5.3.5 Net Primary Production

• During photosynthesis, primary producers (such as plants and algae) convert light energy into chemical energy in biological molecules
• Gross primary production (GPP) can be defined as the amount of chemical energy stored in the carbohydrates within plants (during photosynthesis)
  • In other words, the quantity of energy stored in the glucose produced represents the gross primary production

• Net primary production (NPP) refers to the amount of energy available to herbivores in the plant’s biomass after plant respiratory losses
  • Of total energy trapped in glucose during photosynthesis, 90% of this energy will be released from glucose to create ATP for the plant
  • The plant uses most of its glucose to fuel active cellular processes
  • A significant percentage of the energy originally captured will not be used to create new plant cells and therefore will not be able to be passed on to herbivores via eating

• The chemical energy that is leftover in a plant after respiratory loss is known as the net primary production (NPP)
• Net primary production can therefore be calculated using the equation below:

NPP = GPP - R

• This means that NPP, like GPP, is expressed in units of energy per unit area or volume, for example:
  • Using area: J m^–2 (joules per square metre)
  • Using volume: J m^–3 (joules per cubic metre)

• NPP is important because it represents the energy that is available to organisms at higher trophic levels in the ecosystem, such as primary consumers (herbivores and omnivores) and decomposers

Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account

Net primary productivity

• Net 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)