View source for 15N-ρNO3/chl a (N assimilation rate)

{{BreadcrumbsNutrients}}

* [[Page authors|Page authors]]: [[PRIMO]]
* [[Responsible curator|Responsible curator]]:  [[User:Hagi BucknWise|Hagen Buck-Wiese]]
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{| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;"
! N assimilation rate (Chl-normalised)
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| '''Approach:''' <sup>15</sup>N tracer incubation, EA-IRMS, chlorophyll normalisation
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| '''Context:''' incubation, simulated ''in situ''
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| '''Spatial scale:''' point sample
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| '''Temporal scale:''' 12–24 h
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| '''Units:''' (x)mol N µg Chl<sup>-1</sup> d<sup>-1</sup>
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| '''Community captured:''' all (usually > 0.2 µm)
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| '''Co-measurements:''' background δ<sup>15</sup>N (PON), ambient [NO<sub>3</sub><sup>-</sup>], temperature, PAR, Chl, initial blank
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== Method Overview ==

This is a normalised variant of the <sup>15</sup>N-nitrate uptake method. The incubation and EA-IRMS measurement proceed identically to the bulk nitrate uptake method: <sup>15</sup>N-labelled nitrate is added at tracer concentrations, samples are incubated under simulated ''in situ'' conditions, and the particulate fraction is filtered onto GFF filters for <sup>15</sup>N enrichment analysis. The uptake rate is then normalised to the chlorophyll-a concentration measured in the same sample<ref name="Dugdale1986">Dugdale, R. C., & Wilkerson, F. P. (1986). The use of <sup>15</sup>N 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</ref>. Chlorophyll normalisation removes the effect of varying biomass concentrations across samples, providing the chlorophyll-specific nitrogen assimilation rate (V<sub>NO3</sub>), which is directly comparable across environments and seasons without biomass correction.

=== Scale of measurement ===

Each incubation provides a point measurement. Incubation durations of 12–24 h yield daily-integrated rates. Chlorophyll normalisation does not change the temporal or spatial scale but improves ecological interpretability by removing biomass variability.

=== Data generated ===

Chlorophyll-specific nitrate uptake rate (V<sub>NO3</sub>, (x)mol N µg Chl<sup>-1</sup> d<sup>-1</sup>). This rate reflects the physiological nitrogen acquisition capacity of the phytoplankton community per unit biomass and is used to compare nutrient acquisition across contrasting environments.

=== Units & currency ===

Units are (x)mol N µg Chl<sup>-1</sup> d<sup>-1</sup>. The currency is nitrogen.

=== Sample size ===

Typical samples are 0.5–2 L in volume.

=== Repositories & databases ===

== Limitations ==

In addition to the standard assumptions of the <sup>15</sup>N tracer method (steady-state system, negligible nitrification), the chlorophyll normalisation assumes that chlorophyll-a is an accurate and consistent proxy for phytoplankton biomass. This assumption is violated when chlorophyll:carbon ratios vary substantially due to photoacclimation, nutrient stress, or taxonomic shifts. Chl-specific rates cannot be directly converted to volumetric or areal uptake without a separate biomass estimate.

== Example Applications & Protocols ==

=== Classic examples ===
* Dugdale & Goering (1967) ''Uptake of new and regenerated forms of nitrogen in primary productivity'' <ref name="Dugdale1967">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</ref>
* Dugdale & Wilkerson (1986) ''The use of <sup>15</sup>N to measure nitrogen uptake in eutrophic oceans'' <ref name="Dugdale1986" />

=== Recent applications ===
* Kafrissen et al. (2022) ''An 8-year record of phytoplankton productivity and nutrient distributions from surface waters of Saanich Inlet'' <ref name="Kafrissen2022">Kafrissen, M. E., Giesbrecht, K. E., Cronin, K., Strang, J., Quiring, W., & Varela, D. E. (2022). An 8-year record of phytoplankton productivity and nutrient distributions from surface waters of Saanich Inlet. ''Scientific Data'', 9, 434. https://doi.org/10.1038/s41597-022-01434-y</ref>

=== Common calculations/conversions ===
* V<sub>NO3</sub> ((x)mol N µg Chl<sup>-1</sup> d<sup>-1</sup>) = ρNO<sub>3</sub><sup>-</sup> / [Chl].
* To recover volumetric rate: ρNO<sub>3</sub><sup>-</sup> = V<sub>NO3</sub> × [Chl].

== References ==

[[Category:Main Pages|Model types]]