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  Teaching about Sustaining Life Under the Ice

Marlene Schoeneck initiated this lesson

This activity introduces students to the affect of winter ice cover on the temperature, dissolved oxygen (DO), and pH content of a lake.

Most northern lakes are sealed off from terrestrial influences in the winter by a lid of ice. The ice prevents gas exchange at the lake / air interface, and reduces light levels as snow cover accumulates. Life processes go on, but decreased photosynthesis, accumulated waste products, and continued respiration reduce DO levels. In extreme cases of shallow lakes and ponds with high amounts of organic matter, DO can be totally depleted (anoxia) and toxic concentrations of hydrogen sulfide (rotten eggs) gas can accumulate. The buildup of repiratory carbon dioxide, while not harmful, leads to decreased pH (for more details and a specific example take a look at the Ice Lake overview). This makes survival in the self-contained environment more difficult. Prolonged exposure to such conditions may create problems in managed systems where maintenance of fish populations is an important factor to consider since mitigation (e.g. aeration) is expensive and not necessarily effective.

Students can meet the goals for this lesson by completing either a studying or investigating lesson. The directed study lesson guides students through the process of experimental design and data collection. The student inquiry lesson provides more student choice and asks students to plan design experiments and data collection.

Each lesson contains two parts. In part one, students create simulated lakes. One is placed under a bright light (summer) and the other is kept dark and chilled (winter). Students measure temperature, DO, and pH readings five times over a week-long study period. They graph these data and compare the two lakes. Part two places students in the role of lake biologists asked to answer a question posed by a local Sportsmen's group: should aerators be placed in a study lake to help fish survive the winter? Using RUSS data, students take weekly readings of DO, temperature and pH in a real lake over one winter. They organize and interpret this data, and devise a recommendation for the Sportsmen's Club.

Both lessons are found in the appropriate student section of WOW under the title "Sustaining Life Under the Ice."

1. Identify the components of lake chemistry affected by ice cover
2. Diagram the relationships between pH, DO and temperature and ice cover
3. Explain how winter ice cover may affect lake organisms
4. Use lake data to make reasonable recommendations for wildlife management

temperature, dissolved oxygen, pH

Basic knowledge of physical, chemical, and biological processes occurring in lakes, and experience with water testing devices will help students complete this lesson successfully.

Internet access for students

1 refrigerator or other method of chilling a class set of 250 bottles

Sensors or kits for measuring pH, DO and temperature (sensors are preferred)

Class set of each:
1. 2, 250ml clear bottles or jars with essentially airtight lids
2. Pond, lake, or aged tap water
3. Organic/mucky sediment, 75 ml per jar
4. 6 sprigs of Anacharis (Elodea) or other aquatic plant

Time Required
Part 1: Lake Simulation-1 hour set up plus 15 minutes each day for five days
Part 2: Sportsmen's Club Recommendation-2 hours for data gathering and management, at least 2 nights for preparing presentations, and at least 1 class period for presentations.

Curriculum Connections
Biology, chemistry, ecology, environmental science, wildlife management

WOW Curriculum Links
Aquatic Respiration, Chemistry of Oxygen Solubility, Data Interpretation, Effect of pH on Aquatic Organisms, Effect of Photosynthesis and Respiration on Aquatic Chemistry

WOW Understanding Links
Lake Ecology Primer
RUSS parameters; temperature, dissolved oxygen, pH
Lake overviews on Ice Lake and Lake Independence


Part I - Winter Lake Laboratory Simulation

Knowledge Base
Facilitate as lab group, or as a class, discussions of the wildlife management problems that have developed in area lakes from winter conditions. What physical, chemical and biological factors determine the severity of ice effects? What types of management practices may improve or limit the negative impacts of ice cover? Reference to specific cases of winterkill in a lake may help to engage students' interests. Some examples include: Illinois EPA, Southern Wisconsin.

Experimental Design
You will need the following equipment:
5. 2 , 250ml clear bottles or jars with essentially airtight lids
6. Pond, lake, or aged tap water
7. Organic/mucky sediment, 75 ml per jar
8. 6 sprigs of Anacharis (Elodea) or other aquatic plant
9. A refrigerator or other method of keeping one of the microcosms chilled. (The colder the better, without freezing the jar solid!)

You may want to divide students into groups of 3-4. Refer groups to instructions posted in lesson plans. Look to the studying plan for a step-by-step example of the simulation.

During the experiments, you should expect that DO will decrease steadily, the rate dependent on the biochemical oxygen demand of the water (including macro and microorganisms) and the amount of sediment oxygen demand (a function of the rate of bacterial and invertebrate respiration occurring in the sediments and the amount of sediment contact with overlying water; essentially the area of sediment to water volume ratio). pH will typically decrease under the ice, as it does in most of the hypolimnion over the course of the summer in thermally stratified lakes. This is caused by a buildup of CO2 from respiration. The magnitude of the observable change in pH is controlled by the rate of respiration as well as the buffering capacity of the water (as measured by its alkalinity which is also called its acid neutralizing capacity or ANC).

Data Collection

Refer students to questions 1 and 2. Have students in groups split the tasks of data collection.

Follow the same directions as studying plan; however be sure to have students explain their experimental design before proceeding with data collection. There should be at least two groups that choose to measure pH, DO and temperature.

Data Management and Analysis

Have students refer to directions. They will create one graph for each factor measured (temperature, DO, pH), which compares data collected from each of the simulated "lakes" (date on x-axis; temperature, DO, or on y-axis). They may graph data by hand or use the excel spreadsheet, following instructions for the template on the WOW site.

Refer students to instructions. They should graph their data by hand or use an Excel spreadsheet--follow instructions for the template on the WOW site. Ask them to explain their rationale for data presentation.

Interpretation of Results

Have students answer the questions posted under the lesson plans. These are the same for both studying and investigating lessons.

1. What changes occurred in your "lakes" during the course of the study?

2. How do you account for the changes in your "lakes"? (Think about the content and covering of your "lakes".)

3. How are these changes related to conditions in an actual lake in winter? What ramifications do they have for lake life?

4. What aspects of this simulation are not realistic? Suggest some revisions that would more accurately simulate winter lake conditions?

Reporting Results
Groups should keep results to turn in with part 2.

Part II. - Changes in a Winter Lake

Knowledge Base
Facilitate groups or class discussion of the results of the winter lake simulations. What lake chemistry changes might they expect to see in an actual lake? Recall the discussion of the physical, chemical, and biological components of lakes, considering their influence on a specific lake and its winter dynamics.

Experimental Design
Guide the students in imagining that their local Sportsmen's Club has asked to purchase aerators for the local lake to reduce winter fish kills. The Club first would like to be sure that winter conditions in the lake justify the installation of these units. Since your science class has been monitoring the lake for several years, the club has come to your school in search of needed evidence.

Directions ask students to use the DVT Toolkit, located in the data section of the WOW site under launch new DVT Toolkit, to look at weekly Ice Lake profiles for the winter of 1998-1999. The data plotter tool allows them to create a graphic representation of each sample (see example below). Students use these graphs to compare changes in temperature, DO and pH and over time.

See the WOW DVTools Index for example. Check the Ice Lake box, and then click on Get this Data. Click Profile Plotter. Try plotting a few different dates. Winter data sets alos exist for Lake Independence and Lake Minnetonka.

Students use RUSS data on the WOW site to determine a study lake with relevant data. Ask them to consider how they will determine ice formation and ice out on their lake. Typically water is densest at 4 degrees Celsius. Lakes that are frozen would be expected to have bottom temperatures of 4 degrees Celsius, with water temperature decreasing toward the surface to 0 degrees Celsius (freezing) at the top. Surface temperatures warm above 0 at ice out. Keep in mind that the RUSS units are pulled off the lakes shortly before ice-over, and again at ice-out. There will be "holes" (missing data) at these points, but you will still have an indication of when each event occurred.

Students should also explain the reasoning behind their experimental design.

Data Collection
Refer students to directions.

Data Management and Analysis
Refer students to directions. They will use the data visualization tools or an Excel spreadsheet on WOW to graph and interpret their data.

Students answer the following questions:

1. What changes did you observe in the parameters you measured over the winter at your lake?

2. What physical characteristics of your lake might have had an influence on the results that you observed in your data? How?

3. What knowledge do you have of the biological characteristics and trophic state of your lake? How might these have affected the life sustaining ability of your lake?

4. Based on the above research, what recommendations would you have for the Sportsmen's Club in regards to installing (or not installing) aerators on the lake?

5. What other management practices might help prevent fish kills in the lake besides installing aerators?

Reporting Results
Directions ask students to prepare posters or graphs for an oral presentation to the class. This is a good opportunity to engage students in comparative debate. Did each group get similar results? Did they make the same recommendation to the Sportsmen's Club? However, you probably want to limit presentations to 10-12 minutes each.

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date last updated: Wednesday March 03 2004