# 5.3.4 Gross Primary Production

### Gross Primary Production

• During photosynthesis, primary producers (such as plants and algae) convert light energy to chemical energy in biological molecules
• The storing of this chemical energy as plant biomass in a given area is known as the gross primary production (GPP)
• This means gross primary production is expressed in units of mass per unit area, for example:
• g m–2 (grams per square metre)
• kg km-2 (kilograms per square kilometre)
• In this case, ‘area’ refers to the area of land that is being studied (this land contains the primary producer(s) that are producing the biomass – if there are no primary producers present in this area of land, there will be no gross primary production)
• Gross primary production can also be expressed in terms of how much stored chemical energy this plant biomass represents
• This means gross primary production can also be expressed in units of energy per unit area, for example:
• J m–2 (joules per square metre)
• kJ km-2 (kilojoules per square kilometre)
• In aquatic environments, it may be more suitable to measure gross primary production per unit volume
• For example, for aquatic algae, gross primary production could be given in:
• kg m-3 (kilograms per cubic metre)
• kJ m-3 (kilojoules per cubic metre)

### Maths Skill: Calculating Gross Primary Production

• Given the appropriate data, it is possible to calculate gross primary production and to derive the appropriate units

#### Worked Example

The total biomass of the grass that grows in a 200 m2 field is found to be 1,000 kg. Calculate the gross primary production of the grass field. Give appropriate units.

Step 1: Calculate the total yearly biomass of grass in 1 m2 of the field

1,000 ÷ 200 = 5 (kg)

Step 2: Give the appropriate units

5 kg m-2

#### Worked Example

On average, a patch of rainforest covering an area of 1 km2 is estimated to contain 1,500 kg of biomass. Calculate the gross primary production of this rainforest patch. Give your answer in g m-2.

Step 1: Calculate the average biomass of 1 m2 of the rainforest patch (1 km2 = 1,000,000 m2)

1,500 ÷ 1,000,000 = 0.0015 (kg)

Step 2: Convert this into grams

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 5,440 joules of chemical energy. The tank has a volume of 4 m3. Calculate the gross primary production of this aquatic algae. Give appropriate units.

Step 1: Calculate the chemical energy of the biomass of aquatic algae in 1 m3

5,440 ÷ 4 = 1,360 (J)

Step 2: Give the appropriate units

1,360 J m-3 OR 1.36 kJ m-3

#### Exam Tip

Be careful what units are being used to describe gross primary production and when answering questions on GPP, make sure you give the appropriate units. GPP can either be expressed in terms of biomass (per unit area) or chemical energy (per unit area). The biomass of an organism is effectively a measure of how much chemical energy is stored within it! ### Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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