Explain productivity and its types.

(N/A) constant input of solar energy is the basic requirement for any ecosystem to function and sustain.
$(i)$ Primary productivity: It is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
It is expressed in terms of weight $(g \cdot m^{-2})$ or energy $(kcal \cdot m^{-2})$.
The rate of biomass production is called productivity. It is expressed in terms of $g \cdot m^{-2} \cdot yr^{-1}$ or $(kcal \cdot m^{-2}) \cdot yr^{-1}$. It is divided into Gross Primary Productivity $(GPP)$ and Net Primary Productivity $(NPP)$.
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 $(NPP)$.
$GPP - R = NPP$
Net primary productivity is the available biomass for consumption by heterotrophs (herbivores and decomposers).
$(ii)$ Secondary Productivity: It is defined as the rate of formation of new organic matter by consumers.
Primary productivity depends on the plant species inhabiting a particular area, environmental factors, availability of nutrients, and the photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems.
The annual net primary productivity of the whole biosphere is approximately $170$ billion tons (dry weight) of organic matter. Despite occupying about $70$ percent of the surface, the productivity of the oceans is only $55$ billion tons, while the rest is on land.