• 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

   

• • 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)

     

    Gross primary production can be expressed in:

• 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