27.3:
Primary Production
Primary productivity refers to the rate at which autotrophs convert carbon dioxide into organic material during photosynthesis, and the total primary production within an ecosystem is the gross primary production, or GPP.
However, autotrophs spend some of their energy during cellular respiration, leaving less for consumption by heterotrophs. The usable energy leftover in the ecosystem is called net primary productivity, or NPP, and is usually expressed in units of energy per unit area per unit time.
In marine ecosystems, algae are the primary producers of energy, but are limited by the amount of sunlight that penetrates the ocean surface, and the amount of nutrients available.
In terrestrial ecosystems, vascular plants are responsible for the majority of primary production. Because NPP is driven by temperature and moisture in these systems, tropical rainforests have the highest productivity, while desert ecosystems have the lowest.
27.3:
Primary Production
The total amount of energy acquired by primary producers in an ecosystem is called gross primary production (GPP). However, of this energy, producers use some for metabolic processes, and some is lost as heat, decreasing the amount of energy available to the next trophic level. The remaining usable amount of energy is called the net primary productivity (NPP). In terrestrial ecosystems, NPP is driven by climate, while light penetration and nutrient availability drive NPP in aquatic ecosystems.
Gross Primary Production
Energy can be acquired by organisms in three ways: photosynthesis, chemosynthesis and by the consumption of other organisms. Autotrophs, or producers, synthesize their food. Chemoautotrophs occur in ecosystems where sunlight is unavailable and use chemicals as an energy source—like hydrogen sulfide, H2S, from deep ocean hydrothermal vents—while photoautotrophs transform energy from sunlight into usable energy for the rest of the organisms in an ecosystem. The rate at which these producers obtain and transform this energy is known as the ecosystem’s gross primary production (GPP), which is also a measure of the total amount of energy accumulated by primary producers in an ecosystem.
Net Primary Production
However, not all of the energy obtained by producers is available for use by other organisms in the ecosystem. During both chemosynthesis and photosynthesis, energy is used by primary producers to fuel their cellular respiration, and some is lost as heat as a byproduct of metabolic processes. The energy left over after respiration and metabolism by primary producers is known as net primary production (NPP), which is then available to primary consumers on the next trophic level.
NPP of Ecosystems
Ecosystems with the highest NPP are tropical wet rainforests and estuaries influenced by warm temperatures, high humidity and an influx of nutrients. Low productivity ecosystems include deserts and the Arctic, which are dry and either too hot or too cold for high rates of plant growth.
In aquatic ecosystems, quantities of both light and nutrients control primary production. The depth of light penetration drives high primary productivity in both shallow coastal waters, and at the surface of deep-ocean and lakes. Phytoplankton, which produces almost 40% of the earth’s oxygen, thrives at the surface of deep marine and fresh waters, while in shallow waters, vastly diverse coral reefs and aquatic plants thrive.
Areas of nutrient inflow have exceptionally high levels of primary production. Examples are estuaries where nitrogen-rich freshwater mixes with saltwater, or ocean upwellings where the organic matter of the deep ocean circulates to the surface. The inflow of macronutrients, such as phosphorus and nitrogen, increases primary production as these are otherwise limiting factors in the growth of photosynthesizing organisms. An excessive influx of these nutrients from agricultural runoff can cause exponential growth in algal and phytoplankton populations, depleting the water of oxygen, and negatively affecting the aquatic flora and fauna—a process known as eutrophication.
Suggested Reading
Ask, Jenny, Owen Rowe, Sonia Brugel, Mårten Strömgren, Pär Byström, and Agneta Andersson. “Importance of Coastal Primary Production in the Northern Baltic Sea.” Ambio 45, no. 6 (October 2016): 635–48. [Source]
Orcutt, Beth N., Jason B. Sylvan, Nina J. Knab, and Katrina J. Edwards. “Microbial Ecology of the Dark Ocean above, at, and below the Seafloor.” Microbiol. Mol. Biol. Rev. 75, no. 2 (June 1, 2011): 361–422. [Source]