Biomass
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of food chains from GCSE. You can test your knowledge on this below.
What is meant by a trophic level in a food chain?
A trophic level refers to the position an organism occupies in a food chain, such as producer, primary consumer, or secondary consumer.
What is the role of producers in an ecosystem?
Producers, such as green plants and algae, convert light energy into chemical energy through photosynthesis and provide energy for all other trophic levels.
What is the ultimate source of energy for most ecosystems?
The Sun is the ultimate source of energy for most ecosystems.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains biomass or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Is Biomass?
- Biomass refers to the total mass of living material in a specific area at a given time. Mass of carbon is assumed to be 50% of the biomass.
- It represents the chemical energy stored in organic compounds such as carbohydrates, proteins, and lipids within organisms.
- Biomass provides an estimate of the potential energy available to organisms in the next trophic level of a food chain.
- Biomass is often used to assess productivity in ecosystems and to evaluate how efficiently energy is transferred between trophic levels.
How Is Biomass Measured?
There are two main methods used to measure biomass:
1. Measuring Dry Mass
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Organisms are first dried in an oven at around 80°C until they reach a constant mass. This ensures all water has been removed.
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The dry mass of several individuals is usually taken to calculate a mean dry mass.
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This method provides the mass of organic material, excluding water, which varies between organisms and environmental conditions.
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Dry mass is more reliable than wet mass for comparing energy stores, as water content can fluctuate significantly.
2. Using Calorimetry
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In bomb calorimetry, a sample of dry biomass is burned completely in a sealed container called a bomb calorimeter.
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The energy released heats a known volume of water, and the temperature rise is used to calculate the chemical energy in the biomass.
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The result is expressed in kilojoules per gram (kJ g⁻¹) of dry biomass.
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This method allows scientists to determine how much energy is available at a particular trophic level.
Bomb calorimeter
Sources of Error in Biomass Calculations
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Some water may remain in samples if drying is not complete, leading to an overestimation of dry mass.
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Heat losses during combustion in calorimetry (e.g. heat escaping from the system) may result in energy being underestimated.
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Sample size may be too small or not representative of the population, reducing reliability.
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Variation in the metabolic state or life stage of organisms (e.g. young vs adult) may introduce inconsistencies.
Why Biomass Is Given per Unit Area and per Unit Time
- Biomass is usually expressed as grams per square metre per year (g m⁻² yr⁻¹) for land ecosystems or grams per cubic metre per year (g m⁻³ yr⁻¹) in aquatic ecosystems.
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“Per unit area” allows fair comparison of ecosystems of different sizes, making it easier to standardise data (e.g. comparing a forest to a meadow).
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“Per unit time” (usually per year) accounts for seasonal variation and shows how much biomass is produced or transferred over time.
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This standardisation helps scientists measure productivity, such as net primary productivity (NPP), and compare energy flow across different ecosystems.
Key Terms
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Biomass: The total mass of living organisms in a given area at a given time.
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Dry mass: The mass of biological material once all water has been removed.
- Calorimetry: A method used to determine the chemical energy stored in biomass by measuring heat released during combustion.
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Productivity: The rate at which energy is stored in biomass.
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Exam Tips
Be prepared to explain why calorimetry may lead to a lower value of the productivity of a trophic level.
When answering biomass questions, refer to units such as g m⁻² yr⁻¹ Remember MAT = Mass per Area per Time
Be ready to link biomass to concepts like energy transfer efficiency and productivity in food chains.
No answer provided.
Describe how you would measure the chemical energy stored in biomass and explain why the result is expressed in kilojoules per square metre per year. (4 marks)
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The sample of biomass would be dried to a constant mass to remove all water content.
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A known mass of the dry sample would then be combusted in a bomb calorimeter.
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The energy released would be used to heat a known volume of water, and the temperature rise would be measured to calculate energy content.
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The result is expressed in kilojoules per square metre per year (kJ m⁻² yr⁻¹) to standardise comparisons between ecosystems and account for seasonal changes.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!