primary productivity

primary productivity(primary production) The rate at which biomass is produced by photosynthetic and chemosynthetic autotrophs (mainly green plants) in the form of organic substances some of which are used as food materials. Gross primary productivity (GPP) is the total rate of photosynthesis and chemosynthesis, including that portion of the organic material produced which is used in respiration during the measurement period. Net primary productivity (NPP) is the rate of production 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, which is a rate) 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 or gross and to primary or community productivity, or to some portion of these, as with a harvested crop. In the oceans, photosynthesis by phytoplanktonic algae in the upper 100 m (the euphotic zone) accounts for most primary production. Waters of tropical areas are less productive than those of temperate regions, because in tropical areas the water column undergoes no seasonal vertical mixing and so becomes oligotrophic. Areas of upwelling of nutrient-rich deep waters have high productivity.