Pond Dip – Bring an Ecosystem into the Classroom

Studying aquatic microorganisms is engaging and very educational.

Here I share how I teach a 7th grade lab on finding protists and microscopic animals in pond water. This may seem daunting if you are not familiar with the organisms or haven’t had the time to organize the activity. I hope that I’ll give you all the information you need for a successful lab.

aquatic beautiful bloom blooming
Photo by Diego Madrigal on Pexels.com

Why take the time for this lab? Well, no one is going to be excited about algae by looking at a picture of it. Algae in the macroscopic world is called POND SCUM, but under the microscope it’s BEAUTIFUL.

Also, many standards can be addressed in a pond dip lab:

  • modeling ecosystems and ecosystem interactions
    • food chains, food webs and trophic levels
    • nonliving and living factors within an ecosystem
  • biological diversity
    • taxonomy and evolutionary relationships among species:
      • single-celled protists
        • algae – single-celled organisms with plant-like characteristics
        • protozoans – single-celled organisms with animal-like characteristics
      • microscopic animals – all multicellular
        • crustaceans, aquatic worms and rotifers
  • cells
    • single-celled organisms are microscopic, but have all of the characteristics of living things
    • microscopic multi-celled organisms such as microscopic crustaceans and worms
  • Using dichotomous keys and technical language – meet Common Core standards for using “key terms” and “domain-specific words and phrases” used in a “scientific and technical context”.
green toad in water
Photo by Darius Krause on Pexels.com

The supplies are simple and usually found in the typical middle school or high school biology classroom. The only things you will need to gather otherwise is the microorganism-rich water and organism keys; I provide you with information on both.

Supply List:

  • microscopes with at least 100x magnification
  • containers (3 or more) to hold the pond water
  • pipettes (3-5 per container)
  • concavity slides (designed to hold a drop of water). Example here.
  • microscope cleaning tissues (such as Kimwipes®)micro key
  • dichotomous keys to aquatic microorganisms
    • Guide to Identification of Fresh Water Microorganisms – this one I print enough copies for each group
    • Pond Life Identification Kit – this source has a lot of links that are useful to the teacher and can be used by students if internet access is possible during the lab
    • Protozoans and Small Animals – this website is also helpful; click on the phylum for more drawings and information – I print the Ciliophora page and multicellular animals page for extra reference materials.

      Plastic container with pond water. The little leaves on top or the decayed plant matter on the bottom are the best places to find microorganisms.
  • pond water – this might be the one supply that keeps teachers from doing this lab. Micro-rich water is not too hard to find in most places – any relatively natural fresh water source will do, such as ponds, streams and lakes. Artificial ponds such as a Koi ponds work too. Even well-established fish tanks have a micro-community within them!
    • How much water? I collect about one gallon (or less). Make sure the water has some debris in it – the debris provides food and hiding places for microorganisms. Most are found within the debris (dead plant matter and silt).

Warning:  Do NOT add tap or bottled water to your pond water. Drinking water usually has some chlorine in it – this will kill your organisms! If I need additional water for rinsing slides I use water that is treated with water conditioner for fish tanks. The conditioner removes chlorine.

Preparing Students for this Lab

I have my students do this lab after we have studied the basic characteristics of the 6 kingdoms of life. In this lab we can study the protist kingdom as well as microscopic animals. Since we always are lucky to see crustaceans, worms and rotifers (amazing little animals!), my students are able to observe the differences between the animals and the single-celled protists.

The key, I think, is that your students should become a little familiar with the organisms before beginning. I show some videos on actual pond water organisms (always creates excitement for the lab) in addition to some other videos about the protist kingdom.

flagella/cilia video                        microscopic life video (shows many types)

pond life video (fast forward past the tadpole to learn about protists)

Daphnia video (microscopic crustacean)           rotifer video

pond dipAdditionally, I review the possible types of microscopic animals we hope to find. Lastly, my students complete my Virtual Pond Dip WebQuest a few days before.

Showing Students What To Do

First, students will need to know how to use the microscopes. With the concavity slides, my students are able to use the low magnification (100x), but not the high because the lens would go into the water.

Second, model how to make the slide, use the slide to find organisms and clean up the slide.

  • Model making the slide – show how the ONE drop of water goes into the depression within the slide. Show how to put a cover on the slide if you are using them.
    • Model using the pipette to get a sample from the “dirty” bottom of the container. Most organisms will be among the debris, not in the more clear water.
  • Model carefully carrying the slide to the microscope and carefully placing it on the stage.
  • Model how the student will need to move the slide and change the focus routinely to find the organisms. The water drop is 3D so changing the focus is required to see through different levels of the drop.

Third, explain that if what they see is not green and/or not moving it is not alive. Students naturally want every little thing they see to be an amoeba or hydra or leech! I give a clear direction – don’t call me over to verify that you are looking at sand particles or dead plant matter. If it isn’t moving or green, then just keep on looking.

Fourth, Model how to return the water drop to the container (I prefer to preserve my organisms for others to see as much as possible). I demonstrate that the drop should be poured into the container and then the slide should be gently tapped on the side of the plastic container to save as much of the water as possible. Another option is to rinse the drop into the container using a small squirt of conditioned tap water. Afterwards, dry the slide with a tissue before adding a new drop.

Fifth, Verification. All identifications have to be verified by me. However, students must tell me what they think they are seeing before I will check the microscope. Otherwise, students will use me as their ID key rather than the actual keys.

Please share your experiences or ideas with pond dips in the comments section below.

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