(15N-ρNH4+) uptake Regenerated Production
| Ammonium uptake (regenerated 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 [NH4+], temperature, PAR, Chl, initial blank; dark uptake; can couple with 13C uptake |
Method Overview
15N-labelled ammonium (15NH4Cl) is added to seawater samples at tracer concentrations and incubated under simulated in situ conditions. In the framework of Eppley & Peterson (1979), ammonium-based nitrogen uptake corresponds to regenerated production — the recycling of previously fixed nitrogen within the euphotic zone[1]. After incubation, the particulate fraction is collected on GFF filters; 15N enrichment in PN is measured by EA-IRMS, and the absolute uptake rate (ρNH4+) is calculated using the constant-flux model[2].
Scale of measurement
Point sample; 12–24 h incubation integrating over a diel cycle.
Data generated
Ammonium uptake rates in µmol N L-1 d-1. Combined with nitrate and urea uptake, the f-ratio and the relative contributions of different nitrogen sources to total nitrogen nutrition are calculated.
Units & currency
Units are µmol N L-1 d-1. The currency is nitrogen.
Sample size
Typical samples are 0.5–2 L in volume.
Repositories & databases
Limitations
A critical limitation is that ambient [NH4+] in the euphotic zone is often very low (nM range), which can violate the tracer requirement: adding sufficient 15NH4+ to achieve measurable enrichment may perturb the system. Additionally, ongoing microbial regeneration of NH4+ from unlabelled organic matter during the incubation dilutes the 15N pool, causing underestimation of true uptake rates (isotope dilution). Bottle effects apply.
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 [2]
Recent applications
- Varela et al. (2013) Pelagic primary productivity and upper ocean nutrient dynamics across subarctic and Arctic seas [4]
Common calculations/conversions
- ρNH4+ is calculated identically to ρNO3-; the isotope dilution correction of Glibert et al. should be applied when regeneration is significant.
References
- ↑ 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
- ↑ 2.0 2.1 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
- ↑ 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
- ↑ 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