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1PLANS II for the Chesapeake Bay – A Teacher’s Guide

Nutrient Limitation of Phytoplankton in the Chesapeake Watershed

Preparation Phase

OverviewStudents begin their participation in the PLANS II program by taking a pre-survey. Students will be evaluated to determine their current knowledge of eutrophication, nutrient limitation and related Chesapeake Bay Watershed issues. Students will be introduced to these topics by their teacher. Additionally, students will be introduced to PLANS II 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 CBNERR 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 CBNERR staff) Gain knowledge of Chesapeake Bay eutrophication and nutrient limitation

(resource provided by the PLANS II website) Introduce students to the PLANS II program (resources provided by PLANS II

website and CBNERR staff) 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

Figure 1. Diagram showing the flow of energy and the cycling of nutrients. (Reproduced from Picsdigger: www.picsdigger.com)


the heterotrophic 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 fixes during photosynthesis. The ratio of 106 carbon atoms:16 nitrogen atoms:1 phosphorus 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

Figure 2. Cycling of nutrients from autotrophs and heterotrophs. (http://www.nature.com)


without nutrient additions to its water and measuring how rapidly it grows. These types of experiments are known as nutrient limitation bioassays.

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 slightly larger types of plankton. Those are consumed, in turn, by 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?

Figure 3. Plankton exhibit a size range of over 7 orders of magnitude (reproduced from the Center for Microbial Oceanography: http://cmore.soest.hawaii.edu/cruises/operex/sutton_blog.htm)


Plankton can be divided into three major classes: phytoplankton; zooplankton; and bacteria and viruses.

Phytoplankton (Phyto – meaning “plant”)

Major groups of phytoplankton in the Chesapeake Bay and the freshwaters leading to the Bay include:

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.

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 and its larger tributaries because they are sensitive to changes in ecosystem health. Scientists can get a good picture of current conditions by looking at the amount and diversity of different species of zooplankton.

Bacteria

Bacteria play an important function in the all aquatic ecosystems:

Bacteria are 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.

Bacteria are natural residents of the aquatic ecosystems; however, some may also 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:

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.


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.

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 co-exist.

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 PLANS II website The PowerPoint presentation on Chesapeake Bay Watershed eutrophication

and nutrient limitation, available from the PLANS II website The PowerPoint presentation Introduction to the PLANS II program,

available from the PLANS II website 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 CBNERR staff Microscope slides and coverslips Pipettes and bulbs Paper towels Students can bring a digital camera to class and photograph their samples

Preparation

Ask your students to complete the online pre-survey (one per student) Review the PowerPoint presentations on Chesapeake Bay Watershed

eutrophication and nutrient limitation and the introduction to the PLANS II


program. Modify these presentations as you would like to best convey this information to your students.

Review the Student Reading: Understanding Nutrient Limitation: Questions and Answers, found on the PLANS II website. This reading could be assigned as homework to introduce the subject of nutrient limitation and the use of bioassays.

Review a second Student Reading: An Introduction to Plankton in the Chesapeake Bay, found on the PLANS II website.

Make copies of the Check for Understanding: Plankton page at the end of this document for each student

Make copies of the student worksheet: Observing a Natural Plankton Sample. Students will fill out this worksheet as they view the plankton sample provided by the CBNERR staff

Prepare the classroom computer and projector by loading the plankton video to be viewed. The YouTube link to the video can be found on the PLANS II website.

Ensure each pair of students has one compound microscope, slides and cover slips, pipettes, bulbs, and paper towels

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 rivers and estuaries.

4. Have teams of students brainstorm ways in which aquatic systems receive the nutrients they need. Again, if your students are familiar with the nitrogen cycle, you can suggest they consider how it functions in different aquatic ecosystems (lakes, streams, rivers, estuaries). Have them discuss and/or record responses to the following questions:

How do aquatic systems get necessary nutrients?


Are these nutrients obtained and simply used up or are they cycled through various aquatic systems?

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 nutrients in different 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 PLANS 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 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?

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 set up a microscope


3. CBNERR 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 slideb. 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.

5. Ask the students to stop here and watch the CBNERR 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

Figure 4. Preparing a wet mount. (Image from: http://tes.geog.tamu.edu/kdk/)


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:_____________________________________________



What eats me? ____________________________________


3. What am I?Group:_____________________________________________



What eats me? ____________________________________


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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