Interpreting a Fertilizer Runoff Experiment with a Confounded Control
A student studies algal growth in pond water. She fills four 500 mL beakers with pond water and adds fertilizer solution: 0 mL, 2 mL, 4 mL, and 8 mL. She places all beakers on a sunny windowsill for 10 days and measures the final chlorophyll concentration as a proxy for algal biomass. Results: 0 mL gives 12 units, 2 mL gives 25 units, 4 mL gives 41 units, and 8 mL gives 38 units. She concludes that fertilizer increases algal growth up to 4 mL, after which it becomes toxic. A classmate notes that the 8 mL beaker was placed near an open window, so its water was several degrees cooler than the others. The student also did not measure the fertilizer's phosphorus content or the starting chlorophyll level.
1. Evaluate whether the data support the conclusion that fertilizer above 4 mL is toxic to algae.
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The data do not support a toxicity claim. The 8 mL beaker had a different temperature due to its location, so the drop from 41 to 38 units could reflect temperature, not fertilizer toxicity. The design confounds fertilizer volume with position/temperature. To support toxicity, the student would need to control temperature (same location or a controlled incubator) and test multiple replicates at each volume. Also, the difference (3 units) is small and may be within measurement variability. A safer interpretation: growth increased from 0 to 4 mL, but the 8 mL result is ambiguous because of the temperature confound.
2. The student wants to know whether phosphorus or some other component of the fertilizer drives the growth. Describe a control or measurement that would help distinguish these possibilities.
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She could add a phosphorus-only treatment (e.g., a phosphate solution matched to the phosphorus content of the 4 mL fertilizer dose) and compare it with the same volume of complete fertilizer. If the phosphorus-only beaker shows similar growth, phosphorus is likely the driver. Alternatively, she could measure dissolved phosphorus at the start and end and correlate it with chlorophyll. She should also measure starting chlorophyll to establish a baseline and use replicates to estimate variability. Without knowing fertilizer composition, she cannot attribute the effect to phosphorus specifically.