1. 1
   A coastal fish farm keeps Atlantic salmon in large net cages in a sea loch. Waste food and faeces from the salmon sink to the loch bed beneath the cages. Local residents have noticed that the water near the cages sometimes turns green and cloudy, and dead fish occasionally wash ashore further down the loch.

   (a)

   Which term describes the enrichment of water with nutrients from waste, leading to excessive growth of algae?

   [1 mark]

   • AEutrophication
   • BBioaccumulation
   • CDesertification
   • DDenitrification

   (b)

   The dead fish found further down the loch were later found to have died from a lack of dissolved oxygen in the water. Which of the following best explains why decaying algae reduces the dissolved oxygen available to fish?

   [1 mark]

   • AAlgae absorb oxygen only during the day, so fish suffocate at night regardless of algae
   • BAlgae release carbon dioxide directly into the fishes' gills as they decay
   • CDead algae block sunlight so fish cannot photosynthesise
   • DDecomposing bacteria feeding on the dead algae respire aerobically and use up the dissolved oxygen

   (c)

   Describe two negative impacts that this fish farm could have on the biodiversity of the sea loch, other than the eutrophication event described above.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In the 1930s, American mink were brought to the UK to be farmed for their fur. Some mink escaped from fur farms and established wild populations in British rivers. Since then, numbers of the native water vole, a small mammal that lives along riverbanks, have fallen sharply in areas where mink are present.

   (a)

   Which term describes the American mink in this situation?

   [1 mark]

   • AA non-indigenous (introduced) species
   • BA keystone species
   • CAn indicator species
   • DA decomposer

   (b)

   Which of the following is the most likely biological reason why the introduction of American mink has caused water vole numbers to fall so sharply?

   [1 mark]

   • AMink release toxins into the river water that poison water voles
   • BMink compete with water voles for the same plant food, starving the voles
   • CMink are an effective predator of water voles and voles have no evolved defences against them
   • DMink destroy the riverbank vegetation that water voles use for shelter, but do not prey on them directly

   (c)

   Suggest two reasons why maintaining the biodiversity of British river ecosystems, including species such as the water vole, is important.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hillside in Wales was cleared of native oak woodland 60 years ago to create grazing pasture for sheep. A conservation charity has now bought the land and plans to reforest it by planting native oak, birch and rowan saplings.

   (a)

   Explain how reforesting this hillside could improve local biodiversity.

   [3 marks]

   (b)

   The charity is deciding between two reforestation strategies: planting only fast-growing non-native conifers, or planting a mixture of native broadleaf species as described above. Evaluate which strategy is likely to be better for local biodiversity, and explain why.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A shallow freshwater lake used for recreational fishing has, over the past ten years, received run-off from surrounding farmland containing fertiliser rich in nitrates and phosphates. Anglers report that the lake now regularly develops thick green algal blooms in summer, the water has become cloudy, and the number of fish they catch has fallen considerably. A local council wants a full explanation of the biological process that has caused the decline in fish, from the arrival of fertiliser run-off to the death of fish, in order to plan a response.

   (a)

   Explain, step by step, how the fertiliser run-off has led to the fall in fish numbers.

   [6 marks]

   (b)

   The council is considering two options to reduce future eutrophication of the lake: (1) planting a strip of grass and shrubs (a buffer zone) between the farmland and the lake shore, or (2) regularly dredging the lake to physically remove algae once blooms appear. Evaluate the effectiveness of each option in preventing eutrophication and protecting fish numbers in the long term, and recommend which option the council should prioritise.

   [6 marks]

   Total for question 4: 12 marks

End of questions