Marine biologist Dr. Elena Morales argues that establishing dense kelp forests along temperate coastlines can lessen local ocean acidity (raise pH) and thereby improve the calcification of shell-forming organisms such as mussels and oysters. To test her idea, Morales’s team selected two nearly identical coves: an experimental cove, where thousands of juvenile kelp plants were attached to submerged lines, and a control cove, which was left unchanged. For one month before the kelp was installed and six months afterward, the researchers recorded seawater pH every week. They also caged mussels in both coves at the start of the post-installation period and measured shell thickness monthly.
Which outcome of the pilot project, if verified, would most strongly support Morales’s claim?
For evidence questions involving a proposed mechanism, separate the claim into an intervention, an intermediate effect, and a final response. Prefer evidence that establishes comparable baselines, shows the intermediate effect after the intervention, and places the final response after that intermediate effect; this helps rule out preexisting differences and reversed temporal order.
Hints
Trace the causal sequence
Identify the intervention, the proposed change in water chemistry, and the biological response that is supposed to follow.
Use the baseline data
Ask whether any difference between the coves was already present before the kelp was installed or before mussel growth was measured.
Examine temporal order
For one change to explain another, consider which change should appear first in the measurements.
Step-by-step Explanation
Identify the complete causal claim
Morales proposes a sequence: adding kelp raises local pH, and this reduction in acidity then improves mussel calcification. Strong evidence should support both effects and the proposed connection between them.
Check the baseline comparisons
If the future experimental cove already had higher pH before the kelp was installed, a later pH difference cannot confidently be attributed to the kelp. Similarly, a difference in final shell thickness is weak evidence if the mussels began with different shell thicknesses.
Consider the timing of the results
Because Morales claims that reduced acidity improves calcification, the pH difference should arise before the shell-growth advantage. If greater shell growth appeared first, the acidity change could not explain the earlier growth difference.
Select the strongest evidence
The strongest outcome reports similar baseline pH and shell thickness, a kelp-cove pH increase after installation, and greater shell growth that developed only after the pH values diverged. This pattern supports the effect of kelp, the calcification result, and Morales’s proposed mechanism.