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 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).
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.
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.
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 cant
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
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 worlds 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.