The amount of biomass produced per unit area over a period of time is called primary productivity expressed in weight $(\mathrm{g}–2)$ or energy $\left(\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{ }{\mathrm{m}}^{–2}\right)$ . The rate at which this biomass is produced is called productivity. To compare the productivity of different ecosystems it is expressed by units $\mathrm{g}–2\mathrm{ }\mathrm{y}\mathrm{r}\mathrm{ }–1$ or $\left(\mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{ }\mathrm{m}–2\right)\mathrm{ }\mathrm{y}\mathrm{r}\mathrm{ }–1$ . It is divided into gross primary productivity and net primary productivity. The rate at which organic matter is produced during photosynthesis is gross primary productivity and the remaining biomass after the respiratory loss is net primary productivity.