Plankton are floating or weakly swimming organisms. All types of plankton are at the mercy of tides, currents, and waves for transportation. Phytoplankton require light for photosynthesis and need to be near the surface; however, many phytoplankton are heavier than water. So they have evolved adaptions to float at least as long as it takes to reproduce themselves. Individuals only live for a few hours to a few days. The greater the surface area of an organism relative to its volume, the greater its friction as it moves through the water, and the slower it sinks. Just being small makes the surface-to-volume relationship advantageous. Some cells have a large cavity at their center, reducing their mass and density relative to their surface area. Another way to increase relative surface area is to grow spines and other projections help to distribute the organism's weight over a large surface area. In many phytoplankters, stores of oil and fat give them buoyancy.
  • Consider possible mechanisms by which phytoplankton remain suspended in ocean water.
    • How do they slow down the rate at which they sink?
  • Design a phytoplankter
1 cup cooking oil tinted with food coloring; 1 cup water tinted with another food coloring; Clear glass beaker or jar; Top from a jar (at least two inches in diameter and capable of sinking); Ball of clay equal in weight to the jar top; Scale; Gallon jar or aquarium filled with water; Sheets of paper; Scissors; Balance (optional)
  1. Select two students to help with the demonstration. Show the class the two cups of colored liquids. Ask the helpers to carefully pour the liquids into the clear glass beaker or jar. Ask the class why the liquids did not mix. Discuss density (weight per unit volume). Explain that one of the liquids (oil) is less dense than water and therefore it floats on water. How does this relate to plankton?
  2. Next, hold up the ball of clay and the jar top. Ask the class to predict which will sink faster in water (the clay ball) and why? If they say because it is heavier, show them with a simple balance or tell them that they have the same weight. Now drop both objects into an aquarium filled with water. Discuss the results.
  3. Challenge the class to design a simple test to determine how body shape affects sinking using two pieces of paper. (Tear a sheet of paper in half. Squeeze or roll one half into a ball. Hold one half in each hand and extend your arms to shoulder height and drop at the same time. Have students closely watch how each falls.)
  4. Based on this demonstration, discuss how the shape of an organism affects its rate of sinking.
  1. When designing a phytoplankton, consider: How will it stay afloat? What shape will it be? Will it be able to swim? How?
  2. Be creative! Maybe your organism sinks to the bottom at night and swims up to the light during day. Maybe it acts like an animal at night and catches prey, then photosynthesizes during the day like a plant. Sound strange? This is typical behavior for some dinoflagellates.
  3. Remember, most phytoplankton end up as food for zooplankton. Any ideas on how a phytoplankter could avoid being eaten?
  4. Draw your phytoplankton. Label the parts and explain their functions.
  5. Name Your Plankter: Give it a common name and scientific name. Use a dictionary or root-word book.
  6. Build a three-dimensional model of the plankter to hang from the classroom ceiling.
From Sea Soup Teacher's Guide: Discovering the Watery World of Phytoplankton and Zooplankton, Copyright 1999, by Betsy T. Stevens, Tilbury House, Publishers. Inquiry-based activities for use with Sea Soup: Phytoplankton and Sea Soup: Zooplankton, children's picture books by Mary R. Cerullo, photography by Bill Curtsinger, Tilbury House, Publishers
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