Purpose: Students will learn about the impact of climate change on aquatic ecosystems.
Procedural overview: After students read the Science News article “Climate change could turn some blue lakes to green or brown,” they will identify how sediment, organic matter and algae determine lake color. Using this information, students will explore Colors of Lakes, an interactive online map that displays lake coloration worldwide, and will explain how latitude influences lake temperature and algal growth.
Using the online map and information provided by the teacher, students will then study the color of a specific lake and design an experiment that investigates lake coloration. As an extension of this activity, students can present their proposed investigations to the class, and teachers can use student hypotheses and investigations to introduce a water-quality lab.
Approximate class time: 1 class period
Directions for teachers:
Assign students to read “Climate change could turn some blue lakes to green or brown” from Science News a few days before the start of this activity. (A version of the article titled “Warming puts blue lakes at risk” appears in the Nov. 5, 2022 print issue.) Have students annotate the article as they read, identifying any new vocabulary and concepts.
If you ask students to summarize the article, suggest they first review “How to write a summary” from Science News Learning.
Prior to starting the activity, verify that the interactive online map of lake colors is loading and can be projected. Preload the sections of the map you plan to use in separate tabs to minimize loading time during class.
Want to make it a virtual lesson?
If students are working remotely, they can do the activity using a conference and screen-sharing platform such as Zoom.
Before you begin the article analysis with your class, review the following terms: sediment, algae and organic matter.
Sediment: Small particles made from rock or soil that are carried by water.
Algae: Organisms that, like plants, photosynthesize to create energy, releasing oxygen in the process.
Organic matter: Any material that was part of or made from a living organism. This can include things that are in the process of decomposing, like leaf litter.
Use the following questions for the article analysis.
1. How is temperature related to lake color?
Warmer temperatures can help algae grow, and algae can give lakes a green color. Cooler lakes will have less algae and can be blue in color.
2. How can sediments enter lakes? How does this affect lake color?
Rain can carry sediments into lakes through runoff. This can make lakes brown.
3. How can organic matter enter lakes? How does this affect lake color?
Rain can carry organic matter into lakes through runoff, or organisms in the lake can die or produce waste, increasing decomposition. This can make lakes brown.
4. Sediment, organic matter and algae can affect lake color. What other factors can influence lake color? (Think outside the box!)
Student answers will vary. Salts, minerals or plant growth can influence color.
5. What is a threshold? What is the threshold air temperature for algal growth?
A threshold is the amount of something (for example, the minimum air temperature) required for a change to occur (for example, a significant increase in algal growth). For lakes, the threshold air temperature for algal growth is 19° Celsius.
6. How would lake colors change if the average summer air temperature were to increase by 3 degrees C? Why?
Many lakes are near the threshold temperature of 19° C. This means that if the air temperature were to increase by 3 degrees C, lakes would pass the temperature threshold for algal growth and would begin to grow more algae. The increased algal growth would make blue lakes turn green or brown.
Before moving into the lake exploration, preload a section of the interactive online map that shows how lake colors vary with changes in latitude. South America and the eastern coast of the United States are ideal sections of the map to preload as they show that lakes closer to the equator are more often green or brown, and lakes further south or further north of the equator are more often green and blue.
Have the students answer these questions during the class discussion.
1. What happens to air temperature as you move from the poles toward the equator?
Air temperatures are highest at the equator, so air temperature would increase as you move from the poles to the equator.
2. How is air temperature related to water temperature? What does this mean for algal growth?
Air temperatures can influence water temperatures. Higher air temperatures will result in higher water temperatures. This means that lakes in areas with high air temperatures will be better suited for algal growth and may contain more algae.
3. Look at the middle of the map. What happens to lake color as you go toward the equator? Why?
Lakes are mostly green and brown. This is because air temperatures are higher at the equator, so there will be more algae growth.
4. What happens to lake color as you move away from the equator? Why?
More blue lakes are present. This is because air and water temperatures are lower as you move closer to the poles, so lakes are less likely to have significant algal growth, making blue lakes more common.
5. If latitude heavily influences temperature, then, in theory, lakes at the same latitude should experience similar air temperatures. However, not all lakes at the same latitude are the same color. What might change from location to location?
Student answers will vary. Changes in elevation, pollution, sediment levels or level of organic matter could all influence lake color.
6. If blue, green and brown lakes can all be healthy, how could we define a healthy lake?
A healthy lake contains an abundance of life (has high biodiversity) and are relatively stable, meaning they aren’t experiencing dramatic changes in temperature, pH, nutrient abundance or population sizes.
7. What might we look for to identify if a lake is unhealthy?
We might meaure the characteristics of the lake, like measuring its pH and oxygen levels. We can also test for the amount of pollution present in the lake.
8. If a lake were to change color, how could we tell if the color change was the result of a natural process or human activities?
We could look at the area surrounding the lake and see whether there are any potential sources of pollution or human-made changes such as clearcutting. If there aren’t any potential pollution sources or changes made to the nearby area, it is likely the result of a natural process.
Before moving into the lake investigation, preload a section of the interactive online map that shows a lake affected by human activities. Have students answer questions 1 and 2 before telling them anything about the lake they are viewing. Before asking students to answer question 3, describe the lake and the surrounding environment in more detail. Be sure to tell students enough about activities in the local watershed that they can predict possible environmental problems.
Once students have answered these questions, tell them more about the lake you have chosen and its watershed. You can choose a lake from your part of the country or another. One option is Lake Oconee in Greensboro, Ga.
Lake Oconee is dark brown, but it is surrounded by blue and green lakes. This area used to be part of the Oconee River and became a lake when Wallace Dam was built. Poor farming practices in the nearby area caused a lot of erosion, which changed the lake’s characteristics.
1. Looking at the lakes on the map, are there any that are a different color than the surrounding lakes? Why might this lake be a different color?
Answers will vary. Lake Oconee is different. It is brown, possibly because of sediments.
2. How could you determine what causes this lake’s color to differ?
The lake could be tested for the levels of algae, sediment or organic matter, and those levels could be compared with those of the surrounding lakes. If the levels of algae, sediment and organic matter in this lake are similar to levels in the surrounding lakes, it hints that something else is changing the lake’s color.
3. How can we fix the environmental problem that causes this lake’s color?
Student answers will vary. Lake Oconee’s color is caused by poor farming practices and erosion. Farmers can switch to more sustainable farming practices that don’t cause as much erosion or find a way to prevent sediment from entering the watershed.
Design an experiment
Students will use the knowledge they have gained about lake coloration to develop a testable hypothesis.
If students need a review, remind them of the meaning of hypothesis — a testable statement that explains the relationship between two variables/groups — and point them to the Science News Learning lesson “Using Science News to develop a research question and hypothesis.”
To formulate a hypothesis, have students answer the following questions.
1. What do you find most interesting about lake colors?
Student answers will vary. I think it is interesting that temperature influences algal growth, which influences lake color.
2. Ask a scientific question that could be realistically answered through observation or experimentation.
Student answers will vary. Can water temperature change the amount of organic matter in the lake? Do lakes with more algae have higher biodiversity? Can pollution increase algal growth?
3. Rephrase your scientific question into a testable hypothesis.
Student answers will vary. Higher water temperatures will increase decomposition, decreasing organic matter in lakes. Lakes with high levels of algal growth will have more fish species than lakes without significant algal growth. Lakes exposed to agricultural pollution will have higher levels of algae than unpolluted lakes.
4. What data will you collect to test your hypothesis? Make sure the data are quantifiable.
Student answers will vary. We could take measurements of the amount of algae, sediment and organic matter in the water.
5. Imagine that the data you collect do not support your hypothesis. What will you do next?
Student answers will vary. I could form a new hypothesis or perform the experiment a second time to ensure that the data were correctly collected and are accurate.
Activity extension: Investigation presentations
After students have done their investigations and developed hypotheses, students can present their hypotheses to the class. In presenting each testable hypothesis, students should identify the difficulty of testing their hypothesis. After all students have presented, the class should discuss the hypotheses and identify one hypothesis that could be realistically tested. The class can then conduct the selected investigation, or teachers can use the student presentations to develop a water-quality testing lab.