Some students investigate the impact of treated sewage effluent on the growth of floating aquatic plants. They select water lettuce (Pistia stratiotes), a free-floating plant with prominent rosettes that are easy to count.
The students use the following procedure:
Their experimental results are summarised below:
| Beaker | Contents | Number of water lettuce plants after 141414 days |
|---|---|---|
| 1 | 500 cm3500\text{ cm}^3500 cm3 of deionised water | 666 |
| 2 | 450 cm3450\text{ cm}^3450 cm3 of deionised water + 50 cm350\text{ cm}^350 cm3 of effluent | 121212 |
| 3 | 400 cm3400\text{ cm}^3400 cm3 of deionised water + 100 cm3100\text{ cm}^3100 cm3 of effluent | 242424 |
| 4 | 350 cm3350\text{ cm}^3350 cm3 of deionised water + 150 cm3150\text{ cm}^3150 cm3 of effluent | 484848 |
Explain why the students adjusted the volume of deionised water in each beaker.
To optimize their experimental setup, the students want to ensure that carbon dioxide availability does not limit the rate of photosynthesis. State how they could achieve this.
Water lettuce reproduces vegetatively by producing stolons (runners) that form daughter plants. State the term used for this mode of reproduction.
In beaker 4, starting with 333 plants, determine how many rounds of population doubling occurred to reach 484848 plants.
An ecological report claims that effluent discharge causes massive phytoplankton blooms which block light, thereby severely inhibiting the growth of benthic (rooted, bottom-dwelling) pond weeds.
Explain the results of the students' experiment with water lettuce, and discuss why using a floating plant like water lettuce yields results that differ from the harmful effects on benthic weeds reported in the literature.
The students decide to repeat their investigation using a benthic pond weed, such as Canadian pondweed (Elodea canadensis).
Describe one method they could use to quantitatively measure the growth of this submerged plant.
25 exam-style questions on OCR GCSE Biology 6.1 Monitoring and maintaining the environment, covering 6.1.1 Field investigation of distribution and abundance, 6.1.2 Human interactions and biodiversity, 6.1.3 Maintaining biodiversity, and 6.1.4 Environmental change and distribution of organisms. Each one has a worked solution and a mark scheme showing where the marks go.