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  Teaching about the Effect of Photosynthesis and
Respiration on Aquatic Chemistry
 

Credits
Glenn Merrick developed this lesson.

Goals
This activity introduces students to the concept that aquatic organisms alter the chemistry of their environment. Students also use microcosms to monitor rapid changes in water chemistry caused by photosynthesis and respiration.

Introduction
When teaching units on photosynthesis and respiration or the carbon cycle, direct evidence of their occurrence is sometimes difficult to provide in a compelling form the laboratory. This exercise allows students to work directly with organisms and, via RUSS, an entire lake ecosystem. Increasing oxygen concentrations in student built microcosms tested with water chemistry kits and daytime RUSS data provide evidence for photosynthetic activity. Decreasing oxygen concentrations in student built microcosms with fish and nighttime RUSS data provide evidence of respiration. The net balance of photosynthesis and respiration plays a major role in determining the level of oxygen present in lakes, particularly at depth in thermally stratified lakes. Oxygen levels at different depths differ if a lake is thermally stratified. Furthermore, as a byproduct of their larger biomass eutrophic lakes will generally illustrate changes in oxygen concentration more dramatically than lakes which are less rich in nutrients and biomass.

Collecting and interpreting data both in lab and from a lake will help student in understanding the connections that exist between cellular processes, individual organisms, and entire ecosystems with regard to the ongoing exchange of inorganic matter that occurs during the carbon cycle.

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

The directed study lesson guides students through the lab activity and the "Photosynthesis and Respiration Worksheet." Students need a printed copy of the worksheet and may want to print directions for the activity. They will either need to read the "Introduction to Photosyntheis and Respiration" on the web, or get a printed copy of that as well.

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

The student inquiry lesson asks students to demonstrate the effects of photosynthesis and respiration. Students decide which variables to analyze in a microcosm study related to photosynthesis and respiration. They produce a written paper, oral presentation, poster, or multi-media presentation instead of a worksheet. The teacher specifies the format. It is helpful if students can refer to printed directions.

The student inquiry lesson is found in the student section of WOW under the title: "Investigating the Effect of Photosynthesis and Respiration on Aquatic Chemistry." Students may want to print directions for the activity. They will either need to read the "Introduction to Photosyntheis and Respiration" on the web, or get a printed copy of that as well.

Outcomes
Students will:

  1. Develop an understanding of the changes that may occur in oxygen and pH levels in lakes at the surface on a daily basis and at depth in thermally stratified lakes.
  2. Manage, graph, and interpret data.

Keywords
Photosynthesis, respiration, carbon cycle, primary production, oxygen, metabolism, pH, carbon dioxide

Prerequisites

  1. A basic understanding of eukaryotic and prokaryotic cell structure, chemical formulas, and simple chemical reactions are recommended for students in courses in which this exercise is being used to illustrate photosynthesis and respiration.
  2. Introductory exposure to the carbon cycle is recommended when this exercise is linked to units in ecology.
  3. If sampling of oxygen levels is planned for depths below a lake’s thermocline, students should be familiar with topics related to water density and thermal stratification.

Materials/Resources/Software

  1. Microcosm Materials/student group
    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
  2. Excel Spreadsheet (optional)
  3. Photosynthesis and Respiration Worksheet for the directed study lesson
  4. Copies of "Introduction to Photosyntheis and Respiration"

Time Required
Two hours are required to complete the microcosm work if using water chemistry kits. If using oxygen or pH meters less than 2 hours is sufficient. Computer work may be completed in a computer lab in small groups or independently by students.

Curriculum Connections
Biology - photosynthesis, respiration, metabolism, enzyme/temperature dependence, nutrient cycling
Chemistry - chemical equilibrium, oxidation-reduction, Law of Conservation of Matter, pH, conductivity

WOW Curriculum Links
Diel Temperature Variation, Aquatic Respiration, Effect of pH on Aquatic Organisms

Procedure

Part 1 - Microcosm Set-up

Knowledge Base

Directed Study and Student Inquiry
Students should read the "Introduction to Cell Respiration and Photosynthesis" before beginning the exercises.

Discuss students’ knowledge of photosynthesis and respiration. How do these processes affect oxygen concentrations in lakes? Discuss the measures to be analyzed as indicators of photosynthesis and respiration.

Experimental Design

Directed Study
Introduce the microcosm study. Ask students to make predictions for oxygen, pH, and CO2 in each of the three microcosms. What is the rationale for their predictions?

Student Inquiry
Students are challenged to demonstrate that photosynthesis and respiration occur in lakes. They begin by attempting to demonstrate that photosynthesis and respiration can be measured in lab microcosms. Students need to choose a water quality measure(s) to analyze in laboratory microcosms. They should begin by researching the effects that respiration can have on water temperature, pH, CO2, and DO, using the "Understanding" portion of the WOW website. They should choose a measure(s) that they feel correlates strongly to photosynthesis and aquatic respiration. Students should write a procedural plan for testing their hypothesis.

Data Collection

Directed Study
Students should 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 the jars on a window sill (the brighter the better.) They should sit for approximately 30 minutes from the time they were sealed.

Students should 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. They need to 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. students should record your results.

Student Inquiry
Students need to organize a way to record the results they collect.

Data Management and Analysis

Directed Study
Refer students to the "Photosynthesis and Respiration 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 the "Photosynthesis and Respiration Worksheet"

Student Inquiry
Ask students to share possible explanations for their experimental results. Do the results indicate photosynthesis and/or respiration? Compare results of different student groups. What might account for similarities or differences among results?

Reporting Results

Directed Study
Students should turn in the worksheet after they complete the WOW data analysis.

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.

Part 2 - Investigating Photosynthesis and Respiration in Lakes

Knowledge Base
The WOW data visualization tools can help illustrate changes in DO, temperature, and pH. These changes could be demonstrated by advancing the Profile Plotter through several sampling periods or by creating graphs in Excel (see Figures 1 & 2). You may want to use 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. Ice Lake DO at Selected Depths (8/18-8/19/99)

 

 

Figure 2. Ice Lake Profiles 8/18/99

 

Directed Study and Student Inquiry
Discuss lab results with students. 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.

Experimental Design

Directed Study and Student Inquiry
Ask students make predictions about oxygen concentrations, temperature, and pH in a lake. Ask them to explain the rationale for their predictions. Do they expect a different response? Why?

Student Inquiry
Students will need to write a procedural plan for using WOW data to demonstrate that photosynthesis and respiration occur in a lake. The plan should be detailed enough for others to follow.

Data Collection

Directed Study and Student Inquiry
Students will collect their data from the WOW website.

Data Management and Analysis

Directed Study
Ask students to inspect the oxygen concentration at one meter intervals over a 48 hour period. Is there a pattern in the data?

Students should 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?

Have students create a graph of temperature, oxygen, and pH profile data. Remind students to title the graph and label legends and axes.

Student Inquiry
Have students inspect the data. They need to analyze the data for changes in the laboratory variable(s) they studied. They can also look for other changes that might correlate to photosynthesis or respiration. Is there a pattern in the data?

Have students create a graph of their data. Remind students to title the graph and label legends and axes.

Interpretation of Results

Directed Study
Refer students to the questions on the Photosynthesis and Respiration Worksheet

Student Inquiry
Ask students to reflect on the results of the microcosm set-up and data analysis as they prepare for the presentation. They should compare their results with the equations for photosynthesis and respiration and consider the relationships among data.

Reporting Results

Directed Study
Students should turn in worksheets and graphs.

Student Inquiry
Specify the format for students' presentations: written paper, oral report, poster, or multi-media presentation.

Evaluation
Answers on the student worksheet indicate students' understanding of the conceptual material presented in the lesson.

Teacher Notes
Monitor conductivity change along with oxygen, carbon dioxide, and pH to give advanced students a more complete look at chemical equilibrium and buffering in aquatic environments. Conductivity is monitored using a meter.

Direct students to complete the reading (Introduction to Cell Respiration and Photosynthesis) prior to carrying out the exercises and answering questions.

Extensions

  1. Low oxygen levels in a lake's deeper water may eliminate it as usable habitat at certain times of the year. Using RUSS data establish which times of year fish can use the deep water habitat.
  2. Explore photosynthesis and respiration at the Biology Place http://www.biology.com/home/home.html. (Use of this site requires a subscription.)
  3. Sample other RUSS monitored lakes and determine if trends in oxygen and pH levels differ from the lake originally sampled.

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date last updated: Wednesday March 03 2004