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. This lesson is divided into two parts. In the laboratory part, you are challenged to develop two distinct layers of water in a glass. In the technology application, you will determine whether or not a specific WOW lake is stratified during the summer.

Your parents own a cabin on a WOW lake. Your mother claims that the lake remains stratified throughout the summer, while your father is positive that the opposite is true. As a biology student you have decided to settle this argument by proving whether or not the lake remains stratified throughout the summer.

Part I - Laboratory Demonstration

Knowledge Base
Review your knowledge of thermal stratification. Have you 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
Can you demonstrate thermal stratification in the laboratory? Your teacher will supply materials.

You need to write a protocol for your demonstration. The protocol should provide clear, step-by-step directions for other researchers to follow. Be ready to explain the rationale for decisions about experimental design.

Data Collection
Proceed with your experimental plan. Consider variables that might affect the outcome of the experiment. Take notes about your observations and, if possible, repeat your experiment to substantiate the results.

Data Management and Analysis
Analyze your results in a way suited to your experimental design. If multiple demonstrations were performed or if data were measured over time, a chart or graph is helpful. For other designs a narrative analysis best communicates results.

Interpretation of Results
Consider the following questions as you prepare to complete a final report:

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

Reporting Results
You should be prepared to orally present your experimental design, results, and your interpretations of results. Which groups or designs in your class were most successful? Why?

Part II - Researching Thermal Stratification in Lakes

Knowledge Base
Consider the following questions as you prepare to research thermal stratification in a lake:

How do your laboratory experiences relate to a lake environment?

Will stratification be more or less pronounced in a lake setting?

Experimental Design
You need to demonstrate whether or not your parents' lake is stratified. Consider what measures indicate stratification. How many measures are necessary to decide whether or not the lake is stratified? How many dates are necessary? Be prepared to explain your experimental design decisions.

Data Collection
Collect the data necessary to decide whether or not your parents' lake is stratified. Use as many dates and measures as you feel are necessary to prove whether or not the lake remains stratified.

Data Management and Analysis
Graph the data you collected for your parents' lake. Based on your graphs, does the lake remain stratified throughout the summer? If stratification is evident, label each of the layers (epilimnion, thermocline, hypolimnnion).

Interpretation of Results
Consider the following questions as you prepare for your final report:

• 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
It is time to settle the argument. Use your graphs and knowledge of thermal stratification to prove whether or not the lake remains stratified. Your teacher will specify whether you should make an oral argument, written presentation, a poster, or a multi-media presentation.