Changing Planet: The Warming of Our Large Lakes - Reasons for Concern
|Students observe models of water stratification related to density differences to understand the mechanisms of thermal stratification. Students then analyze and interpret temperature profile data from the Great Lakes in order to locate key thermal layers in the water column and determine whether lake mixing has occurred. They apply their interpretations to the lake ecosystem.||Materials:
|Adapted by NESTA/Windows to the Universe team members Missy Holzer, Jennifer Bergman, and Roberta Johnson from resources on Windows to the Universe as well as from a Great Lakes Lesson (Great Lakes Lessons, Teaching with Great Lakes Data, Michigan Sea Grant, www.greatlakeslessons.com). Data was acquired from the NOAA Great Lakes Environmental Research Lab, Ann Arbor, MI, with special thanks to Gregory Lang for his assistance.|
|7-9, although it may be adapted to higher grade levels using the extensions below.|
| Introduction: 10 minutes
Part 1: Teacher demonstration of a lake temperature model: 30 minutes
Part 2: Temperature profile analysis and interpretation: 50 minutes
Total lesson time: 90 minutes
Student Learning Outcomes:
|Hands-on inquiry & modeling activity, including analysis and interpretation of graphs|
DIRECTIONS:1. For background information on the warming of large lakes watch the Changing Planet episode, Temperature of Large Lakes. Also, explore these topics on the Windows to the Universe website.
2. Gather materials and print out the worksheet and temperature profiles. As an alternative, this lesson could be completed in student journals. Use your current journal format, and assist students in setting up a clear method for collecting observations. Have them leave space next to their observations so they can interpret and explain what they are observing throughout the lesson.
3. As an entry point to the lesson, probe student understanding of density. Use the demonstration called "Exploring Density with Salt and Fresh Water: Par 5" found on the Windows to the Universe website. In the discussion, be sure to mention that stratification not only occurs in oceans, but it also occurs in lakes and ponds.
4. Introduce students to the concept of lake stratification and the terms used to define the 3 major layers of water: epilimnion (warm surface layer), metalimnion (middle layer where temperature rapidly drops and is sometimes called the thermocline), and hypolimnion (cold bottom layer).
5. Distribute student worksheet. To model the above terms, fill one of the jars almost to the top with ice cold water, and a second jar almost to the top with warm water, add yellow food coloring to the cold water, and blue food coloring to the warm water. Ask students to write a hypothesis about what would happen if you placed the open end of one jar on top of the open end of the other jar. Allow students to share their hypotheses. Cover the top of each jar with an index card, and invert one jar over the other and carefully remove the index card. Ask students to make observations and to include a labeled drawing, with the epilimnion, metalimnion, and hypolimnion correctly identified. Carefully place the two stacked jars on the counter, and use the other two jars to repeat this procedure using warm water in both jars, and then cool water in both jars. Once these two explorations are completed, return to the original two jars and allow students to make additional observations after time has elapsed. After completing this demonstration, show students an image of lake stratification similar to the one below, and discuss how the model they just observed is similar to a large lake.
Image courtesy of Michigan Sea Grant, www.miseagrant.umich.edu
6. Students will be analyzing 2010 Lake Superior temperature profile data in order to locate layers and mixing. Lake Superior is a deep lake and therefore is an excellent example to use for identifying the lake layers and instances of mixing. Show students a picture of Lake Superior using an image from Google Earth or another similar program. The coordinates for the data collection site are 48.05479°N and 87.74523°W, and the depth at this location is 255m. Explain to the students that scientists are studying lake temperatures in a water column by taking the temperature at different depths throughout the year, and then they create profile graphs of the data.
7. Students may have never seen temperature profile graphs. Therefore, it is important to assess their abilities to do so before beginning the next part of this lesson. Distribute the profiles and ask students to identify the parameters on the X and Y axes. Ask them to make general observations of all the profiles. Use their comments to gauge their understanding of how to interpret the profiles, and correct any errors.
8. Have students work in pairs or threes to interpret the profiles using the questions on the worksheet as a guide. Again, this may be adapted for use with student journals.
9. Students may have the misconception that lakes do not contain oxygen, and that organisms in lakes do not require oxygen to survive. The seasonal change in water temperature and weather patterns in these large lakes influences the quantity and availability of oxygen for organisms living in the water. Lake water mixing assists in distributing oxygen from the surface to the hypolimnion but during the summer the depth of the warm epilimnion prevents new supplies of oxygen from reaching the hypolimnion. Therefore, the marine life in the hypolimnion must rely on reserve suppliesof oxygen which can be depleted quickly by marine life and with the decomposition of organic matter. This lack of oxygen in the hypolimnion coupled with depletion of oxygen in the epilimnion can lead to dead zones that are either hypoxic (below 2 mg/l) or anoxic (no oxygen). Students will be considering these concepts in the final interpretation of the temperature profiles. It is also important to note that the dynamics of the lake (how and when they turn over) varies for reasons such proximity to the equator, and the depth of the lake.
10. Review the responses to the questions, and spend time discussing the potential impacts of delayed mixing of the lake water on the lake ecosystem. Have student teams research the oxygen requirements for various marine species.
ASSESSMENT:As an assessment, create an activity using the Nowcast data available from the Great Lakes Coastal Forecasting System. Seek out current events related to this topic, and assess the students' abilities to interpret the issues put forth in the articles based on what they learned from this lesson.
LAB SAFETY:Use safe laboratory practices at all times.
CLEAN-UP:Discard water down the drain, and store dry materials for a future lab activity.
EXTENSIONS:Visit the Great Lakes Lessons website and click on the "Dead Zone" module for additional lesson activities related to this topic.
BACKGROUND INFORMATION:There is a scientific consensus to the fact that the global climate is warming because of the addition of heat-trapping greenhouse gases which are increasing dramatically in the atmosphere as a result of human activities. A lot of focus has been placed on the effects of global warming on the ocean -- sea level is rising, sea water is becoming more acidic, and ocean circulation is changing due to melting sea ice. But did you know that the Earth's freshwater lakes are being affected by global warming as well?
A NASA study that was released in November 2010, surveyed the surface temperature trends in 167 large lakes worldwide. They reported an average warming rate of 0.45 degrees Celsius (0.81 degrees Fahrenheit) per decade, with some lakes warming as much as 1 degree Celsius (1.8 degrees Fahrenheit) per decade. The warming trend was global. While the greatest increases were found in lakes found in mid- to high-latitudes of the Northern Hemisphere, increases were seen in North America, South America, Europe, Asia, Africa and Australia.
Scientists are just starting to study and understand the implications of rising temperatures on lake ecosystems. One area of concern is the fact that rising lake temperatures result in increased algal blooms. Algae is naturally found in lake ecosystems and is in fact the base of the aquatic food web. But when the numbers of algae in a lake rise dramatically, a bloom results. Some algal blooms are harmless to life, but are simply unappealing. Water in that area might look terrible, smell foul or even taste bad (when water is drawn for drinking and purification from that source). Other times, algal blooms can be toxic to fish, other aquatic organisms, wild and domestic animals that use that source of water, and humans. Humans can experience gastroenteritis (if the toxin is ingested), lung irritations (if the toxin becomes aerosolized) or skin irritation (if the algae/toxin is touched for instance while swimming) .
Rising lake temperatures have also been shown to favor invasive species found in lakes. In the Great Lakes region, two examples of invasive species under scrutiny are zebra mussels and lampreys. Zebra mussels have been seen to thrive in warmer and warmer waters, which means they can extend their living range to higher and higher latitudes. Lampreys seem to thrive in warmer waters growing bigger and bigger and are staying active for more of the year. Both of these invasive species are extreme pests that are killing off native species, eating the food of native species, or in the case of zebra mussels, causing billions of dollars of damage to structures and aquatic vehicles.
NASA was able to survey a large number of lakes all in one study with the help of satellite data. These findings are in line with what is being reported 'on the ground'. For instance, Russian and American scientists have been studying the Siberian Lake Baikal for over 60 years. Lake Baikal contains 20 percent of the world's freshwater, and it is large enough to hold all of the water in the U.S. Great Lakes. It is the world's deepest lake as well as its oldest (25 million years old). The data from the lake shows that the surface waters have warmed significantly and that the food web in this lake has already experienced dramatic changes. Many are concerned that global warming has reached this most remote of locations, especially since Lake Baikal is home to over 2,500 plant and animal species, with most, including the freshwater seal, found nowhere else in the world.
In about as different a location as you could find from Siberia, scientists have found more evidence of global warming -- in East Africa's Lake Tanganyika. Geologists have determined that the lake has experienced unprecedented warming during the last century; in fact according to core samples, it is the warmest the lake has been for 1,500 years. Scientists expect that as the lake gets warmer, fish productivity will decline. This has been attributed in a large part to water stratification that many large lakes are experiencing. The lake, one of the richest freshwater ecosystems in the world, is divided into two levels. Most of the animal species live in the upper 100 meters, including valuable sardines which are a source of food for many living in the area. The lake depends on wind to churn its waters and send nutrients like nitrogen and phosphorous from the depths toward the surface. These nutrients are food for algae, which supports the lake's entire food web. But as Lake Tanganyika warms, the difference in temperature (and density) increases and the mixing of waters is lessened; fewer nutrients are funneled from the depths to the surface. An estimated 10 million people live near the lake, and depend on it for drinking water and for food. Temperature rises in Lake Tanganyika will change how people eat, live and make money.
Many of the same issues are being studied by scientists studying Lake Superior, the deepest, the coldest, and the largest of the Great Lakes. By surface area, it is the biggest freshwater lake in the world. Dr. Jay Austin leads a team of researchers from the Large Lakes Observatory (University of Minnesota) who are studying Lake Superior. One of the ways they study the lake is with buoys. The buoys house instruments that measure things like air temperature, water temperature, cloud cover, humidity, wind speed and direction, all the things that control how energy is transferred from the atmosphere to the lake. Scientists then access the buoy data remotely. Austin says the summer temperatures of Lake Superior jumped 4-4.5 degrees Fahrenheit over the last 30 years. In this complex system, there are obviously many interconnected factors to gauge. One basic assumption is that warmer temperatures mean less winter ice in Lake Superior. With less ice, the lake surface is actually darker, the albedo is lower, more solar energy is absorbed, and less is reflected (than it would have been by the white ice). Holding onto the energy causes more ice to melt, which, in turn, lowers the albedo, causes more energy to be absorbed and more warming. With this warming comes evaporation of the lake's waters. This in turn, results in lower water levels for the lake overall. Low lake levels are a concern to property owners on lakes, those in the shipping industry, and low lake levels affect water outlets and inlets into the lake (even groundwater seepage), and wildlife too.
Lake Baikal, Lake Tanganyika and Lake Superior aren't isolated cases - global warming affects the temperature of lakes around the world. Many of these lakes are experiencing the undesirable effects of global warming such as an increase in algal blooms, the rise of invasive species, decreased fish productivity and lowered lake levels. Obviously, more study and attention is due these important areas where so many people live, work and make their homes.
RELATED SECTIONS OF THE WINDOWS TO THE UNIVERSE WEBSITE:Earth's Climate and Global Change
Effects of Climate Change Today
Rising Temperature in Large Lakes
Global Warming Affects World's Largest Freshwater Lake
East Africa's Lake Tanganyika is Warming More Than Ever Before
OTHER RESOURCES:Nowcast data for the Great Lakes
Seasonal Lake Stratification
Water on the Web
Oxygen Depletion Animation