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XI. PLANKTON STATION
INTRODUCTION
Plankton are microscopic plants and animals that form the base
of the aquatic food web in the Great Lakes. Plant plankton are
known as phytoplankton (primarily diatoms and other algae).
Animal plankton are known as zooplankton (copepods,
cladocerans, rotifers, and others).
Most of what students observe on the Schoolship are
zooplankton due to the mesh size of the net used to collect the
samples and the reduced level of magnification on the
Schoolship microscope (10x and 30x). Some zooplankton are
herbivores (eat only plants), while others are carnivores (eat only
animals) or omnivores (eat plants and animals). Zooplankton
generally consume diatoms, algae, and detritus. These
zooplankton are, in turn, eaten by predatory zooplankton, small
forage fish like spottail shiners and sticklebacks, and early lifestage fish like salmon and trout.
LEARNING OBJECTIVES
Students will be able to:
1. Identify the collection device used to sample plankton.
2. Define phytoplankton as suspended plants and zooplankton as suspended animals.
3. Recognize plankton as the basis of the aquatic food web.
4. Describe the feeding (trophic) relationships among phytoplankton, zooplankton, and fish.
5. Identify zooplankton seen on the video monitor and record their findings.
6. Discuss the source and role of micro-plastics being found in the Great Lakes.
TOOLS
Demonstration plankton net
Eye droppers
Food web diagram
Forceps
Microvideo system
Penny
Petri dish
Plankton identification poster
Plankton Station manual
Sample jar
Wash bottle
TEACHING THE STATION
During the large group sampling, the lead instructor will hand the plankton sample to you. It is helpful to
preview a live sample before the first group. This ensures the microvideo system is working properly and
helps you know which species to look for in the sample.
It is important to begin the Plankton Station with an introduction to plankton using the Great Lakes food
web diagram– what they are, the distinction between phytoplankton and zooplankton, their importance in
the food web, and the impact of invasive species. Point out the main groups of zooplankton shown on the
poster – copepods, cladocerans, rotifers, ostracods, mysids, and exotics. You may ask students to predict
the kinds of organisms they will see. It is important to mention that phytoplankton are not typically seen
on-board the schoolship because they are much smaller than the zooplankton and are not typically
present in the sample. One exception is colonial phytoplankton which can sometimes be viewed.
Before having the students look at actual plankton, have them look at the back of a penny with the naked
eye and then also under the microscope. They will be able to see Abe Lincoln in the monument, with the
assistance of the microscope, which will provide them with a scale of size. Have a student put a few drops
of the plankton sample in the Petri dish. Using the video microscope, scan the sample and have students
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identify organisms and estimate their relative abundance (abundant, common, or rare). Students should
record this data in their student log books. In addition to the life history and food web dynamics of the
zooplankton, you should also cover the connection between phytoplankton and dissolved oxygen and the
impacts of exotic species (such as zebra and quagga mussels and Bythrotrephes). The concept of
bioaccumulation can also be discusses with high school students. A good time to introduce
bioaccumulation is when you see the orange-red oil deposits in the Calanoid copepods, because many
organic chemicals are lipid or fat-soluble.
Stewardship Component
The recent discovery of micro-plastics in the Great Lakes has brought attention to the connection between
the lakes and the surrounding shore-line. The high densities of plastic particles found floating on the
surface of the Great Lakes have caused major concerns because it indicates large supplies of plastics
entering the lakes unchecked. Larger pieces of plastic break down over time into the micro-plastic beads
that have been collected from the lakes’ surfaces. Scientists are concerned about these plastic beads being
vectors for contaminants and entering the food web through accidental and/or intentional ingestion by fish
and other organisms.
Teaching Tip
To give students an idea of the true size of these organisms, pass around the plankton sample
before showing the species on the microvideo system. The net mesh is 153μm (0.153 mm). A
human hair is about 60 μm wide; the holes in the net are just slightly wider than 2 human hairs.
You can also place the demonstration plankton net under the microscope to show the mesh size.
DATA COLLECTION
Students should note the relative abundance of plankton types as abundant (A), common (C), or rare (R).
When discussing the density of zooplankton, it is helpful to talk about the volume of water filtered. The
approximate calculation is 1 foot of vertical tow = 16 gallons of water filtered.
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VOCABULARY
Aphotic zone: area in the water column where there is not enough sunlight for photosynthesis to occur.
Bioaccumulation: accumulation of contaminants in the tissues of organisms (also known as
bioaccumulation).
Biomagnification: increase in the concentration of contaminants as you move through the levels of a
food web.
Carnivores: consume only animals.
Consumer: organisms that do not produce their own food (must consume other organisms to produce
energy).
Dissolved oxygen: amount of oxygen molecules dissolved in water.
Ecosystem: group of living and non-living things that interact with each other.
Exotic species: species introduced beyond its native range.
Food web: complex network of feeding relationships between organisms in an ecosystem.
Herbivores: consume only plants.
Hydrophobic: attracted to lipids (fats).
Invasive species: exotic species that causes or is likely to cause economic or environmental harm or harm
to human health.
Omnivores: consume both plants and animals.
Photic zone: area in the water column where there is enough sunlight for photosynthesis to occur.
Photosynthesis: converting energy from sunlight into energy used by organisms to function.
Phytoplankton: microscopic plants that float freely in the water.
Plankton: suspended plants and animals that float freely in the water column.
Plankton net: device used to collect plankton samples.
Producer: plant that produces its own energy through photosynthesis using sunlight and nutrients.
Zooplankton: microscopic animals that float freely in the water.
BACKGROUND INFORMATION
Plankton
Aboard the Schoolship, the plankton sample is collected during the Large Group Sampling. A plankton
net with a diameter of 20 inches and a mesh size of 153 μm (μm = microns; 1 μm = 1/1000 mm) is used
for the sample collection. This mesh size allows most of the phytoplankton to pass through the net, while
retaining the zooplankton. Many zooplankton are less than 1 mm in length, although some can be up to 25
mm long (ex: Mysis relicta).
Plankton are microscopic plants (phytoplankton) and animals (zooplankton) that are free-floating or
suspended in the water. The most common phytoplankton are diatoms, which often have geometrical
shapes and silica shells. Phytoplankton must regulate their buoyancy in order to stay in the photic zone
(where there is enough light for photosynthesis to occur).
Phytoplankton are the foundation of the aquatic food web. Zooplankton represent the next level of the
food web, and many zooplankton feed upon phytoplankton. While some zooplankton are herbivores
(eating only phytoplankton), others are carnivores (eating only animals) or omnivores (eating both plants
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and animals) and there is an entire food web in the plankton alone. As the base of the food web, the
plankton (both phytoplankton and zooplankton) are very important to the Great Lakes ecosystem. If there
is a decline in the abundance of plankton, for example, that will be reflected throughout the entire food
web – all the way up to game fish like lake trout and salmon.
Picoplankton are a special group of plankton, defined by a size less than 3 microns. They include small
phytoplankton, bacteria, and protozoa. Picoplankton account for 44% of primary production in Lake
Michigan, but are far too small to be seen on the ship’s microscope.
We collect our plankton sample using a vertical tow of the plankton net from close to the bottom of the
lake up to the surface. There are two reasons for this. The first is to filter a large volume of water in order
to increase the number and species of zooplankton captured (we filter approximately 16 gallons of water
per foot of vertical tow). The other reason is that different species of zooplankton may be found at
particular depths in the water column, and many species of zooplankton actually migrate vertically
throughout the water column each day. During daylight hours, these plankton avoid predation by moving
below the zone of light penetration (aphotic zone), while during the evening hours they migrate to
shallower waters for food.
Different zooplankton emerge at different times of the year. Calanoid and Cyclopoid copepods with eggs
are especially abundant in May. Cladocerans such as Daphnia and Bosmina become more abundant in
late May and June as the water warms. Rotifers are more common in the summer and the exotic spiny
water flea (Bythotrephes) doesn’t appear in zooplankton samples until mid-August. The seasonal
succession of zooplankton abundance can be found in the Plankton Station section of this manual (on
page 120).
Bioaccumulation
Bioaccumulation refers to the accumulation of contaminants in the tissues of organisms. Many
contaminants are hydrophobic (water-hating or lipid-loving). This means they prefer to be in the lipids or
fats of an organism rather than in the water, and will partition themselves there. Because these
contaminants are lipid soluble and are stored in the lipids of organisms, they are not easily excreted. The
risk of toxicological effects to the organism increases as more and more contaminants accumulate in their
tissues.
If the compounds are not metabolized as fast as they are consumed, there can be significant accumulation
of contaminants in an organism’s tissues. Organisms at higher trophic (feeding) levels on food webs tend
to have greater concentrations of bioaccumulated contaminants stored in their bodies than those lower on
food webs. The increase in the concentration of contaminants in each successive trophic (feeding) level is
called biomagnification). The concern surrounding bioaccumulation and biomagnification comes mainly
from experience with chlorinated compounds, especially pesticides and PCBs, and their deleterious
effects on vulnerable species of birds, frogs, and fish.
Common Zooplankton Found in Grand Traverse Bay
The main groups of zooplankton found in Grand Traverse Bay are: Copepods, Cladocerans, Rotifers,
Ostracods, Mysids, and exotic species.
A. Copepods
1. Calanoid Copepods
- Long antennae (as long as the body)
- Single egg sac (if present)
- Eye not visible
- Numerous caudal setae (hairs) on tail
- Filter-feeder
- Example: Diaptomus sp.
- Very common aboard the Schoolship
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2. Cyclopoid Copepods
- Short antennae (less than half of the length of the body)
- Two lateral egg sacs (if present)
- Single eye
- Four caudal setae (hairs) on tail
- Single eye
- Raptorial feeder
- Example: Cyclops sp.
- Very common aboard the Schoolship
3. Harpacticoid Copepods
- Very short antennae
- Metasome (head and thorax) and urosome (abdomen
and genital segment) are not distincly separate
- Benthic copepod
4. Copepod nauplii
- Early life history stage (larval stage) of all copepods
- Unable to distinguish at this stage which type of copepod
- Copepod nauplii go through several molts before reaching an "adult-like"
stage called a copepodite, which molts several more times before
reaching adulthood (adults do not molt)
- Observed in Schoolship samples in early summer
B. Cladocerans
1. Bosmina sp.
- Body enclosed in a folded shell or carapace
- Large first antennae
- Appears to have a very long beak
- Two short spines on posterior
- Often becomes trapped on the surface (surface tension)
- Filter-feeders
- Very common aboard the Schoolship
2. Daphnia sp.
- Single, long posterior spine
- Head in the shape of a helmet
- First antennae small or inconspicuous
- Diurnal migration in water column
- Filter-feeders
- Very common aboard the Schoolship
3. Chydorus sp.
- Very spherical or round in appearance
- Lacks long ‘beak’ of Daphnia
- Lacks spines on posterior that are evident on Daphnia
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4. Holopedium gibberum
- Large first antennae that end in 3 long hairs (setae)
- Very humpbacked
- Gelatinous sheath may cover animal
5. Leptodora kindtii
- Long, transparent body (up to 18 mm.)
- Carapace does not cover body
- Very large swimming antennae
- Legs clearly segmented
6. Polyphemus pediculus
- Very large compound eye that dominates head
- Small swiimming antennae
- Carapace does not cover body
- Legs clearly segmented
7. Diaphanosoma birgei
- Rounded head
- Large second antennae
C. Rotifers
Rotifers are microscopic animals that are transparent and are often mistaken for single-celled animals.
Their name comes from the rotating movement of their hair-like projections (cilia) that create a current to
bring food into their mouths. Rotifers feed on a variety of things – some feed on algae, some pierce plant
stems and suck out the juices, and others are predators. The three common types of rotifers found in the
Schoolship samples are Asplanchna, Keratella, and Conochilus. Asplanchna looks like a miniature plastic
baggy floating through the sample. Keratella is much smaller and pointed than Asplanchna. Conochilus is
a colonial rotifer – although it appears to be a single organism, it is actually a collection of tube-like
animals joined together by mucus secreted from their tails.
Asplanchna
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Conochilus (colonial)
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D. Ostracods
- Body enclosed by two oval shells
- Limbs emerge from shells when swimming
- Head not distinct
E. Mysids
Mysis relicta
- Shrimp-like appearance
- Obvious segmentation
- Deep water glacial relict
- Very important food source for forage fish and game fish
- Diurnal migrations through the water column
- Although common in Grand Traverse Bay, mysids are
rarely found in the zooplankton sample because they
are large, proficient swimmers and can more easily
avoid the plankton net
F. Exotic Species
1. Bythotrephes cederstroemi (spiny water flea)
Bythotrephes cederstroemi, referred to as the spiny water flea, is a carnivorous plankton
introduced into the Great Lakes from Europe. This crustacean was most likely brought here in the
ballast water of European freighters. Bythotrephes has a small head with a large eye filled with
black pigment. It also has four pairs of legs, and the first pair is much longer than the others. The
most obvious physical feature of Bythotrephes, however, is its long, spiny tail. Like other
crustaceans, Bythotrephes sheds its exoskeleton, but it keeps the portion that covers the tail spine.
This means that it is never without its long spiny tail. Scientists believe this fact suggests the tail
has an important protective function. Young fish have great difficulty swallowing Bythotrephes
because of the spine. As a result, Bythotrephes are rarely found in the stomachs of fish less than 5
cm. long. Bythotrephes are predators of Daphnia, rotifers, and other zooplankton. They directly
compete with small fish for food. Bythotrephes are common in plankton samples aboard the
Schoolship in late August or September.
2. Cercopagis pengoi (fish hook water flea)
Cercopagis pengoi, referred to as the fish hook water flea because its spine hooks at a right angle
from its body, was first discovered in the Great Lakes in 1998 (Lake Ontario). It was first
discovered in Lake Michigan by the Schoolship in 1999. Cercopagis has spread to the Finger
Lakes in New York, Grand Traverse Bay, and southern Lake Michigan near Waukegan. Like
Bythotrephes, Cercopagis is thought to have entered the Great Lakes in the ballast water of a ship
and feeds upon other zooplankton. Cercopagis can reproduce at a very fast rate – in 7-10 days, a
female can produce a brood of 8-13 young.
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3. Zebra and Quagga Mussel Veligers
- Larval form of the zebra mussel
- After drifting for 3-4 weeks, they settle onto firm substrates
and associate with other zebra mussels in clumps
Note – all illustrations are by:
Balcer, M.D., N.L. Korda, and S. Dodson. 1984. Zooplankton of the Great Lakes: A Guide to the
Identification and Ecology of the Common Crustacean Species. Reprinted by permission of the
University of Wisconsin Press).
Remy Champt (ISEA).
REFERENCES
Auer, M.T., R.P. Canale, and P.L. Freedman. 1976. The Limnology of Grand Traverse Bay, Lake
Michigan. University of Michigan Sea Grant Program, Ann Arbor, MI.
Balcer, M.D., N.L. Korda, and S.I. Dodson. 1984. Zooplankton of the Great Lakes. The University of
Wisconsin Press, Madison, WI.
Needham, J.G., and P.R. Needham. 1962. The Guide to the Study of Fresh Water Biology. Holden-Day,
Inc., San Francisco, CA.
Pennak, R.W. 1989. Freshwater Invertebrates of the United States. John Wiley & Sons, Inc., New York,
NY.
Raven, P.H., L.R. Berg, and G.B. Johnson. 1998. Environment 2nd Edition. Saunders College Publishing.
Reid, G.K. 1987. Pond Life (Golden Guide). Golden Press, New York, NY.
Internet Sites of Interest
http://www.enchantedlearning.com/subjects/invertebrates/crustacean/Copepod.html (basic copepod
biology)
http://www.epa.gov/glnpo/monitoring/data_proj/glenda/species_list/zooplankton_species.pdf (results of
EPA's zooplankton sampling)
www.sgnis.org/kids/factsheets.html
http://www.seagrant.umn.edu/exotics/spiny.html (Minnesota Sea Grant)
http://www.redpath-staff.mcgill.ca/ricciardi/cercopagis.html
DIAGRAMS & RELEVANT DATA
Common Zooplankton Found in Grand Traverse Bay (page 119)
Seasonal Succession of Zooplankton Abundance (page 120)
Zooplankton Pronunciation Key (page 121)
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ZOOPLANKTON PRONUNCIATION KEY
This key was produced in response to several volunteer requests. It is by no means a formal
pronunciation key – different scientists may pronounce these names in slightly different ways. However, it
is an attempt at a guide to help new instructors with some of the unusual names.
The pronunciation ISEA created is in the first parentheses, where each syllable is separated by a dash,
and the emphasis is on the syllable written in capital letters. When available, the official pronunciation
(obtained from the dictionary) is in the second parentheses.
Cladocerans (cla-DAH-sir-ans) (klə däs′ər ən)
Bosmina (bos-MINE-a)
Bythotrephes (bith-o-TREF-ees)
Cercopagis (sir-co-PAY-gus)
Chydorus (keye-DOOR-us)
Daphnia (DAFF-knee-a)
Diaphanasoma (die-a-FAN-a-so-ma)
Holopedium (whole-a-PED-ee-um)
Leptodora (lep-ta-DOOR-a)
Polyhemus (paul-ee-FEEM-us)
Copepods (KO-pe-pods) (kō′pə päd′)
Calanoid copepods (CAL-a-noid KO-pe-pods)
Copepod nauplii (Ko-pe-pos NOP-lee-eye)
Cyclopoid copepods (SIGH-clo-poid KO-pe-pods)
Harpacticoid copepods (har-PACK-ti-coid KO-pe-pods)
Mysids (MICE-ids) (mī′sid)
Mysis relicta (MICE-is re-LICK-ta)
Ostracods (OS-tra-cods) (äs′trə käd′)
Rotifers (WROTE-i-furs) (rōt′ə fər)
Asplancha (as-PLANCH-a)
Conochilus (con-o-CHILL-us)
Keratella (CARE-a-tell-a)
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