Q.Define the terms:

GPP is the total organic matter fixed by producers, while NPP is what remains after subtracting the producers' own respiratory consumption.

(a) Gross Primary Productivity (GPP) is the total rate at which the producers (autotrophs) of an ecosystem synthesise organic matter (biomass) through photosynthesis per unit area per unit time; it represents the total amount of solar energy fixed and converted to chemical energy.

…

Gross primary productivity (GPP) is the total rate of organic-matter production by producers through photosynthesis; it is expressed as biomass (or energy) per unit area per unit time, e.g. g m$^{-2}$ yr$^{-1}$ or kcal m$^{-2}$ yr$^{-1}$.

Gross primary productivity (GPP) is the rate at which the producers (green plants and other autotrophs) of an ecosystem capture and fix energy and produce organic matter (biomass) during photosynthesis, per unit area, per unit time. It represents the total amount of organic matter produced, including the part that the producers later consume in their own respiration.

A substantial fraction of the GPP is used up by the plants themselves in respiration ($R$). The organic matter that is left after respiratory losses is the net primary productivity (NPP):

$\text{NPP} = \text{GPP} - R$

NPP is the biomass actually available to the next (consumer/heterotroph) trophic level.

…