# 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

> 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.

## 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

- [biomass](https://www.gradenine.co.uk/glossary/biomass)
- [producer](https://www.gradenine.co.uk/glossary/producer)
- [tertiary consumer](https://www.gradenine.co.uk/glossary/tertiary-consumer)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-marine-ecosystem-contains-phytoplankton-producers-03301705) · Published by Druglandscape Ltd.