Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled
export production in the VAHINE mesocosm experiments

Knapp, Angela N.; Fawcett, Sarah E.; Martínez-Garcia, Alfredo; Leblond, Nathalie; Moutin, Thierry; Bonnet, Sophie

doi:https://doi.org/10.5194/bg-13-4645-2016

Articles | Volume 13, issue 16

https://doi.org/10.5194/bg-13-4645-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Biogeochemical and biological response to a diazotroph bloom...

https://doi.org/10.5194/bg-13-4645-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 16

Research article

|

18 Aug 2016

Research article |

| 18 Aug 2016

Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in the VAHINE mesocosm experiments

Angela N. Knapp, Sarah E. Fawcett, Alfredo Martínez-Garcia, Nathalie Leblond, Thierry Moutin, and Sophie Bonnet

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Final revised paper (published on 18 Aug 2016)
Supplement to the final revised paper
Preprint (discussion started on 14 Dec 2015)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC C8769: 'Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in VAHINE mesocosm experiments', Anonymous Referee #1, 30 Dec 2015
RC C10130: 'Peer Review of Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in VAHINE mesocosm experiments by A.N. Knapp et al.', T.B. Meador, 09 Mar 2016
RC C10337: 'Peer Review of “Nitrogen isotopic evidence for a shift from nitrate- to diazotroph-fueled export production in VAHINE mesocosm experiments” by Knapp et al', Anonymous Referee #3, 28 Mar 2016
AC C10518: 'Author comments', Angela Knapp, 26 Apr 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (28 Apr 2016) by François Lacan

AR by Angela Knapp on behalf of the Authors (13 Jun 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Jun 2016) by François Lacan

RR by Anonymous Referee #3 (06 Jul 2016)

RR by Travis Blake Meador (07 Jul 2016)

Suggestions for revision or reasons for rejection

The issue of publishing alternative DON concentrations for the same samples/experiment is still awkward, but the authors have handled this by providing some methodological explanation and discussing how this may have impacted their results.

That the mesocosm was isolated from external inputs of N and thus N-fixation was the only N source by P2, it remains puzzling (to me) that the d15N-PNsusp would remain constant and consistent with that observed previously in the turbulent and diffuse open ocean. The manuscript has been improved by the addition of a clear estimate of the timescale for the acquisition of N via N-fixation to export as sinking particles (< 1 day), such that DDN does not accumulate as PNsusp and sinking particles are the only bulk geochemical tracer of N-fixation. If I understand correctly, this would be the estimate for the turnover time of DDN in the mesocosm; the study would benefit from directly stating or calculating this value, if possible.

The authors note that particles in the ocean have a distinct d15N value and the measured value of d1N-PNsusp thus records their mass weighted average (page 12 lines 20-24). This same concept also applies to d15N-PNsink. As pointed out in the initial review, the current study provides only indirect evidence to support the claim that DDN was transferred to non-diazotrophic phytoplankton before sinking into sediment traps. This is indeed one explanation of their data, but this conclusion should reside with the companion study that directly measured this flux (Bonnet et al., 2016). Moreover, since the previous revision of the manuscript, Hunt et al. (2016) has revealed that zooplankton graze on unicellular diazotrophs, thus providing another mechanism to explain the depleted values of d15N-PNsink. The particles that collected in the sediment trap were neither characterized by the current study nor have been detailed by any companion study. It is thus not clear how the relative abundance or d15N value of particles in the sediment trap varied during the experiment (UCYN-aggregates, non-diazotrophic phytoplankton, fecal pellets; please provide a reference for these data if available). The Bonnet et al. (2016) study would suggest that there was significant variability in UCYN-aggregates, both temporally and between replicate mesocosms, yet the authors have not attempted to provide a 15N mass balance of the various components of the sinking material. Therefore, despite the bulk isotopic evidence indicating that N derived from N2-fixation was rapidly channeled into sinking particles, there is no direct evidence introduced by the current study to interpret, suggest, or support a mechanism for this flux. While appropriate to include in the Discussion section (page 18, lines 22-26), the principal finding of Bonnet et al. (2016) should not be reasserted in the Conclusions of the current study (page 22, lines 2-25 can be removed), which should instead focus on the timing of this flux, as nicely stated (page 21 thru page 22, line 2).

References:
Bonnet et al., Biogeosciences, 13, 2653–2673, 2016
Hunt et al., Biogeosciences, 13, 3131–3145, 2016

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ED: Publish subject to minor revisions (Editor review) (11 Jul 2016) by François Lacan

AR by Angela Knapp on behalf of the Authors (21 Jul 2016)

ED: Publish as is (29 Jul 2016) by François Lacan

AR by Angela Knapp on behalf of the Authors (29 Jul 2016) Author's response Manuscript

Short summary

The goal of this manuscript was to track the fate of newly fixed nitrogen (N) in large volume mesocosms in the coastal waters of New Caledonia. We used a N isotope ("δ¹⁵N") budget and found a shift in the δ¹⁵N of sinking particulate N over the 23-day experiment, indicating that nitrate supported export production at the beginning of the experiment, but that nitrogen fixation supported export at the end. We infer that nitrogen fixation supported export production by a release of dissolved N.