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Size-fractionated (15N-ρurea) uptake Regenerated Production

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Urea uptake, size-fractionated
Approach: 15N tracer, size-fractionated filtration, 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: 0.7 µm (GFF), 5 µm, and sometimes 20 µm size fractions
Co-measurements: background δ15N (PON), ambient [urea], temperature, PAR, Chl, initial blank

Method Overview

The size-fractionated variant of the urea uptake method. After a 15N-urea tracer incubation, samples are filtered sequentially through 0.7, 5, and sometimes 20 µm filters. EA-IRMS measurement of 15N enrichment on each fraction yields size-class-specific urea uptake rates, allowing the contribution of different phytoplankton size classes to urea-based regenerated production to be resolved[1].

Scale of measurement

Point sample; 12–24 h incubation.

Data generated

Size-class-specific urea uptake rates (µmol N L-1 d-1).

Units & currency

Units are µmol N L-1 d-1 per size fraction. The currency is nitrogen.

Sample size

Typical samples are 0.5–2 L in volume.

Repositories & databases

Limitations

Same as the bulk urea uptake method. Additional limitation: isotope dilution may be particularly severe in size fractions dominated by heterotrophic bacteria, which remineralise organic nitrogen to urea rapidly.

Example Applications & Protocols

Classic examples

  • Dugdale & Goering (1967) Uptake of new and regenerated forms of nitrogen in primary productivity [2]
  • Dugdale & Wilkerson (1986) The use of 15N to measure nitrogen uptake in eutrophic oceans [1]

Recent applications

  • Meyer et al. (2022) Phytoplankton size-class contributions to new and regenerated production during EXPORTS [3]

Common calculations/conversions

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. ↑ 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
  3. ↑ 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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