"Tracing the Toxins"

Note to teachers: Click here to see the printable version of this content.

Harmful algal blooms (HABs) can negatively impact organisms in a variety of ways that can range from cell and tissue damage to organism death. Some HABs are not toxic; in other words, they are caused by algae that do not produce a toxin that directly kills organisms, but they are harmful in that they create unhealthy conditions in the environment (e.g., too little oxygen, decreased sunlight). The impacts of these algae are discussed further in "Help! It's an HAB!" In contrast, toxic blooms are caused by algae that produce potent toxins that can cause massive fish kills, marine mammal deaths, and human illness.

There are several types of toxins produced by these harmful algae. Commonly, the toxins affect the functioning of nerve and muscle cells. Other toxins affect proteins or act like amino acids. Toxic blooms have been responsible for causing respiratory irritation and distress, diarrhea, vomiting, numbness, dizziness, paralysis, and even death. For more information on individual toxins and toxin biochemistry, visit "Toxic and Harmful Algal Blooms".

How can a toxin in phytoplankton cells cause such serious illnesses in so many different organisms? The key is how the toxins move through the food web.

Although a phytoplankton cell may only contain a tiny amount of toxin, imagine how much toxin a copepod would contain if it ate dozens of phytoplankton cells a day! Now imagine how much toxin a right whale would contain if it ate thousands of those copepods a day!

An important component involved in our scenario above is bioaccumulation. Bioaccumulation is the process by which compounds accumulate or build up in an organism at a rate faster than they can be broken down. Several organisms, including copepods, krill, mussels, anchovies, and mackerel, have been found to retain toxins from phytoplankton in their bodies. These organisms are often not affected by the toxins, but act as vectors and transport the toxins up the food web. There have been several cases of whale and sea lion illness and death attributed to this process.

In many cases the toxins can be transported through the food web to humans, often through contaminated shellfish. The toxins can impact humans in different ways leading to mild symptoms or even death. The toxins cause many illnesses, including Ciguatera Fish Poisoning, Diarrhetic Shellfish Poisoning, Neurotoxic Shellfish Poisoning, Paralytic Shellfish Poisoning, and Amnesic Shellfish Poisoning.

Visit "Toxic and Harmful Algal Blooms" to learn more about the impact of toxins on the food web, and the specific human ailments caused by individual blooms.

  • Understand the differences between toxic and harmful algal blooms
  • Appreciate the role of bioaccumulation of toxins in the food web

The students should already be familiar with the integral role of algae in the food web. This may have been accomplished by completing “Fitting Algae into the Food Web” and “Building a Bloom.” Other valuable resources include NOAA's State of the Coast Report on Harmful Algal Blooms and Bigelow's "Toxic and Harmful Algal Blooms."

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You Are What You Eat

Note to teachers: Click here to see the printable version of this activity.

Students will participate in a food web game where their feeding decisions determine their survival in the environment. The Antarctic food web that we created in "Fitting Algae Into the Food Web" will be the basis for the game. If you don't have Adobe Acrobat Reader or another way to view the PDF file containing the "life cards", click here to download the software for free.

Materials: 1 pound of M&M regular candies (can be substituted with poker chips if one color constitutes 30% of the total number), paper or plastic sandwich bags, name tags, markers, cut out “life cards”, rope or another boundary marker, stopwatch or other timing device, optional extra M&M’s for student consumption


1. Divide the class into trophic groups. If you have 28 students, assign roles as follows: 1 killer whale, 3 seals, 9 fish, 15 krill. Have each student write his/her role on his/her name tag.

2. Without the students watching, randomly distribute the M&M’s in a grassy area. Boundaries of the area should be made clear with the use of rope or something similar.

3. Have the students gather around the grassy area. Distribute a food bag to each of the students. Explain to them that there has been an “M&M” algal bloom in the area. The organisms that are able to eat the M&M algae are the krill. Remind the students of the food web involving algae, krill, fish, seals, and whales. Distribute the appropriate “Life Card” to each student and explain that each “animal” is being given specific directions as to how they should respond to this bloom.

4. Step 1 – The Krill: Tell the “krill” that they will have 30-60 seconds (depending on the number of students involved and the size of the area) to “graze.” They should move around the area feeding on the algae (collecting M&M’s and placing them in their food bags). Remind the students to refrain from truly eating their food, since they will need to assess their feeding success at the end of the game. At the end of the timed period, the “krill” should remain where they are in the area, but stop collecting M&M’s.

5. Step 2 – The Fish: Tell the fish to enter the feeding area and do what is indicated on their “Life Card.” Explain to the class that in this simulation, predators “eat” their prey by tagging their prey’s elbow. Once an organism is “eaten,” it relinquishes its food bag to its predator and sits down in the feeding area. During this time, the living krill can continue to graze on the algae.

6. Step 3 – The Seals: Once the fish accomplish what they were guided to do by their “Life Cards,” tell the seals to enter the feeding area and do the same.

7. Step 4 – The Killer Whale: Once the seals complete their tasks, allow the killer whale to enter the feeding area and accomplish what is indicated on the killer whale “Life Card.”

8. Once the killer whale has eaten, review what occurred by having the students share what was written on their “Life Card” starting with the krill and ending with the killer whale.

9. At this time, inform the students that some of the algae that they ate were toxic! If the krill consumed red or orange M&M’s, they consumed toxic algae!

10. Have the students go through their food bags and sort their food. Have them count the total number of M&M’s they collected and the number of red and orange M&M’s they collected. Have them calculate the amount of toxic algae they consumed as a percent of their total consumption.

100 x [# red + # orange]/[total #]

11. If there are any krill still alive that “ate” ANY red or orange M&M algae, they are now dead.

12. If there are any fish still alive that “consumed” 20% or more red and orange M&M algae, they are now dead.

13. If there are any seals still alive that “consumed” between 20%-30% red and orange M&M algae, they are now sick. If there are any seals still alive that “ate” over 30% red and orange M&M algae, they are now dead.

14. If the killer whale “ate” between 20%-30% red and orange M&M algae, it is now sick. If the killer whale “ate” over 30% red and orange M&M algae, it is now dead.

15. Discuss the conclusions that can be made from this activity. This can be accomplished through a group discussion, through individual reflective papers, or through individual journal entries.

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HABs in the News

Note to teachers: Click here to see the printable version of this extension.

Articles on harmful algal blooms have become commonplace in newspapers, magazines, and other media. The articles serve to make the public aware of bloom related phenomena, and in many cases offer scientific explanations of the event and its related consequences.

Your job is to investigate some recent articles on harmful algal blooms. The first thing you will have to do is find the articles; they can be from a newspaper, a magazine, or the internet. When you read the articles, take some notes. You should be sure to note the following:

    • Date the article was published
    • Medium in which the article was published (e.g. NY Times, National Geographic)
    • Purpose for the article (e.g. to describe an event, to detail a new technique)
    • Summary of the article
    • Audience targeted by the article
    • Your thoughts (Did the article accomplish its purpose? Was it well written? How could you improve the article? Were you left with any unanswered questions?)

Regroup as a class and discuss your findings with your classmates.

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Can You See the Link?

Note to teachers: Click here to see the printable version of this extension.

The table below provides a summary of a few research studies that investigated the transfer of algal toxins through the food web. The organisms in red are ones that were either made ill or died as a result of the toxin. Look back at the articles your class reviewed in "HABs in the News." Do any of your articles discuss a toxin food web path similar to one in the table below? For the articles that don't suggest a food web path, can you form a hypothesis as to how the algal toxin might have affected the different organisms discussed in your article?

Food Web Path
Diatom > Anchovy > Sea Lion Scholin, C, et al. 2000. Mortality of sea lions along the central California coast linked to a toxic diatom bloom. Nature 403:80-84.
Dinoflagellate > Zooplankton > Mackerel > Whale Geraci, JR, et al. 1989. Humpback whales (Megaptera novaeangliae) fatally poisoned by dinoflagellate toxin. J. Fish. Aquat. Sci. 46:1895-1898.
Diatom > Krill Bargu, S, et al. 2002. Krill: a potential vector for domoic acid in marine food webs. Mar. Ecol. Prog. Ser. 237:209-216.


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This table summarizes the McREL science standards that are met through this lesson. To see a detailed list of standards that this lesson addresses, please click here.

Grade Level
Primary (K-2)
Elementary (3-5)
Middle (6-8)
High School (9-12)

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