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| {{other uses|Primary production (economics)}}
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| [[image:Seawifs global biosphere.jpg|400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September 1997 to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it. Provided by the [[SeaWiFS]] Project, [[NASA]]/[[Goddard Space Flight Center]] and [[ORBIMAGE]].]]
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| [[File:MOD17A2 M PSN.ogv|thumb|400px|The maps above show Earth's monthly net primary productivity from February 2000 to December 2013. The data come from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Values range from near 0 grams of carbon per square meter per day (tan) to 6.5 grams per square meter per day (dark green). A negative value means decomposition or respiration overpowered carbon absorption; more carbon was released to the atmosphere than the plants took in. In mid-latitudes, productivity is obviously tied to seasonal change, with productivity peaking in each hemisphere’s summer. The boreal forests of Canada and Russia experience high productivity in June and July and then a slow decline through fall and winter. Year-round, tropical forests in South America, Africa, Southeast Asia, and Indonesia have high productivity, not surprising with the abundant sunlight, warmth, and rainfall. However, even the tropics, there are variations in productivity over the course of the year. For example, in the Amazon, productivity is especially high from roughly August through October, which is the area’s dry season. Because the trees have access to a plentiful supply of ground water that builds up in the rainy season, they actually grow better when the rainy skies clear and allow more sunlight to reach the forest.<ref>http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MOD17A2_M_PSN</ref>]]
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| '''Primary production''' is the synthesis of [[organic chemistry|organic compounds]] from atmospheric or aqueous [[carbon dioxide]]. It principally occurs through the process of [[photosynthesis]], which uses light as its source of energy, but it also occurs through [[chemosynthesis]], which uses the oxidation or reduction of chemical compounds as its source of energy. Almost all life on earth is directly or indirectly reliant on primary production. The organisms responsible for primary production are known as ''primary producers'' or [[autotroph]]s, and form the base of the [[food chain]]. In [[terrestrial ecoregion]]s, these are mainly [[plant]]s, while in [[aquatic ecoregion]]s [[algae]] are primarily responsible. Primary production is distinguished as either ''net'' or ''gross'', the former accounting for losses to processes such as [[cellular respiration]], the latter not.
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| ==Overview==
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| [[Image:Calvin-cycle4.svg|thumb|The [[Calvin cycle]] of photosynthesis]]
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| Primary production is the production of [[chemical energy]] in organic compounds by living [[organism]]s. The main source of this energy is [[sunlight]] but a minute fraction of primary production is driven by [[lithotroph]]ic organisms using the chemical energy of [[inorganic]] molecules.
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| Regardless of its source, this energy is used to synthesize complex [[organic molecule]]s from simpler inorganic compounds such as [[carbon dioxide]] (CO<sub>2</sub>) and [[water]] (H<sub>2</sub>O). The following two equations are simplified representations of photosynthesis (top) and (one form of) [[chemosynthesis]] (bottom):
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| ::: CO<sub>2</sub> + H<sub>2</sub>O + ''light'' <math>\rightarrow</math> CH<sub>2</sub>O + O<sub>2</sub>
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| ::: CO<sub>2</sub> + O<sub>2</sub> + 4 H<sub>2</sub>S <math>\rightarrow</math> CH<sub>2</sub>O + 4 S + 3 H<sub>2</sub>O
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| In both cases, the end point is [[Redox|reduced]] [[carbohydrate]] (CH<sub>2</sub>O), typically molecules such as [[glucose]] or other [[sugar]]s. These relatively simple molecules may be then used to further synthesise more complicated molecules, including [[protein]]s, [[complex carbohydrates]], [[lipids]], and [[nucleic acids]], or be [[cellular respiration|respired]] to perform [[work (thermodynamics)|work]]. Consumption of primary producers by [[heterotroph]]ic organisms, such as [[animal]]s, then transfers these organic molecules (and the energy stored within them) up the [[food web]], fueling all of the [[Earth]]'s living systems.
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| ==GPP and NPP== | |
| ''Gross primary production'' (GPP) is the amount of chemical energy as [[Biomass (ecology)|biomass]] that primary producers create in a given length of time. (GPP is sometimes confused with Gross Primary ''productivity'', which is the rate at which photosynthesis or chemosynthesis occurs.) Some fraction of this fixed energy is used by primary producers for [[cellular respiration]] and maintenance of existing tissues (i.e., "growth respiration" and "[[maintenance respiration]]").<ref>Amthor, J.S. and Baldocchi, D.D. (2001). Terrestrial Higher Plant Respiration and Net Primary Production. In ''Terrestrial Global Productivity'', Academic Press, 33-59</ref> The remaining fixed energy (i.e., mass of photosynthate) is referred to as ''net primary production'' (NPP).
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| ::: NPP = GPP - respiration [by plants]
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| Net primary production is the rate at which all the plants in an ecosystem produce net useful chemical energy; it is equal to the difference between the rate at which the plants in an ecosystem produce useful chemical energy (GPP) and the rate at which they use some of that energy during respiration. Some net primary production goes toward growth and reproduction of primary producers, while some is consumed by herbivores.
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| Both gross and net primary production are in units of mass per unit area per unit time interval. In terrestrial ecosystems, mass of carbon per unit area per year (g C m<sup>−2</sup> yr<sup>−1</sup>) is most often used as the unit of measurement.
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| ==Terrestrial production==
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| [[Image:Raunkiaer.jpg|thumb|right|An [[oak]] tree; a typical modern, terrestrial autotroph]]
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| On the land, almost all primary production is now performed by [[vascular plant]]s, with a small fraction coming from algae and [[non-vascular plant]]s such as [[moss]]es and [[Marchantiophyta|liverworts]]. Before the [[evolution]] of vascular plants, non-vascular plants likely played a more significant role. Primary production on land is a [[function (mathematics)|function]] of many factors, but principally local [[hydrology]] and [[temperature]] (the latter covaries to an extent with light, specifically photosynthetically active radiation (PAR), the source of energy for photosynthesis). While plants cover much of the Earth's surface, they are strongly curtailed wherever temperatures are too extreme or where necessary plant resources (principally water and PAR) are limiting, such as [[desert]]s or [[polar region]]s.
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| Water is "consumed" in plants by the processes of photosynthesis (see above) and [[transpiration]]. The latter process (which is responsible for about 90% of water use) is driven by the [[evaporation]] of water from the [[leaf|leaves]] of plants. Transpiration allows plants to transport water and [[mineral]] [[nutrient]]s from the [[soil]] to growth regions, and also cools the plant. Diffusion of water vapour out of a leaf, the force that drives transpiration, is regulated by structures known as [[stoma]]ta. These structure also regulate the diffusion of carbon dioxide from the atmosphere into the leaf, such that decreasing water loss (by partially closing stomata) also decreases carbon dioxide gain. Certain plants use alternative forms of photosynthesis, called [[Crassulacean acid metabolism]] (CAM) and [[C4 carbon fixation|C4]]. These employ [[physiology|physiological]] and [[anatomy|anatomical]] adaptations to increase water-use efficiency and allow increased primary production to take place under conditions that would normally limit carbon fixation by [[C3 carbon fixation|C3]] plants (the majority of plant species).
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| ==Oceanic production==
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| [[Image:Diatoms through the microscope.jpg|thumb|left|Marine [[diatom]]s; an example of [[plankton]]ic microalgae]]
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| In a reversal of the pattern on land, in the oceans, almost all photosynthesis is performed by algae, with a small fraction contributed by [[vascular plants]] and other groups. Algae encompass a diverse range of organisms, ranging from single floating cells to attached [[seaweeds]]. They include photoautotrophs from a variety of groups. [[Bacteria|Eubacteria]] are important photosynthetizers in both oceanic and terrestrial ecosystems, and while some [[archaea]] are [[phototroph]]ic, none are known to utilise oxygen-evolving photosynthesis.<ref name=Schafer>{{cite journal |author=Schäfer G, Engelhard M, Müller V |title=Bioenergetics of the Archaea |journal=Microbiol. Mol. Biol. Rev. |volume=63 |issue=3 |pages=570–620 |date=1 September 1999|pmid=10477309 |pmc=103747 |url=http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=10477309 }}</ref> A number of [[eukaryote]]s are significant contributors to primary production in the ocean, including [[green alga]]e, [[brown alga]]e and [[red alga]]e, and a diverse group of unicellular groups. Vascular plants are also represented in the ocean by groups such as the [[seagrass]]es.
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| Unlike terrestrial ecosystems, the majority of primary production in the ocean is performed by free-living [[microorganism|microscopic organisms]] called [[phytoplankton]]. Larger autotrophs, such as the seagrasses and macroalgae ([[seaweed]]s) are generally confined to the [[littoral]] zone and adjacent shallow waters, where they can [[holdfast|attach]] to the underlying substrate but still be within the [[photic zone]]. There are exceptions, such as ''[[Sargassum]]'', but the vast majority of free-floating production takes place within microscopic organisms.
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| The factors limiting primary production in the ocean are also very different from those on land. The availability of water, obviously, is not an issue (though its [[salinity]] can be). Similarly, temperature, while affecting [[metabolism|metabolic]] rates (see [[Q10 (temperature coefficient)|Q<sub>10</sub>]]), ranges less widely in the ocean than on land because the [[heat capacity]] of seawater buffers temperature changes, and the formation of [[sea ice]] [[Thermal insulation|insulates]] it at lower temperatures. However, the availability of light, the source of energy for photosynthesis, and mineral [[nutrients]], the building blocks for new growth, play crucial roles in regulating primary production in the ocean. Available Earth System Models suggest that ongoing ocean bio-geochemical changes could trigger reductions in ocean NPP between 3% and 10% of current values depending on the emissions scenario.<ref name="Mora">{{cite journal |last1=Mora |first1=C. et al. |title=Biotic and Human Vulnerability to Projected Changes in Ocean Biogeochemistry over the 21st Century |journal=Plos Biology |volume=11 |pages=e1001682 |year=2013 |doi=10.1371/journal.pbio.1001682}}</ref>
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| ===Light===
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| [[Image:Kelp forest Otago 1s.JPG|thumb|right|A [[kelp forest]]; an example of attached [[macroalgae]]]]
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| The sunlit zone of the ocean is called the [[photic zone]] (or euphotic zone). This is a relatively thin layer (10–100 m) near the ocean's surface where there is sufficient light for photosynthesis to occur. For practical purposes, the thickness of the photic zone is typically defined by the depth at which light reaches 1% of its surface value. Light is [[attenuation|attenuated]] down the water column by its [[absorption (optics)|absorption]] or [[scattering]] by the water itself, and by dissolved or particulate material within it (including phytoplankton).
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| Net photosynthesis in the water column is determined by the interaction between the photic zone and the [[mixed layer]]. [[Turbulence|Turbulent mixing]] by [[wind]] energy at the ocean's surface homogenises the water column vertically until the turbulence [[dissipation|dissipates]] (creating the aforementioned mixed layer). The deeper the mixed layer, the lower the average amount of light intercepted by phytoplankton within it. The mixed layer can vary from being shallower than the photic zone, to being much deeper than the photic zone. When it is much deeper than the photic zone, this results in phytoplankton spending too much time in the dark for net growth to occur. The maximum depth of the mixed layer in which net growth can occur is called the [[critical depth]]. As long as there are adequate nutrients available, net primary production occurs whenever the mixed layer is shallower than the critical depth.
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| Both the magnitude of wind mixing and the availability of light at the ocean's surface are affected across a range of space- and time-scales. The most characteristic of these is the [[season|seasonal cycle]] (caused by the [[Effect of sun angle on climate|consequences]] of the Earth's [[axial tilt]]), although wind magnitudes additionally have strong [[Wind#Winds by spatial scale|spatial components]]. Consequently, primary production in [[temperate]] regions such as the [[Atlantic|North Atlantic]] is highly seasonal, varying with both incident light at the water's surface (reduced in winter) and the degree of mixing (increased in winter). In [[tropics|tropical]] regions, such as the [[gyre]]s in the middle of the major [[oceanic basin|basins]], light may only vary slightly across the year, and mixing may only occur episodically, such as during large [[storm]]s or [[hurricane]]s.
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| ===Nutrients===
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| [[File:WOA09 sea-surf NO3 AYool.png|thumb|Annual mean sea surface nitrate for the [[World Ocean]]. Data from the [[World Ocean Atlas]] [http://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html 2009].]]
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| Mixing also plays an important role in the limitation of primary production by nutrients. Inorganic nutrients, such as [[nitrate]], [[phosphate]] and [[silicic acid]] are necessary for phytoplankton to [[Organic synthesis|synthesis]]e their cells and cellular machinery. Because of [[gravity|gravitational]] sinking of particulate material (such as [[plankton]], dead or fecal material), nutrients are constantly lost from the photic zone, and are only replenished by mixing or [[upwelling]] of deeper water. This is exacerbated where summertime solar heating and reduced winds increases vertical stratification and leads to a strong [[thermocline]], since this makes it more difficult for wind mixing to entrain deeper water. Consequently, between mixing events, primary production (and the resulting processes that leads to sinking particulate material) constantly acts to consume nutrients in the mixed layer, and in many regions this leads to nutrient exhaustion and decreased mixed layer production in the summer (even in the presence of abundant light). However, as long as the photic zone is deep enough, primary production may continue below the mixed layer where light-limited growth rates mean that nutrients are often more abundant.
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| ===Iron===
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| Another factor relatively recently discovered to play a significant role in oceanic primary production is the [[micronutrient]] [[iron]].<ref>{{cite journal | doi = 10.1038/331341a0 | last1 = Martin | first1 = J. H. | last2 = Fitzwater | first2 = S. E. | year = 1988 | title = Iron-deficiency limits phytoplankton growth in the Northeast Pacific Subarctic | url = | journal = Nature | volume = 331 | issue = 6154| pages = 341–343 |bibcode = 1988Natur.331..341M }}</ref> This is used as a [[cofactor (biochemistry)|cofactor]] in [[enzyme]]s involved in processes such as [[nitrate reductase|nitrate reduction]] and [[nitrogen fixation]]. A major source of iron to the oceans is dust from the Earth's [[desert]]s, picked up and delivered by the wind as [[Aeolian processes|aeolian dust]].
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| In regions of the ocean that are distant from deserts or that are not reached by dust-carrying winds (for example, the [[Southern Ocean|Southern]] and [[Pacific|North Pacific]] oceans), the lack of iron can severely limit the amount of primary production that can occur. These areas are sometimes known as [[HNLC]] (High-Nutrient, Low-Chlorophyll) regions, because the scarcity of iron both limits phytoplankton growth and leaves a surplus of other nutrients. Some scientists have suggested [[Iron fertilization|introducing iron]] to these areas as a means of increasing primary productivity and sequestering carbon dioxide from the atmosphere.<ref name=cooper96>{{cite journal | last=Cooper | first=D.J. | coauthors=Watson, A.J. and Nightingale, P.D. | year=1996 | title=Large decrease in ocean—surface {{co2}} fugacity in response to ''in situ'' iron fertilization | journal=Nature | volume=383 | pages=511–513 | doi=10.1038/383511a0 | issue=6600|bibcode = 1996Natur.383..511C }}</ref>
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| == Measurement ==
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| The methods for measurement of primary production vary depending on whether gross vs net production is the desired measure, and whether terrestrial or aquatic systems are the focus. Gross production is almost always harder to measure than net, because of respiration, which is a continuous and ongoing process that consumes some of the products of primary production (i.e. sugars) before they can be accurately measured. Also, terrestrial ecosystems are generally more difficult because a substantial proportion of total productivity is shunted to below-ground organs and tissues, where it is logistically difficult to measure. Shallow water aquatic systems can also face this problem.
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| Scale also greatly affects measurement techniques. The rate of carbon assimilation in plant tissues, organs, whole plants, or plankton samples can be quantified by [[photosynthetic meter|biochemically based techniques]], but these techniques are decidedly inappropriate for large scale terrestrial field situations. There, net primary production is almost always the desired variable, and estimation techniques involve various methods of estimating dry-weight biomass changes over time. Biomass estimates are often converted to an energy measure, such as kilocalories, by an [[empirical]]ly determined conversion factor.
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| ===Terrestrial===
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| In terrestrial ecosystems, researchers generally measure net primary production (NPP). Although its definition is straightforward, field measurements used to estimate productivity vary according to investigator and biome. Field estimates rarely account for below ground productivity, herbivory, turnover, [[litterfall]], [[volatile organic compounds]], root exudates, and allocation to [[symbiotic]] microorganisms. Biomass based NPP estimates result in underestimation of NPP due to incomplete accounting of these components.<ref name="clark">{{cite journal | title = Measuring net primary production in forests: Concepts and field methods | first=D.A. | last=Clark | coauthors=Brown, S., Kicklighter, D.W., Chambers, J.Q., Thomlinson, J.R. and Ni, J. | year=2001 | journal=Ecological Applications | volume=11 | pages=356–370 | url=http://www.esajournals.org/doi/abs/10.1890/1051-0761%282001%29011%5B0356:MNPPIF%5D2.0.CO%3B2 | doi = 10.1890/1051-0761(2001)011[0356:MNPPIF]2.0.CO;2 | format = <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3AClark+intitle%3AMeasuring+net+primary+production+in+forests%3A+Concepts+and+field+methods&as_publication=Ecological+Applications&as_ylo=2001&as_yhi=2001&btnG=Search Scholar search]</sup> | issn = 1051-0761 | issue = 2 }}</ref><ref name="olson">{{cite journal | title = Estimating net primary productivity from grassland biomass dynamics measurements | first=J.M.O. | last=Scurlock | coauthors=Johnson, K. and Olson, R.J. | year=2002 | journal=Global Change Biology | volume=8 | pages=736–753| doi=10.1046/j.1365-2486.2002.00512.x | issue = 8 }}</ref> However, many field measurements correlate well to NPP. There are a number of comprehensive reviews of the field methods used to estimate NPP.<ref name="clark"/><ref name="olson"/><ref>{{cite book | title = Primary Productivity of the Biosphere | first=H. | last=Leith | coauthors=Whittaker, R.H. | year=1975 | publisher = [[Springer-Verlag]] | isbn= 0-387-07083-4 | location=New York }}</ref> Estimates of [[ecosystem respiration]], the total carbon dioxide produced by the ecosystem, can also be made with gas flux measurements.
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| The major unaccounted for pool is belowground productivity, especially production and turnover of roots. Belowground components of NPP are difficult to measure. BNPP (below-ground NPP) is often estimated based on a ratio of ANPP:BNPP (above-ground NPP:below-ground NPP) rather than direct measurements.
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| Gross primary production can be estimated from measurements of net ecosystem exchange (NEE) of carbon dioxide made by the [[Eddy covariance|eddy covariance technique]]. During night, this technique measures all components of ecosystem respiration. This respiration is scaled to day-time values and further subtracted from NEE.<ref name="Reichstein">{{cite journal | title = On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm | first=M. | last=Reichstein | coauthors=Falge, E., Baldocchi, D., Papale, D., Aubinet, M., Berbigier, P., Bernhofer, C., Buchmann, N., Gilmanov, T., Granier, A., Grünwald, T., Havránková, K., Ilvesniemi, H., Janous, D., Knohl, A., Laurila, T., Lohila, A., Loustau, D., Matteucci, G., Meyers, T., Miglietta, F., Ourcival, J.-M., Pumpanen, J., Rambal, S., Rotenberg, E., Sanz, M., Tenhunen, J., Seufert, G., Vaccari, F., Vesala, T., Yakir, D. and Valentini, R. | year=2005 | journal=Global Change Biology | volume=11 | pages=1424–1439 | url=http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2005.001002.x/abstract | doi=10.1111/j.1365-2486.2005.001002.x}}</ref>
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| ====Grasslands====
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| [[Image:Konza1.jpg|thumb|left|The [[Konza Prairie|Konza]] [[tallgrass prairie]] in the [[Flint Hills]] of northeastern [[Kansas]]]]
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| Most frequently, peak standing biomass is assumed to measure NPP. In systems with persistent standing litter, live biomass is commonly reported. Measures of peak biomass are more reliable in if the system is predominantly annuals. However, perennial measurements could be reliable if there were a synchronous phenology driven by a strong seasonal climate. These methods may underestimate ANPP in grasslands by as much as 2 ([[temperate]]) to 4 ([[tropical]]) fold.<ref name="olson"/> Repeated measures of standing live and dead biomass provide more accurate estimates of all grasslands, particularly those with large turnover, rapid decomposition, and interspecific variation in timing of peak biomass. [[Wetland]] productivity (marshes and fens) is similarly measured. In [[Europe]], annual mowing makes the annual biomass increment of wetlands evident.
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| ====Forests====
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| Methods used to measure forest productivity are more diverse than those of grasslands. Biomass increment based on stand specific [[allometry]] plus litterfall is considered a suitable although incomplete accounting of above-ground net primary production (ANPP).<ref name="clark"/> Field measurements used as a proxy for ANPP include annual litterfall, diameter or basal area increment ([[Diameter at breast height|DBH]] or BAI), and volume increment.
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| ===Aquatic===
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| In aquatic systems, primary production is typically measured using one of six main techniques:<ref>Marra, J. (2002), pp. 78-108. In: Williams, P. J. leB., Thomas, D. N., Reynolds, C. S. (Eds.), Phytoplankton Productivity:Carbon Assimilation in Marine and Freshwater Ecosystems. Blackwell, Oxford, UK</ref>
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| # variations in oxygen concentration within a sealed bottle (developed by Gaarder and Gran in 1927)
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| # incorporation of inorganic [[carbon-14]] (<sup>14</sup>C in the form of [[sodium bicarbonate]]) into organic matter<ref>{{cite journal | last=Steeman-Nielsen | first=E. | year=1951 | title=Measurement of production of organic matter in sea by means of carbon-14 | journal=[[Nature (journal)|Nature]] | volume=267 | pages=684–685 | doi=10.1038/167684b0 | pmid=14826912 | issue=4252|bibcode = 1951Natur.167..684N }}</ref><ref>{{cite journal | last=Steeman-Nielsen | first=E. | year=1952 | title=The use of radioactive carbon (C14) for measuring organic production in the sea | journal=J. Cons. Int. Explor. Mer. | volume=18 | pages=117–140 }}</ref>
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| # Stable isotopes of Oxygen (<sup>16</sup>O, <sup>18</sup>O and <sup>17</sup>O)<ref>{{cite journal | last=Bender, | year=1987 | title=A Comparison of 4 Methods for Determining Planktonic Community Production | first9=Gary | last9=Hitchcock | first8=Chris | last8=Langdon | first7=Michael | last7=Pilson | first6=John | last6=Sieburth | first5=Peter J. LEB. | last5=Williams | first4=John | last4=Marra | first3=Kenneth | last3=Johnson | journal=[[Limnology and Oceanography (journal)|Limnology and Oceanography]] | first2=Karen | volume=32 | last2=Grande | pages=1085–1098 | doi=10.4319/lo.1987.32.5.1085 | first1=Michael | display-authors=1 | issue=5 }}</ref><ref>{{cite journal | last=Luz and Barkan | year=2000 | title=Assessment of oceanic productivity with the triple-isotope composition of dissolved oxygen | journal=Science | volume=288 | pages=2028–2031 | doi=10.1126/science.288.5473.2028 | pmid=10856212 | first1=B | last2=Barkan | first2=E | issue=5473 |bibcode = 2000Sci...288.2028L }}</ref>
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| # fluorescence kinetics (technique still a research topic)
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| # Stable isotopes of Carbon (<sup>12</sup>C and <sup>13</sup>C)<ref>{{cite journal | last=Carvalho and Eyre | year=2012 | title=Measurement of planktonic CO2 respiration in the light | journal=Limnology and Oceanography: methods | volume=10 | pages=167–178 | doi=10.4319/lom.2012.10.167 }}</ref>
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| #[[Oxygen/Argon Ratios]] <ref>{{cite journal | last=Craig and Hayward | year=1987 | title=Oxygen supersaturations in the ocean: biological vs. physical contributions | journal=Science | volume=235 | pages=199–202 }}</ref>
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| The technique developed by Gaarder and Gran uses variations in the concentration of oxygen under different experimental conditions to infer gross primary production. Typically, three identical transparent vessels are filled with sample water and [[Stopper (plug)|stoppered]]. The first is analysed immediately and used to determine the initial oxygen concentration; usually this is done by performing a [[Winkler test for dissolved oxygen|Winkler titration]]. The other two vessels are incubated, one each in under light and darkened. After a fixed period of time, the experiment ends, and the oxygen concentration in both vessels is measured. As photosynthesis has not taken place in the dark vessel, it provides a measure of [[ecosystem respiration]]. The light vessel permits both photosynthesis and respiration, so provides a measure of net photosynthesis (i.e. oxygen production via photosynthesis subtract oxygen consumption by respiration). Gross primary production is then obtained by adding oxygen consumption in the dark vessel to net oxygen production in the light vessel.
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| The technique of using <sup>14</sup>C incorporation (added as labelled Na<sub>2</sub>CO<sub>3</sub>) to infer primary production is most commonly used today because it is sensitive, and can be used in all ocean environments. As <sup>14</sup>C is [[radioactive decay|radioactive]] (via [[beta decay]]), it is relatively straightforward to measure its incorporation in organic material using devices such as [[scintillation counter]]s.
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| Depending upon the incubation time chosen, net or gross primary production can be estimated. Gross primary production is best estimated using relatively short incubation times (1 hour or less), since the loss of incorporated <sup>14</sup>C (by respiration and organic material excretion / exudation) will be more limited. Net primary production is the fraction of gross production remaining after these loss processes have consumed some of the fixed carbon.
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| Loss processes can range between 10-60% of incorporated <sup>14</sup>C according to the incubation period, ambient environmental conditions (especially temperature) and the experimental [[species]] used. Aside from those caused by the physiology of the experimental subject itself, potential losses due to the activity of consumers also need to be considered. This is particularly true in experiments making use of natural assemblages of microscopic autotrophs, where it is not possible to isolate them from their consumers.
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| The methods based on stable isotopes and O<sub>2</sub>/Ar ratios have the advantage of providing estimates of respiration rates in the light without the need of incubations in the dark. Among them, the method of the triple oxygen isotopes and O<sub>2</sub>/Ar have the additional advantage of not needing incubations in closed containers and O<sub>2</sub>/Ar can even be measured continuously at sea using equilibrator inlet mass spectrometry (EIMS)<ref>{{cite journal | last=Cassar | first=N. | coauthors=B.A. Barnett, M.L. Bender, J. Kaiser, R.C. Hamme, B. Tilbrooke | year=2009 | title=Continuous high-frequency dissolved O<sub>2</sub>/Ar measurements by equilibrator inlet mass spectrometry | journal=Anal. Chem. | volume=81(5) | pages=1855–1864 }}</ref> or a membrane inlet mass spectrometry (MIMS).<ref>{{cite journal | last=Kaiser | first=J. | coauthors=M. K. Reuer, B. Barnett, M.L. Bender | year=2005 | title=Marine productivity estimates from continuous O‐2/Ar ratio measurements by membrane inlet mass spectrometry | journal=Geophys. Res. Lett. | volume=32(19) | doi=10.1029/2005GL023459 }}</ref> However, if results relevant to the carbon cycle are desired, it is probably better to rely on methods based on carbon (and not oxygen) isotopes. It is important to notice that the method based on carbon stable isotopes is not simply an adaptation of the classic <sup>14</sup>C method, but an entirely different approach that does not suffer from the problem of lack of account of carbon recycling during photosynthesis.
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| ===Global===
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| As primary production in the [[biosphere]] is an important part of the [[carbon cycle]], estimating it at the global scale is important in [[Earth science|Earth system science]]. However, quantifying primary production at this scale is difficult because of the range of [[habitat (ecology)|habitat]]s on Earth, and because of the impact of [[weather]] events (availability of sunlight, water) on its variability. Using [[satellite]]-derived estimates of the [[Normalized Difference Vegetation Index]] (NDVI) for terrestrial habitats and sea-surface [[chlorophyll]] for the oceans, it is estimated that the total (photoautotrophic) primary production for the Earth was 104.9 [[tonne|Gt]] C yr<sup>−1</sup>.<ref name=behrenfeld98>{{cite journal | last=Field | first=C.B. | coauthors=Behrenfeld, M.J., Randerson, J.T. and Falkowski, P. | year=1998 | title=Primary production of the Biosphere: Integrating Terrestrial and Oceanic Components | journal=[[Science (journal)|Science]] | volume=281 | pages=237–240 | doi=10.1126/science.281.5374.237 | pmid=9657713 | issue=5374 |bibcode = 1998Sci...281..237F }}</ref> Of this, 56.4 Gt C yr<sup>−1</sup> (53.8%), was the product of terrestrial organisms, while the remaining 48.5 Gt C yr<sup>−1</sup>, was accounted for by oceanic production.
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| Scaling ecosystem-level GPP estimations based on [[eddy covariance]] measurements of net ecosystem exchange (see above) to regional and global values using spatial details of different predictor variables, such as climate variables and remotely sensed [[fAPAR]] or [[Leaf Area Index|LAI]] led to a terrestrial gross primary production of 123±8 Gt carbon (NOT carbon dioxide) per year during 1998-2005<ref name="Beer">{{cite journal | title=Terrestrial Gross Carbon Dioxide Uptake: Global Distribution and Covariation with Climate | first=C. | last=Beer | coauthors=Reichstein, M. , Tomelleri, E., Ciais, P., Jung, M., Carvalhais, N., Rödenbeck, C., Arain, M.A., Baldocchi, D., Bonan, G.B., Bondeau, A., Cescatti, A., Lasslop, G., Lindroth, A., Lomas, M., Luyssaert, S., Margolis, H., Oleson, K.W., Roupsard, O., Veenendaal, E., Viovy, N., Williams, C., Woodward, F.I. and Papale, D. | journal=Science | year=2010 | volume=329 | pages=834–838 | url=http://www.sciencemag.org/content/329/5993/834.abstract | doi=10.1126/science.1184984 | issue=5993|bibcode = 2010Sci...329..834B }}.</ref>
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| In [[area]]l terms, it was estimated that land production was approximately 426 g C m<sup>−2</sup> yr<sup>−1</sup> (excluding areas with permanent ice cover), while that for the oceans was 140 g C m<sup>−2</sup> yr<sup>−1</sup>.<ref name=behrenfeld98/> Another significant difference between the land and the oceans lies in their standing stocks - while accounting for almost half of total production, oceanic autotrophs only account for about 0.2% of the total biomass.
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| == Human impact and appropriation ==
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| Extensive human [[land use]] results in various levels of impact on ''actual NPP'' (NPP<sub>act</sub>). In some regions, such as the [[Nile]] valley, [[irrigation]] has resulted in a considerable increase in primary production. However, these regions are exceptions to the rule, and in general there is a ''NPP reduction due to land changes'' (ΔNPP<sub>LC</sub>) of 9.6% across global land-mass.<ref name=haberl07>{{cite journal
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| | last=Haberl
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| | first=H.
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| | coauthors=Erb, K.H., Krausmann, F., Gaube, V., Bondeau, A., Plutzar, C., Gingrich, S., Lucht, W. and Fischer-Kowalski, M.
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| | year=2007
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| | title=Quantifying and mapping the human appropriation of net primary production in earth's terrestrial ecosystems
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| | journal=[[Proceedings of the National Academy of Sciences|Proc. Natl Acad. Sci. USA]]
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| | volume=104
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| | issue=31
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| | pages=12942–12947
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| | doi=10.1073/pnas.0704243104
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| | pmid=17616580
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| | pmc=1911196
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| | bibcode=2007PNAS..10412942H}}</ref> In addition to this, end consumption by people raises the total ''human appropriation of net primary production'' (HANPP)<ref>{{cite journal
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| | last=Vitousek
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| | first=P.M.
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| | coauthors=Ehrlich, P.R., Ehrlich, A.H. and Matson, P.A.
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| | year=1986
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| | title=Human appropriation of the products of photosynthesis
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| | journal=BioScience
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| | volume=36
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| | pages=368–373
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| | url=http://www.biology.duke.edu/wilson/EcoSysServices/papers/VitousekEtal1986.pdf
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| | doi=10.2307/1310258
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| | jstor=1310258
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| | issue=6
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| | publisher=BioScience, Vol. 36, No. 6}}</ref> to 23.8% of ''potential vegetation'' (NPP<sub>0</sub>).<ref name=haberl07/> It is estimated that, in 2000, 34% of the Earth's ice-free land area (12% [[arable land|cropland]]; 22% [[pasture]]) was devoted to human agriculture.<ref>{{cite journal
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| | last=Ramankutty
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| | first=N.
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| | coauthors=Evan, A.T., Monfreda, C. and Foley, J.A.
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| | year=2008
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| | title=Farming the planet: 1. Geographic distribution of global agricultural lands in the year 2000
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| | journal=Global Biogeochemical Cycles
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| | volume=22
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| | doi=10.1029/2007GB002952
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| | pages=GB1003
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| | bibcode=2008GBioC..22.1003R}}</ref> This disproportionate amount reduces the energy available to other species, having a marked impact on [[biodiversity]], flows of carbon, water and energy, and [[ecosystem services]],<ref name=haberl07/> and scientists have questioned how large this fraction can be before these services begin to break down.<ref>{{cite journal
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| | last=Foley
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| | first=J.A.
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| | coauthors=Monfreda, C., Ramankutty, N. and Zaks, D.
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| | year=2007
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| | title=Our share of the planetary pie
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| | journal=[[Proceedings of the National Academy of Sciences|Proc. Natl Acad. Sci. USA]]
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| | volume=104
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| | pages=12585–12586
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| | doi=10.1073/pnas.0705190104
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| | pmid=17646656
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| | issue=31
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| | pmc=1937509|bibcode = 2007PNAS..10412585F }}</ref> Reductions in NPP are also expected in the ocean as a result of ongoing climate change, potentially impacting marine ecosystems and goods and services that the oceans provide <ref name="Mora"/>
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| ==See also==
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| * [[Biological pump]]
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| * [[f-ratio]]
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| * [[Productivity (ecology)|Productivity]]
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| * [[Hydrogen sulfide]] (H<sub>2</sub>S)
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| ==References==
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| {{Reflist|2}}
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| {{DEFAULTSORT:Primary Production}}
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| [[Category:Plants]]
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| [[Category:Ecology]]
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