Primary Productivity (Gross and Net)
Primary productivity (gross and net)
Primary producers (or autotrophs) are organisms that synthesize their own biochemical constituents using simple inorganic compounds and an external energy source to drive the process. The amount of energy fixed by autotrophs is known as primary production, and the rate of fixation is primary productivity. Both primary productivity and primary production may be measured in units of carbon dioxide (CO2) fixed, energy fixed (as calories or joules), or biomass produced. Productivity in terrestrial ecosystems is often expressed in such units as kilograms of dry biomass (or its energy equivalent) per hectare per year (e.g., kg/ha.yr or kJ/ha.yr), while aquatic productivity is often measured on a volumetric basis (e.g., kg/m3.yr). Many studies have been made of the primary productivity of various kinds of ecosystems.
Most primary producers are photoautotrophs that use sunlight as the external source of energy to drive photosynthesis . Photoautotrophs capture solar radiation using photosynthetic pigments, such as chlorophyll. Green plants are the most abundant photosynthetic organisms, along with algae and some bacteria. A much smaller number of autotrophs are chemoautotrophs, which capture some of the energy content of certain inorganic chemicals to drive their chemosynthesis . For example, Thiobacillus thiooxidans is a bacterium that oxidizes sulfide minerals to sulfate, utilizing some of the energy liberated during this reaction to drive its chemosynthesis.
The total fixation of solar (or chemical) energy by primary producers within an ecosystem is known as gross primary production (or GPP). Some of this production is utilized by autotrophs in support of their own respiration (R). Respiration involves physiological functions needed to maintain organisms in a healthy condition. Its complex reactions involve the metabolic oxidation of biochemicals, which requires a supply of oxygen and releases carbon dioxide and water as waste products. Net primary production (NPP) refers to the fraction of gross primary production that remains after primary producers have utilized some of their GPP for their own respiration. In other words, NPP = GPP - R.
For example, studies of an oak-pine forest in New York found that the total fixation of solar energy by vegetation (i.e., the gross primary productivity) was about 48.1-thousand kilojoules per hectare per year. This fixation rate was equivalent to less than 0.01% of the input of solar radiation to the forest. Because the plants utilized 27.2-thousand kJ/ha.yr in support of their respiration, the net primary productivity was 20.9-thousand kJ/ha.yr, occurring as the accumulating biomass of trees.
The NPP of primary producers supports the productivity of all other organisms, known as heterotrophs, in ecosystems. Heterotrophs can only utilize living or dead biomass as food, and they rely on other organisms to supply this fixed energy. Animal heterotrophs that feed on plants are known as herbivores (or primary consumers). Animals that kill and eat other animals are known as carnivores (or secondary consumers). Animals that feed on both plant and animal biomass are known as omnivores. Many heterotrophs feed on dead organic matter, and are called decomposers or detritivores .
[Bill Freedman Ph.D. ]
Begon, M., C. R. Townsend, and J. L. Harper. Ecology: Individuals, Populations and Communities. London: Blackwell Science Inc., 1998.
Lieth, H. and R. H. Whittaker, eds. Primary Productivity of the Biosphere. New York: Springer-Verlag.
Odum, E. P. Basic Ecology. New York: Saunders College Publishing.