(a) What is the primary productivity of an ecosystem and how is it expressed?

(b) Explain what does the equation given below show:
NPP = GPP - R

(a) Primary productivity is defined as the amount of biomass or organic matter produced per unit area over a time period by
plants during photosynthesis. The rate of biomass production is called primary productivity.
It is expressed in terms of $g^{-2}{\mathrm{yr}}^{-1}$
where g = weight
Or$\left(\mathrm{kcal}{m}^{-2}\right){\mathrm{yr}}^{-1}$
(b) Gross primary productivity (GPP) of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).