primary productivity

primary productivity (primary production) The rate at which energy is stored by photosynthetic and chemosynthetic autotrophs (mainly green plants) in the form of organic substances that may be used as food materials. Gross primary productivity (GPP) is the total rate of photosynthesis and chemosynthesis, including that portion of the organic material produced that is used in respiration during the measurement period. Net primary productivity (NPP) is the rate of production of surplus organic material after some has been lost to plant respiration during the measurement period. Net community productivity is the rate of accumulation of organic material, allowing for both plant respiration and heterotrophic predation during the measurement period. Rates of storage at higher trophic levels are termed secondary productivities. Strictly, the primary production of an ecosystem (as distinct from its productivity) is the amount of organic material accumulated. It is usual to indicate a period of time, however, since otherwise such data have limited value. Thus, in practice the terms ‘production’ and ‘productivity’ are often used interchangeably. It is also vital to indicate whether production figures relate to net primary or net community productivity, or to some portion of these, as with a harvested crop.