PNI Preparation Phase – A Teacher’s Guide
Nutrient Limitation of Phytoplankton in the Chesapeake Estuary
Preparation Phase
Overview
Students begin their participation in the PNI program by taking a pre-survey.
Students will be evaluated to determine their current knowledge of eutrophication,
nutrient limitation and related Chesapeake Bay issues. Students will then be
introduced to these topics by their teacher. Students will then be introduced to PNI
and what their role will be in the program. Further, students will become familiar
with the different groups of plankton by viewing a video prepared by PNI staff. In
addition students will be introduced to using microscopes and spend time viewing
live organisms. Students will be provided with an optional reading.
Objectives
 Establish baseline student knowledge using an online Pre-Program survey
(provided by PNI staff)
 Gain knowledge of Chesapeake Bay eutrophication and nutrient limitation
(resource provided by PNI)
 Introduce students to the PNI program (resources provided by PNI)
 Become familiar with plankton groups
 Learn how to use a microscope
 View live organisms under the microscope
Background Information
General Background
All ecosystems require a constant input of energy to maintain their structure and
complexity. In the vast majority of ecosystems the sun is the source of this energy
and it enters the biotic community via autotrophs. Autotrophs (Producers such as:
land plants, algae and some bacteria), which contain the solar energy-trapping
compound chlorophyll,
are capable of “fixing” the
carbon in CO2 to form
carbohydrates – a
process known as
photosynthesis. Thus,
solar energy is converted
into potential energy,
stored within the bonds
of these carbohydrates,
and is now available to
the heterotrophic
Figure 1. Diagram showing the flow of energy and the cycling of
nutrients. (Reproduced from Picsdigger: www.picsdigger.com)
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PNI Preparation Phase – A Teacher’s Guide
organisms (consumers) of the ecosystem that feed directly (herbivores and
decomposers) or indirectly (carnivores) on the autotrophs (Figure 1).
Since autotrophs often provide the only avenue for importing energy into an
ecosystem, the rate at which they photosynthesize is critical to the rate of energy
supply to all other trophic levels. In other words, the rate at which algae grow in the
Chesapeake Bay in part determines the mass of animals (for example fish and crabs)
and decomposers (fungi and bacteria) that can also exist in that system. As you
might imagine, measuring photosynthetic rates and understanding the factors that
control them are of fundamental importance to the study of energy flow through an
ecosystem.
What do autotrophs require for photosynthesis and growth? Certainly light, CO2, and
water are necessary for photosynthesis. However, plants are made up of more than
just carbohydrates. They need to be able to make nucleic acids, proteins, pigments
and a vast array of other molecules to produce a functioning autotrophic organism
that can grow and reproduce. These compounds require a wide range of nutrients
(mostly elements) for their production. For example, nucleic acids contain
relatively large amounts of phosphorus and nitrogen; likewise, chlorophyll and
proteins are nitrogen-rich. Certain autotrophs also have additional elemental needs.
For example, diatoms, a type of algae, produce a glass-like case around each cell.
They need silica to produce their case in addition to other more commonly used
nutrients. Thus, the rate of photosynthesis and
growth of plants is not only dependent on the
availability of materials used directly in fixing
carbon (i.e. light, CO2, and H2O) but also depends
on the availability of all these other nutrients as
well (Figure 2).
Even though many different nutrients are required
for plant growth, they are not all required in equal
proportions. This concept is stated formally as
Liebig’s Law of the Minimum. Liebig’s Law
states that “Plant growth is limited by that factor
that is shortest supply relative to its need by the
plant.” For example, on average a single-celled alga
requires about one atom of phosphorus and 16
atoms of nitrogen for every 106 atoms of carbon it
Figure 2. Cycling of nutrients from
fixes during photosynthesis. The ratio of 106
autotrophs and heterotrophs.
carbon atoms:16 nitrogen atoms:1 phosphorus
(http://www.nature.com)
atom is known as the Redfield Ratio. Even
elements that are needed in trace amounts, such as iron, can sometimes limit
growth. Therefore, determining what nutrient limits plant production is not as
simple as testing the soil or water to determine the concentrations of each nutrient.
Instead, nutrient limitation is usually determined by growing the algae with and
without nutrient additions to its water and measuring how rapidly it grows. These
types of experiments are known as nutrient limitation bioassays.
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PNI Preparation Phase – A Teacher’s Guide
In freshwater and estuarine systems such as the Chesapeake Bay, light intensity or
concentrations of nitrogen or phosphorus are often limiting the growth of most
algae. Light intensity depends largely on water depth, the amount of vertical mixing,
and water clarity. Nitrogen and phosphorus concentrations typically reflect the
geology and soil types in the watershed. However, human activities (for example
changes in population, land-use, sewage and stormwater outflows, and acid
precipitation) can alter nutrient concentrations considerably. Additionally, the
seasonal variation in nutrient inputs into the Bay and the cycling of nutrients within
the Bay’s sediments can affect which nutrient is controlling algal growth during any
particular time of the year.
Plankton Background
(excerpted, in part, from http://www.chesapeakebay.net and Richard Lacouture powerpoint)
What are plankton?
The name plankton, is derived from a Greek word that means "wanderer" or
“drifter”. Plankton are ubiquitous in almost all aquatic environments. Plankton are
subject to the movements of the water in which they live for example, marine
plankton are at the mercy of ocean currents.
Plankton is any organism, plant or
animal, which lives either part
(meroplanktonic) or all
(holoplanktonic) of its life in the water
column. Plankton ranges in size from a
single celled organism to large
multicellular organisms like a jellyfish.
As a group they collectively exhibit a
tremendous size range, from bacteria
less than a micron to jellyfish over a
meter in diameter. The smallest
plankton are often consumed by
Figure 3. Plankton exhibit a size range of over 7 orders of
slightly larger types of plankton.
magnitude (reproduced from the Center for Microbial
Those are consumed, in turn, by
Oceanography:
http://cmore.soest.hawaii.edu/cruises/operex/sutton_blog.htm)
slightly larger planktonic forms. The
resulting estuarine food chain is often
longer and more complex than those seen in terrestrial systems.
What types of plankton live in the Chesapeake Bay?
Plankton can be divided into three major classes: phytoplankton; zooplankton; and
bacteria and viruses.
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PNI Preparation Phase – A Teacher’s Guide
Phytoplankton (Phyto – meaning “plant”)
Major groups of phytoplankton in the Chesapeake Bay include:
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Diatoms (Bacillariophyta)
Golden brown algae (Chrysophyta)
Green algae (Clorophyta)
Blue-green algae (Cyanophyta)
Dinoflagellates (Pyrrophycophyta)
Cryptomonads (Cryptophyta)
Microflagellates (Prasinophyta, Euglenophycota, Protozoa)
Pytoplankton are mostly microscopic, single-celled plants that live in aquatic
habitats. As we have learned previously, they require sunlight and nutrients to
produce food via photosynthesis. Phytoplankton are vital organisms in the
environment because, they serve as the base of the food chain and produce vast
quantities of oxygen.
Zooplankton (Zoo – meaning “animal”)
Zooplankton are animals that range in size from single-celled protozoa to tiny fish
larvae to larger jellyfish. One gallon of water can contain more than a half-million
zooplankton.
The zooplankton community is composed of both primary consumers (which eat
phytoplankton) and secondary consumers (which feed on other zooplankton).
Nearly all fish depend on zooplankton for food during their larval phases, and some
fish continue to eat zooplankton their entire lives. One herring may consume
thousands of copepods — the most abundant type of zooplankton found in the Bay.
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The smallest zooplankton are able to recycle nutrients found in the water
column. Because of this, they are often closely tied to nutrient pollution
measurements.
Larger zooplankton are important food for forage fish and fish larvae. They
also link the primary producers (phytoplankton) with larger or higher
trophic level animals.
Zooplankton also feed on bacteria and particulate plant matter.
Tiny larvae of fish and invertebrates, which feed on copepods, are also
considered zooplankton. Although this planktonic stage is only temporary
(meroplanktonic), larvae are a significant part of the zooplankton community
because they are a food source for larger animals.
Zooplankton are distributed according to salinity and the availability of
phytoplankton, their main food source. Like phytoplankton, zooplankton are
excellent indicators of environmental conditions within the Chesapeake Bay
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PNI Preparation Phase – A Teacher’s Guide
because they are sensitive to changes in its health. Scientists can get a good picture
of current Bay conditions by looking at the amount and diversity of different species
of zooplankton.
Bacteria
Bacteria play an important function in the Chesapeake Bay:
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Bacteria are the Bay's decomposers, breaking down dead matter. Through
this process, nutrients in dead plant and animal matter again become
available for growing plants.
Bacteria are food for zooplankton and other filter-feeding organisms in the
Bay.
Bacteria can be residents of the Bay or be introduced through various pathways,
including human sewage and polluted runoff from the land.
Why are plankton communities so diverse?
The water column habitat of plankton appears quite uniform. In other words, from
the standpoint of an ecological niche, the water column seems to have few attributes
that would contribute to a diversity of niches. However, there is a large diversity of
plankton co-existing in that habitat.
Most observations indicate that there is high plankton diversity, even in single
groups such as phytoplankton. This runs counter to the “Competitive Exclusion
Hypothesis” - a famous cornerstone of ecology that states that over the long-term
only one species can occupy a single niche, while all other species are excluded as a
result of competition.
This seeming contradiction was expressed formally by G. E. Hutchinson in 1961. He
termed it “The Paradox of the Plankton”. Much research has occurred since then to
try to explain this paradox. Here are some of the more recent findings that have
shed light on why the Paradox may occur:
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Competitive exclusion takes place in environments that are uniform in time
and space. Even though the water column may seem be a fairly uniform
environment, a closer examination indicates that it is not.
o Physical and chemical factors in water columns vary with depth and
season of the year. Water mixing during storms can also bring about
environmental changes in the water column over even shorter time
scales.
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PNI Preparation Phase – A Teacher’s Guide
o However, water mixing is surprisingly non-uniform. Even after a
mixing event, the water column may have isolated “pockets” that
differ in chemical or physical attributes.
o Light penetration – both quantity and quality (color) - always vary
with depth, making the water column as a whole non-homogenous.
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Even in a homogenous environment multiple species of plankton can co-exist
indefinitely IF they have differing abilities to compete for different resources.
For example, phytoplankton species “A” may be a better competitor for
nitrogen than species “B”. However, “B” could be a better competitor for
phosphorus than “A”. This could set up a situation in which there are
oscillations in the populations of both “A” and “B”, but they continue to coexist.
Why Are Plankton Important?
Plankton communities form the base of the Chesapeake Bay food web, acting as food
for fish, shellfish and other upper trophic level organisms. All fish and shellfish
depend on plankton for food during their larval phases, and some species continue
to consume plankton their entire lives.
Plankton are often used as indicators of environmental and aquatic health because
of their short life span and high sensitivity to environmental change.
Materials Needed
 The student Pre-Program survey, available from the PNI website under
“Teacher Resources”
 The PowerPoint presentation on Chesapeake Bay eutrophication and
nutrient limitation, available from the PNI webpage under “Teacher
Resources”
 The PowerPoint presentation Introduction to the PNI program, available
from the PNI website under “Teacher Resources”
 Access to a computer and projector to view plankton video online
 One compound microscope for each pair of students
 Sample of live plankton - provided by PNI staff
 Microscope slides and coverslips
 Pipettes and bulbs
 Paper towels
 Students can bring a digital camera to class and photograph their samples for
the photography competition at the PNI summit
Preparation
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Ask your students to complete the online pre-survey (one per student)
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PNI Preparation Phase – A Teacher’s Guide
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Review the PowerPoint presentations on Chesapeake Bay eutrophication and
nutrient limitation and the introduction to the PNI program, available from
the PNI webpage under “Teacher Resources”. Modify these presentations as
you would like to best convey this information to your students.
Review optional Student Reading I: An Introduction to the Chesapeake Bay
and the Patuxent River Estuary. This reading could be assigned as homework
to introduce the subject of eutrophication and nutrient limitation in the Bay
and Patuxent River.
Review optional Student Reading 2: Understanding Nutrient Limitation:
Questions and Answers. This reading could be assigned as homework to
introduce the subject of nutrient limitation and the use of bioassays.
Make copies of the Check for Understanding: Plankton page of this document
for each student
Make copies of the student worksheet: Observing a Natural Plankton Sample
students will fill this worksheet in as they view the plankton sample
provided by the PNI staff
Prepare the classroom computer and projector by loading the video to be
viewed. The video can be found on the PNI webpage under Teacher
Resources - Honors Teacher Resources - Plankton Video
Ensure each pair of students has one compound microscope, slides and cover
slips, pipettes, bulbs and paper towels
Review Optional Student Reading: An Introduction to Plankton in the
Chesapeake Bay
Procedure
Conduct the Pre-Assessment:
1. Ensure all students have completed the online survery.
Begin a Discussion:
2. Ask students why estuaries are one of the most productive ecosystems in the
world. What conditions exist in the estuary that would make it particularly
productive? (Nutrients from land via runoff, surface tributaries, and
groundwater, mixing and circulation of nutrients and oxygen by tides,
abundance of food sources and protective habitats make the estuary a good
place for rearing of many types of juvenile organisms and for diversity of
species.)
3. Ask students what types of nutrients estuaries need to support high
productivity. (Plants and animals need nitrogen and phosphorous, as well as
many other trace nutrients. Nitrogen is a component of amino acids, enzymes,
DNA, and proteins. Phosphorus is found in DNA and most cell membranes) If
your students are familiar with the nitrogen cycle, you can suggest they
consider how it functions in estuaries.
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PNI Preparation Phase – A Teacher’s Guide
4. Have teams of students brainstorm ways in which the estuary receives the
nutrients it needs. Again, if your students are familiar with the nitrogen
cycle, you can suggest they consider how it functions in estuaries. Have them
discuss and/or record responses to the following questions:
 How do estuaries get necessary nutrients?
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Are these nutrients obtained and simply used up or are they cycled
through the estuary?
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Are equal amounts of all nutrients necessary for proper plant growth?
Why or why not?
5. Discuss the teams’ brainstormed ideas and answers. Ask if it is possible for
an ecosystem to get too many nutrients. Is it possible to get different
nutrients in proportions than are needed by plants? Can the availability of a
single nutrient control the growth of a plant?
Provide some Content:
6. Present the information found in the two PowerPoints: Chesapeake Bay
Nutrient Limitation and an Introduction to the PNI program. (Both of these
PowerPoints can be modified to suit your class and time constraints.)
Checking for Understanding
To assess content learning, students could answer one or two of these questions as
an in-class assignment or homework.
1. How could an over enrichment of nutrients in the estuary lead to the death of
oysters in the deep waters of the Chesapeake Bay? Explain the series of
events that link these two occurrences.
2. What land-use activities in Calvert County do you think contribute the
greatest amounts of nutrients to the Bay? How do you think this list might
be different if this list were being made 100 years ago? One hundred years
from now? Provide the reasoning for your choices.
3. If the concentrations of nitrogen and phosphorus in a water sample are
equal, which nutrient is most likely to control the amount of algae that can
grow in the water? Why?
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PNI Preparation Phase – A Teacher’s Guide
Viewing Plankton Sample Procedure
1. View the video on plankton and have a brief discussion about plankton
diversity
2. Ask students to pair up and sit at microscope
3. PNI staff and the teacher will demonstrate the correct way to prepare a wet
mount
4. Have students prepare their wet mount as follows (Figure 2):
a. Place a small drop of the sample in the center of the slide
b. While holding a cover slip upright, carefully place one edge of the
cover slip next to the water.
c. Slowly lower the upper edge of the cover slip onto the water. The
objective is to minimize or eliminate air bubbles under the cover slip.
You might find it helpful to use one toothpick to hold the lower edge
in place, while using another to carefully lower the slip into place.
Figure 4. Preparing a wetmount. (Image from:
http://tes.geog.tamu.edu/kdk/)
5. Ask the students to stop here and watch the PNI staff demonstrate the
correct way to view a slide under the microscope
6. Have the students place the slide on the microscope stage and begin
examining the slide on the lowest magnification
7. As the students are examining their slide, ask them to complete the student
worksheet: Observing a Natural Plankton Sample
8. Some students may find different groups of plankton, so ensure each student
has had the chance to view all the groups in the plankton sample. It may help
to keep a list on the classroom board as students find things
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PNI Preparation Phase – A Teacher’s Guide
Checking for Understanding: Plankton
1. What am I?
Group:_____________________________________________
Do I photosynthesize?
Yes / No
How do I get my nutrition?_______________________
What eats me? ____________________________________
Am I phytoplankton or zooplankton? (circle choice)
2. What are we?
Group:_____________________________________________
Do I photosynthesize?
Yes / No
How do I get my nutrition?_______________________
What eats me? ____________________________________
Am I phytoplankton or zooplankton? (circle choice)
3. What am I?
Group:_____________________________________________
Do I photosynthesize?
Yes / No
How do I get my nutrition?_______________________
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PNI Preparation Phase – A Teacher’s Guide
What eats me? ____________________________________
Am I phytoplankton or zooplankton? (circle choice)
Illustrations from www.biodidac.com
Preparing for next class
The next class introduces the concept of experimental design, directs the students to
set up their nutrient limitation bioassay and record initial readings for their
phytoplankton culture.
Depending on how familiar the students are with experimental design and whether
or not you have a finished light box, you may want to divide this class period into
two.
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