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Primary production (changes)

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Contents

Idea

From Wikipedia

Primary production is the production of organic compounds from atmospheric or aquatic carbon dioxide, principally through the process of photosynthesis, with chemosynthesis being much less important. 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 autotrophs, and form the base of the food chain.

In terrestrial ecoregions, these are mainly plants, while in aquatic ecoregions 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.

Details

GPP and NPP

Gross primary production (GPP) is the rate at which an ecosystem’s producers capture and store a given amount of chemical energy as biomass in a given length of time. 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”).[1] The remaining fixed energy (i.e., mass of photosynthate) is referred to as net primary production (NPP).NPP = GPP - respiration [by plants]

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.

Both gross and net primary production are in units of mass / area / time. In terrestrial ecosystems, mass of carbon per unit area per year (g C/m2/yr) is most often used as the unit of measurement.

In Townsend et al. there is a table showing annual NPP per biome:

MarineNPPTerrestrialNPP
Tropical or subtropical oceans13.0Tropical rainforests17.8
Temperate oceans16.3Broadleaf deciduous forests1.5
Polar oceans6.4Mixed broad/needle leaf forests3.1
Coastal10.7Needleleaf evergreen forests3.1
Salt march/estuaries/seaweed1.2Needleleaf deciduous forests1.4
Coral reefs0.7Savannas16.8
--Perennial grasslands2.4
--Broadleaf shrubs with bare soil1.0
--Tundra0.8
--Desert0.5
--Cultivation8.0
Total48.3Total56.4

The unit used are petagram , Pg which is 10^15 gram, the same as 10^9 tonnes or Gt, which non-SI countries tend to write as billion tonnes.

Human impact and appropriation

From Wikipedia:

Extensive human land use results in various levels of impact on actual NPP (NPPact). 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 (ΔNPPLC) of 9.6% across global land-mass.[14] In addition to this, end consumption by people raises the total human appropriation of net primary production (HANPP)[15] to 23.8% of potential vegetation (NPP0).[14] It is estimated that, in 2000, 34% of the Earth’s ice-free land area (12% cropland; 22% pasture) was devoted to human agriculture.[16] 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,[14] and scientists have questioned how large this fraction can be before these services begin to break down.

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 system science. However, quantifying primary production at this scale is difficult because of the range of habitats 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 Gt C yr−1.[13] Of this, 56.4 Gt C yr−1 (53.8%), was the product of terrestrial organisms, while the remaining 48.5 Gt C yr−1, was accounted for by oceanic production. In areal terms, it was estimated that land production was approximately 426 g C m−2 yr−1 (excluding areas with permanent ice cover), while that for the oceans was 140 g C m−2 yr−1.[13] 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.

References

category: ecology