Stream Sampling Below an Orchard: Correlation, Cause, and Controls
Students sample a stream above and below an orchard. Above the orchard: pH 7.2, nitrate 6 mg/L, dissolved oxygen 8.5 mg/L, temperature 14°C. Below the orchard: pH 6.4, nitrate 42 mg/L, dissolved oxygen 5.0 mg/L, temperature 17°C. It rained heavily the day before sampling. The orchard owner says he applied fertilizer two weeks earlier. One student concludes, 'The fertilizer caused the oxygen drop, so the stream is dead.' Another says, 'The rain makes the data useless.' The class has one sample per site and no data from other seasons.
1. Which two measurements most strongly support fertilizer runoff, and which measurement weakens a simple 'fertilizer alone' explanation? Explain.
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The strongest supports for fertilizer runoff are nitrate rising from 6 to 42 mg/L and pH falling from 7.2 to 6.4. Fertilizer adds nitrate, and runoff can acidify the stream. The measurement that weakens a simple 'fertilizer alone' explanation is temperature rising from 14°C to 17°C: warmer water holds less dissolved oxygen, so part of the oxygen drop from 8.5 to 5.0 mg/L could be physical rather than caused only by fertilizer. The heavy rain is also a confounder because it can wash in materials and change flow.
2. Evaluate the claim 'the stream is dead.' What additional biological measurement would you need, and what oxygen level would make the claim clearly unsupported?
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'The stream is dead' is not supported. One oxygen reading of 5.0 mg/L is low compared with 8.5 mg/L, but it is not zero, and 'dead' is a biological claim. The class would need a biological measurement: counts of sensitive invertebrates, fish, or algae, plus repeated oxygen readings at different times of day. No single oxygen level proves death; if oxygen were back above about 6–7 mg/L and a diverse invertebrate community were present, the claim would be clearly unsupported. As it stands, the data show a stressor and a possible impact, not a dead stream.
3. Design a better two-week sampling plan to test whether the orchard causes the change. Include at least one control and one way to reduce rain confounding.
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A better two-week plan: sample three sites - above the orchard (reference), below the orchard, and a nearby tributary not fed by the orchard (control). At each site, take three replicate samples on at least three dates: before rain, during or after rain, and on a dry day. Measure nitrate, pH, dissolved oxygen, temperature, and flow at the same time of day. Add a kick-net survey for invertebrates. This reduces rain confounding by comparing wet and dry conditions, and the upstream and tributary controls help separate orchard runoff from other sources.