Carbon dioxide’s ecological footprint

This exercise is a part of Educator Guide: Rising Carbon Dioxide Threatens Lake Food Webs / View Guide

These questions are based on the article Rising CO2 threatens lake food webs.”

1. How are dissolved carbon dioxide levels and pH levels in oceans and lakes changing? Explain why these changes are occurring.

Possible student response: Dissolved CO2 concentrations in oceans and lakes have been increasing, causing the pH to decrease. As the partial pressure of CO2 has increased in the Earth’s atmosphere, more CO2 has dissolved into oceans and lakes. A greater concentration of dissolved CO2 in water makes the solution more acidic, decreasing the pH.

2. What is new about this study versus previous water acidification studies?

Possible student response: Most previous studies investigated how rising CO2 levels affect oceans and organisms living in them, whereas this study examined how rising CO2 levels affect freshwater bodies and some of their organisms.

3. According to the article, how much did CO2 and pH change in the lakes?

Possible student response: From 1981 to 2015, the average CO2 level across all four lakes rose by more than 500 microatmospheres. Two of the lakes experienced about a fourfold increase in CO2 levels. The overall average pH dropped from 8.13 to 7.82.

4. What data are the graphs displaying and what do the trend lines show? Be specific about the units of measurement used and the increments of measurement on each axis.

Possible student response: The upper graph plots the partial pressure of CO2 in lakes from 1981 to 2015, and the lower graph plots the pH levels of the lakes over that time period. Data is plotted for four different lakes, shown in four different colors. There are a lot of fluctuations in the data, but the trend lines indicate an increase in the partial pressure of CO2 and a decrease in pH for all four lakes. In both graphs, time is measured in years, and each line on the x-axis represents five years. Partial pressures of CO2 are measured in microatmospheres and each line on the y-axis of the upper graph represents 100 microatmospheres. Measurements of pH are unitless (by definition) and each line on the y-axis of the lower graph represents 0.2 pH units.

5. What are Daphnia, and what defenses do they have against predators?

Possible student response: Daphnia are a genus of water fleas. They are miniature crustaceans that are at the base of many freshwater food webs. When the water fleas detect chemical signals from predators feeding on other Daphnia, they display defensive behavior for protections. These behaviors include raising neck spikes and growing giant “helmet” crests that make the Daphnia harder to swallow.

6. What effect did increased CO2 levels have on Daphnia in the lab?

Possible student response: Raising the dissolved CO2 level from 2,000 to 11,000 or 16,000 microatmospheres made two species of Daphnia less defensive in response to predators’ chemical cues. The Daphnia displayed fewer neck spikes and developed smaller head crests.

7. Do scientists attribute the effect to increased CO2 levels, decreased pH or both? Explain.

Possible student response: Experiments showed that the effect was due to the elevated CO2 levels, not a reduction in pH. There is no definitive answer yet known for why an increase in CO2 level decreases flea defenses, but the researchers theorized that CO2 makes the Daphnia drowsy and therefore less sensitive to predator danger signals.

8. What uncertainties and unknowns does the article acknowledge?

Possible student response: Different lakes can vary greatly in terms of their setting, their chemistry, how much they are warming and whether they are absorbing CO2 or are already saturated with it. Freshwater organisms other than the two Daphnia species might be affected differently, depending on the CO2 levels and environment.

9. What other questions do you still have after reading the article?

Possible student response: How exactly do increased CO2 levels in the water affect Daphnia or other aquatic organisms? How do the effects on Daphnia affect the whole food web? How could harmful effects be minimized?

10. Summarize the article using the following key words: CO2, pH, acidity, Dapnia, fleas, reservoirs and defenses. 

Possible student response: Data from the last 34 years show that concentrations of dissolved CO2 in four German reservoirs increased and pH decreased (acidity increased). Scientists found that elevated levels of CO2 interfered with the defenses of two species of water fleas of the genus Daphnia by suppressing some of their normal defense responses to predators.