# A freshwater lake ecosystem contains the following food chain: aquatic plants → water fleas → small fish → pike. Scientists measured the energy content at each trophic level. The aquatic plants contained 200,000 kJ of energy per unit area. Only 10% of energy is transferred from one trophic level to the next. Explain why only a small percentage of energy is transferred between trophic levels in this food chain, and state two reasons why energy is lost at each stage.

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

> In a freshwater lake ecosystem, energy enters through photosynthesis in aquatic plants and passes along the food chain: aquatic plants → water fleas → small fish → pike. Only around 10% of the energy stored in one trophic level is passed on to the next.

## Mark scheme (5 marks)

1. Much of the energy is used in respiration by organisms at each trophic level
2. Energy is lost as heat to the surroundings during respiration
3. Not all of the organism is eaten / some biomass is not consumed
4. Energy is lost in excretion / faeces / urine
5. Therefore only the energy stored in the biomass of the organism is available to the next trophic level

## Key terms

- [trophic level](https://www.gradenine.co.uk/glossary/trophic-level)
- [food chain](https://www.gradenine.co.uk/glossary/food-chain)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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-freshwater-lake-ecosystem-contains-the-4811036d) · Published by Druglandscape Ltd.