Interpreting a fertiliser and algal bloom dataset
Students monitor a lake receiving runoff from farmland. They sample four sites weekly for 10 weeks, measuring nitrate, phosphate, chlorophyll a and water clarity. Results show Site 1, nearest the inflow, has nitrate of 4.8 mg/L and chlorophyll a of 62 µg/L in weeks 6 to 8. Site 4, at the outflow, has nitrate of 0.9 mg/L and chlorophyll a of 11 µg/L over the same period. Phosphate stays near 0.02 mg/L at all sites. A student writes: "Nitrate causes algal blooms, so reducing nitrate will fix the lake." The class also notes that water clarity at Site 1 fell sharply in week 7 after a calm, hot spell.
1. Critique the student's causal claim using the evidence available and identify what would strengthen the inference.
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Correlation is not causation. The data show an association between high nitrate and high chlorophyll a at Site 1, but this is spatial and temporal correlation, not a demonstrated cause.
Alternative explanations.
- Site 1 is nearest the inflow, so it also receives sediment, organic matter and possibly other nutrients. The pattern may reflect proximity to the inflow generally rather than nitrate specifically.
- Phosphate is uniformly low at about 0.02 mg/L, so phosphorus may be limiting. Adding nitrate alone might not drive growth if phosphorus is scarce, which weakens a simple nitrate-limits-blooms story.
- The week 7 clarity drop followed a calm, hot spell, pointing to stratification or temperature effects that the student ignores.
What would strengthen inference. A controlled mesocosm experiment adding nitrate alone, nitrate plus phosphate, and a no-addition control, with chlorophyll measured over time. Also useful: continuous logging to establish whether nitrate rises before chlorophyll, and sampling after a nutrient-reduction intervention to see whether chlorophyll falls. Without these, the claim is plausible but unproven.
2. Explain why the calm, hot spell in week 7 matters for interpreting the clarity data.
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The calm, hot spell introduces a confounding variable. In calm, hot conditions a lake can stratify, with a warm surface layer separated from cooler bottom water. Stratification reduces mixing, so algae that would otherwise be dispersed can accumulate near the surface, and clarity measured at the surface can fall sharply without any change in total algal biomass.
Heat can also favour certain cyanobacteria and increase growth rates, so temperature and nitrate effects become entangled. If the clarity drop coincides with stratification rather than with a nitrate peak, attributing the drop to nutrients is unsafe.
To separate these effects, students should record temperature profiles, dissolved oxygen and mixing depth alongside nutrients and chlorophyll, and compare stratified and mixed periods. Sampling at a fixed depth and time of day also reduces artefacts. The week 7 observation is therefore a warning that the dataset needs physical measurements before causal claims about nutrients are made.