View source for 14-Carbon uptake (NPP)

{{BreadcrumbsPrimaryProduction}}

* [[Page authors|Page authors]]: [[PRIMO]]
* [[Responsible curator|Responsible curator]]:  [[User:Hagi BucknWise|Hagen Buck-Wiese]]
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! Net primary production
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| '''Approach:''' radiotracer (<sup>14</sup>C) uptake
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| '''Context:''' incubation, simulated ''in situ''
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| '''Spatial scale:''' point sample
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| '''Temporal scale:''' 12–24 h
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| '''Units:''' µmol C L<sup>-1</sup> d<sup>-1</sup>; (x)mol C biomass<sup>-1</sup> d<sup>-1</sup>
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| '''Community captured:''' all (usually > 0.2 µm)
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| '''Co-measurements:''' temperature, PAR (simulated ''in situ''), DIC, biomass (Chl, cells, POC, cell volume)
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== Method Overview ==

Radioactive bicarbonate (NaH<sup>14</sup>CO<sub>3</sub>) is added to seawater samples incubated under simulated ''in situ'' conditions of temperature and photosynthetically active radiation (PAR). Phytoplankton incorporate <sup>14</sup>C into organic matter during photosynthesis. At the end of the incubation, samples are filtered onto membrane filters, acidified to remove dissolved inorganic <sup>14</sup>C, and the organically fixed <sup>14</sup>C is quantified by liquid scintillation counting. The resulting rate reflects net primary production (NPP), as some of the fixed carbon is respired back to CO<sub>2</sub> during the incubation period<ref name="Nielsen1952">Steemann Nielsen, E. (1952). The use of radio-active carbon (C<sup>14</sup>) for measuring organic production in the sea. ''ICES Journal of Marine Science'', 18(2), 117–140. https://doi.org/10.1093/icesjms/18.2.117</ref>.

The method can be applied to bulk community samples, size-fractionated fractions, or individual sorted cells, and is routinely performed across depth profiles to characterize the vertical distribution of NPP.

=== Scale of measurement ===

Each incubation provides a point measurement in space. Standard incubation durations of 12–24 h integrate photosynthetic activity over a full light cycle, yielding a daily-integrated NPP estimate. Depth profiles require separate incubations at each target depth under the appropriate irradiance level.

=== Data generated ===

The method yields NPP expressed as carbon fixed per unit volume and time. When normalized to biomass (chlorophyll, cell carbon, or POC), it gives the carbon-specific production rate. The fraction of fixed carbon released as dissolved organic carbon (DOC) is not captured by standard filtration and requires additional measurements.

=== Units & currency ===

Units are µmol C L<sup>-1</sup> d<sup>-1</sup>, or (x)mol C biomass<sup>-1</sup> d<sup>-1</sup> when normalized to biomass. 

=== Sample size ===

Typical samples are 0.5-1 L in volume; multiple replicate bottles are incubated at each depth.

=== Repositories & databases ===

== Limitations ==

Bottle confinement excludes larger grazers and alters the physico-chemical environment relative to ambient conditions. Incubation duration determines the balance between GPP and NPP captured: short incubations approach GPP, while 24 h incubations approach NPP. Excretion of labelled dissolved organic carbon is not retained on the filter and is lost from the measured rate. Small bottle volumes discriminate against larger, less abundant phytoplankton taxa.

== Example Applications & Protocols ==

=== Classic examples ===
* Steemann Nielsen (1952) ''The use of radio-active carbon (C<sup>14</sup>) for measuring organic production in the sea'' <ref name="Nielsen1952" />

=== Recent applications ===
*

=== Common calculations/conversions ===
* NPP (µmol C L<sup>-1</sup> d<sup>-1</sup>) = (<sup>14</sup>C<sub>POC</sub> / <sup>14</sup>C<sub>added</sub>) × DIC<sub>initial</sub> × (1.05/incubation time) ; the factor 1.05 corrects for isotopic discrimination against <sup>14</sup>C relative to <sup>12</sup>C.

== References ==

[[Category:Main Pages|Model types]]