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GPP & NPP

01GPP & NPP

Gross primary production

This section covers gross primary production as the chemical energy stored in plant biomass and the appropriate units for terrestrial and aquatic ecosystems.

What GPP represents

Gross primary production (GPP) is the chemical energy stored in plant biomass within a specified area or volume.

Because biomass represents stored chemical energy, GPP may be expressed using either an energy measurement or a mass measurement.

Environment How GPP is expressed Example units
Terrestrial Energy or biomass per unit area J m−2 or g m−2
Aquatic Energy or biomass per unit volume kJ m−3 or kg m−3

GPP describes production, not a rate of production. Its units therefore do not require a time term.

Exam Tip: Check the units before identifying a value as production or productivity. GPP as a measure of production should include an area or volume, but not a time unit.

Calculating GPP

Worked example: A grass field covering 200 m2 contains 1000 kg of plant biomass. Calculate the gross primary production per unit area.

Step 1: Divide the total biomass by the total area.

1000 kg200 m2=5 kg m−2\frac{1000\ \mathrm{kg}}{200\ \mathrm{m^2}} = 5\ \mathrm{kg\,m^{-2}}

The gross primary production is 5 kg m−2.

Converting units

Calculation questions may require both the area and the biomass to be converted before giving GPP in the requested units.

Worked example: A 1 km2 area of rainforest contains 1500 kg of biomass. Calculate GPP in g m−2.

Step 1: Convert the area into square metres.

1 km2=1000 m×1000 m=1 000 000 m21\ \mathrm{km^2} = 1000\ \mathrm{m}\times1000\ \mathrm{m} = 1\,000\,000\ \mathrm{m^2}

Step 2: Find the biomass per square metre.

1500÷1 000 000=0.0015 kg m−21500 \div 1\,000\,000 = 0.0015\ \mathrm{kg\,m^{-2}}

Step 3: Convert kilograms to grams.

0.0015×1000=1.5 g m−20.0015\times1000 = 1.5\ \mathrm{g\,m^{-2}}

The gross primary production is 1.5 g m−2.

For aquatic producers, use volume instead of area. If algae in a 4 m3 tank contain 5440 J of chemical energy:

5440÷4=1360 J m−3=1.36 kJ m−35440 \div 4 = 1360\ \mathrm{J\,m^{-3}} = 1.36\ \mathrm{kJ\,m^{-3}}

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