Both photosynthesis and respiration occur in lake settings. In this lesson you use microcosms and RUSS data to investigate changes in oxygen concentrations that result from photosynthesis and respiration.

Knowledge Base
Read the Introduction to Cell Respiration and Photosynthesis before beginning the lesson.

Review your knowledge of photosynthesis and respiration. Consider how these processes might affect oxygen concentrations in lakes.

Part I - Microcosm Set-up

Experimental Design
You need the following materials for the microcosm set-up.

• 3 - 8 oz or 250 ml jars
• 10 - 8 cm sprigs of Elodea sp.
• 8 - 5 cm fathead minnows
• oxygen, carbon dioxide, and pH kits (meters may be substituted)
• a sunny window or a grow light

Make predictions for oxygen, pH, and C0₂ in each of the three microcosms. You need to be prepared to explain the rationale for your predictions.

Data Collection
Label the jars A, B, and C. Fill Jar A to overflowing from the supply of aged water. Add 4 fathead minnows. (Do not worry about spilling water.) Note the time jar is sealed. Fill Jar B to overflowing with aged water. Add five 3 inch sprigs of elodea. (Do not worry about spilling water.) Note the time jar is sealed. Fill Jar C to overflowing with aged water. Add 4 fathead minnows and five 3 inch sprigs of elodea. Note the time jar is sealed. Place all of your jars on a window sill (the brighter the better.) They should sit for approximately 30 minutes from the time they were sealed.

Test the oxygen, pH, and carbon dioxide 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. Perform the tests rapidly to minimize the development time between microcosms.

After 30 minutes retest the carbon dioxide, oxygen, and pH in each of the 3 microcosms. Record your results on your Photosynthesis and Respiration Worksheet.

Data Management and Analysis and Interpretation of Results
Refer to your worksheet for these lesson components.

Reporting Results
After Part II is complete, turn your worksheet in to your teacher.

Part II - Using WOW Technologies

Knowledge Base
Participate in the discussion of lab results. How might photosynthesis and aquatic respiration affect water chemistry in a lake environment? Consider how photosynthesis and respiration might affect oxygen concentrations, temperature, and pH in a lake.

Make predictions about oxygen concentrations, temperature, and pH in a lake. What is the rationale for your predictions? Do you expect a different response? Why?

Experimental Design
Select a lake to study from the WOW web site. Open a data file from midsummer, 1998.

Data Collection
Access the WOW data from the web site. If using a spreadsheet select a file in the Excel format. Otherwise, select the HTML format. You will need to either download the Excel file or copy and paste the HTML data into a spreadsheet for analysis.

Data Management and Analysis
Inspect the oxygen concentration at one meter intervals over a 48 hour period. Is there a pattern in the data?

Inspect the data from top to bottom trends in temperature, dissolved 0₂ (mg/l), dissolved oxygen (% saturation), and pH levels. Are there any patterns in the data?

Create a graph of temperature, oxygen, and pH profile data.

Interpretation of Results
Complete the questions on the Photosynthesis and Respiration Worksheet.

Reporting Results
Turn your worksheet and graph in to your teacher.