The rate of synthesis of organic matter, or biomass produced at any trophic level during a given period of time is called productivity. For productivity three things are necessary – (i) Rate of production, (ii) Per unit area and (iii) Unit time. Productivity is of two types– (1) Primary productivity and (2) Secondary Productivity. It is measured as weight (Ex- g/m/yr) or energy (Ex-Kcal/m2/yr).
Primary Productivity – Primary productivity 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–2) or energy (K cal m–2). It can be divided into two types – (a) gross primary productivity and (b) net primary productivity.
(a) Gross Primary Productivity – Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. It is equal to addition weight of stored organic material present in body of producer and loss during different activity happens in its body.
(b) Net Primary Productivity – Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers). Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
G.P.P – R = N.P.P.
Primary productivity depends on the plant species inhabitating a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 percent of the surface, the productivity of the oceans are only 55 billion tons.
