A farm changes how it stores livestock waste to reduce run-off into a stream. The table shows mean results from three samples at each site, taken with the same sampling effort before and after the change.
| Measurement | Upstream before change | Upstream after change | Downstream before change | Downstream after change |
|---|---|---|---|---|
| Dissolved oxygen in water, in mg per dm³ | 8.4 | 8.3 | 4.1 | 7.6 |
| Bloodworms, mean number per sample | 2 | 2 | 26 | 9 |
| Freshwater shrimps, mean number per sample | 16 | 15 | 3 | 13 |
| Stonefly nymphs, mean number per sample | 12 | 12 | 0 | 7 |
Explain whether the results provide strong evidence that reducing livestock-waste run-off improved water quality downstream.
101 exam-style questions on Edexcel GCSE Biology 9.3 Material cycles and decomposition, covering 9.3.1 Cycling of materials through ecosystems, 9.3.2 The carbon cycle, 9.3.3 The water cycle, 9.3.4 The nitrogen cycle, 9.3.5 Indicator species and pollution, 9.3.6 Decomposition in preservation and composting, and 9.3.7 Rate changes in decay of biological material. Each one has a worked solution and a mark scheme showing where the marks go.