A power station takes water from a river to cool its machinery. It returns the warm water to the river.
Scientists measured some factors at site A, upstream of the power station, and at site B, downstream of the power station.
The Table shows their results.
Table
| Factor | Site A (upstream) | Site B (downstream) |
|---|---|---|
| Water temperature in ∘C^\circ\text{C}∘C | 12 | 19 |
| Concentration of dissolved oxygen in mg per dm3\text{dm}^3dm3 | 10.5 | 6.0 |
| Number of species of invertebrate | 18 | 7 |
| Number of fish caught per 100 m of river | 45 | 12 |
Calculate the percentage decrease in the concentration of dissolved oxygen from site A to site B.
Warm water holds less dissolved oxygen than cold water.
Explain why there are fewer species of invertebrate at site B.
Suggest two reasons why fewer fish were caught at site B.
111 exam-style questions on OCR GCSE Biology 4.1 Ecosystems, covering 4.1.1 Cycling of materials in ecosystems, 4.1.2 Microorganisms in cycling materials, 4.1.3 The carbon and water cycles, 4.1.4 Factors affecting decomposition, 4.1.5 Levels of organisation in an ecosystem, 4.1.6 Abiotic and biotic factors affecting communities, 4.1.7 Interdependence and competition, 4.1.8 Trophic levels, 4.1.9 Pyramids of biomass, and 4.1.10 Efficiency of biomass transfers. Each one has a worked solution and a mark scheme showing where the marks go.