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Chesapeake Bay

Activity 4 (Explain): Algae: The Silent Killer

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Activity Overview

Activity Summary

In this activity, students analyze graphs of data from Sykes Creek, a stream in Florida that was the site of a fish kill in 2016 (the fish kill isn’t revealed to students until the end). After looking at the graphs of algae and dissolved oxygen, they read about the connection between algae, dissolved oxygen, and decomposers. Finally, they use what they have learned from the graphs and the reading to explain what caused the fish kill.

Activity Objectives & Materials

Approximate Time: 60 minutes​


  • Students will interpret a graph and read a text to learn how algal blooms result in dead zones.



  • Computer & projector

  • Word wall words: dissolved oxygen, algae, algae bloom, decomposer


  • Reading: Algae & Dissolved Oxygen (double-entry journal)

Standards Connection

DCI: LS2.C: Ecosystem Dynamics, Functioning, and Resilience

SEP: Constructing Explanations & Designing Solutions

CCC: Cause & Effect



What happens to the bodies of animals and plants when they die? What other living things help out with this process?

  • Answer: they decompose, break down, etc.; this is done mainly by bacteria and fungi, but also detritivores like worms and insects

  • The purpose of this warmup is to gauge students’ background knowledge on decomposition to prepare for the activity


1. Frame the Activity

Tell students that as a class they need to do some research about algae to see how they could be connected to the dead fish in the Bay. To do this, they’re going to analyze some graphs for algae growth from a place called Sykes Creek, and read a text about algae.

Step 1

2. Graph 1: Algae

Hand out the Algae at Sykes Creek sheet to students and show it on a projector so students can see the color.

ORCA algae graph.png

Make sure students understand that this is a graph of the amount of blue-green algae in a creek. Ask students what they notice about the graph.


Students should notice that the graph goes up and down a lot, and that it eventually goes down to a very low amount starting on March 15. Use questioning to help them realize that it goes up and down on a day/night cycle (up during the day, then back down at night). They will come back to this graph to think about why it went down so much at the end.

Teacher Tip: The “algae” in these graphs is actually a bacterium (cyanobacterium) which is called blue-green algae because it shares a lot like of characteristics with algae. You can bring this up with students, but don’t let it distract from the overall purpose of the activity.

Discussion Techniques: For each of the graph discussions, consider using a turn-and-talk to have students share ideas with a partner before sharing with the whole class

Source Data: The data source for these graphs is ORCA (Ocean Research & Conservation Association) based in Florida. The data comes from their public monitoring website. More information about the March 2016 fish kills can be found here.

3. Graph 2: Dissolved Oxygen

Display Graph 2 (Dissolved oxygen). Remind students that dissolved oxygen is just like oxygen in the air that is mixed into the water. Fish and other animals need dissolved oxygen to breathe underwater (they will learn more about it later in the activity). Have students share what they notice. They should see that it also goes up and down a lot, and then goes down to a very low level starting on March 15.

ORCA DO graph.png
Step 4

4. Graphs 1 & 2 Combined

Show students Graph 3, which is Graphs 1 and 2 combined. Ask students what they notice. They should recognize that the two graphs fit together (the amount of blue-green algae and the amount of dissolved oxygen seem to be related up until the end). Ask students why they might be related.

ORCA combined graph.png

Students may remember that algae make oxygen, so that could be one reason they are related. However, that doesn’t explain why they both went down starting on March 15th. Depending on where students are with their Algae in a Bottle experiment, they may also be seeing similar results in their own data. Use discussion to help students reach the question: “Why do the algae and dissolved oxygen both go down at the same time?”

5. Building Background Knowledge Reading

Hand out the double-entry journal reading “Algae & Dissolved Oxygen.” Have students read and answer the questions.

6. Sensemaking Discussion

Have students share out key information they learned from the reading. Put up the word wall words dissolved oxygen, algae, algae bloom, and decomposers. Key ideas that students should share from the reading:

  • Algae are unicellular or multicellular living things

  • Algae can produce their own food and oxygen using sunlight

  • Algae can increase the amount of dissolved oxygen in the water

  • When there are too many algae, they die out and get broken down by decomposers

  • Decomposers use up the dissolved oxygen

7. Return to Sykes Creek

Have students go back to their graphs of Sykes Creek. Remind them of the drop on March 15th. Have them turn to a partner and talk about what they think happened starting on that day. Students should be able to connect their reading to the data to recognize that there were too many blue-green algae, and so they died. When they decomposed, the decomposers used up the dissolved oxygen, causing it to drop. After their turn and talk, have groups share out their conclusions.

8. March 20, 2016

Remind students of their original mystery (the dead fish in the Chesapeake Bay). Point out key clues that they’ve learned:

  • Dead fish in the Chesapeake Bay

  • Lots of algae appeared in the Bay before the fish died


Have students add any more clues from today’s activity such as:

  • When there is too much algae, they die, and the amount of dissolved oxygen goes down

Tell students that the reason they’ve been studying Sykes Creek in March 2016 is that it was the site of another mystery.  Ask students if they can predict what happened at Sykes Creek on March 20th 2016 (point out the data on the graph). They may or may not be able to guess what happened. In either case, show them this photo:

ORCA fishkill.png

Use questions and discussion to help students realize that when the dissolved oxygen level gets too low, it will kill the fish. Have them add a clue to their clues board such as:


  • Low dissolved oxygen can kill fish

9. Formative Assessment: Conclusion

Have students answer the prompt on their graphs sheet as a narrative (or Claim-Evidence-Reasoning argument) explaining what caused the fish to die in Sykes Creek based on what they learned today. For example:

  • Claim: The fish died because there was not enough oxygen due to too many blue-green algae.


  • Evidence: On March 15, the amount of blue-green algae went down a lot. The amount of dissolved oxygen also went down a lot.


  • Reasoning: The amount of blue-green algae went down because there were too many and they blocked out the sunlight so they could not make enough food. When they decomposed, the decomposers used up the oxygen in the water. Because there was not enough oxygen, the fish couldn’t breathe and they died.

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