What are plankton?

The word plankton is from the Greek word for "wandering" (MARE: Marine Activities, Resources & Education). 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 (MARE, 1995). Most meroplankton are the larvae of animals which spend their adult lives on the bottom or free swimming (MARE, 1995).

Phytoplankton:

Phytoplankton are a flora of freely floating, often minute organisms that drift with water currents ("Phytoplankton", 415). Like land vegetation, they produce much of our oxygen, are an important absorber of carbon dioxide (responsible for global warming), and convert minerals to a form that animals can use ("Phytoplankton", 415). Phytoplankton is the primary food source, directly or indirectly, of all sea organisms ("Plankton", 497). Diatoms and dinoflagellates are among the most important members of the phytoplankton (MARE, 1995). Diatoms are housed in beautifully decorated glass skeletons shaped like petri dishes (MARE, 1995). Some diatom species form long chains, which help them float and avoid being eaten (MARE, 1995). Dinoflagellates share both animal and plant traits. Like plants, most photosynthesize, but some eat other organisms. They can also swim using tiny whip-like flagella. Some dinoflagellates are bioluminescent and create light when disturbed by waves, boat wakes or predators (MARE, 1995). Other dinofalgellates produce toxins, which they release into the water. During blooms, they may become so abundant that the water turns red (MARE, 1995). These "red tides" can cause fish kills due to poisoning and oxygen depletion. During some months, mussels and other filter-feeding shellfish are unsafe to eat due to concentrated dinofalgellate toxins which cause Paralytic Shellfish Poisoning (MARE, 1995).

Zooplankton:

Most major animal groups have representatives in the zooplankton. Arthropods of the class Crustacea are the most numerous zooplankton. Some, like the copepods spend their entire lives as plankton (holoplankton) (MARE, 1995). Copepods graze on phytoplankton, and, as the most numerous animals on earth, are critically important to the ocean ecosystem (MARE, 1995). Some crustaceans, like crab larva, are temporary members of the plankton community, and settle to the bottom to live their adult lives. Shrimp-like krill are among the most well known plankton because they are the major food source for some of the great whales. Other common zooplankton groups include the adults and larvae of the phyla Cnidaria (jellyfish), Mollusca (snails, clams, etc.), Chaetognatha (arrow worms), Ctenophora (comb jellies), and Chordata (e.g., fish larvae, sea squirts, salps) (MARE, 1995). With nowhere to hide in the open sea, many plankton species are transparent, and nearly invisible. In addition, many have long spines to help repel predators and to help with flotation.

Sinking

All plankton must avoid sinking. Phytoplankton need sunlight for photosynthesis, so they must stay within the photic zone, usually the top 100 meters (MARE, 1995). 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 (MARE, 1995). 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 (MARE, 1995). In addition, water currents caused by convection and upwelling can stir the water and help keep plankton from sinking (MARE, 1995).

Surface-to-volume (S/V) ratio

The S/V ratios obtained by dividing the surface area of an animal by its volume. Small organisms are overwhelmed by waster viscosity (Milne, 113). 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 (Milne, 113). 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.

Migration

While plankton are too weak to swim against a current, many do swim relatively huge distances vertically each day. Great numbers of zooplankton commute up to 1,300 feet toward the surface (at night) and back down each day. That is the equivalent of a person walking 25 miles to and from work each day (MARE, 1995)! There are several possible reasons for this amazing daily migration. Migrating plankton can take advantage of greater densities of food near the surface at night when they can’t be as easily seen by predators, then move to deeper, cooler waters where they move more slowly (MARE, 1995). Also, if the organisms were to stay at the surface, they would remain in the middle of their food supply and exhaust it. Another theory is that, since horizontal current directions vary with depth, plankton can catch rides to other areas by moving vertically. For example, by descending, they enter waters that carry them beneath new surface waters by the following evening (Milne, 301). When they return to the surface to feed, they enter water that they have not previously fed upon (Milne, 301).

Why are they important?

An estimated 90% of all photosynthesis and production of usable oxygen takes place in the oceans (MARE, 1995). Marine phytoplankton are the first link in the large marine food chain. Larger animals like fish and the blue whale then consume the zooplankton, which feeds on the phytoplankton. The food material from living and dying plankton may sink to the bottom and become food for organisms living on the bottom.

About 90% of the world’s fisheries occur in rich coastal areas because of the high densities of plankton that grow in areas with many nutrients in the water. The high protein content of plankton is causing them to be considered as a potential food source for people. There is also discussions about using phytoplankton in space missions. The personnel would give the plankton carbon dioxide and it would in turn give oxygen and a food source to the people ("Plankton", 497).

This lesson plan was tested in a classroom.

I tested this lesson plan in two classes. Both the fifth and sixth graders were in a gifted program. The fifth graders were really interested in the photos and had a lot of comments and questions. On the other hand, the sixth graders were not as interested in the pictures. Both classes always wanted more time to create their plankton, but some groups had trouble working together. Everyone loved racing them as well. Overall, this lesson was easy to do. It has a chance of running overtime if the pictures/video introduction and or building of plankton gets too much time. Otherwise, it went very well.

Introduction: 10-20 minutes

• Questions, video/pictures

Building Plankton: 15-20 minutes

Actual Race: 5-10 minutes

Conclusion: 10-15

• Filling out lab sheet, answering questions, etc.

-Large pictures, slides and/or video of various plankton species.

See references for video

-Large aquarium (20 gallons or more, if possible)

-Several gallon jars (e.g. clear Mayonnaise jar, small fish bowls, etc.)

-Two-four stopwatches

-Recycled Styrofoam packing "peanuts" (the non-biodegradable kind) and/or corks

-Toothpicks, paperclips, metal washers, yarn, fishing sinkers, popsicle sticks, toothpicks, straws (regular and/or coffee stirrers), feathers, etc.

-Plastic Baggies

-Lab worksheet (PDF file for download and printing)

1. Ask the students questions.

• What are plankton?
• What are some types of plankton?
• Why are they important?

1. Pass out lab worksheet
2. Have students observe photos, slides of various plankton, then record observations like colors, shapes, spines, and motion.
3. Have students brainstorm possible advantages to observed adaptations
4. Brainstorm ways that plankton could reduce sinking rates

• Flattened appendages, small bodies, large surface area relative to volume, reduced density, oil or gas floats, chains, etc.

1. Tell students that they will be creating their own plankton.
2. Have them get into groups around the supplies.
3. Let them know that the objective is to have the slowest one to sink to the bottom.

• Why does the slowest one win?

1. At the group table, there will be a bag of materials and bowl to practice sinking in. At the end of the time, students will choose the one they like the best to race.
2. Have one person from each group describe their plankton and the adaptations it has to sink slowly.

• Record the hypothesis of why it will sink the slowest.

1. 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.

• Record the depth of the tank and the times of each group

1. Repeat if the races if time permits.
2. Have the students figure out the average time and the sinking rates for each group.
3. Declare the slowest one the winner.

• Record the conclusions and give reasons why they think one plankton was the slowest.

Phytoplankton

Algae

Dinoflagellates

Diatoms

Zooplankton

Copepod

1st question: We conducted this experiment with fresh water, would there be a difference if we used seawater (salt water)? Why or why not?

2nd question: Why did we repeat the races several times?

3rd question: Why is it important for plankton to sink slowly?

4th question: What would you do differently now if you were going to create another plankton to race? Why?

For students in higher grade levels

1. Have the students graph their sinking times on a frequency histogram on the blackboard (or graph sinking rates in cm/sec)
2. As a class, 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).

For students in lower grade levels

1. Have models of plankton already made.
2. Have them predict which they think will be the slowest before doing a race.
3. Then gather the class around and race them.
4. Compare the findings of the race with the predictions of the students.
5. Ask them why the winner was the slowest.
6. Have students build their own plankton and discuss why they built them the way they did.
7. Have everyone make predictions beforehand on which will sink the slowest.
8. Race them and record times on the board.
9. Compare the findings of the race with the predictions of the students.
10. Ask them why the winner was the slowest.
11. (If time allows, repeat the whole process of building a plankton, again using what they have learned.)

Other activities:

1. Create a bottle habitat of plankton.
2. Look at plankton under a microscope and draw what you see.
3. Find an article about plankton (red tides, etc) and write a report on it.