(15N-ρurea) uptake Regenerated Production
| Urea 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 [urea], temperature, PAR, Chl, initial blank; can couple with 13C uptake |
Method Overview
15N-labelled urea (CO(15NH2)2) is added at tracer concentrations to seawater incubated under simulated in situ conditions. Urea is an important dissolved organic nitrogen (DON) source for phytoplankton, particularly in low-nitrate environments. After incubation, the particulate fraction is filtered onto GFF filters; EA-IRMS measures the 15N enrichment in PN, and the urea uptake rate (ρurea) is calculated using the constant-flux model[1]. In the Eppley & Peterson framework, urea uptake is classified as regenerated production[2].
Scale of measurement
Point sample; 12–24 h incubation.
Data generated
Urea uptake rates in µmol N L-1 d-1. When measured alongside nitrate and ammonium uptake, urea provides the third component of the total dissolved inorganic and organic nitrogen uptake budget.
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
Ambient urea concentrations in the euphotic zone are often very low (nM), which can violate the tracer requirement and cause underestimation. As with ammonium, microbial regeneration of urea during the incubation can dilute the 15N pool (isotope dilution), leading to underestimation. 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 [1]
Recent applications
- Varela et al. (2013) Pelagic primary productivity and upper ocean nutrient dynamics across subarctic and Arctic seas [4]
Common calculations/conversions
- ρurea is calculated identically to ρNO3- using the 15N-urea enrichment in PN and the initial PN concentration.
References
- ↑ 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
- ↑ 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
- ↑ 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