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Bacteria in Pond Water Science Fair Project

Postby HopePressler1 » Tue Oct 20, 2020 9:35 am

My daughter is interested in learning about the amount of bacteria in our natural garden pond. She wants to sample the water once a week for the next 8-10 weeks. There should be a significant change in the air and water temperature where we live in the coming weeks. Would "Does the amount of bacteria change in the pond water when the temperature of the water changes?" be a good question? Would it be better to grow bacteria on an agar plate and examine it under the microscope or would it be better to examine the bacteria directly in the water under the microscope? We have access to a powerful microscope. What is the best way to determine how much bacteria is present if there is a lot of bacteria in the samples?

Thank you,

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Re: Bacteria in Pond Water Science Fair Project

Postby probiotics » Tue Oct 20, 2020 10:39 am


This seems like a great project idea! When working with an unknown amount of bacteria, it may be useful to do some kind of serial dilution on the water you gather from the lake. A serial dilution works by diluting the sample by a factor of 10. For example, you would take .1 mL of lake water and mix it with 9.9 mL of distilled water, and plate 1 mL of that solution. The point of a serial dilution is to reduce the amount of bacteria to the point where you can count the number of colonies present on an agar plate without worrying about causing a lawn of bacteria. You would need to experiment with the number of times you repeat a serial dilution until a proper amount of bacteria is present.

I think that plating it in an agar plate would be the best idea, because it provides a more controlled way of examining the samples. Once you plate and allow it to grow for 1-2 weeks, you can count the number of colonies in one quadrant of the plate and multiply by four for an estimate of the bacterial load. Another way you could test bacteria is by purchasing a coliform bacteria kit. These are very easy to use, and will tell you if there are fecal pathogens in your water and at what level.

I like your current question, but I am slightly worried that your results may not show any differences that you may expect. Bacteria in lake water isn’t uniform. Perhaps another interesting question would be testing water from different locations (either from the same lake or a different one) and seeing if there are any differences. Or, you could think about anything you add to the water (fertilizer, color) and test the impact of that material on the bacteria present.

Hope this helps, and good luck on your project!

- probiotics

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Re: Bacteria in Pond Water Science Fair Project

Postby Cherylreif » Wed Oct 21, 2020 2:19 pm

Hi Hope,

This sounds like a terrific project idea! Probiotic has great suggestions to how to determine the number of bacteria present in your water samples using serial dilutions. If you decide to culture bacteria on agar plates, as suggested, make sure to follow the safety precautions for handling microorganisms (see https://www.sciencebuddies.org/science- ... roBio_p007).

I also agree that you might not see much of a difference in the numbers of bacteria in your water samples over the course of 8-10 weeks--but if that's the case, a negative result is still a result! To make interpreting her results easier, your daughter might want to collect some other data when collecting her water samples. For example, what is the water temperature when she collects a sample? What is the air temperature? How deep is the pond water? These factors could affect the number of bacteria in the water.

Similarly, she will want to make sure that--as much as possible--she keeps everything the same when taking water samples. That is, she should make sure she collects water at the same time of day; collects it at the same location in the pond; and collects it from the same depth under the water's surface.

I hope this is helpful! Best of luck on your project!


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