Net Primary Productivity
- During photosynthesis organisms such as plants convert light energy into chemical energy stored in biological molecules
- Organisms that do this are known as producers
- The rate at which producers convert light energy into chemical energy is known as primary productivity
- Gross primary productivity, or GPP, can be defined as the rate at which chemical energy is converted into carbohydrates during photosynthesis
- Net primary productivity, or NPP, is the GPP minus plant respiratory losses
- Of the total energy stored in glucose during photosynthesis, 90 % will be released from glucose to create ATP for the plant during respiration
- 90% of the energy originally converted by the plant will therefore not be stored as new plant biomass and will not be available to be passed on to herbivores, also known as primary consumers
- The NPP can therefore be defined as the rate at which energy is stored in plant biomass
- NPP is important because it represents the energy that is available to organisms at higher trophic levels in the ecosystem, such as primary consumers and decomposers
- Net primary productivity can be calculated using the equation
NPP = GPP - R
- NPP is expressed in units of energy per unit area or volume per unit time e.g.
- Using area: J m–2 yr-1 (joules per square metre per year)
- Using volume: J m–3 yr-1 (joules per cubic metre per year)
- Volume would be used when calculating NPP in aquatic habitats
Net primary productivity, or NPP, is the rate at which energy is stored in plant biomass and made available to primary consumers.
Worked example
The grass in a meadow habitat converts light energy into carbohydrates at a rate of
17 500 kJ m-2 yr-1. The grass releases 14 000 kJ m-2 yr-1 of that energy during respiration. Calculate the net primary productivity of the grass in the meadow habitat.
Step 1: Work out which numbers correspond to which parts of the equation
The meadow grass converts 17 500 kJ m-2 yr-1 into carbohydrates; this is its GPP
The meadow grass releases 14 000 kJ m-2 yr-1 of that energy in respiration; this is R
Step 2: Substitute numbers into the equation
NPP = GPP - R
NPP = 17 500 - 14 000
Step 3: Complete calculation
17 500 - 14 000 = 3 500
NPP = 3 500 kJ m-2 yr-1
Examiner Tip
The worked example above uses the equation in its basic form, but you may also be expected to rearrange the equation e.g. to calculate GPP or R
- If a question provides you with the NPP and R and asks you to calculate GPP, you will need to use the equation
GPP = NPP + R
- If a question provides you with the NPP and the GPP and asks you to calculate R, you will need to use the equation
R = GPP - NPP
