The photic zone is the sunlit upper ocean where the ocean food web begins. It is populated with many types of microscopic organisms -- some plants, some animals, others in-between -- all of which depend on one another. Interestingly, key members at the base of the food web range in size from less than one micrometer (i.e., one millionth of one meter) to well over a thousand times larger!

This two-part activity help students better understand major groups of plankton and their food web relationships. The first part, "Plankton Rummy," introduces the characteristics of diatoms, dinoflagellates, coccolithophores, blue-green algae, ciliates and zooplankton. The second part involves making 1:5000 scale models of individuals from these key groups. Together these activities lay the foundation for student discussion of trophic levels within the microscopic marine food web.
  • Learn specific characteristics about key groups of marine plankton
  • Use knowledge of plankton groups to better understand relationships at the base of the marine food web
  • Build scale models of plankton to observe the great diversity each organism's shape, size and function

Index cards, poster paper, colored pens, thumbtacks, wall space to hang plankton models

1. Introduce these six common types of marine plankton by playing "Plankton Rummy." Each round has up to six players; teacher may choose to have more than one group of students playing at a time.

2. Create five cards for each type of plankton listed below, following the descriptions given below. The sixth card should be its "group" name (i.e., DINOFLAGELLATES, DIATOMS, etc.):

  • DIATOMS (Example images with scale bars)
    1. These phytoplankton commonly form chains or colonies
    2. Get energy from the sun (i.e., photosynthesize)
    3. Non-swimmers: projections (such as spines) and stored oils can help them float
    4. Have external "skeletons" made of silica (glass)
    5. Two common examples: Chaetoceros, Thalassiosira
  • DINOFLAGELLATES (Example images with scale bars)
    1. These common phytoplankton usually exist as single cells
    2. Have two "whip-like" tails (flagella) and can swim weakly
    3. Some species get energy from the sun AND by eating other organisms
    4. Many species are toxic
    5. Protoperidinium and Dinophysis are "armored" examples; Ceratium is an "unarmored" example
  • COCCOLITHOPHORES (Example image with scale bar)
    1. Have two "whip-like" tails (flagella) but only at certain life stages
    2. Get energy from the sun (i.e., photosynthesize)
    3. Spherical organism is covered with plates of calcium carbonate
    4. Fossils are used to make chalk
    5. Very large blooms can occur, turning the ocean surface "milky"
  • BLUE-GREEN ALGAE (Example image with scale bar)
    1. Also known as "Cyanobacteria"
    2. Group has the smallest known photosynthetic organisms
    3. Synechococcus was not discovered until 1977 although it is found throughout the world's oceans
    4. Although individuals are less than 1 micrometer in size, this group is a major contributor to primary production
    5. Blooms of one genus (Trichodesmium) were documented by Captain Cook in the 1700s
  • CILIATES (Example images with scale bars)
    1. Have short, hairlike structures that cover the cell surface
    2. These protists are a very diverse group with about 10,000 species
    3. Eat bacteria and small phytoplankton and are eaten by zooplankton
    4. Must steal chloroplasts from their prey in order to photosynthesize
    5. Laboea and Mesodinium are two examples of these very active swimmers
  • MARINE ZOOPLANKTON (Example image with scale bar)
    1. Swimming, one-celled animal plankton that live in seawater
    2. Energy is primarily gained through eating other organisms including plants, other animals or both
    3. Cannot photosynthesize and thus are "heterotrophic" ("other" + "energy")
    4. Copepods are this throughout their lifetimes
    5. Lobsters, crabs, clams, sea urchins, barnacles and starfish begin their lives as this

3. Shuffle cards and pass out six cards to each player. Players may or may not get a "group" card. They should look at their other cards to choose which group they like'd to continue collecting.

4. When signaled, each player will pass one card to the left and pick up a card from the player at right. Play continues until one player has six cards from the same group and calls out "Rummy." Other players should verify that the set is truly complete (and accurate).
5. Continue playing until all players have completed their sets.

6. After students have become familiar with these groups, they are ready to make 5000:1 scale models on poster paper. Be sure to include at least one example from each of the six groups listed in Step #2. [NOTE: The final models will range in size from 5 millimeters (blue-green algae) to 5 meters for the example copepod; thus you may wish to make the copepod model at 2500:1. This "smaller" copepod option is valid because there is a lot of variation in copepod size.] Students can either use the "Example images" (links given above) or research their own examples from these groups. ("Checklist of phytoplankton" is a great place to find phytoplankton photos!)

7. When completed, hang models on classroom walls. It is a good idea to have the sun depicted somewhere, as well. As a class, discuss the food web relationships between these organisms (i.e., prey, predators, etc.). You may want draw arrows to delineate these relationships, as well.
8. "Trophic level" is defined as "position in the food chain, determined by the number of energy transfer steps to that level." Discuss trophic level in light of what students have learned during "Plankton Rummy" and from their scale models. Related questions might include:
  • Which groups are "photosynthetic only"?
  • Which groups are "heterotrophic only"?
  • Which groups are both? What are the advantages and disadvantages of doing both?
  • Does the size of organisms play an important role in predator-prey relationships?
  • How does swimming speed (or lack of motility) affect the lifestyles of these groups?
    • How might shape affect organisms' ability to swim and/or float?
  • Does trophic level depend on "who's eating whom"? Which groups shift among trophic levels?
  • How might non-photosynthetic bacteria fit in with the food web the students have created?
  • Have students extend their research and scale models to the greater marine food web.
    • Students can investigate larger organisms "up the food chain" including larger zooplankton, fish larvae, fish, etc.
    • Or students may wish to study the smallest end of the food web: the "microbial loop" that includes interactions between small phytoplankton, microflagellates, ciliates, bacteria and dissolved organic material (DOM).
Copyright Bigelow Laboratory for Ocean Sciences, 2000