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(15N-ρNO3-) uptake New Production

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Nitrate uptake (new production)
Approach: 15N tracer incubation, EA-IRMS
Context: incubation, simulated in situ
Spatial scale: point sample
Temporal scale: 12–24 h
Units: µmol N L-1 d-1; µmol N m-2 d-1 (depth-integrated)
Community captured: all (usually > 0.2 µm)
Co-measurements: background δ15N (PON), ambient [NO3-], temperature, PAR, Chl, initial blank (spiked and filtered immediately); dark uptake; can couple with 13C uptake in same assay

Method Overview

15N-labelled nitrate (K15NO3) is added to seawater samples at tracer concentrations and the samples are incubated under simulated in situ conditions of temperature and photosynthetically active radiation (PAR). Phytoplankton and other organisms take up the labelled nitrate and incorporate 15N into particulate nitrogen. At the end of the incubation, the particulate fraction is collected on pre-combusted GF/F filters (nominal pore size 0.7 µm); the 15N enrichment of particulate nitrogen (PN) is measured by elemental analyzer–isotope ratio mass spectrometry (EA-IRMS). The absolute nitrate uptake rate (ρNO3-) is calculated from the isotope enrichment using the constant-flux model of Dugdale & Wilkerson (1986)[1], based on the PN concentration at the start of the incubation. In the framework of Eppley & Peterson (1979), nitrate-based uptake corresponds to new production[2].

Scale of measurement

Each incubation provides a point measurement. Standard incubation durations of 12–24 h integrate nitrogen uptake over a diel cycle, yielding a daily-integrated rate. Depth profiles require separate incubations at each target depth under appropriate irradiance levels. Rates can be depth-integrated (µmol N m-2 d-1) for comparison with other production estimates.

Data generated

The method yields nitrate uptake rates in µmol N L-1 d-1; conversion to carbon units uses the measured POC:PON ratio (or the Redfield C:N ratio of 6.6 as a default). Combined with ammonium and urea uptake rates, the f-ratio (new production / total N uptake) can be calculated as an indicator of ecosystem nutrient status.

Units & currency

Units are µmol N L-1 d-1 (volumetric) or µmol N m-2 d-1 (depth-integrated). The currency is nitrogen.

Sample size

Typical samples are 0.5–2 L in volume.

Repositories & databases

Limitations

The method assumes steady-state conditions during the incubation: nitrate concentration does not change significantly, and nitrification does not introduce unlabelled nitrate that would dilute the 15N pool. Bottle confinement can alter community composition and activity. Short incubations (< 6 h) minimize bottle effects but may miss diel uptake patterns; 24 h incubations integrate over the full light cycle but assume constant rates. Excretion of labelled dissolved organic nitrogen is not retained on the filter and is lost from the measured rate.

Example Applications & Protocols

Classic examples

  • Dugdale & Goering (1967) Uptake of new and regenerated forms of nitrogen in primary productivity [3]
  • Dugdale & Wilkerson (1986) The use of 15N to measure nitrogen uptake in eutrophic oceans [1]
  • Eppley & Peterson (1979) Particulate organic matter flux and planktonic new production in the deep ocean [2]

Recent applications

  • Varela et al. (2013) Pelagic primary productivity and upper ocean nutrient dynamics across subarctic and Arctic seas [4]
  • Meyer et al. (2022) Phytoplankton size-class contributions to new and regenerated production during EXPORTS [5]

Common calculations/conversions

  • ρNO3- (µmol N L-1 d-1) = [(atom% 15NPN,final − atom% 15NPN,initial) / (atom% 15NNO3,added − atom% 15Nnatural)] × [PN] / incubation time.
  • f-ratio = ρNO3- / (ρNO3- + ρNH4+ + ρurea).

References

  1. ↑ 1.0 1.1 Dugdale, R. C., & Wilkerson, F. P. (1986). The use of 15N to measure nitrogen uptake in eutrophic oceans; experimental considerations. Limnology and Oceanography, 31(4), 673–689. https://doi.org/10.4319/lo.1986.31.4.0673
  2. ↑ 2.0 2.1 Eppley, R. W., & Peterson, B. J. (1979). Particulate organic matter flux and planktonic new production in the deep ocean. Nature, 282, 677–680. https://doi.org/10.1038/282677a0
  3. ↑ Dugdale, R. C., & Goering, J. J. (1967). Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography, 12(2), 196–206. https://doi.org/10.4319/lo.1967.12.2.0196
  4. ↑ Varela, D. E., Crawford, D. W., Wrohan, I. A., Wyatt, S. N., & Carmack, E. C. (2013). Pelagic primary productivity and upper ocean nutrient dynamics across subarctic and Arctic seas. Journal of Geophysical Research: Oceans, 118, 7132–7152. https://doi.org/10.1002/2013JC009211
  5. ↑ Meyer, M. G., Davis, P. B., Vogt, M., & Tortell, P. D. (2022). Phytoplankton size-class contributions to new and regenerated production during the EXPORTS Northeast Pacific Ocean field deployment. Elementa: Science of the Anthropocene, 10(1). https://doi.org/10.1525/elementa.2021.00068
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