5.3.4 Gross Primary Production

• During photosynthesis, primary producers (such as plants and algae) convert light energy to chemical energy stored within biological molecules
• Gross primary production (GPP) can be defined as the amount of chemical energy stored in the carbohydrates within plants (during photosynthesis)
  • Roughly only 1% of the light falling on a plant is used in photosynthesis to produce glucose
  • 99% of the light either passes through the leaf without hitting chloroplasts, is reflected off of the leaf, or is transferred to heat energy
  • After that 1% is successfully absorbed and used to form glucose, the quantity of energy now stored in glucose is the gross primary production

• Gross primary production can 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 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)

  • Units of mass per unit area, for example
    • g m^–2 (grams per square metre)
    • kg km^-2 (kilograms 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)

Gross primary productivity

• The rate at which plants are able to store chemical energy via photosynthesis is referred to as gross primary productivity
• Gross primary productivity is expressed using units of energy/mass per unit area per unit time
  • Time must be included as it is a rate

• For example
  • Mj m^–2 y^-1 (megajoules per square metre per year)
  • kg km^-2y^-1 (kilograms per square kilometre per year)

• Given the appropriate data, it is possible to calculate gross primary production and to derive the appropriate units
  • Remember that gross primary production is not a rate so does not need to include time

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

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

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