Stream Sampling Below a Golf Course and a Housing Development
Students sample a stream at three sites on the same sunny day. Site A is 200 m upstream of a golf course pond outfall. Site B is in the pond discharge channel. Site C is 500 m downstream, below a housing development with septic systems. They measure nitrate (1.0, 12.0, and 8.0 mg/L), dissolved oxygen (9.0, 5.0, and 6.0 mg/L), algal cover (10%, 80%, and 55%), and water temperature (14 °C, 22 °C, and 18 °C). Sampling times are 8:00 a.m. at A, 11:00 a.m. at B, and 2:00 p.m. at C. A homeowner says the golf course is clearly the only source of pollution because nitrate and algae peak at B. A student says the low oxygen at B proves the pond is dead. The teacher asks the class to identify the strongest supported conclusion, the most serious confound, and a better sampling plan.
1. What conclusion about nitrate and algae is best supported, and what is the most serious confound?
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The best-supported conclusion is an association: nitrate and algal cover are much higher at the pond outfall (B) and remain elevated downstream (C) than upstream (A). That pattern is consistent with nutrient enrichment from the pond, but it does not prove the golf course is the only source because the housing development also lies upstream of C. The most serious confound is time of day: samples were taken at 8 a.m., 11 a.m., and 2 p.m. Temperature and dissolved oxygen change diurnally, and algal photosynthesis and respiration change with light. Site differences could partly reflect when each sample was taken rather than where.
2. Evaluate the claim that low dissolved oxygen at B proves the stream is dead.
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The low oxygen reading does not prove the stream is dead. At 5 mg/L, oxygen is low for many fish and invertebrates, but it is not zero, and a single daytime sample cannot describe the whole day. Site B is also 8 °C warmer than A, and warm water holds less dissolved oxygen. Algae at B produce oxygen in sunlight but consume it at night, so the reading could be higher or lower at dawn. To evaluate biological harm, sample at dawn and afternoon, measure oxygen repeatedly, and survey organisms rather than relying on one number.
3. Design a sampling plan to test whether the golf course pond raises nitrate and algae downstream, controlling for time of day and isolating sources.
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Sample A, B, and C at the same time of day, on several days, and at both dawn and afternoon for oxygen. Measure flow, temperature, and nitrate in triplicate, and add sites upstream and downstream of the housing development to separate its contribution from the pond's. A stronger design would sample the pond outflow and a nearby reference tributary before and after fertilizer application or rain, and use a tracer or isotope signature to link nitrate to a source. If nitrate and algae rise only below the pond outfall under matched conditions, the case for the golf course is stronger.