Of the several thousand species of marine phytoplankton that have been identified, approximately seventy-five are known to produce powerful toxins that contaminate coastal areas around the world. Although many of these species cause problems when they bloom or accumulate in large numbers, some types are so toxic that even a few hundred of these minute cells can be deadly. Some forms cause massive die-offs of fish and other marine life, while others are eaten benignly by marine animals, which then accumulate the toxins. When these toxins are passed through the food web they can be dangerous to humans and other animals.

In this activity, students investigate ways in which a toxic species of phytoplankton might avoid predation or outcompete other species in the ecosystem. This may help them understand that even though only a few species of phytoplankton are toxic, they can have very widespread effects.
  • Discover how a toxic species might be concentrated within an ecosystem
  • Consider how filter feeders may consume toxins without harm but poison those who eat them


Red and black licorice or jellybeans (one piece of each for every student)

Review the marine food chain , making sure that students understand that in the oceans matter and energy move from the producer level through the various consumer levels. A simple marine food chain starts with phytoplankton which are eaten by herbivores (e.g., zooplankton) which are in turn eaten by carnivores, and so on. In most case, food chains overlap and interconnect, forming the marine food web.
  1. Pass a bowl of licorice to students, asking each to select one piece. In the end, you'll probably have many "leftover" black licorice pieces.
  2. Tell the class they have been zooplankton preying on licorice-flavored "phytoplankton." How might preferential grazing concentrate "bad tasting" toxic phytoplankton in an ecosystem?
  3. Consider how some non-discriminating filter feeders -- oysters, mussels and clams -- might be affected by toxic dinoflagellates left in the water column. (Studies show that some shellfish can retain poisons up to two years without harm.) What might happen to humans who eat these shellfish? (Note that cooking the shellfish does not destroy potent neurotoxins.)
  • Again, assume that toxic phytoplankton are not the "tastiest treat" available to predators. So, why would they get eaten? Perhaps the toxic phytoplankton have developed ways to outcompete other species, making them readily available for hungry and/or non-discrimating feeders.
    • Give the students the scenario that there are two factories; one that makes black licorice and another that makes red licorice. They are competing for the same shelf space at two grocery stores.
    • Divide the students into four groups: one "red licorice manufacturer" and one "black licorice manufacturer" per store. Challenge them to come up ways that their factory can "win" the shelf space. (Examples of solutions might be to get additional production machines, using faster shipping routes between the factory and the store, interfere with the other factory's process, etc.)
      • Did the groups come up with similar strategies?
      • What if the ingredients needed to manufacture either red or black licorice were in short supply?
    • Using ideas from these strategy sessions, discuss as a group how certain species of phytoplankton might outcompete others. You can learn more about common Harmful Algal Bloom species by linking to Toxic & Harmful Algal Blooms or The Harmful Algae Page
Bigelow Laboratory for Ocean Sciences, Copyright 2000