Size-fractionated (15N-ρNO3-) uptake New Production
| Nitrate uptake, size-fractionated |
|---|
| Approach: 15N tracer incubation, 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 [NO3-], temperature, PAR, Chl, initial blank; can couple with 13C uptake |
Method Overview
This is the size-fractionated variant of the bulk 15N-nitrate uptake method. 15N-labelled nitrate (K15NO3) is added at tracer concentrations to seawater samples incubated under simulated in situ conditions. At the end of the incubation, samples are passed sequentially through filters of different pore sizes (typically 0.7 µm GFF, 5 µm polycarbonate, and sometimes 20 µm polycarbonate), partitioning the nitrogen uptake signal among size classes corresponding broadly to bacteria and picophytoplankton, nanophytoplankton, and microphytoplankton. 15N enrichment on each filter is measured by EA-IRMS, and size-class-specific uptake rates are calculated using the constant-flux model[1].
Scale of measurement
As for the bulk method, each incubation provides a point measurement. The size-fractionated approach adds the dimension of size class resolution, revealing which plankton size fractions dominate new production at the sampling location.
Data generated
Size-class-specific nitrate uptake rates (µmol N L-1 d-1) for each filter fraction. The sum of all fractions gives total community nitrate uptake. The relative contributions of each size class to total nitrate uptake can reveal taxonomic or ecological differences across regions and seasons.
Units & currency
Units are µmol N L-1 d-1 per size fraction, or µmol N m-2 d-1 (depth-integrated). The currency is nitrogen.
Sample size
Typical samples are 0.5–2 L in volume per incubation.
Repositories & databases
Limitations
The same assumptions as the bulk method apply. An additional limitation is that cells may pass through or be retained on filters imprecisely depending on their morphology, cell aggregation state, or chain formation, leading to misassignment of uptake to the wrong size fraction.
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
- Giesbrecht et al. (2019) A decade of summertime measurements of phytoplankton biomass, productivity and assemblage composition in the Pacific Arctic Region [3]
- Meyer et al. (2022) Phytoplankton size-class contributions to new and regenerated production during EXPORTS [4]
Common calculations/conversions
- Size-fraction-specific ρNO3- = ρNO3-(fraction X) / Σ ρNO3-(all fractions) gives the fractional contribution.
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
- ↑ 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
- ↑ Giesbrecht, K. E., Varela, D. E., Wiktor, J., Grebmeier, J. M., Kelly, B., & Long, J. E. (2019). A decade of summertime measurements of phytoplankton biomass, productivity and assemblage composition in the Pacific Arctic Region from 2006 to 2016. Deep-Sea Research Part II, 162, 93–113. https://doi.org/10.1016/j.dsr2.2018.06.010
- ↑ 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