Science Buddies: "Ask an Expert"

My 7th grade daughter is modifying the experiment slightly and I would appreciate your feedback. She will collect sediment samples from four locations - commericial parking lot, residential building site, residential yard and mountain stream and have a control. Her independent variable will be location (treatment).... and the dependent variable will be viability of the daphnia. Her question is does runoff from different locations/sources affect aquatic life differently. She will have three reps per treatment. She will compare the viability of daphnia between the treatments and control. Thank you for the guidance.

Here are my questions.

What dilution factor (g/L) should I use to simulate a rain event? This should be the same across treatments.

Can I use a smaller number of daphia since I will have 15 containers total?

Here is the original procedure as listed on Science Buddies.

Dilute the sediment samples with spring water at different concentrations, to simulate rainwater run-off after a storm. Label each container with concentration (in grams per liter, or g/L) of sediment in the water.
Transfer 30–50 Daphnia into each container. Keep a population of Daphnia in sediment-free water as a control. Keep all containers in the same environmental conditions (same temperature, light level, feeding, etc.).
Count and record the number of Daphnia in each container at 8, 16, 24, and 48 hours after the start of the experiment.
Calculate the percentage of viable Daphnia in each container for each time point.
Graph the percentage of viable Daphnia (y-axis) vs. sediment concentration (x-axis), for each time point.
Does the sediment have any effect on Daphnia viability?
If there is a decrease in viability, can you estimate the LC50 for the sediment?

Hi and welcome to Scibuddies.
Your questions are excellent and thoughtful ones and I wish I could give you a simple, unequivocal answer. The question of how much 'sediment' to use to simulate runoff has come up before and there is no straightforward answer. The concentration of whatever toxins happen to be in rainwater from a particular location will depend on the location and the amount of rain and neither of these variables can be easily defined. I suppose that you could bury a can next to a parking lot and wait for it to rain and then test the water in the can.

I have not done this Daphnia experiment, but with other tests of water pollutants we always use a series of dilutions of the alleged toxic sample. So, you should start with a fairly concentrated runoff sample--say 10 g of sweepings put into 100 ml of spring water. You could then add a measured amount of this simulated runoff water to a culture of Daphnia and compare their behavior to Daphnia in just spring water. If you have a microscope, you can count the heart rate of the Daphnia which is one quantitative way of doing the experiment. If you see no effect then that means that either the area you sampled is clean or you did not get a large enough sample--maybe 50 g of detritus per 100 ml of water.

If you see a strong effect on the Daphnia, then you can test smaller amounts of the runoff and hopefully you will see a 'dose' effect.

Try to use as many Daphnia as possible per experiment and do the test at least three times so that you can take the average and get a statistically more accurate answer.

I hope this helps. If you have more questions, please post again.

Sybee