The primary productivity of an ecological system is defined as the rate at which radiant energy is converted by the photosynthetic and chemosynthetic activity of producer organisms (chiefly green plants) to organic substances. In effect, it is the rate at which plants can use photosynthesis to make more plant material (biomass). However, to stay alive, grow, and reproduce, an ecosystem's producers must use some of the total biomass they produce for their own respiration. Only what is left, called net primary productivity (NPP), is available for use as food by other organisms (consumers) in an ecosystem.
NPP, usually reported as the energy output of producers in a specified area over a given time (typically as kilocalories per square meter per year), is the basic food source or 'income' for an ecosystem's consumers. It is the rate at which energy is stored in new biomass - cells, leaves, roots, and stems are available for use by consumers.
An estimated 59% of Earth's annual NPP occurs on land; the remaining 41% is produced in oceans and other aquatic systems. Estuaries, swamps and marshes, and tropical rainforests are highly productive; open ocean, tundra (arctic and alpine grasslands), and deserts are the least productive.
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