How does gross primary productivity differ from net primary productivity?

Hint: In a given geographical area, the biotic factors (plants, animals, etc.) and the abiotic factors (climate, water availability, etc.) interact with one another to form an ecosystem. An ecosystem may be terrestrial or aquatic. The stability of any ecosystem depends on four parameters- productivity, decomposition, energy flow, and nutrient cycle.

Complete answer:
The biotic factors of an ecosystem include plants, animals, and microorganisms. The green plants (terrestrial ecosystem) and phytoplankton (algae; aquatic ecosystem) are the producers i.e. that are able to prepare their own food. Animals and other microorganisms are consumers. Consumers may be primary, secondary, or tertiary. Consumers prey on other consumers or they eat the plants or phytoplankton.

This consumption pattern forms a food chain. The producers occupy the first trophic level as they begin the food chain. The producer is consumed by a primary consumer. The primary consumer occupies the second trophic level and so on.

Productivity is one of the factors that affect the working of an ecosystem. Productivity refers to the rate at which biomass is produced per unit area at each trophic level. Biomass is the total mass of the organic matter produced.

Plants and phytoplankton can perform photosynthesis to prepare food i.e. carbohydrates. Primary productivity refers to the organic matter synthesized by the producers. The sum total of all the organic matter synthesized by producers is called gross primary productivity (GPP).

The producers also expend energy to perform other cellular functions such as respiration. This requires the breakdown of the organic matter. The organic matter producers are able to save or store is called net primary productivity (NPP). In other words, the difference between gross primary productivity and respiration is the net primary productivity.

GPP depends on factors that affect photosynthesis i.e. intensity of sunlight, the amount of nutrients received, etc. NPP depends on the rate of photosynthesis and the respiration rate.

Note: In the case of producers, the respiration rate is the only cellular function that is taken into consideration. This is because this activity takes the most energy. Also producers do not move or perform other complex cellular functions as animals do.
Secondary productivity is the rate of biomass production by consumers in a given area.