Ecosystem

Net primary productivity (NPP) is the biomass energy that remains with producers after subtracting the energy consumed in their own respiration from the gross primary productivity (GPP). The relationship is NPP = GPP − R, where R is respiratory loss. Substituting the given values, NPP = 2,000 − 1,200 = 800 g per square metre per year. This NPP is the amount of organic matter actually available to support consumers (herbivores and decomposers). The value 3,200 is wrong because it adds GPP and respiration instead of subtracting, which is conceptually invalid. The value 1,200 simply restates the respiratory loss, not the net production. The value 2,000 equals the gross productivity and ignores respiratory consumption altogether. As described in NCERT Class 12, Chapter 14 (Ecosystem), GPP is the total rate of organic matter production during photosynthesis, while NPP is what is left for the next trophic level. Productivity also depends on the photosynthetic capacity of producers and on environmental factors such as light, temperature, and nutrient availability, which is why oceans, despite covering most of the planet, have a relatively low annual net productivity. Plausibility check: since respiration must always consume part of the gross production, NPP must be smaller than GPP, and 800 being less than 2,000 confirms the answer is biologically consistent.