"Fitting Algae Into the Food Web"

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

Food chains and food webs show the flow of energy through an ecosystem. Food chains are linear depictions of energy flow, while food webs show the multiple interactions among the different types of organisms. Food webs are generally a more realistic portrayal of the energy flow in the system. After all, most organisms eat more than one type of food and can be eaten by more than one type of predator.


To the left is a typical food chain in a deciduous forest. Grasshoppers eat the grass; grasshopper mice eat the grasshoppers; and owls eat the grasshopper mice.

To the right is a more complex portrayal of the same system as a food web. Despite the visual complexity of the diagram, many species and many links between species are not shown.

In both images, the arrows indicate the direction of energy flow.


So where does energy come from? The fundamental energy source for most of the environment is the sun. Photoautotrophs capture the sun’s energy and use it to make organic compounds through photosynthesis. The process of photosynthesis transforms carbon dioxide and water into simple carbohydrates. The photoautotrophs then use the simple carbohydrates to build other more complex organic molecules (proteins, lipids and starches) that are either used as building blocks for their cells or are stored for later use. Photoautotrophs are often also called primary producers because they establish the basis for most other production; they create organic material from inorganic, or non-living, sources.


How do algae fit into the marine food web? The photoautotrophs with which we are most familiar are the trees and flowers that we see everyday on land. However, there are a substantial number of photoautotrophs in the marine environment as well – most of which we can’t see without a microscope! Despite their small size, these microscopic primary producers, marine algae and cyanobacteria, are vital to our planet's productivity since they are at the base of the marine food web.


What happens further up the food web? Each level of a food web or a food chain is called a trophic or feeding level, and the organisms in the food web are classified by whether they are primary producers or consumers. The consumers in food webs are called heterotrophs and they consume the organic material made by the autotrophs. Heterotrophs cannot make their own food so they are dependent on the autotrophs for survival.


A simple marine food chain might look like the one to the left. The salmon is the top consumer; the herring are the secondary consumers; and the copepods are the primary consumers. The phytoplankton are the producers.


A more complex marine food web might look like the one to the right. Despite the visual complexity of the diagram, many species and many links between species are not shown. Organisms may have more than one trophic role because they eat a variety of food types.

  • Understand food chains and food webs
  • Recognize the importance of and need for primary producers
  • Appreciate the interconnectedness and interdependence of organisms
  • Understand the role of bacteria in the food web
  • Realize the geographic variation in food webs

To receive the greatest benefit from the activity, students should have a basic familiarity with food webs. This can be accomplished by having the students read the above content.

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The Food Web Matching Game

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Let’s see if we can build a food web! This is an Antarctic food web. The organisms that fit into the web are listed below it. The trophic table below tells you a little bit about each organism’s role in the food web. Your job is to place the organisms in their correct places on the food web. Remember that the arrows indicate the direction of energy flow!

Click here to get started! (You will need Flash Player to play the game on-line. If your browser doesn't load the player automatically, you can download the free player from the Macromedia Website).

Trophic Type
algae primary producer --- krill, fish, blue whales
birds carnivorous consumer krill, fish seals, killer whales
blue whales planktivorous consumer algae, krill killer whales
fish omnivorous consumer algae, krill birds, seals, killer whales
killer whales top consumer blue whales, fish, birds, seals ---
krill herbivorous consumer algae fish, blue whales, birds
seals carnivorous consumer fish, birds killer whales


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A Missing Link

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Note to teachers: This extension exercise can be done in a variety of ways. Some suggestions are listed here: (1) Divide the class into three groups and assign each group a question (1-3) from below (if you have more than 3 groups, add questions about removing other organisms). Have the groups present their hypotheses to the entire class. Afterwards, ask and discuss questions 4 and 5 with the entire class. (2) Provide the questions below to guide students in writing an essay on the importance of primary producers in the food web.

Now let’s take a closer look at the food web you created. What would happen if you removed one of the organisms from the food web? Are there other organisms that could take its place? How would the organism’s predators/grazers respond? How would the organism’s prey/food respond? Let’s start at the top and try a few scenarios:

1) What would you hypothesize might happen to the other organisms in the web, if killer whales went extinct?

2) What might happen if we exhausted the fishery (through over fishing), and fish were no longer a component of our food web?

3) What might happen if we removed the algae from the food web?

4) Which scenario above affected the most organisms?

5) What does this tell you about the importance of the base of the food web?

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Tiny but Mighty

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The food web you created in the food web game included producers and consumers, but it didn’t include all of the types of organisms that play a role in the food web. Decomposers also play an extremely important role in the food web. What are the decomposers? Bacteria! That’s right, just like the bacteria in every terrestrial environment, bacteria also live in the marine environment. And just like terrestrial bacteria, marine bacteria are EVERYWHERE and impact EVERYTHING! In their role as decomposers, they break down dead plant and animal matter and release simple molecules that can be absorbed and used by autotrophs and some heterotrophs.

Now that you know what the decomposers are and what they do, place them on your food web and draw arrows to show how they interact with the other organisms in the web.

Bacteria play other roles in the marine environment as well. They are integral to nutrient cycling in the marine environment. Also, some bacteria are producers; they produce organic matter from inorganic matter through photosynthesis and chemosynthesis. To learn more about the importance of bacteria in nutrient cycling, and production, visit Bigelow's web site, "Cycling Through the Food Web."

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Geographic Variation

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

The food web you created in the food web game was one that is found in the Antarctic. As you might imagine, a food web in the tropics looks very different from the one you created. Environmental conditions (temperature, light, nutrients) are different in the two areas, so it would make sense that the organisms that live in those areas might be different also. What might a food web from a tropical marine environment look like? How about one from a temperate coastal area?

Divide into groups to create your own food web games. Use books and the internet to find information about the areas listed below or think of some on your own. Create food webs that you might find in those areas. Give your completed game to the other groups to play. Don’t forget to come up with clues to be provided if a player guesses incorrectly!

1) The west coast of South Africa - This is an area often associated with phytoplankton blooms. Upwelling brings nutrient-rich water from deep, dark depths to the sunlit surface. Phytoplankton are quick to utilize these upwelled nutrients, making this among the most productive areas in the world. However, there are also recurrent harmful and toxic blooms along the South African coast that threaten some members of the ecosystem, along with the fishing and shellfish industries.

2) The middle of the Sargasso Sea - The Sargasso Sea in the North Atlantic is named for the sargassum seaweed that is found throughout it. Due to the convergent circulation pattern of the sea, new nutrients are not upwelled from deeper waters, so the Sargasso is often called a marine desert. Despite its relatively low productivity, the warm, clear waters of the sea are home to entire communities built around mats of floating sargassum.

3) A hydrothermal vent environment on the East Pacific Rise - Unlike the food webs we have discussed thus far, hydrothermal vent communities are not dependent on solar energy. These communities are found in waters deeper than 1500m, where sunlight cannot penetrate. Instead, the hydrothermal vent food webs are driven by geothermal energy. Hydrogen sulfide (H2S) is released in the fluid that is forced from the vents, and is then utilized by bacteria to form organic matter. This process is called chemosynthesis, and the bacteria that chemosynthesize are the primary producers of the food web.

4) Lake Erie - Lake Erie is a freshwater lake that has experienced major changes in its food web over the past few decades. The lake's populations of primary producers (phytoplankton and cyanobacteria) have varied with reductions in phosphorus loading, and the introduction of exotic species (i.e. zebra mussel). There are also recurrent harmful and toxic blooms that threaten some members of this freshwater ecosystem.

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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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Phytoplankton drawings from: "Visit to an Ocean Planet" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory.