosm E: +₂, -CO₂, +pH

Pondweed in buffered water/ Microcosm F: +O₂, -CO₂, less change in pH

Review the relationship of bicarbonate buffering equilibrium with the general equations for respiration and photosynthesis.

Student Inquiry
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?

Reporting Results

Directed Study
Groups should present their results to the class. Were results consistent among all groups? If not, why?

Student Inquiry
Students should begin to consider organization for their final presentation. The final presentation is completed after Part 2.

Part 2 - Effect of pH in Lakes

Knowledge Base
The DxT 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.

Figure 1. pH Changes in Ice Lake (missing data has been interpolated)

Directed Study
Discuss 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?

Student Inquiry
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?

Experimental Design

Directed Study
Assign each student group a WOW lake. Ask them for predictions about changes in their lake’s pH and dissolved oxygen by depth.

Student Inquiry
Students 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?

What WOW measures should be used to demonstrate acid deposition?

Data Collection

Directed Study
Refer students 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.

Student Inquiry
Students should develop a plan for recording measurements. Ask them to collect and record their measurements.

Data Management and Analysis

Directed Study
Based on their experience with the microcosms, have student groups hypothesize what each lake’s oxygen profile may look like. Can students determine if the lake is thermally stratified?

Students should finish by adding oxygen data to the table and graph.

Student Inquiry
Students 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?

Interpretation of Results

Directed Study
Explain 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.

Student Inquiry
What are 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?

Reporting Results

Directed Study
Have the data recorder from each group reproduce their lake’s profile on the blackboard. Compare and discuss the results of the groups.

Student Inquiry
Students 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.

Teacher Notes
As a measure of water quality, pH estimates the activity (concentration) of hydrogen ions 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 lake’s watershed and the buffering capacity (acid neutralizing capacity or ANC) of the watershed’s soils.

The amount of acid deposition falling within a lake’s watershed is a function of the lake’s 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.

A lake’s 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 pH’s near 7, but not the buffering capacity to neutralize acidic deposition.

In most lakes bicarbonate buffering systems operate to maintain pH between 6 and 9.

In aquatic 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.

Resources
Twaroski, C. et al. 1989

Extensions

1. 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.
2. Find additional background on the environmental effects of acid rain on the Environmental Protection Agency's Acid Rain Program website at http://www.epa.gov/acidrain/effects/enveffct.html.
3. 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?
4. Try some of the laboratory exercises related to acid rain found on the Woodrow Wilson Leadership Program in Chemistry.
5. Find out what the Minnesota Pollution Control Agency does to monitor, regulate, and educate Minnesotans about acid rain.
6. 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.