A coral reef ecosystem contains the following food chain: phytoplankton → zooplankton → small reef fish → reef sharks. Scientists estimate that phytoplankton in this ecosystem fix 200,000 kJ of energy per year. Explain why reef sharks receive so little of the energy originally fixed by phytoplankton, and describe what happens to the energy that is not passed on at each trophic level.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.2 Energy flow and ecosystems · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In a coral reef ecosystem, energy is transferred through a food chain: phytoplankton → zooplankton → small reef fish → reef sharks. Phytoplankton fix 200,000 kJ of energy per year through photosynthesis.

Model answer (5 marks)

Only about 10 % of the energy fixed by phytoplankton is transferred to the next trophic level, so the energy that reaches zooplankton is roughly 20 000 kJ. Of this, most is used for respiration (movement, growth, maintenance) and the rest is lost as heat. The remaining energy is excreted as faeces, urine and other waste, which are not eaten by the next level. The same pattern repeats when small reef fish eat zooplankton and when reef sharks eat small fish. Because energy is lost at each of the three transfers, the cumulative loss is very large, leaving only a tiny fraction of the original 200 000 kJ for the sharks. The energy that is not passed on is released into the environment mainly as heat and is also utilised by decomposers that break down dead organisms and waste products.

Examiner tips

  • State the 10 % transfer efficiency and explain the losses at each level; mention respiration, waste and heat; show the cumulative effect; link to decomposers and heat release.

Common mistakes

  • Saying all energy is lost as heat; ignoring waste products; not recognising the cumulative loss across multiple trophic levels.

Mark scheme (5 marks)

  1. Only approximately 10% of energy is transferred from one trophic level to the next (energy transfer is inefficient)
  2. Much of the energy is lost through respiration (used for movement, growth, maintaining body temperature etc.)
  3. Energy is lost in faeces / urine / waste materials that are not consumed by the next organism
  4. Because energy is lost at each of the three transfers (phytoplankton → zooplankton → small reef fish → reef sharks), the cumulative loss means reef sharks receive a very small fraction of the original energy
  5. The energy not transferred enters the environment as heat / is used by decomposers breaking down dead organisms

Key terms in this question

trophic level · food chain

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