Credits
Rich Axler, Glenn Merrick, and Bruce Munson developed this lesson.

Goals
Students will investigate the effects of respiration on water chemistry and a lake ecosystem.

Introduction
Respiration by visible organisms (minnows or aquaria fish) and by "invisible" organisms — bacteria — uses oxygen to oxidize organic materials and releases energy, water, and carbon dioxide. In this activity students will use laboratory microcosms and WOW data to explore the affects of respiration on the water chemistry of lakes. In this lesson students learn that the simple act of respiration can have profound effects on an ecosystem.

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

In the directed study lesson students set up a microcosm. Dissolved oxygen (DO) and pH are used as indicators of the complex chemical interactions that occur. Students complete a worksheet corresponding to the laboratory experiment. They also analyze RUSS data for DO and pH and complete a corresponding worksheet. They should print the worksheet and microcosm set-up directions.

The directed study lesson is found in the student section of WOW under the title: "Studying Aquatic Respiration."

The student inquiry lesson asks students to research water quality measures with respect to aquatic respiration. They choose to analyze one parameter (measured by RUSS) that correlates to aquatic respiration. They analyze that measure using a microcosm and RUSS data. The instructor specifies a format for the final presentation: written paper, oral report, poster, or multi-media presentation. Students need to print directions for this lesson.

The student inquiry lesson is found in the student section of WOW under the title: "Investigating Aquatic Respiration."

Outcomes
Students will be able to:

1. Provide data collected from microcosms and RUSS monitored lakes as evidence that respiration occurs and can be quantified in aquatic settings.
2. Describe the effects of respiration on a body of water.
3. Design an appropriate water sampling regimen for assessing pH and DO (or the selected water quality measure in the student inquiry lesson) in the hypolimnion of a lake.
4. Explain how organisms affect pH and DO (or the selected water quality measure in the student inquiry lesson) in the hypolimnion of a lake.

Keywords
Respiration, pH, dissolved oxygen, conductivity, detritus, hypolimnion

Materials/Resources/Software

Part I - Lab

Water Quality Testing Equipment and Materials (per student group)

1. Hach or Lamotte water quality testing materials or meters for each group's workstation (for analyzing DO and pH). Each group will make up to 5 pH and DO analyses with a meter or two analyses with chemical reagent kits.
2. 250 ml bottle with a cap
3. Each group should receive at least one of the following three options below to place in their jar:
   1. 4 - two-inch minnows (fathead minnows are readily available and tolerant of low oxygen levels)
   2. 75 -100 ml of sandy sediment
   3. 75 -100 ml of organic/mucky sediment
4. Respiration in a Microcosm Worksheet for groups doing the directed study lesson
5. Thermometers (if water temperature is the selected measure for the student inquiry lesson)
6. Conductivity pens (if conductivity is the selected measure for the student inquiry lesson)

Part II - Technology Application

1. Aquatic Respiration in a Lake Worksheet for groups doing the directed study lesson
2. Access to WOW data on the Internet

Time Required
The lesson is completed in two parts. Generally, the lesson can be completed in two class periods, but students will need some time to finish homework before the final results are handed in. Part I requires one hour in a lab setting. Part II also requires about one hour. Student will need access to computers with Internet capabilities or a data file that has been downloaded from the website in order to complete Part II.

Curriculum Connections
Biology - respiration, lake stratification, detritus

Chemistry -chemical equilibrium, pH, dissolved oxygen, carbon cycle

WOW Curriculum Links
Effect of Photosynthesis and Respiration on Aquatic Respiration, Effect of pH of Aquatic Organisms

Procedure

Part I - Lab Microcosm Studies

Knowledge Base
Discuss students’ observations of events such as aquaria that have been overfed or left untended for prolonged periods of time, or minnow buckets that have been left without fresh water for a prolonged period. What happens? Why do the animals often die in these situations? What water quality measures might correlate to this situation? Why do students think similar situations might, or might not, occur in lakes? What water quality measures might correlate to aquatic respiration?

Notes: Respiration by visible organisms (minnows or aquaria fish) and by "invisible" organisms — bacteria — uses oxygen to oxidize organic materials and releases energy, water, and carbon dioxide. In a system where respiration exceeds the input of oxygen (through photosynthesis or aeration), there will be a measurable decline in dissolved oxygen and a corresponding decline in pH due to the addition of CO₂ to the system. A lack of dissolved oxygen or a significant decline in pH can kill aquatic life.

Student Inquiry
Ask students to research the effects that respiration can have on water quality, pH, DO, conductivity, and turbidity (see RUSS Unit section). Students should use the WOW page and links. They should select the water quality measure that they feel most strongly correlates to aquatic respiration.

Experimental Design
Divide students into lab groups of two to four and ask them to complete either the directed study ("Studying Aquatic Respiration") or student inquiry ("Investigating Aquatic Respiration") lesson. Explain that when conducting scientific research, scientists need to carefully design experiments to answer the relevant questions. Students should reflect on the relationships among the questions being asked in both parts of this lesson.

Suggest students divide chemical testing work within their group so that everyone gets hands-on experience doing the testing. Note that test kit instructions need to be followed precisely. (A practice test with water of known dissolved oxygen content and pH may help build students’ skills and confidence.)

Data Collection
Introduce the microcosm study. Discuss procedural variables that could influence the outcomes of students' measurements.

Directed Study
Have one third of the class work with minnows in their lab microcosms. One third will work with sandy sediment, and the remaining third will work with an organic sediment. Review the procedures for using the water test kits or meters. Ask students to make predictions for DO and pH in each of the three microcosms.

Test the DO and pH concentrations in samples of the remaining aged water using the water chemistry kits or meters. If using Hach or Lamotte kits, proceed by testing the oxygen first in each bottle. They should perform the tests rapidly to minimize the development time between microcosms. After 30 minutes, retest the DO and pH in each of the 3 microcosms. Have students record their results on the worksheet.

Student Inquiry
Students need to choose a water quality measure to analyze. They should research the effects that respiration can have on at least one other WOW parameter (water temperature, pH, DO, and conductivity) using the WOW page and links. They should choose a measure that they feel correlates strongly to aquatic respiration. You may want to limit the number of variables (sediments and fish) and amount of those variables the students can use for their lab experiment.

Data Management and Analysis

Directed Study
Refer students to this section of the "Respiration in a Microcosm Worksheet."

Student Inquiry
Students should graph their data and summarize their results. Remind students to title their graphs and label legends, axes, and units of measure.

Interpretation of Results

Directed Study
Refer students to this section of the "Respiration in a Microcosm Worksheet."

Student Inquiry
Ask students to share possible explanations for their experimental results.

Notes: Organisms are respiring in all of the microcosms. The minnows are obvious. In the other microcosms bacteria are respiring. In all cases organisms are using up the available oxygen and giving off carbon dioxide. The number of organisms available in each microcosm and the metabolism of those organisms will affect the rate and amount of DO used, carbon dioxide given off, and resulting changes in pH. The buffering capacity of the water will also affect the decline of pH in the microcosms.

Reporting Results

Directed Study
Students should turn in their completed "Respiration in a Microcosm Worksheet."

Student Inquiry
Students should develop a presentation using the format specified by the teacher: written paper, oral presentation, poster, or multi-media presentation. The final presentation should reflect both parts of the lesson.

Part II - The Effects of Respiration in a Lake

Knowledge Base
The WOW data visualization tools can help illustrate changes in DO and pH (see Figures 1 & 2). These changes could also be demonstrated by advancing the Profile Plotter through several sampling periods or by creating graphs in Excel (see Figure 3). 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.

Discuss students’ perspectives on the possible relationships between the lab microcosms and what might be found in a lake.

Figure 1. DO Profiles for Ice Lake

Figure 2. pH Profiles for Ice Lake

Figure 3. Monthly Records for Ice Lake DO and pH at 10 m

Experimental Design

Directed Study
Direct students to the Aquatic Respiration in a Lake Worksheet. Students can begin work on Part II while completing the microcosm study if they have time, but they will need additional time and access to WOW data in order to complete Part II. (If Internet access is difficult, you could provide students with data files downloaded from the WOW web site.)

Student Inquiry
Students are placed in the role of consulting biologists trying to answer questions from a lake association. The students should determine how they will analyze WOW data for the water quality measure they selected for Part I. A written plan should be developed that describes how the students will answer the Lake Association’s questions.

Data Collection
Remind students of the need to carefully collect and record their data in completing Part II. Students also need to reflect on the quality of the data. There is always the possibility that inaccurate data have been collected or posted to the WOW web site. Scientists need to reflect on the quality of any data that are used.

Directed Study
Students complete the table in the Aquatic Respiration in a Lake Worksheet.

Student Inquiry
Students should create a table to record data for their selected water quality measure. They should also write a description of their data collection plan as a protocol that others could follow. Remind students they only need to record measurements from the hypolimnion.

Data Management and Analysis

Directed Study
Students will graph dissolved oxygen and pH for their sampling periods. They should be reminded to label the axes and include proper titles, legends, and units of measure.

Student Inquiry
Students should create a graph that illustrates changes in the selected water quality measure.

Interpretation of Results
Discuss the students’ results. (Brief oral presentations by the student lab groups can be useful in comparing group approaches and results).

Discuss the following questions:

• Over the course of the summer, did pH and DO continuously decline near the bottom of the lake?
• What similarities and differences are observed in comparing the group graphs and results?
• Why might some lakes have sudden changes in DO and pH near the lake bottom during the summer?

Notes: Lakes that remain stratified and are highly productive will usually demonstrate a continuing decline in pH and DO during the summer months due to bacterial action in the sediments. Some lakes may go anoxic (lacking any DO in the depths). Other lakes may have sudden increases in DO and pH if the water is mixed through events such as high winds (this also depends on basin shape and lake depth).

Directed Study
Refer students to this section of the Aquatic Respiration in a Lake Worksheet.

Student Inquiry
Students should write a summary of the changes in the selected measure.

Reporting Results

Directed Study
Students should turn in their worksheet and graphs.

Student Inquiry
Students should develop a presentation using the format specified by the teacher: written paper, oral presentation, poster, or multi-media presentation. The final presentation reflects both parts of the lesson.

Teacher Notes
A steady "rain" of detritus (dead stuff, mostly algae and particulate material washed into the lake from the watershed) falls to the bottom of lakes. This "rain" of detritus is greatest during the most productive time of year. This productivity coincides with the period when lakes are thermally stratified for long periods of time (in Minnesota this might be May-November depending on the basin shape, lake depth and weather.) In the sediments at the bottom of the lake (at the base of the hypolimnion), the detritus is decomposed by bacteria through the process of respiration. The bacterial processes of respiration release the potential energy stored in the chemical bonds of the organic carbon compounds, consuming oxygen in oxidizing the compounds, and releasing carbon dioxide (CO₂). This CO₂ rapidly dissolves in water to form carbonic acid (H₂CO₃), bicarbonate ions (HCO₃^-) and carbonate ions (CO₃^--). The relative amounts of these depends on the pH of the water. The newly formed carbonic acid gradually decreases the pH of the water. The ions produced in the water as CO₂ dissolves increase the TDS (total dissolved solids), and therefore, increase the electrical conductivity (EC) in the water.

CO₂ + H₂0 <-----> H₂CO₃ <-----> H⁺ + HCO₃^-

Resources

1. Behar, S., Dates, G., Byrne, J. (1996). Testing the Waters. River Watch Network: Montpelier, VT.
2. Cole, G.C. (1988). Textbook of Limnology. Waveland Press: Prospect Heights, IL.
3. Munson, Bruce A. (1992). A Primer on Limnology (Second Edition). Public Report Series #6. Water Resources Research Center, St. Paul, MN.

Extensions

1. Compare DO and pH of lakes during different seasons.
2. Use a similar lesson approach to see if you can observe pH and DO changes in a lake during a 24-hour period.