May/June 2026 Paper 21

2026 · 40 questions · 40 parts · 40 marks

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39Human Impacts EnvironmentMedium1 mark
The graph shows the relative concentrations of three substances in the water of a river. The three substances are oxygen, algae and mineral ions. The graph also shows where fertiliser is released into the river.
Graph of relative concentration of three substances against distance along the river, with the fertiliser outflow marked; after the outflow, line X (solid) rises to a peak and then falls, line Y (dashed) falls to a minimum and then rises again, and line Z (dashed) falls steadily
Which substances do lines X, Y and Z represent?
XYZ
Aalgaeoxygenmineral ions
Bmineral ionsalgaeoxygen
Coxygenalgaemineral ions
Doxygenmineral ionsalgae