Read the following passage carefully and answer the given questions.
Productivity in the Ecosystem
The flow of energy through a terrestrial ecosystem starts with the harnessing of sunlight by autotrophs. The radiant energy is converted by photosynthesis to organic compounds is known as productivity. The productivity is expressed in units of energy per unit of area per unit time. The amount of organic matter present at any given time is biomass. The productivity of any ecosystem is affected by many parameters like weather conditions, nutrient availability etc. The productivity of any ecosystem does vary seasonally and yearly.
Identify the correct statement in relation to productivity in an aquatic ecosystem.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The GPP is always more than NPP
Option (1) The GPP is always less than NPP — Incorrect. NPP is what is left of GPP after respiration, so it can never be larger than GPP.
Option (2) The GPP is always more than NPP — Correct. Plants always use part of what they make for their own respiration, in water and on land alike. So GPP minus respiration (NPP) is always smaller than GPP.
Option (3) The GPP is always equal to NPP — Incorrect. The two would be equal only if plants did not respire, which never happens.
Option (4) No relation between GPP and NPP — Incorrect. They are linked by a simple equation, NPP = GPP − R.
NCERT Extract:
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. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
GPP – R = NPP
Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers).”