Size-fractionated 15N-ρNO3/chl a (N assimilation rate)

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N assimilation rate, size-fractionated (Chl-normalised)
Approach: ¹⁵N tracer, size-fractionated filtration, Chl normalisation
Context: incubation, simulated in situ
Spatial scale: point sample
Temporal scale: 12–24 h
Units: (x)mol N µg Chl^-1 d^-1
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

Method Overview

This variant combines size-fractionated filtration with chlorophyll normalization. After a ¹⁵N-nitrate tracer incubation under simulated in situ conditions, samples are passed sequentially through filters of different pore sizes (0.7, 5, and sometimes 20 µm). The ¹⁵N enrichment on each filter fraction is measured by EA-IRMS, and the resulting size-class-specific uptake rate is normalized to the chlorophyll-a concentration in the corresponding size fraction^[1]. This yields the chlorophyll-specific nitrate assimilation rate for each plankton size class, enabling biomass-independent comparisons of size class nitrogen acquisition.

Scale of measurement

Point sample; 12–24 h incubation.

Data generated

Chlorophyll-specific nitrate uptake rate per size fraction (V_NO3, (x)mol N µg Chl^-1 d^-1). Reveals whether the chlorophyll-specific assimilation rate differs between large and small phytoplankton.

Units & currency

Units are (x)mol N µg Chl^-1 d^-1. The currency is nitrogen.

Sample size

Typical samples are 0.5–2 L in volume.

Repositories & databases

Limitations

Inherits limitations of both the size-fractionated and chlorophyll-normalized variants: cells may straddle filter size cuts, and Chl:C ratios vary with photoacclimation and nutrient status. Chlorophyll in each size fraction must be measured independently for reliable normalization.

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

V_NO3(fraction X) = ρNO₃^-(fraction X) / [Chl]_{fraction X}.

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
↑ 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

[Dugdale1986-1] 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

[Dugdale1967-2] 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

[Meyer2022-3] 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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