Ecosystems and material cycles (Extended response)

Question 1

Explain why farmers are not advised to spread fertilisers on their crops when heavy rain is due. [6]

Answer 1

• fertilisers contain nitrates and other mineral ions that plants need for healthy growth
• mineral ions in fertilisers dissolve in water
• absorbed through the soil by plant roots
• heavy rain will wash the mineral ions away from the crops and into nearby water, such as a river
• this means there will be fewer mineral ions available for the crops so they will not grow as well
• it will be a waste of money for the farmer
• extra minerals in the water courses will cause eutrophication
• rapid growth of algae and water plants
• extra growth blocks light to plants deeper underwater
• these plants die
• bacteria that decompose dying plants will multiply in number as they decompose the dead plant material
• they will take more oxygen from the water
• a lack of oxygen causes fish and other animals to die
• reduces biodiversity

Question 2

Wild salmon take up to five years to reach adult size. Farmed salmon are kept in conditions so they reach this size in less than two years. Explain the impact of fish farming on ecosystems. [6]

Answer 2

• farmed fish specially bred for rapid growth
• farmed fish treated with antibiotics and other medicines to keep them healthy
• farmed fish supplied with plenty of food
• some of the food and medicines given to farmed fish get out into the environment
• mineral ions from food will encourage growth of water plants which could change the environment

-medicines that kill organisms (e.g. parasites) could reduce biodiversity in the environment

• growing farmed fish reduces impact of fishing wild salmon, so maintains biodiversity
• growing salmon produces food for growing human population