A marine ecosystem contains phytoplankton (producers), krill (primary consumers), fish (secondary consumers) and seals (tertiary consumers). Explain why the biomass available to seals is much less than the biomass of the phytoplankton.

OCR A-Level Biology B — Advancing Biology (H422) — 4.3 Energy budgets and ecosystems · Explain · 5 marks · View as Markdown

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

Phytoplankton are microscopic photosynthetic organisms that form the base of many ocean food chains. Despite phytoplankton being extremely abundant, the seals that feed at the top of the food chain are far fewer in number and represent a much smaller total biomass.

Model answer (5 marks)

At each trophic level a large part of the biomass is used for respiration, so only a small fraction is retained. In addition, most of the remaining biomass is egested as faeces and is lost from the food chain. Consequently, only a small proportion of the phytoplankton biomass is transferred to krill, and an even smaller proportion is transferred from krill to fish. This loss at every step means that the biomass that reaches the tertiary consumers (seals) is very little compared with the original phytoplankton biomass.

Examiner tips

  • Use the word ‘loss’ or ‘reduction’ to show understanding of energy transfer. Mention respiration and egestion explicitly. Show the cumulative effect across three trophic levels. Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Failing to mention both respiration and egestion as sources of loss. Assuming that all phytoplankton biomass is available to seals. Writing a long paragraph instead of a short, point‑style answer.

Mark scheme (5 marks)

  1. Organisms use energy/biomass for their own respiration at each trophic level
  2. Biomass is lost through egestion (as faeces) at each trophic level
  3. Only a small proportion of biomass is transferred from one trophic level to the next
  4. This reduction in biomass occurs at each stage/trophic level along the food chain
  5. By the time biomass reaches the seals (tertiary consumers), very little of the original phytoplankton biomass remains available

Key terms in this question

biomass · producer · tertiary consumer

Related

More Energy budgets and ecosystems questions

▶ Try answering this question with AI marking (free) →