Lab: The Great Plankton Race
What are plankton?
The word plankton is from the Greek word for "wandering"They drift or wander the oceans at the mercy
of the currents. They are generally unable to move against currents. This lack of mobility separates
plankton from the nekton, which are organisms that can propel themselves through the water (such as
fish). Some planktonic organisms can be quite large (like jellyfish); however, most are smaller than
nekton, and small enough that they have to be viewed under a microscope. The plankton that
photosynthesizes are called phytoplankton and are made up of organisms called algae. The plankton
that eat other plankton are called zooplankton, and are made up of tiny animals and single-celled
protozoans. Organisms that spend their whole lives drifting are called holoplankton; those spending only
part of their lives as plankton are called meroplankton. Most meroplankton are the larvae of animals
which spend their adult lives on the bottom or free swimming.
Sinking
All plankton must avoid sinking. Phytoplankton need sunlight for photosynthesis, so they must stay
within the photic zone, usually the top 100 meters. Zooplankton depend on phytoplankton and other
zooplankton for food, so they must avoid sinking as well. Plankton avoid sinking by increasing their
surface area and/or decreasing their density. Most plankton are quite small and so have a larger surface
area to volume ratio than do larger organisms. Flattened bodies and appendages, spines, and other
body projections also slow sinking by adding surface area without increasing density. Some diatoms
resist sinking by forming chains. The use of low-density substances like oil or fat helps increase buoyancy
and can serve as food reserves. In addition, water currents caused by convection and upwelling can stir
the water and help keep plankton from sinking.
Surface-to-volume (S/V) ratio
The S/V ratio is obtained by dividing the surface area of an animal by its volume. Small organisms are
overwhelmed by waster viscosity. Small animals experience too much friction relative to their muscle
strength to go fast. To them, water feels like molasses would to a human swimmer. Although small
organisms cannot swim very fast, neither can they sink very fast. Spines, flattened bodies, etc., have so
much surface area for viscosity to work on, that such organisms hardly sink at all. With the benefit of
slowly sinking at no cost from streamlining, most tiny swimmers in the sea have ragged, irregular
profiles.
Objectives
1. Students will have a basic understanding of what plankton are.
2. Students will learn about the different adaptations used to slow the sinking of plankton.
3. Students will be able to create plankton and describe why they designed it that way. (i.e.
Describe the adaptations that will make their plankton sink slowly.).
4. Students will understand the relationship between surface area and volume and how it affects
density and the sinking and floating of objects.
Procedures:
Before the Race:
1. Brainstorm certain plankton body shapes, and forms and their advantages. What are some ways
that plankton can slow down their sinking rates?
2. Begin building a planktonic form out of any of the materials provided (or brought with you). The
goal is to have a plankton that sinks very slowly (it cannot just float).
3. At your group’s table, there will be a bag of materials and clear container to practice sinking in.
4. Measure the depth of your container and the time (in seconds) it takes to sink to the bottom.
Get the average of 3 trials
5. Calculate the average speed of your plankton.
6. At the end of the class, we will “race” 2 plankton at a time.
At the Race:
1. Have one person from each group describe their plankton and the adaptations it has to sink
slowly.
2. Record a hypothesis on your student worksheet on sinking rates. (decide what your DV and IV
will be)
3. Have a couple of people be timers. If the aquarium in front of the class is large enough, have the
groups race against each other at the same time. Repeat if the races if time permits.
4. Record the depth of the tank and the times of each group.
After the Race:
1. Calculate out the average time and the sinking rates (speed) for each group.
2. Graph the sinking times on a frequency histogram for the whole class on a sheet of paper.
3. Graph the sinking rates in (cm/sec) on a frequency histogram for the whole class on a sheet of
paper.
4. Estimate the time it would take the slowest to sink below the photic zone (25 cm sinking time x
4 = sinking time per meter x 100 = sinking time through photic zone).