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{{TS|lingua=inglese|argomento=ecologia|data=ottobre 2007}}
[[Immagine:Seawifs_global_biosphere.jpg|thumb|400px|right|Abbondanza globale di fotoautotrofi sia terrestri sia marini, dal [[settembre]] [[1997]] all'[[agosto]] [[2000]]. Essendo una stima della [[biomassa]], questa può essere considerata un indicatore della potenzialità della produzione primaria e non una stima di essa.]]
La '''Produzione primaria''' è la produzione di [[composti organici]] dalla [[Anidride carbonica|CO<sub>2</sub>]] presente nell'[[atmosfera]] o in acqua che avviene principalmente mediante processi [[fotosintesi|fotosintetici]] o, in misura minore, [[chemiosintesi|chemiosintetici]].
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Un'altra differenza con la terraferma è data dal fatto che la maggior parte della produzione primaria degli oceani è dovuta ad [[microorganismi|organismi microscopici]], il [[fitoplancton]]. Gli autotrofi più grandi, come le fanerogame e le macroalghe, sono confinati nella zona litoranea e in acque relativamente basse, dove possono attaccarsi al substrato ma essere ancora all'interno della [[zona fotica]]. Un'eccezione viene dalle alghe brune appartenenti al [[genere (tassonomia)|genere]] ''[[Sargassum]]''.
 
<!-- 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.
 
===Light===
[[Immagine:Kelp_forest_Otago_1s.JPG|thumb|A [[kelp forest]]; an example of attached [[macroalgae]]]]
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).
 
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.
 
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.
 
===Nutrients===
[[Immagine:AYool WOA surf NO3.png|thumb|Annual mean sea surface nitrate for the [[World Ocean]]. Data from the [[World Ocean Atlas]] [http://www.nodc.noaa.gov/OC5/WOA01/ 2001].]]
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 [[synthesis]]e their cells and cellular machinery. Because of [[gravity|gravitational]] sinking of particulate material (such as [[plankton]], dead or faecal 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.
 
===Iron===
Another factor relatively recently discovered to play a significant role in oceanic primary production is the [[micronutrient]] [[iron]]<ref>Martin, J. H. and Fitzwater, S. E. (1988) Iron-deficiency limits phytoplankton growth in the Northeast Pacific Subarctic. ''Nature'' '''331''', 341-343</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 [[eolian|eolian dust]]. 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.
 
== Human impact and appropriation ==
Extensive human [[land use]] results in various levels of impact on ''actual NPP'' (NPP<sub>act</sub>). In a few regions, such as the [[Nile]] valley, [[irrigation]] has resulted in a considerable increase in primary production. This is an exception to the rule, which is that there is a ''NPP reduction due to land changes'' (ΔNPP<sub>LC</sub>) of 9.6% across global land-mass. In addition to this, end consumption by people raises the total ''human appropriation of net primary production'' (HANPP) to 23.8% of ''potential vegetation'' (NPP<sub>0</sub>). This disproportionate amount reduces energy available to other species, having a marked impact on [[biodiversity]], flows of carbon, water and energy, and [[ecosystem services]].<ref>
{{cite journal
| author=H. Haberl, ''et al.''
| month=
| year=2007
| title=Quantifying and mapping the human appropriation of net primary production in earth's terrestrial ecosystems
| journal = [[Proc. Natl Acad. Sci. USA]]
| volume=
| issue=online early edition
| pages=
| doi=10.1073/pnas.0704243104
}}</ref>
 
== Measurement ==
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.
 
Scale also greatly affects measurement techniques. While biochemically-based techniques are appropriate for plant tissues, organs, whole plants, or plankton samples, they 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.
 
===Terrestrial===
In terrestrial ecosystems, researchers generally measure net primary production. A variety of field methods are used to estimate 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, [[decomposition]], turnover, [[litterfall]], volatile organic compounds, root exudates, and allocation to [[symbiotic]] microorganisms. As discussed<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; Ni, J|
year=2001|
journal=Ecological Applications|
volume=11|
pages=356-370|
url=http://www.esajournals.org/esaonline/?request=get-document&issn=1051-0761&volume=011&issue=02&page=0356
}}</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; Olson, R. J.|
year=2002|
journal=Global Change Biology|
volume=8|
pages=736|
doi=10.1046/j.1365-2486.2002.00512.x|
}}
</ref>, biomass based NPP estimates result in underestimation of NPP due to incomplete accounting of these components. However, many field measurements correlate well to NPP. Comprehensive reviews of field methods used to estimate NPP can be found<ref name="clark"/>, <ref>{{cite book|
title = Primary Productivity of the Biosphere|
first=Helmut|
last=Leith|
coauthors=Robert Harding Whittaker|
year=1975|
publisher = [[Springer-Verlag]]|
isbn= 0387070834|
location=New York
}}</ref>.
 
The major unaccounted for pool is belowground productivity, especially production and turnover of roots. Belowground components of NPP are difficult to measure. BNPP is often estimated based on a ratio of ANPP:BNPP rather than direct measurements.
 
Grasslands: 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, or when perennial, if there was 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, e.g.; of marshes and fens, is similarly measured. In [[Europe]], annual mowing makes the annual biomass increment of wetlands evident.
 
Forests: 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.
 
===Aquatic===
In aquatic systems, primary production is typically measured using one of three 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>:
 
# variations in oxygen concentration within a sealed bottle (developed by Gaarder and Gran in [[1927]])
# incorporation of inorganic [[carbon-14]] (in the form of sodium bicarbonate) into organic matter <ref>Steeman-Nielsen, E. (1951) Measurement of production of organic matter in sea by means of carbon-14. ''Nature'' '''267''', 684–685</ref> <ref>Steeman-Nielsen, E. (1952). The use of radioactive carbon (C14) for measuring organic production in the sea. ''J. Cons. Int. Explor. Mer.'' '''18''', 117-140</ref>
# fluorescence kinetics (technique still a research topic)
 
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 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 subtracting oxygen consumption in the dark vessel from net oxygen production in the light vessel.
 
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.
 
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.
 
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.
 
===Global===
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 [[NDVI|normalised 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 <ref>Field, C. B., Behrenfeld, M. J., Randerson, J. T. and Falkowski, P. (1998) Primary production of the Biosphere: Integrating Terrestrial and Oceanic Components. ''Science'' '''281''', 237-240</ref>. Of this, 56.4 Gt C/yr (53.8%), was the product of terrestrial organisms, while the remaining 48.5 Gt C/yr, was accounted for by oceanic production.
 
In [[area|areal]] terms, it was estimated that land production was approximately 426 g C/m<sup>2</sup>/yr (excluding areas with permanent ice cover), while that for the oceans was 140 g C/m<sup>2</sup>/yr. 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. -->
{{Portale|ecologia e ambiente}}
 
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