Sinking Races2021-12-18T17:23:17-05:00

Introduction: The tiny animals and plants of the ocean’s plankton are the basis of ocean food chains. The word “plankton” comes from the Greek word planktos meaning “drifting.” Planktonic plants are called phytoplankton. These plants, like plants on land, have the ability to photosynthesize. Using chlorophyll, they capture the energy of the sun to make food, releasing oxygen in the process. Virtually all aquatic life depends upon these microscopic single-celled organisms for food. Phytoplankton is the main source of food for zooplankton. Phytoplankton also contributes a significant portion of the oxygen found in the air we breathe.

Planktonic animals are referred to as zooplankton (“oo” is pronounced as in toe). Many zooplankton are able to move up and down in a water column, pursuing food and escaping predators. However, their small size prevents them from moving against the currents.

Phytoplankton must remain in the photic (light) zone in order to receive enough sunlight energy to carry on photosynthesis. However, if they are too close to the surface, photosynthesis is less efficient. Phytoplankton are found in a fantastic array of shapes, incorporating adaptations which help keep them from sinking. For example, phytoplankton typically have a very large surface area relative to body size. They can be round and flat, have long spines or bristles, or join single-celled units in long chains. Many zooplankton need to stay in the same zone because the phytoplankton are their food source. Zooplankton are often wide and flat, and many have long spines and bristles, but they have an advantage over phytoplankton – they can swim. Tiny movements of their appendages can propel them and keep them in the food-filled zone of the ocean. Some zooplankton also rely on accumulating tiny amounts of oils, whch help serve as floatation.

What to Expect: This activity should take about three class periods: one to introduce and begin designing creatures, one to build them, and one to race them. Have a variety of materials on hand. Students can bring in materials to use and share. The marbles and size restrictions help standardize the races.

This activity is likely to produce splashes and spills, so it should be done in a location where this is not a problem.


– Pictures of phytoplankton and zooplankton

– Live or preserved plankton if available with microscope and magnifier

– Marbles of a standard size

– An assortment of materials for constructing plankton models

– Dish pans or similar tubs for testing models

– 5 or 10 gallon aquarium for sinking races

– Stopwatches or digital watches which read in seconds (optional)

– Small, inexpensive prizes


1. Start with observations of zooplankton and phytoplankton. Use pictures, photos, and samples to allow students to observe their shapes, projections, and behaviors.

2. Next ask students what phytoplankton and zooplankton need to survive (light for photoplankton; water, food, space, protection).

3. Most plankton are heavier than water and tend to sink. Ask how they might stay up in the water.

4. Make a list of the students’ observations. Encourage connections to what they know about density.

5. Some of the students should notice that many plankton have long projections or antennae or hairs. Have them speculate on how these would affect movement through water. Could the students run through water faster with their own arms spread out or folded up?

6. After observing and discussing shapes, tell students they will make a model phytoplankton or zooplankton organism which will sink slowly. Since “thrashing” or swimming is not possible in a non-mechanical model, they must concentrate on designing a plant or animal that is just barely heavier that water and that slows its rate of sinking by increasing its resistance to movement through water.

7. Have a variety of materials and clear containers of water available around the room for design and testing.

8. The creatures must meet the following criteria. They must be denser than water (not float at the surface) – density can be measured as mass per unit volume, and must be higher than 1 gm/cm3, the density of water. They must be no larger than 15 cm x 5cm or they will be seen and eaten! Each phytoplankton must contain one standard sized marble, green if possible (representing body structures and chlorophyll). Each zooplankton must contain 2 standard sized marbles (representing body structures). NOTE: phytoplankton will be raced against other phytoplankton; zooplankton will be raced against other zooplankton. If students wish to race the finalists in each category, they may.

9. Have students use stopwatches to time the speed of sinking. Set a time limit for experimentation and announce a contest for the slowest sinking animal or plant at the end of that time.

10. For the contest, have the class gather around a large glass aquarium where everyone can see. You can time each trial separately, but it will be more exciting if pairs of phytoplankton or zooplankton are released to “reverse race” their way down.

11. Put both on a sheet of cardboard so they can be tipped in at the same time for a fair start. The SLOWEST from each pair goes into a second heat and so on until the last tow models winners. Then have them vote on which they think will win the grand prize for slowest overall based on their analysis. Do the fianl test and distribute prizes.

Evaluation: Students should write or orally present the adaptations they incorporated to slow the sinking rate of their organisms. Race results can be grounds for some prizes, but students might come up with creative awards for “also-swam” contenders. Students might receive extra credit for modeling their creature after a real organism.

Extensions: Have students write descriptions of their creature, then photograph them with a digital camera and add the gallery of creatures to the school’s web page.

Copyright 1998-2008 by Sea Education Association, all rights reserved. Compiled and edited by Pat Harcourt & Bill Meyer.

This project was supported, in part, by the National Science Foundation (Proposals # TEI-8652383, TPE-8955214, and ESI-925324), the Henry L. and Grace Doherty Foundation, the Donner Foundation and the Pew Charitable Trusts. Opinions, findings, conclusions or recommendations expressed are those of the authors and not necessarily of the Foundations.

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