Credits
Jon Rowe developed this lesson.

Goals
Students will build an understanding of lake stratification and the characteristics of lake stratas.

Introduction
Thermal stratification, or layering, occurs in many Minnesota lakes. Whether or not a lake stratifies depends on a number of factors: the shape and depth of the lake, the amount of wind, and the orientation of the lake (lakes that are oriented east-west are more affected than lakes oriented north-south). When layering occurs the upper, warmer layer is referred to as the epilimnion, and the colder, deeper layer is referred to as the hypolimnion. The boundary between the layers where the rate of temperature change is most rapid is referred to as the thermocline. Temperature stratification is often paralleled by stratification of other water quality measurements such as pH and dissolved oxygen.

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

The directed study lesson consists of a brief demonstration of stratification, turnover and an analysis of WOW data. The six lesson components are divided among the demonstration and analysis of WOW data. Students may want to print directions for the lesson. The directed study lesson is found in the student section of WOW under the title: "Studying Thermal Stratification."

The student inquiry lesson is divided in two parts. The first part challenges students to demonstrate thermal stratification in the laboratory. Students develop a protocol, complete their experimental demonstration, and orally present their results. In the second part, students use WOW data to provide evidence of thermal stratification. Students develop a written paper, oral presentation, poster, or multi-media presentation based on their research. Each part of the lesson contains the six components. They may want to print directions for the student inquiry lesson. The student inquiry lesson is found in the student section of WOW under the title: "Investigating Thermal Stratification."

Outcomes
Students will:

1. Graph WOW data to determine whether or not a lake is stratified.
2. Label the stratified layers of lakes.
3. Identify variables that affect temperature stratification in lakes.
4. Describe and explain the process of turnover in stratified lakes.

Keywords
Temperature, stratification, thermocline, hypolimnion, epilimnion

Prerequisites
Students need basic graphing skills, and they need to know how to use the computer to retrieve WOW data before beginning this lesson.

Materials/Resources/Software

• Two clear cups or glasses per group
• One colored ice cube per group (add food coloring to water before freezing)
• One gallon of cold (the colder the better) colored water
• Access to the Internet for temperature profile data of a lake (or handouts of temperature data that have been printed from the WOW web site).

In the student inquiry lesson students are challenged to demonstrate thermal stratification. The above materials may be provided, teachers can supply additional materials, or students can generate an original demonstration of thermal stratification.

Time Required

Directed Study
This lesson requires approximately one hour.

Student Inquiry
The lesson requires approximately two hours.

Curriculum Connections
Biology - thermocline, lake stratification, turnover
Physics - temperature, density, convection

WOW Curriculum Links
Diel Temperature Variation, Heat Budgets of Lakes

Procedure

Part I - Laboratory Demonstration

Knowledge Base
The WOW website resources for teachers and students includes several movies. The movies will take awhile to download, but are useful illustrations of lab demonstrations showing thermal stratification and mixing in water. Check out the following links to see the kinds of thermal effects your students can demonstrate using simple lab materials. You will need the Quicktime Plug-in from Apple to view these movies.

• Movie 1 - Here's what happens when warmer water (green) enters the surface of a lake in winter. The second addition shows that the warm water is buoyant (less dense) than the cold water and therefore rises.
• Movie 2 - Here's what happens when colder water enters a summer-stratified lake.
• Movie 3 - Same movie 2 without the dyed green epilimnion.
• Movie 4 - See what happens to the epilimnion (mixed layer) and thermocline during a storm. Did the lake mix?
• Movie 5 - Same as movie 4, but with increased turbulence. See what starts to happen when the class 5 tornado hits.
• Movie 6 - Shows how stream sediment entering a lake or reservoir deposits its load. Why does some material stay in the upper layer and some crash to the bottom?
• Movie 7 - An estuary is a 2-layer system with freshwater overlying salt water. Here we see how freshwater behaves when added to each layer.
• Movie 8 - Same as movie 7, but here we introduce water that is saltier than the upper freshwater layer. Example: Hurricanes can "throw" huge amounts of saltwater into coastal lakes. What happens to this water and what might its impact be?

Directed Study
Discuss students’ observations about the temperature of lake water. Compare the temperature at the surface to the temperature near the bottom of the lake. Introduce this lesson as an activity that will investigate temperature relationships in lakes.

Student Inquiry
Introduce the concept of thermal stratification. Have students observed distinct temperature layers in lakes? What might cause these layers? Might it be possible to replicate these layers in a laboratory setting? How?

Experimental Design

Directed Study
Divide students into groups of two, and distribute a clear glass to each group. Ask for predictions about what will happen after the ice cube is placed in a glass of water. Have each group fill the glass with warm water and place a colored ice cube in the glass. Ask them to observe and record what happens for 5 minutes. Each group should brainstorm possible explanations for what happened.

Questions

• Why did the colored water sink?
• What are convection currents?
• Could you see convection currents in the glass?
• Why do you think convection currents might/might not be found in lakes?

Notes: The colored water is colder and denser than the water in the glass and sinks to the bottom. Convection currents occur in liquids and gases as colder molecules are pulled downward by gravity, forcing the warmer molecules upward. The convection movement downward of the cold colored water can be seen in the glass. Convection currents occur in many temperate lakes when weather events cool the surface water. The cooled water is pulled downward, forcing the warmer water upward. In lakes this process may occur in the fall and be accompanied by wind driven mixing of the lake. The total process is known as seasonal "turnover."

Students should empty their glasses and fill the glasses halfway with cold colored water. Ask students to carefully pour a 1/4 cup of warm, clear water into their glass (it is often best if the warm water is poured slowly in from the side of a tilted glass to avoid mixing). They should try to create a glass of water that has two distinctly separate layers.

Discuss the similarities and differences between the layered water and what students might expect to find in regional lakes during the summer.

Questions

• How does this demonstration relate to what happens in lakes?
• Why might layering occur in lakes?

Notes: Layering occurs in many Minnesota lakes, depending on the shape and depth of the lakes and the degree to which they are affected by winds. When layering occurs the upper, warmer layer is referred to as the epilimnion, and the colder, deeper layer is referred to as the hypolimnion. The boundary between the layers where the rate of temperature change is most rapid is referred to as the thermocline.

Students completing the directed study lesson proceed to Part II.

Student Inquiry
Challenge students to demonstrate thermal stratification. The materials listed in the Materials/Resources/Software section may be provided. Alternately, students can be challenged to create original demonstrations of thermal stratification.

Students should write a protocol for their demonstration. The protocol should provide clear, step-by-step directions for other researchers to follow. Students should be ready to explain the rationale for decisions about experimental design.

Data Collection

Student Inquiry
Students should proceed with their experimental plan. Remind students to consider variables that might affect the outcome of their experiment. Students should take notes about their observations and, if possible, repeat their experiment to substantiate their results.

Data Management and Analysis

Student Inquiry
Students should analyze their results in a way suited to their experimental design. If multiple demonstrations were performed or data were measured over time, a chart or graph is helpful. For other students a narrative analysis best communicates their results.

Interpretation of Results

Student Inquiry
Ask students to consider the following questions as they prepare to complete a final report:

• Did they succeed in replicating thermal stratification in a lab? Why?
• Were there other materials that might have aided their research?
• Would they expect the same results if another researcher completed the experiment?
• Would they proceed differently if repeating the experiment? Explain.

Reporting Results

Student Inquiry
Ask student groups to orally present their experimental design, results, and their interpretations of results. Which groups or designs were most successful? Why?

Part II - Researching Thermal Stratification in Lakes

Knowledge Base
The WOW data visualization tools can help illustrate temperature profiles in the WOW lakes. You may want to display the profiles (see Figure 1) for the students. This could be done either during your initial discussions of thermal stratification or as part of the closure to the lesson.

Figure 1. Ice Lake Temperatures

Student Inquiry
Ask students to consider the following questions as they prepare to research thermal stratification in a lake:

• How do their laboratory experiences relate to a lake environment?
• Will stratification be more or less pronounced in a lake setting?

Experimental Design

Student Inquiry
Students need to demonstrate whether or not a RUSS lake is stratified. They should consider what measures indicate stratification. How many measures are necessary to decide whether or not a lake is stratified? How many dates are necessary? Students should be prepared to explain their experimental design decisions.

Data Collection

Directed Study
Assign student teams to collect archival data from the WOW website from 6 consecutive dates for a lake. (All student teams should work with the same lake.) Students can choose a set of dates between May 15 and September 15.

Student Inquiry
Students collect WOW data to determine whether or not a lake is stratified. Students need to decide how many dates are necessary to prove whether or not the lake remains stratified.

Data Management and Analysis

Directed Study
Using the data, each group should create temperature profile graphs. If stratification is evident, the students should label each of the layers (epilimnion, thermocline, hypolimnion).

Student Inquiry
Students should graph the data collected. If stratification is evident, the students should label each of the layers (epilimnion, thermocline, hypolimnion).

Interpretation of Results

Directed Study
Discuss observations of temperature profile graphs for the selected lake.

Student Inquiry
Discuss observations of students graphs. Compare results for various measures. Which measure best shows stratification? Do other measures correlate to stratification?

Questions

• Does the lake appear to stratify?
• Does the lake remain stratified throughout the summer?
• What variables might affect whether or not the lake remains stratified throughout the summer?
• How might temperature profiles of other lakes in the region compare to the lake monitored by RUSS?
• Why is this type of information important to people who monitor and manage or use our lakes?

Reporting Results

Directed Study
Each student team should present its graph and interpretations to the entire class. Arrange the graphs in chronological order in the front of the room.

Student Inquiry
Students develop a summary of their data. Remind students to reflect on questions discussed as the class interpreted the data. Specify the final format: a written paper, oral presentation, poster, or multi-media presentation.

Notes: Many deeper Minnesota lakes stratify during the summer. Some lakes remain stratified throughout the summer. Lakes with large, open surfaces may be mixed periodically by the wind and temporarily lose some of their temperature stratification. The amount of wind, depth of the lake, surface area of the lake, and orientation of the lake (lakes that are oriented east-west are more affected than lakes oriented north-south) determine whether or not a lake will remain stratified throughout the summer. Temperature stratification is often paralleled by stratification of other water parameters such as the availability of dissolved oxygen for fish. Anglers often find some types of fish near the thermocline. Lakes that seldom mix are likely to have a lower layer that cannot support fish life due to lack of dissolved oxygen in the hypolimnion.

Evaluation
The completed temperature profile graphs will demonstrate the students’ ability to interpret and use temperature data from a lake. A journal of students’ exploration of temperature relationships in lakes will reveal their understanding of the conceptual material contained in the lesson.

Resources
"Productivity of Waters" by Dave Sonnenburg. Education Section, Bureau of Information and Education, MN Department of Natural Resources: St. Paul, MN.

Extensions

1. Compare temperature profiles of the selected lake during different seasons of the year.
2. Compare temperature profiles of different lakes.
3. Collect and graph temperature profiles of a lake that illustrate the process of turnover.
4. Create and compare graphs that include temperature, dissolved oxygen, and pH at increasing depths.