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Pond Life

EXAMINING A POND ECOSYSTEM

DESCRIPTION OF ACTIVITY

In this activity, students will examine pond water under the microscope and try to identify different microbes that are present. Students will observe the organisms present and their interactions in order to construct a food web and identify examples of predation, competition, or mutualism. An optional extension activity involves setting up two parallel pond terrariums, (1 control and 1 experimental), introducing a new variable (e.g., change in temperature or light, or the addition of a pollutant, like detergent). Students can predict how this change will affect the pond ecosystem and propose ways in which the pond can be restored to its original state.


GRADE LEVELS:

3-5


TEACHER BACKGROUND

The simplest iteration of this lab activity requires teacher familiarity with microscopy, specimen collection from ponds and the diversity and interactions in a typical pond ecosystem. Teachers should be familiar with food webs, in general, and the relationships between autotrophs, herbivores, carnivores and detritivores (decomposers), as well as competition and predation.

Autotrophs

Two types of duckweed. Image by Eric Guinther, from en:Image:Duckweeds.jpg, licensed under GNU
Spirogyra (40x magnification). Image from Wikipedia and released in the public domain.


Duck weed is an aquatic plant that tends to grow in thick mats across the surface of ponds. They do not have stems or leaves. Rather, the majority of the plant is a “thallus” or “frond” composed of just a few layers of cells. Many organisms use duckweed as a source of food or shelter. Another common autotroph (phytoplankton) is spirogyra, a filamentous alga, with spirals of green chloroplasts, that ranges from 0.01-0.1 mm in length. Diatoms, which can range from 0.01-2 mm in length, are also examples of phytoplankton. They have glass-like shells made of silica and interesting, geometric shapes.

Herbivores

Copepods drawn by Ernst Haeckel, 1904 (PD-1923), Wiki Commons.

Copepods are transparent crustaceans (relatives of shrimp, crabs) that generally range from 1-2 mm in length. Many are herbivorous, feeding on phytoplankton. They are the most common component of zooplankton. They are notable for their jointed limbs and feathery projections. Their predators include fish, krill and birds. Daphnia (water fleas) are planktonic crustaceans that range in size from 0.2-5 mm. They are primarily filter feeders that consume mostly algae, but they sometimes also consume detritus and protozoa. Many protozoa (unicellular, animal-like protists) are also herbivorous (e.g., paramecium) or omnivorous (e.g., stentors).

Hydras, by Stephen Friedt, uploaded to Wikipedia by Coveredinsevindust and licensed under Creative Commons CC-BY-SA-3.0. (Approximate size: several mm long)

Carnivores

Some copepods are carnivorous predators, consuming herbivores, including other copepods. Hydras are another predatory component of zooplankton. They belong to phylum cnidaria (includes jellies) and are common in most ponds. They are several millimeters long, and have tube-shaped bodies with tentacles at one end. Their tentacles contain nematocysts (like jellies) for stinging and capturing prey.

Waterbear (tardigrade). Image by Bob Goldstein and Vicky Madden, UNC Chapel Hill, May 2008, licensed under the Creative Commons. (Appoximate size: 1 mm long)

Water bears (also known as tardigrades) are 1 mm long organisms that can be found in most habitats, though the majority live in freshwater ponds. They look like chubby bears, except they have four pairs of legs. They can consume entire organisms like rotifers and other tardigrades, or suck out the internal contents of their prey. They are unusual in that they can withstand extremes of temperature, pressure, dehydration and radiation, and can withstand long periods without food or water.

Rotifers (0.1-0.5 mm). Image by Bob Blaylock, Wiki Commons, licensed under the Creative Commons

Detritivores

Rotifers are multicellular zooplankton that consume organic detritus and dead algae and bacteria. They are usually between 0.1 and 0.5 mm long. They are prey to copepods and fish. Some copepods are detrivores.

References

Rotifers, Wikipedia, http://en.wikipedia.org/wiki/Rotifer#Feeding Copepods, Wikipedia, http://en.wikipedia.org/wiki/Copepod Duckweed, Wikipedia, http://en.wikipedia.org/wiki/Lemnoideae Hydra, Wikipedia, http://en.wikipedia.org/wiki/Hydra_%28genus%29 Tardigrade Facts, Illinois Wesleyan University, http://sun.iwu.edu/~tardisdp/tardigrade_facts.html Spirogyra, Jan Parmentier, Microscopy-UK, http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjan99/gyra.html Daphnia, Wikipedia, http://en.wikipedia.org/wiki/Daphnia Stentor, MicrobeWiki, Kenyon College, http://microbewiki.kenyon.edu/index.php/Stentor


LEARNING GOALS

(Students Will Be Able to): Objectives are aligned to the national Next Generation Science Standards (NGSS)

  • Describe how an environmental change affects an ecosystem and propose solutions for restoring the ecosystem to its original state
  • Identify which organisms can survive well in a pond ecosystem and why others cannot
  • Identify structures on plants and animals that help them survive, grow and behave in a pond ecosystem
  • Use a model (i.e., pond terrarium) to test how animals interact with a natural system Develop a model to describe the movement of matter between plants, animals and decomposers
  • Describe the food web of a pond ecosystem
  • Compare specimen sizes and convert these sizes within the metric system


STUDENT PREREQUISITE KNOWLEDGE

  • Scientific process (Observation and Inference; Variables)
  • Basic microscopy (Parts of the microscope; calculating magnification; focusing)
  • Ecological terms (e.g., ecosystem, community, niche, habitat, population)
  • Ecological relationships and food webs (e.g., producers, herbivores, carnivores, predation, competition, mutualism)


SEQUENCING OF LESSONS

Day 1: Make initial observations of pond water Day 2: Introduce an environmental change to the experimental pond Days 3-5: Observe the experimental pond for changes Day 6: Discuss ways to restore the pond Optional Day 7: Attempt remediation of experimental pond


EXPECTED TIME FOR COMPLETION

Day 1 should take 45-60 minutes Subsequent days may take as little as 20-30 minutes to make observations


MATERIALS

Prepare two identical pond terrariums (1 control and 1 experimental). You can collect specimens from a local pond several weeks in advance to give them time to grow and acclimate. If using tap water, make sure to leave it out for 24 hours for the chlorine to evaporate. Alternatively, you can purchase dehydrated pond organisms from Carolina Biological (Pond Mystery Mix, Cat. #13-2010A, $10.94 each). One or two jars should be sufficient. Mixed with Carolina Spring Water (Cat. 13-2450), swimming creatures will be visible in a week. In either case, it is good to pick up some algae or duck weed from a pond so that there are photosynthetic organisms present in your terrariums. Many of the critters visible in ponds use the algae and aquatic plants for shelter and as a food source.

Ideally, this should be done in student teams of no more than 2 students. This will keep everyone busy and give each student an opportunity to contribute. If you have sufficient materials and equipment, you can even have 1 student per microscope. If necessary, this activity can be done in student teams of 3 or 4. However, this increases the chances that some students will be standing around doing nothing and it will increase the time necessary for everyone to observe and draw their specimens.

For teams of 2 students: 1 microscope, 2 microscope slides, 2 cover slips, transfer pipette or eye dropper, Kimwipes (or tissues or paper towels)


PREPARATION

  • Purchase necessary materials
  • Set up pond terrariums 1-2 weeks in advance to give them time to acclimate
  • Take samples and examine them under the microscope to determine how easy it will be for your students to find and identify specimens. (If organisms are scare or difficult to find under the microscope, you can opt to do this as a whole-class, teacher-led activity)
  • Make copies of any student handouts
  • Preview and cue up any videos you plan to show students
  • Set up student work stations with microscope, slides, cover slips, pipettes


ANTICIPATORY SET

Ask students to describe some ecosystems and record their responses on the board. Ask them to describe some food webs from those ecosystems and record their responses (e.g., in a tropical rainforest there are plants making energy from the sun, animals eating their fruits and leaves, other animals eating these herbivores, and decomposers eating the dead things and wastes). Ask them how we know which organisms are part of a food web? How did scientists figure it out? Answers will vary, but some students should recognize that scientists had to visit the ecosystem and make careful observations of the behavior of the animals. What about very tiny ecosystems made up of microscopic organisms? Explain that they will be doing examining the microscopic food webs of a pond using microscopes.


LESSON PLAN

Note

if you do not have a sufficient number of microscopes or have concerns about your students’ ability to locate and identify species on their own, this activity can be done entirely as a teacher-led lesson, with the teacher preparing the slides and projecting images of the samples onto a screen with an LCD projector.

Day 1-2

1. Review microscope basics and the rules for making biological drawings

2. Explain how to obtain samples. This is a good time to demonstrate how to make a wet mount. If you have a digital microscope and LCD projector, you can project a sample from one of the ponds and demonstrate focusing and how to find organisms in the sample.

3. Using a pipette or eyedropper, transfer 1 drop of pond water to a glass microscope slide

4. Try to obtain a drop that includes some duckweed, algae or whatever photosynthetic material is growing in the pond (generally to be found at the surface), as many organisms use the photosynthetic organisms for food or shelter.

5. Gently lay a cover slip on top of the drop. Start with the cover slip at a 45 ° angle with one side touching the microscope slide and slowly bring the other end of the coverslip down until it makes a glass sandwich, with your specimen in the middle

6. Use the corner of a paper towel, tissue or Kimwipe to suck up any excess liquid that spills or oozes out from the cover slip (The slide should be dry before placing on the microscope stage)

7. Starting at low power, focus and center the specimen Then switch to higher power and focus.

8. Have one student look at the droplet for a minute and describe what is seen, while the teammate records the observations. Then have the students switch. Encourage them to be patient, as microbes tend to swim into and out of the field of view. Also encourage them to scan around the photosynthetic organisms, as many herbivores will be foraging here (and carnivores will be there, too, hunting the herbivores)

9. Their observations should include the shape and structure of each observed organism and its behavior (e.g., swimming around the duckweed; eating algae). If using digital microscopes, it is helpful to take still photos and videos whenever possible to aid in identifying the organisms later

10. Have students make hypotheses about the relationships they observe (e.g., a microbe that spends most of its time crawling on a piece of duckweed could be an herbivore that consumes duckweed; a microbe that chases another microbe and eats it is probably a predator and a carnivore).

11. Circulate around the room and assist as needed. Students will likely need help focusing and centering their specimens, as well as help finding and identifying microbes.

12. Have students clean up and then have a whole-class discussion of their results. If possible, project some of the images taken by students. Record student descriptions of behavior on the board and take suggestions from students about each organisms place in the food web.

13. Use the following websites to help students identify the organisms they observed: http://www.msnucleus.org/watersheds/mission/plankton.pdf http://www.microscopy-uk.org.uk/index.html?http://www.microscopy-uk.org.uk/pond/ http://www.microbiologybytes.com/video/pond.html

14. Optional: Choose how you wish to modify the experimental pond. This can be left to a class vote or determined by the teacher.

Optional Math Extension

15. Have students research the relative sizes of various pond organisms (or provide these measurements) and have them group the organisms by size millimeters and convert to micrometers or centimeters. (See accompanying worksheet)

Optional Days 2-5

16. Apply an environmental change to the experimental pond (e.g., add detergent, salt, garbage, additional light or darkness)

17. Have students repeat steps 2-3 each day until finished. Depending on the environmental change that is used, the differences in microbial diversity and abundance may be quickly noticeable (e.g., an extreme change, like adding large amounts of detergent or salt), or it may take a few weeks if the change is more subtle (e.g., slight change in light or temperature).


EXPECTED RESULTS

Several species of fresh water diatoms. Image by Damian H. Zanette, 2002, Wiki Commons, released into the public domain.

Large photosynthetic organisms (e.g., duckweed) will be common and easy to see and identify, as they are green and large enough to see with the naked eye. With patience, students should also observe herbivores foraging and possibly consuming the photosynthetic organisms. Diatoms are a common autotrophic organism found in pond water. (Look for their clear, symmetrical shapes).

Duckweed root cap, 150x, shot with Celestron handheld, by Michael Dunn, Creative Commons

Because most students have never seen these organisms before and are not confident about what they are looking for, some will accidentally focus on air bubbles, hair or debris, rather than living organisms.

Stalked ciliates attached to duckweed, 400x, shot with OMAX digital microscope by Michael Dunn, Creative Commons.
Protist feeding on San Francisco pond bacteria, 400x, shot with OMAX digital microscope by Michael Dunn, Creative Commons.
Heliozoan from San Francisco pond, 400x, shot with OMAX digital microscope by Michael Dunn, Creative Commons.

If you collect your own specimens from local ponds, the abundance of planktonic microbes may be small and difficult for students to find. In contrast, if you use the Pond Mystery Mix from Carolina Biologicals and keep the total volume low and concentration high, students should have little trouble finding organisms to observe and identify.

QUESTIONS FOR DISCUSSION

1. Which organisms did you observe that were producers (autotrophs)? (Possible Answers: Algae, duckweed, spirogyra) How do you know? (Ans: they were green).

2. Which organisms did you observe that were herbivores? (Possible answers: daphnia, copepods, protozoa, paramecium, stentors). How do you know? (Ans: They were foraging or hanging out near the algae or plants, or they were observed eating the algae or plants).

3. Which organisms did you observe that were carnivorous? (Possible answers: copepods, hydra, water bears, stentors). How do you know? (Ans: they were observed eating other types of non-photosynthetic plankton).


DISCUSSION OF RESULTS

To minimize the risk of students focusing on air bubbles or debris, it is helpful to have a digital microscope connected to an LCD projector and project a sample in front of the class in order to demonstrate the difference between living organisms and debris or bubbles.


STANDARDS

Next Generation Science Standards

3-LS4-3. Construct an argument with evidence that in a particular habitat some organisms can survive well, some survive less well, and some cannot survive at all. [Clarification Statement: Examples of evidence could include needs and characteristics of the organisms and habitats involved. The organisms and their habitat make up a system in which the parts depend on each other.] 3-LS4-4. Make a claim about the merit of a solution to a problem caused when the environment changes and the types of plants and animals that live there may change. [Clarification Statement: Examples of environmental changes could include changes in land characteristics, water distribution, temperature, food, and other organisms.] [Assessment Boundary: Assessment is limited to a single environmental change. Assessment does not include the greenhouse effect or climate change.]

4-LS1-1. Construct an argument that plants and animals have internal and external structures that function to support survival, growth, behavior, and reproduction. [Clarification Statement: Examples of structures could include thorns, stems, roots, colored petals, heart, stomach, lung, brain, and skin.] [Assessment Boundary: Assessment is limited to macroscopic structures within plant and animal systems.] 4-LS1-2. Use a model to describe that animals receive different types of information through their senses, process the information in their brain, and respond to the information in different ways. [Clarification Statement: Emphasis is on systems of information transfer.] [Assessment Boundary: Assessment does not include the mechanisms by which the brain stores and recalls information or the mechanisms of how sensory receptors function.]

5-PS3-1. Use models to describe that that energy in animals’ food (used for body repair, growth, motion, and to maintain body warmth) was once energy from the sun. [Clarification Statement: Examples of models could include diagrams, and flow charts.]

5-LS2-1. Develop a model to describe the movement of matter among plants, animals, decomposers, and the environment. [Clarifcation Statement: Emphasis is on the idea that matter that is not food (air, water, decomposed materials in soil) is changed by plants into matter that is food. Examples of systems could include organisms, ecosystems, and the Earth.] [Assessment Boundary: Assessment does not include molecular explanations.]

Common Core State Standards

CCSS.Math.Content.4.MD.A.1 Know relative sizes of measurement units within one system of units including km, m, cm; kg, g; lb, oz.; l, ml; hr, min, sec. Within a single system of measurement, express measurements in a larger unit in terms of a smaller unit. Record measurement equivalents in a two-column table. For example, know that 1 ft is 12 times as long as 1 in. Express the length of a 4 ft snake as 48 in. Generate a conversion table for feet and inches listing the number pairs (1, 12), (2, 24), (3, 36), . CCSS.Math.Content.5.MD.A.1 Convert among different-sized standard measurement units within a given measurement system (e.g., convert 5 cm to 0.05 m), and use these conversions in solving multi-step, real world problems. CCSS.Math.Content.5.MD.B.2 Make a line plot to display a data set of measurements in fractions of a unit (1/2, 1/4, 1/8). Use operations on fractions for this grade to solve problems involving information presented in line plots. For example, given different measurements of liquid in identical beakers, find the amount of liquid each beaker would contain if the total amount in all the beakers were redistributed equally.

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