1. 1
   The Brazilian Legal Amazon originally held about 4.10 million km² of rainforest. Satellite surveys show that about 3.28 million km² of forest remained in 2022. Air moving westwards from the Atlantic carries water vapour over the forest; rain falls, is taken up by trees and is transpired back into the air, so the same water may fall as rain several times before reaching the Andes. Models suggest that if too much forest is lost, large parts of the remaining forest could turn into dry savanna, but estimates of the forest loss at which this happens vary widely.

   (a)

   What was the percentage change in forest area between the original extent and 2022?

   [1 mark]

   • A−80%
   • B−25%
   • C−20%
   • D+20%

   (b)

   Why is a very large area of rainforest needed to maintain rainfall across the Amazon basin?

   [1 mark]

   • AA large area of trees is needed to produce most of the oxygen in the global atmosphere
   • BTree roots absorb rain directly from clouds
   • CLarge forests block winds from the Atlantic so that rain falls near the coast
   • DTranspiration from a large area of forest generates the water vapour that is recycled as rain further inland

   (c)

   Explain how continued deforestation could push the Amazon rainforest past a tipping point.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A river flows past arable fields that were heavily fertilised with nitrate and phosphate. A week after heavy rain in late summer, samples were taken at site 1, upstream of the fields, and site 2, 2 km downstream of them. At site 1 the nitrate concentration was 1.5 mg dm⁻³, there were few algae, dissolved oxygen was 9.2 mg dm⁻³, the biochemical oxygen demand (BOD) was 2 mg dm⁻³ and mayfly larvae were common. At site 2 the nitrate concentration was 11 mg dm⁻³, thick mats of algae covered the surface, dissolved oxygen was 3.4 mg dm⁻³, the BOD was 14 mg dm⁻³ and no mayfly larvae were found, although red midge larvae (bloodworms) were abundant.

   (a)

   What does the higher BOD at site 2 indicate?

   [1 mark]

   • AMicroorganisms at site 2 consume more oxygen, mainly while decomposing organic matter
   • BAlgae at site 2 are releasing more oxygen by photosynthesis
   • CNitrate ions react directly with dissolved oxygen
   • DWater at site 2 can hold more dissolved oxygen

   (b)

   Which sequence best explains the conditions at site 2?

   [1 mark]

   • ARapid algal growth → leaching of fertiliser → dissolved oxygen rises → algae die
   • BLeaching of fertiliser → rapid algal growth → algae die → aerobic bacteria decompose them → dissolved oxygen falls
   • CLeaching of fertiliser → nitrate poisons algae → algae decompose without bacteria → dissolved oxygen falls
   • DAlgae die → leaching of fertiliser → aerobic bacteria die → dissolved oxygen falls

   (c)

   Explain the absence of mayfly larvae at site 2 compared with site 1.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Mercury released by coal burning and gold mining enters the sea, where bacteria convert it to methylmercury. Methylmercury is absorbed by plankton, stored in body tissues and not readily excreted. In one area of ocean, the mean mercury concentration was 0.01 ppm in phytoplankton, 0.05 ppm in zooplankton, 0.2 ppm in herring, which feed on zooplankton, and 1.2 ppm in tuna, which eat herring and other fish. Methylmercury damages the developing nervous system of a fetus. A health agency advises pregnant women not to eat tuna often, but says that sardines, which feed mainly on plankton, are a safer choice.

   (a)

   Calculate how many times greater the mercury concentration is in tuna than in phytoplankton, and explain why the concentration increases along the food chain.

   [3 marks]

   (b)

   Evaluate the health agency's advice.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The northern cod stock off Newfoundland, Canada, was fished for centuries. In 1962 its spawning stock (mature fish) was estimated at about 1.6 million tonnes. From the late 1960s, factory trawlers dragging large nets along the seabed caught up to 800 000 tonnes of cod a year, including many young fish; cod do not mature until they are 5–7 years old. Stock estimates in the 1980s relied partly on the catch per trawler, which stayed high because trawlers used sonar to find the remaining shoals. By 1992 the spawning stock had fallen to about 22 000 tonnes and a fishing ban was introduced. Thirty years later the stock had recovered only partly. After the collapse, populations of snow crab and northern shrimp, which cod eat, increased greatly.

   (a)

   Using the data, explain why the harvesting of northern cod was not sustainable.

   [6 marks]

   (b)

   Discuss the effects of removing cod on the stability of this marine ecosystem, including whether cod could be considered a keystone species.

   [6 marks]

   Total for question 4: 12 marks

End of questions