Two fish-rearing tanks receive the same amount of food and produce fish waste. Water from tank A passes through a filter containing active decomposer and nitrifying bacteria. Water from tank B passes through an otherwise identical filter without active bacteria. The filters are the only difference between the tanks. After eight weeks, equal volumes of water from each tank are used to water separate batches of identical seedlings. The seedlings are grown under the same conditions, and nitrate is the limiting mineral ion for their growth.
| Time / weeks | Nitrate concentration in tank A water / mg L⁻¹ | Nitrate concentration in tank B water / mg L⁻¹ |
|---|---|---|
| 0 | 4 | 4 |
| 4 | 10 | 5 |
| 8 | 18 | 6 |
Explain why the difference in nitrate concentration between the tank waters changes during the eight weeks.
Use the data to predict which batch of seedlings will have the greater dry mass. Explain your answer.
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.