Stream invertebrates below a new housing estate
A sixth-form class surveys stream invertebrates at two sites. Site U is 200 m upstream of a new housing estate; site S is 50 m downstream of the estate's surface-water outflow. At each site they take one 3-minute kick sample and identify all animals. Upstream they find 8 different taxa, including 34 mayfly nymphs and 5 bloodworms. Downstream they find 4 taxa, including 2 mayfly nymphs and 60 bloodworms. The stream bed upstream is stony; downstream it is silted. The survey follows three days of heavy rain. No replicate samples were taken. The students conclude that the estate is polluting the stream and that biodiversity has fallen by 50%. Their teacher asks them to consider whether the data support both parts of this conclusion.
1. Explain why the claim that biodiversity has fallen by 50% is not well supported, referring to sampling and the meaning of biodiversity.
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Eight taxa versus four is a 50% reduction in taxa richness at these two samples, but that is not the same as a 50% fall in biodiversity. Biodiversity includes richness, abundance and evenness; the downstream sample is dominated by 60 bloodworms, so evenness is low, but one sample per site cannot give a reliable index. The kick sample may miss small or burrowing animals, and sampling effort was not replicated. The difference could be seasonal, caused by the spate after heavy rain, or due to the stony versus silty bed. So the data suggest a difference but do not support a precise 50% claim.
2. Identify two alternative explanations for the difference in mayfly and bloodworm numbers, and explain how the students could distinguish them from pollution.
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Alternative explanations: (1) habitat - mayfly nymphs need a stony, well-oxygenated bed, while silt downstream favours bloodworms, so substrate change alone could explain the pattern; (2) the heavy rain may have washed animals downstream or changed flow, so the survey is a snapshot after an unusual event. To distinguish these from pollution, sample several times before and after the outflow, at several upstream and downstream sites, use the same sampling effort and mesh size, and measure water chemistry (dissolved oxygen, nitrate, phosphate, ammonia, turbidity) and flow. Compare with a nearby unpolluted stream of similar substrate as a control.
3. Assuming the estate is discharging organic matter, explain how the invertebrate data are consistent with organic pollution and why bloodworms increase while mayflies decrease.
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Organic matter from the estate, such as sewage or garden runoff, is decomposed by bacteria, which use dissolved oxygen. Mayfly nymphs are intolerant of low oxygen, so their numbers fall. Bloodworms, which are chironomid larvae, tolerate low oxygen and feed on organic detritus, so they can increase. The combination - sensitive taxa lost and tolerant detritivores abundant - is consistent with organic pollution. However, it is correlational: the silt and the storm are confounders, and one sample cannot prove the estate is the source. Chemical measurements and repeated sampling would strengthen the case.