Gross Primary Production (AQA A Level Biology): Revision Note
Exam code: 7402
Gross primary production
Gross primary production (GPP) can be defined as:
the chemical energy stored in plant biomass, in a given area or volume
Gross primary production in terrestrial plants can be expressed as:
energy per unit area, e.g. J m–2
mass per unit area, e.g. g m–2
biomass = stored chemical energy, so a measure of mass can be used to represent energy
Gross primary production in aquatic environments can be expressed as:
energy per unit volume, kJ m-3
mass per unit volume, e.g. kg m-3
Examiner Tips and Tricks
Remember that GPP is the stored chemical energy or biomass present in a given area or volume, so its units should include energy/mass, and area/volume.
If a unit also includes time, then this is a measure of productivity, which is not the same as production.
Calculating gross primary production
It is possible to calculate gross primary production and to determine the appropriate units
Remember that gross primary production is not a rate and so does not need to include time
Worked Example
The total biomass of the grass that grows in a 200 m2 field is found to be 1000 kg.
Calculate the gross primary production of the grass field. Give appropriate units.
Answer:
Step 1: calculate the biomass of grass in 1 m2 of the field
1m2 is 1/200th of the field, so we divide the total field biomass by 200:
1000 ÷ 200 = 5 kg
Step 2: give the appropriate units
Primary production is a measure of biomass per unit area or volume, so in this instance our units must include a mass and an area
5 kg m-2
Worked Example
A patch of rainforest covering an area of 1 km2 is estimated to contain 1500 kg of biomass.
Calculate the gross primary production of this rainforest patch. Give your answer in g m-2.
Answer:
Step 1: calculate the biomass of 1 m2 of the rainforest patch
1 km2 = 1000 x 1000 = 1 000 000 m2
1500 ÷ 1 000 000 = 0.0015 kg
Step 2: convert this into grams
1 kg = 1000 g
0.0015 × 1 000 = 1.5 g m-2
Worked Example
The biomass of aquatic algae in a tank is estimated to contain a total of 5440 joules of chemical energy. The tank has a volume of 4 m3.
Calculate the gross primary production of this aquatic algae. Give appropriate units.
Answer:
Step 1: calculate the chemical energy of the aquatic algae in 1 m3
5440 ÷ 4 = 1360 J
Step 2: give the appropriate units
Note that we are dealing with aquatic plants, so the units will be volume rather than area
1360 J m-3 OR 1.36 kJ m-3
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