Measuring biomass at Higher Level: The same method as SL, with different real cases: kelp on the Norwegian coast, desert locusts and farmed tilapia. At HL the same dry-mass records are used to find secondary productivity (2.2.24).
Practise this as you read
- Describe drying to constant mass and scaling up.
- Outline the food-minus-faeces method for an animal.
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Weigh it dry: Biomass is the mass of the organic matter in living things. Water is most of the rest, so we dry samples and weigh what is left.
The points to remember
- Biomass is the mass of organic matter in an organism or a trophic level.
- Most of an organism's inorganic mass is water, and the amount of water changes from hour to hour.
- Drying removes the water, so dry mass ≈ biomass (the organic matter).
- Water holds no usable food energy, so dry mass also shows the energy stored.
- Give it per unit area, e.g. g per m², so different places can be compared.
Remember it as: Take out the water; what is left is the biomass.
Real example: a moon jellyfish is about 96% water, so 100 g of fresh jellyfish leaves only about 4 g of dry mass, while 100 g of fresh oak wood leaves about 55 g.
Fresh mass misleads: Fresh mass depends on how much water an organism happens to hold, for example after rain. Always compare biomass as dry mass per m².
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Collect
Take all the organisms in a measured area.
Dry
Oven at about 60-80 °C.
Weigh
Reweigh until the mass stays the same.
Scale up
Mean per m² × total area.
The method, step by step
- Collect a sample: e.g. all the plants in random quadrats.
- Dry it in an oven at about 60-80 °C: hot enough to drive off water, not so hot that it burns.
- Weigh, dry again and reweigh until the mass stays the same: constant mass.
- Work out the mean dry mass per m².
- Extrapolate: multiply by the total area to estimate the whole trophic level.
Real example: off the coast of Norway, divers collect all the kelp in 1 m² frames. A 200 g sample dries to a constant 24.0 g, so the kelp is about 88% water.
Why 'constant mass'?: A sample can look dry and still hold water. Only when two weighings give the same mass has all the water gone.
To build a pyramid of energy (2.2.17) we need the energy in the biomass, not just its mass. The energy is found by burning a small dried sample and scaling up.
Energy from biomass
- Burn a dried sample completely in a calorimeter.
- The heat released warms the water; the temperature rise gives the energy per gram.
- Energy per m² = energy per gram × dry mass per m².
- Scale up (extrapolate) to the whole area or trophic level.
The calculation: Energy per m² = energy released per gram of dry mass × dry mass per m²
Worked example: dry kelp releases 12 kJ per gram when burned. A kelp forest holds 800 g of dry kelp per m², so it stores 12 × 800 = 9 600 kJ per m².
Plant material in class: In school, burn only small dried samples of plant material, such as leaves or seeds, in a simple calorimeter; the energy from a whole field is then an estimate.
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Drying also lets us measure what an animal gains from its food. Its gross productivity is the food it eats minus the faeces it passes out (GP and NP are explained in 2.2.13).
Gross productivity of animals, in the laboratory
- Dry and weigh the food given at the start.
- After a set number of days, dry and weigh the food left: food eaten = given - left.
- Collect, dry and weigh the faeces produced over the same days.
- Gross productivity = food eaten - faeces (the food absorbed).
- Divide by the number of days to give biomass per day.
- Do not weigh the animals themselves: their gain in mass is net productivity.
Worked example: food eaten = 30.0 - 12.0 = 18.0 g. Gross productivity = 18.0 - 5.0 = 13.0 g in 5 days, so 13.0 ÷ 5 = 2.6 g per day for the 30 locusts.
A common slip: Weighing the animals at the start and the end measures their net productivity, not their gross productivity. For gross productivity, weigh the food and the faeces.
Drying is simple and accurate for small plants, but it has real drawbacks.
Limits of the method
- It is destructive: the organisms are killed, which may be unethical, especially for animals.
- Large organisms such as trees cannot be dried whole: only parts are sampled and scaled up.
- Each sample is a snapshot at one time; biomass changes with the seasons.
- Samples may not represent the whole area, and roots or mobile animals are easily missed.
Remember it as: It kills, it cannot take a tree, it is one moment.
Real example: the biomass of the Amazon rainforest is estimated from the trunk widths and heights of trees in sample plots, using equations from a small number of trees that were cut and dried.
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How this comes up: Paper 2, Section B (a): outline a laboratory method to find the gross productivity of a named aquatic animal population [4]. Section A may ask why dry mass is used [1-2].
A fish farm in Kenya keeps young tilapia in a tank and feeds them on pellets.
Outline the procedures the farm could use to find the gross productivity of the tilapia population in terms of biomass per day.
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