Primary productivity measures the rate at which primary producers convert inorganic carbon into new organic matter and chemical energy.

For photosynthetic producers, gross primary productivity (GPP) is the total rate at which photosynthesis captures carbon and energy before subtracting the producers' own respiration.

Primary producers use some of that captured organic matter to power their own metabolism. The remainder is net primary productivity (NPP):

$$\boxed{\mathrm{NPP}=\mathrm{GPP}-R_{\text{producers}}}$$

where $R_{\text{producers}}$ is the rate of producer respiration.

NPP is the rate at which producer biomass becomes available for growth, reproduction and transfer to consumers or detrital pathways.

Worked example

Suppose a grassland fixes carbon equivalent to $2400\ \mathrm{kJ,m^{-2},yr^{-1}}$ through photosynthesis. Producers use $900\ \mathrm{kJ,m^{-2},yr^{-1}}$ in respiration.

Then

$$\mathrm{NPP}=2400-900=1500\ \mathrm{kJ,m^{-2},yr^{-1}}.$$

A common mistake is to confuse productivity with standing biomass. A phytoplankton community can have low biomass at any instant but very high productivity if cells grow and are consumed rapidly. Productivity is a rate of production, whereas biomass is an amount present at a particular time.