Question

Which of the following statement(s) about productivity is/are incorrect?
I. Gross primary productivity of an ecosystem is the rate of production of organic matter per unit area during photosynthesis.
II. Gross primary productivity is measured in terms of kcal/m²/year
III. Net primary productivity of oceans is much higher than that of land masses on earth.
IV. Gross primary productivity = Net primary productivity - Respiration losses (R)

• A. Only III
• B. II and III
• C. III and IV
• D. Only I

Answer 1

The correct option is C III and IV

A self-sustained stable unit formed by the interactions between the biotic (living) and abiotic (non-living) factors present in it, is known as an ecosystem.

The different components of an ecosystem work as a unit to perform some basic functions that help in the sustenance of the ecosystem. These functions include

• productivity
• decomposition of dead decaying organic matter
• energy flow between different trophic levels
• cycling of nutrients between the abiotic and biotic components of the ecosystem

Productivity of an ecosystem refers to the rate at which organic matter/biomass (sum total mass of living organisms in a specific unit of area or volume of habitat) is produced per unit area.

Primary productivity refers to the rate of production of organic matter, by the process of photosynthesis, per unit area over a time period. It is expressed in terms of weight (g/m²/year) or energy (kcal/m²/year).

Primary productivity can be categorised as gross primary productivity and net primary productivity.

Gross primary productivity (GPP) refers to the rate of production of organic matter during photosynthesis.

Net primary productivity (NPP) refers to the remainder of the gross production which is left after its use by the producers in the process of respiration. Net primary productivity is the biomass that is available to the heterotrophs in an ecosystem for consumption.

Hence, NPP = GPP - Respiration losses (R)

The primary productivity of an ecosystem depends on multiple factors such as plant species inhabiting it, nutrient availability, exposure to sunlight and photosynthetic capacity of the plants, etc.

The annual NPP of the entire biosphere is approximately 170 billion tons (dry weight) of organic matter.

Oceans, despite covering almost 70% of the earth’s surface, contribute only 55 billion tons to this. Thus, net primary productivity of oceans is much less than that of land masses on earth. This is because only the upper layers of the oceans receive adequate sunlight for photosynthesis. Moreover, any dead material sinks to the ocean bed and hence the nutrients released during their decomposition are not available to the aquatic plants which are present at higher levels of the ocean. Extremely low temperatures in the oceans of the polar regions also restrict photosynthetic activity due to inactivated enzymes.

Thus, statements III and IV are incorrect.