14-Carbon uptake (NPP)

• Page authors: Robert Strzepek
• Responsible curator: Hagen Buck-Wiese

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Net primary production
Approach: radiotracer (14C) uptake
Context: incubation, simulated in situ
Spatial scale: point sample
Temporal scale: 2–24 h
Units: µmol C L-1 d-1; (x)mol C biomass-1 d-1
Community captured: all (usually > 0.2 µm); size fractions are typically 0.2-2.0, 2.0-20, and >20 µm
Co-measurements: temperature, PAR (simulated in situ), DIC, biomass (Chl, cells, POC, cell volume)

Method Overview

Radioactive bicarbonate (NaH¹⁴CO₃) is added to seawater samples incubated under simulated in situ conditions of temperature and photosynthetically active radiation (PAR). Phytoplankton incorporate ¹⁴C into organic matter during photosynthesis. At the end of the incubation, samples are filtered onto membrane filters, acidified to remove dissolved inorganic ¹⁴C, and the organically fixed ¹⁴C is quantified by liquid scintillation counting. Incubation duration determines the balance between Gross Primary Productivity (GPP) and Net Primary Productivity (NPP) captured: short incubations (e.g. 2h) approach GPP, while 24 h incubations approach NPP as some of the fixed carbon is respired back to CO₂ during the incubation period.

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. For daily-integrated NPP estimates, standard incubation durations of 12–24 h integrate photosynthetic activity over a full light cycle. Depth profiles require separate incubations at each target depth under the appropriate irradiance level.

Data generated

The method yields GPP or 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^-1 d^-1, or (x)mol C biomass^-1 d^-1 when normalized to biomass.

Sample size

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

Limitations

Bottle confinement excludes larger grazers and alters the physico-chemical environment relative to ambient conditions. 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 (C¹⁴) for measuring organic production in the sea

Common calculations/conversions

• NPP (µmol C L^-1 d^-1) = (¹⁴C_POC / ¹⁴C_added) × DIC_initial × (1.05/incubation time) ; the factor 1.05 corrects for isotopic discrimination against ¹⁴C relative to ¹²C.

References

Model types