Authentic Lessons for 21st Century Learning

Clear as Phytoplankton: A Tale of Four Lakes

Nutrients, Genetics, and Plant Growth

Heather Shaffery, Carrie Miller-DeBoer | Published: July 13th, 2022 by K20 Center

  • Grade Level Grade Level 6th, 7th, 8th
  • Subject Subject Science
  • Course Course
  • Time Frame Time Frame 3-5 class period(s)
  • Duration More 200 minutes


In this lesson, students use differences in water clarity among four Oklahoma lakes as a phenomenon. They investigate how local environmental conditions and the survival strategies and growth of phytoplankton (algae) affect water clarity. After designing and conducting experiments that relate water clarity variables to phytoplankton growth, students analyze patterns in phytoplankton community composition. Using the data, students model the cause-and-effect relationships among local conditions, genetic factors, and phytoplankton growth. Finally, students compare their data analysis with findings summarized in official water quality reports before using their models to develop a summative explanation for the relationship between phytoplankton growth and water clarity in a single lake.

Essential Question(s)

How do genetic and environmental factors influence the growth of organisms? How does phytoplankton growth affect water clarity?



Students examine photographs of lake water at macro and micro scales and then create an initial model to explain how phytoplankton growth affects water clarity.


In groups, students plan and carry out investigations that test how phytoplankton growth is affected by different water clarity variables. They also sort phytoplankton species into groups according to physical and functional characteristics.


Based on their data, student groups develop claims about phytoplankton growth. As a class, students create a concept map linking their claims and evidence to the lake water clarity phenomenon. Students refine their models to reflect the cause-and-effect relationships among water clarity variables, phytoplankton survival strategies, and water clarity.


Students use a Venn diagram to compare their previous data analysis with data from official water quality report summaries and then evaluate and revise their models.


Using their final models, students explain the relationship between phytoplankton growth and water clarity in a lake of their choice.


  • Lesson Slides (attached)

  • Detailed Facilitation Guide (attached; for teacher use)

  • Print Student Notebook (attached)

  • Digital Student Notebook (attached)

  • Four Lakes’ Phytoplankton Communities (attached)

  • Procedure Example (attached)

  • Phytoplankton Cards (attached)

  • Four Lakes’ Data Reports (attached)

  • DIY Secchi Disk Instructions (attached; optional)

  • Water bottles

  • Pond water or fast-growing algae culture

  • Nutrient variables

  • Secchi disks (buy or DIY using attached instructions)

  • Fluorescent lights


Introduce the lesson using the attached Lesson Slides. Display slide 3 to share the lesson’s essential questions and slide 4 to go over the lesson objectives. Review these slides with students to the extent you feel necessary.

Display slides 5–9 to engage students in the phenomenon—variation in lake water clarity and what affects it—by presenting them with images of water from four Oklahoma lakes. Have students make observations and ask questions about the images using the I Notice, I Wonder instructional strategy.

As students share, record their ideas and elicit additional details or clarification using tools like STEM Teaching Tools’ Talk Science Resource Cards, which provide sentence and question stems.

Next, pass out the attached Four Lakes’ Phytoplankton Communities, and display slides 10–13 to show students the phytoplankton communities found in each lake, as seen under the microscope. Direct students to look for patterns in the phytoplankton communities, adding these ideas and questions to their previous "I Notice" and "I Wonder" lists.

Go to slides 14–15. Define phytoplankton (also called algae) and water clarity for students.

Go to slide 16. Guide students in discussion to consider what might cause the amount of phytoplankton in the water to increase by asking them to list things that help plants grow. They should mention light, water, carbon dioxide, and nutrients.

Following the discussion, explain that students will focus on answering this question: "What factors explain how phytoplankton affect water clarity?" The question can be broken down into two sub-questions: "What factors cause phytoplankton to grow?" and "How do those factors indirectly affect water clarity?"

Go to slide 17. At this point, ask students to use the ideas from what they’ve observed and discussed so far to draw a model that explains the phenomenon.

Explore 1

Display slide 18. Ask students to look at the factors they included in their models that they think affect phytoplankton growth. Based on this information, ask them how they can test one of their factors to see if their idea is correct.

In pairs or small groups, students will plan and carry out an investigation, choosing a single variable that may affect phytoplankton growth. Make sure that more than one group is assigned to each variable (e.g., two groups test nitrogen, three groups test phosphorous, etc.) so they can later combine their data for analysis. Use slide 19 to provide your class-specific instructions to students.

Make sure that some groups test increased nitrogen, some test increased phosphorus, and some test an addition of fertilizer that contains both nitrogen and phosphorus. If other students want to test light levels, pH, temperature, etc., consider adding these to the factors under investigation.

Students should record phytoplankton abundance data daily; the most efficient method for this is using a modified Secchi disk. For reference, the YouTube video "The Dirty Labcoat: The Secchi Stick" demonstrates how to use a Secchi stick.

Secchi disks can be purchased online or you can make your own using the attached DIY Secchi Disk Instructions.

Go to slide 20. Once students have started their investigation, pass out a set of the attached Phytoplankton Cards to each investigation group. Have students compare the cards with the microscopic views of the four lakes’ phytoplankton communities to identify the number of each type of phytoplankton found in each lake.

Go to slide 21. Students should use the genetic factors listed in the table on each card to sort the plankton into groups. Once the cards are sorted, students should record their sorting factor and list the phytoplankton in each group.

Next, students should choose a different factor, sort the phytoplankton again, and record their factor and list of phytoplankton. Have students sort phytoplankton according to a minimum of three factors.

Explain 1

Display slide 22. After students have finished sorting phytoplankton for a minimum of three factors, they should compare their sorted lists of phytoplankton with the groups of phytoplankton within each lake community, taking note of any patterns or relationships.

Once groups have had time to make their comparisons, bring them together to discuss as a whole class. Have students describe the ecological significance of the genetic factors listed on each card (e.g., being able to use more than one resource means they have more nutrients available than functional groups that cannot use more than one).

Ask students what they can conclude about the phytoplankton in each of the four lakes. If relevant, this is a good opportunity to help students take notes and make connections between the activity and other ecology concepts they may have learned previously.

Next, ask students: In what kind of environment would we find each phytoplankton group (functional group)? Be sure to focus specifically on nutrient conditions (high, low, both conditions) at some point during the discussion.

Go to slide 23. Once pairs and groups have made sense of their data, ask students to revise their initial models based on the evidence they have collected. If genetic factors were not included previously, students should add them at this point.

Explore 2

Display slide 24. Once the water clarity experiments are complete, create experiment focus groups or "factor expert panels" by combining groups that tested the same variable in their investigation. These groups should compile and compare data from their experiments. They should treat their individual experiments as replicates and use them to calculate averages that summarize their collective data.

Use slide 25 to provide your class-specific instructions on grouping students, analyzing and graphing data, and making claims.

Based on the collective data, each group should develop a claim about their results (e.g., "[Our experimental factor] results in [some result]"). Note that water clarity is most likely to be influenced by nutrient resource levels. To prepare for the class discussion, groups should create a graph that displays their results summary and includes their claim in a caption.

Additionally, the "expert panels" should discuss their phytoplankton classifications to identify patterns among phenotypes (e.g., nitrogen fixers tend to be toxic) and comparisons with the phytoplankton community data for each lake. Students also should develop a few claims based on the patterns they observe during the group comparisons.

Explain 2

At this point, the class should work together and use the Concept Card Mapping strategy to synthesize what students have learned so far about the relationships among water clarity, environmental factors, and phytoplankton growth.

Before groups share their ideas for the concept map, post the images of lake water that were used to introduce the phenomenon. Add the nutrient and environmental variables from students’ experiments as concepts to help anchor their ideas. Use slide 26 to provide your class-specific instructions for creating the concept map.

Ask all groups to share their claims and evidence from their experiment results and then add their graphs to the concept map. Next, have students share their claims about phytoplankton functional groups and communities.

As students share their ideas about the relationships among phytoplankton characteristics, environmental variables, and the phenomenon, ask students to use evidence to (1) add new concepts to the map, and (2) identify the relationship(s) among existing concepts.

Once all groups have shared, give them some time to identify any other relationships or concepts that they think should be added to the map to capture what they’ve learned so far.

Display slide 27. Once the class concept map is complete, students should use the compiled information and relationships discussed to revise their model again.

The revisions should explain the cause-and-effect relationships that influence phytoplankton growth and water clarity. Remind students that they should draw a model that shows how changes to the variables they tested in their investigations affect water clarity, even if the connections are indirect.

Go to slide 28. Before moving to the Extend section, ask students to make a prediction about the nutrient conditions for each of the four lakes. If students investigated other factors (e.g., light), they could also use these as part of their predictions.


Once students have a consistent and correct understanding of the environmental and genetic factors that affect phytoplankton growth, which influences water clarity, pass out the attached Four Lakes’ Data Reports. It is recommended that you use one of these water quality reports to model how to interpret the information.

Students will use a Venn diagram to compare and contrast their predictions with the "Parameters" data in the reports. Creating the Venn diagram provides students with the opportunity to evaluate their models.

After students have evaluated their models, have a class discussion about their evaluations. Based on student responses, you may elect to discuss the purpose and limitations of models.

Encourage students to use their evaluation to further revise their model.


Display slide 29. Have students pick one of the lakes other than Tecumseh Lake and then use their model to explain their lake’s water clarity. Emphasize that students must reference the evidence that supports their model.

Students may choose to deliver their explanation as a paragraph, a storyboard, a stop-motion animation, or a video. Have students present their explanations or curate them for other students to watch/read independently (e.g., Gallery Walk, Flip, Padlet, school LMS). After completing their own explanation, students should evaluate one or more peer explanations.

How you formally assess the final explanation (e.g., rubric, checklist, etc.) will depend on the format options students have to choose from, but the scoring should address connections among ideas and use of evidence rather than quality of presentation.