Daphnia Death Data Research Snapshot by Katherine Rupp, Timothea Muljadi, Amina Nur, Mark Malabuen, and Caleb Smith

Figure 1. Daphnia magna habitats 5 days after setup. Note the variation in water color.

In this experiment, we tested the effects of environmental pH on the heart rate of Daphnia magna. We then attempted to use the results to determine the optimum pH for D. magna survival. We initially hypothesized that the pH of an environment would affect the heart rate of D. magna; however, later on we had to revise the experiment to exclude testing the ability of D. magna to become acclimated to different pH levels and include measuring the approximate rate of mortality. The mortality rate was so high that it rendered observation of the long-term effects of pH on living D. magna impossible.

Contrary to our predictions, the data we collected revealed no clear correlation between pH level and D. magna heart rate or mortality rate. In our first set of D. magna populations, the lowest average heart rate occurred in a pH of 6.07, while the highest occurred at pH 9.70. Our first trial involving the second population set revealed that the lowest average heart rate occurred at pH 8.05 and the highest at pH 6.81. We expected that, as the pH deviated further from pH 7, the heart rate of D. magna would increase due to the stress response; however, we observed no trend linking heart rate with deviation from pH 7.  Additionally, we expected that the elevated stress caused by extreme pH levels would result in a higher mortality rate for D. magna in these conditions. Our results did not conform to this prediction either, as the apparent death rate was lowest in both the pH closest to and farthest from pH 7, while the highest death rates occurred in the pH levels intermediate to these two.

The most surprising thing we experienced was finding all of the D. magna dead just a few hours after we set up the experiment. We had to redo the experiment and create a more favorable ecosystem in each jar to see if we could improve the odds of survival. We also narrowed down the pH range so as not to shock the D. magna with excessively extreme conditions. The modifications helped a little bit, since we could still find a few living D. magna after five days of experimentation. Additionally, we expected the pH within each of our jars to remain constant throughout the experiment; but measuring the pH in each jar on most of our experiment days revealed that the pH levels changed significantly over time. We were also surprised to see color change in the solutions, with some remaining relatively clear but others turning a dark-yellow color.

    In the error analysis section of our discussion, we will discuss several factors that contributed to inaccuracies and inconsistencies in our data. The tablets we used to make our buffer solutions of pH 8 and 10 were expired by 10 and 6 years, respectively. All of the buffer solutions except the pond water control were mixed when we revised our pH solutions because we mixed the original solutions to create new solutions of the desired pH. As a result, four of our five environments contained buffer solutions that were made from the expired tablets. Use of these expired chemicals could have negatively affected the health of the D. magna and the plants in each environment. Additionally, our habitats each contained a layer of small, sharp rocks at the bottom. We placed 15 D. magna in each pH solution at the beginning of our experiment; however, over the three days that we collected data, there were several that we could not locate. As a result, several D. magna were unaccounted for in our experiment, so our collected data do not fully reflect the entire sample size. Most likely, the D. magna that died became buried under the sharp rocks at the bottom, making them difficult to locate. Of the corpses that we observed under the microscope, many of them had punctures in their exoskeletons or were ripped apart, which potentially was a result from the sharp rocks in the environment.

    We would like to ask other groups, particularly those who also worked with Daphnia, the following questions. How big was your sample size? How did you eliminate environmental factors other than your independent variable? How long did it take for your organisms to die?