Science Buddies: "Ask an Expert"

I am doing a modified Science Buddies experiment (https://www.sciencebuddies.org/science- ... r-toxicity) using daphnia in a sediment bioassay. I must collect various sediment samples and add water. It says a broom and dustpan will be needed to collect from a parking lot. How much (in grams) should I expect to collect? What should I know before collecting?

Hello and thank you for your patience in waiting for a response!

You ask: How much sediment to collect?

Looking at steps 6 and 7 in the procedure, you will weigh the sediment before you dilute it. You will be making several dilutions. So you won't need to weigh the sediment at the time you are collecting it, but weigh it just before you start making your dilutions.

What should you know before collecting?

You will want to plan the number of different concentrations to use in preparing your sediment samples to follow the procedure steps.

Good luck with your project. Be sure to let us know if you have more questions.

MadelineB wrote:Hello and thank you for your patience in waiting for a response!

You ask: How much sediment to collect?

Looking at steps 6 and 7 in the procedure, you will weigh the sediment before you dilute it. You will be making several dilutions. So you won't need to weigh the sediment at the time you are collecting it, but weigh it just before you start making your dilutions.

What should you know before collecting?

You will want to plan the number of different concentrations to use in preparing your sediment samples to follow the procedure steps.

Good luck with your project. Be sure to let us know if you have more questions.

Thank you for your response. However, I have some more questions:
1. When I asked how much sediment I should use, I was talking about the constant for which I should aim after collecting samples. There will be 10.0 mL of spring water.

2. My independent variable is the sediment location, so I'm using places near drainage, mulch, grass, and trees. Should I be concerned if there's a curb between the mulch and road, for example?

3. Is getting the sediment as easy as sweeping with a broom and dustpan, or will I need to dig?

4. Can the sidewalk be a source for sediment collection?

Hi Okafore,

Answers:

(1) I'm not sure I understand what you mean by "the constant to aim for"?? Since you will weigh each of your sediment
samples before making the dilutions, and you will start the dilutions with 10.0 mL, then the concentrations of all of your dilutions will be expressed in g/L (grams/L), which you can think of as a percentage. You will then be observing the number of living/(or dead) at each concentration. Be sure to make a table to put in your lab book and record the concentrations of each of your dilutions (in g/L). Several of the references in the bibliography in the background section contain examples tables.

(2) Does it matter if there is a curb between the mulch and the road? No, but be sure to document the environment for each of your locations. Maybe use your smartphone to take a picture!

(3) Yes, according to the procedure, just sweep up some sediment!

(4) This project can be done with many variabtions, so sure, a sidewalk could be one of your locations.

I hope this helps, but don't hesitate to ask again if you have more questions!

MadelineB wrote:Hi Okafore,

Answers:

(1) I'm not sure I understand what you mean by "the constant to aim for"?? Since you will weigh each of your sediment
samples before making the dilutions, and you will start the dilutions with 10.0 mL, then the concentrations of all of your dilutions will be expressed in g/L (grams/L), which you can think of as a percentage. You will then be observing the number of living/(or dead) at each concentration. Be sure to make a table to put in your lab book and record the concentrations of each of your dilutions (in g/L). Several of the references in the bibliography in the background section contain examples tables.

(2) Does it matter if there is a curb between the mulch and the road? No, but be sure to document the environment for each of your locations. Maybe use your smartphone to take a picture!

(3) Yes, according to the procedure, just sweep up some sediment!

(4) This project can be done with many variabtions, so sure, a sidewalk could be one of your locations.

I hope this helps, but don't hesitate to ask again if you have more questions!

Thank you for answering my questions, but I still have some more:

(1) My independent variable is the location of the sediment, so the concentration is a constant. What constant concentration would be best to provide a realistic simulation?

(2) Can I order daphnia and immediately start the bioassay, or must I culture it? Data is due by January 11th, but half of that is Christmas break and I must do the project at school. I want to finish experimenting by December 21st so I can use the break to analyze the data.

(3) While the website recommends 30-50 daphnia, I would like to save money. At what number of daphnia would the bioassay become invalid (I'm considering dropping to 20 to 25 daphnia/per trial).

Thanks again for all your help.

Hi Okafore,

You should let the daphnia start eating and growing before you run the experiment, so just follow the instructions that come with the kit. Daphnia magna don't like it too warm or too cold so try to keep the water temperature between 64 and 72F (18-22C). Daphnia pulex and D. moina can tolerate a wider temperature range.

Here's a website that has lots of information about growing daphnia: http://www.aka.org/wako/DaphniaCultureMadeSimple.htm

Be sure you use spring water for culturing Daphnia as tap water can contain chlorine which would be harmful to them. Keep all your cultures at about the same temperature and light level. Also don't forget to include the control with just spring water and NO sediment.

The one piece of information that is lacking from the project procedure (https://www.sciencebuddies.org/science- ... #procedure) is how much of your collected material you should add to a certain volume of water. Yes, the instructions say weigh the material you collected, and yes, it says to use different concentrations--but nowhere does it say how many grams of debris you should put into how many milliliters of water. This is important. The more solid matter per volume of water, the higher the concentration of whatever pollutants or toxic chemicals are present. And also, the dilutions you should use are not specified either.

If some other expert has done this project, maybe they could tell you how many grams of collected material to put into how many milliliters of water. In order to compare debris from different locations, you need to use the same concentration solution in g/mL and add the same volumes to the Daphnia cultures.

I hope this answers your questions. If you need more help, post back and I will answer right away.

Sybee

To Whom It May Concern,
In my analysis before and after my experiment, I talked a lot about the LD50/LC50 (I know that they're different). I needed to explain it when I realized that because the independent variable (IV) is categorical (location of sediment), neither of the two work. EC50 and IC50 do not work either. Is there a similar test that works for a qualitative IV?

Thank you for all your help so far.

Sincerely,
Okafor

Hi Okafor,
I will try to explain the answer to your question about the variables. First let's look at the hypothesis. I don't know what you said was the hypothesis, but I will assume it was something like this:

Daphnia survival is reduced by toxic substances in dirt taken from parking lots.

First off, I would not use the word 'sediment' except to refer to a certain type of material that you collected. To a scientist, sediments are the result of weathering and erosion or the material that settles to the bottom of a body of water. If you used a broom and dust pan to collect material from a parking lot then it was dirt--not sediment.

Where you collected the dirt (or actual sediment if it was muck from the bottom of a pond) is one of your IVs, but another variable, and perhaps one that is even more important is the amount of material that was added to a certain volume of water. I assume that you used the same weight of each material, but how did you decide how much? For a given volume of water, the more material you put into it, the greater will be the concentration of whatever substances that are toxic to Daphnia.

I'm assuming that your DV is survival of the Daphnia, correct? You could also have measured reproduction and heart rate. The material that you exposed the Daphnia to is a mixture of many things that you have no way of determining so it makes no sense to express the survival measurement as an LD50 because you don't know what substance(s) are causing the lethality. Just report the survival as a percent of the starting number of Daphnia and then give some tentative explanations for the results. You could have checked survival several times over a period of days or weeks and maybe been able to show a more rapid affect for some of the 'dirt'.

Sybee

Dear SciB,
My hypothesis was that the dirt from near drainage and grass would kill daphnia the most.

How much dirt I collected from each place is constant, not a variable.

My DV is the survival rate and that was the only thing which I measured. (I didn't do heart rate because that was my friend's idea. Lol.) I checked the rate over 48 hours.

After researching LC50, I know that I cannot use it. I would like if there is something similar which I can use.

Sincerely,
Okafor

I know that the amount of material you used from each place was the SAME. What I am trying to make clear is that how MUCH dirt you added to the water is the variable. The more dirt you add to the SAME volume of water, the more concentrated will be the toxic substances.

What I am trying to find out is this:
How MUCH dirt [in grams or ounces] did you mix with what VOLUME of water [in milliliters]?

And, how did you decide how much dirt to put into a certain volume of water? If I remember right, the project procedure does not say how much dirt to use. I understand that the idea of the project was to mimic conditions of rain washing substances from a parking lot or field into a pond where Daphnia lived and determine how this affected their survival.

The procedure said to use different 'concentrations' but it did not tell you what concentrations to use. How did you decide this?

You said you measured survival of the Daphnia over 48 hours. Did you count them at 8, 16, 24 and 48 hours? What was the survival at each time? Did you test different concentrations of your 'run-off' liquid? The more you dilute the test water the greater the survival should have been.

If you did run 4 or 5 different 'concentrations' of run-off then you could determine the LD50 which would be the concentration at which half the total number of Daphnia were dead. You just would not be able to say what substance caused the Daphnia to die.

Let me know more of the methods and results and I can help yo better.

Sybee

I added the SAME amount of dirt to the SAME amount of water for each experimental group. My IV was from WHERE I collected the dirt. For each experimental group, I added 6 grams of dirt to 50 mL of bottled spring water.
I counted the number of daphnia around 8:00 am and 3:30 pm each day.

Okafor

Why did you use 6 g of dirt to 50 ml of water?

I am just trying to get you to see that the amount of dirt you used IS a variable that you control and that it DOES have an effect on the experiment.

What made you pick 6 g?

The dirt is what kills the Daphnia and the more you use, the greater the potential killing power.

I picked 6 grams for ALL of my trials because I had enough dirt/sediment for it.

Are you sure 6 g is enough to have an effect on the Daphnia?

I hope so. It's too late to go back.

Good luck. Let us know what happens.

Sybee

Dear 'SciB' and To All Whom It May Concern,
At https://www.sciencebuddies.org/science- ... r-toxicity, they talk a lot of about using a broom and dustpan to collect "sediment". Why should I not call what I collected "sediment", what is a better word to call it, and how do I incorporate sediment into my explanations, because it is the basis of my research.

Sincerely,
Okafor

Hi Okafor,

I am glad you are thinking about words because they are very important in science. Scientists have precise meanings for terms and if they are used wrongly then there can be confusion.

Sediment refers to particles that settle out of water in oceans, rivers, lakes, etc. Most non-scientists use words very loosely so that's where the confusion can come in. If you uses the word 'sediment' to mean anything that you can sweep up with a dustpan then it loses its specific meaning and people can misunderstand.

The goal of your science project is to test the effects of rainwater run-off from certain areas on the health of an aquatic test organism--Daphnia. You could try and collect actual run-off during a rain storm but that would be difficult. So, the project procedure has you collect the debris and dirt that is on an area, mix it with water and use it as if it were actual run-off.

There are several words that you can use for the material that you collected--dirt, debris, detritus--but nothing that precisely means the material that you sweep off a parking lot. Dirt is the closest thing in American English, although I suppose the British version would be 'dust' since that is their usual word for 'dirt'.

If English is not your native language, then word choice can be very difficult. English words can have multiple meanings and American and British versions are usually different. Knowing which word to use when is what makes a good writer. I am a writer as well as a scientist and I appreciate precise, clear, readable language. Try to make that your goal in all your writing and it will help you a lot in the years to come.

Best wishes,
Sybee