Modelling the biogeochemical effects of heterotrophic and autotrophic N<sub>2</sub> fixation in the Gulf of Aqaba (Israel), Red Sea

Kuhn, Angela M.; Fennel, Katja; Berman-Frank, Ilana

doi:https://doi.org/10.5194/bg-15-7379-2018

Articles | Volume 15, issue 24

https://doi.org/10.5194/bg-15-7379-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-7379-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 24

Research article

|

14 Dec 2018

Research article |

| 14 Dec 2018

Modelling the biogeochemical effects of heterotrophic and autotrophic N₂ fixation in the Gulf of Aqaba (Israel), Red Sea

Angela M. Kuhn, Katja Fennel, and Ilana Berman-Frank

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Final revised paper (published on 14 Dec 2018)
Supplement to the final revised paper
Preprint (discussion started on 19 Jan 2018)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of "Biogeochemical evidence of heterotrophic N2 fixation in the Gulf of Aqaba (Israel), Red Sea”', Fanny Monteiro, 04 Apr 2018
- AC1: 'Response to comments by Reviewer 1 (F. Monteiro)', Angela Kuhn, 26 May 2018
RC2: 'Review of Kuhn et al', Anonymous Referee #2, 25 Apr 2018
- AC2: 'Response to Reviewer 2', Angela Kuhn, 26 May 2018
RC3: 'Review of Kuhn et al.', Anonymous Referee #3, 26 Apr 2018
- AC3: 'Response to Anonymous Reviewer 3', Angela Kuhn, 26 May 2018
RC4: 'Review of "Biogeochemical evidence of heterotrophic N2 fixation in the Gulf of Aqaba (Israel), Red Sea" by Kuhn et al.', Anonymous Referee #4, 03 May 2018
- AC4: 'Response to Anonymous Reviewer 4', Angela Kuhn, 27 May 2018

Peer-review completion

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

ED: Reconsider after major revisions (28 May 2018) by Jack Middelburg

AR by Angela Kuhn on behalf of the Authors (13 Aug 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (14 Aug 2018) by Jack Middelburg

RR by Anonymous Referee #3 (10 Sep 2018)

Suggestions for revision or reasons for rejection

The authors have done a reasonable job in responding to my previous review. However, I do have a couple of concerns related to two of the more substantive changes which the authors implemented in response to comments previously raised by both myself and the other reviewers.

Major comments

Firstly, many of the reviewers, myself included, commented on the poor agreement between the modelled and the observed N* distributions in the previous version. In response to this the authors have removed direct comparison between observed and modelled N* and instead rely on comparisons between model results and observations of N and P separately. However, the use of N* (or effectively N:P ratios), which will reflect the decoupling between the cycling of N and P (see e.g. Sarmiento and Gruber 1997 and numerous subsequent usage), would still seem like the most obvious and direct constraint on the N cycle in general and N2 fixation in particular within the model. As such I personally found the decision to remove this comparison, rather than, for example, to directly address the prior weakness by investigating how the added sensitivity analyses (see below) influenced model reproduction of N* (and/or N:P ratios) to be a somewhat unsatisfactory response.

Additionally, I was pleased to see the authors perform a sensitivity analysis investigating how changes in the assumed ratios of N:P within both non-N2 fixing phytoplankton and nitrogen fixers influenced their results. However, this sensitivity analysis appeared quite limited. In particular, I was unclear why this analysis was only performed for the H1 version of the model? Also, I note that the sensitivity analysis did not really cover the parameter space of different assumed N:P ratios for phytoplankton and nitrogen fixers very thoroughly, as they were varied independently around their initial assumed values. Wouldn’t it be more appropriate to perform a sensitivity analysis over a matrix of values fully investigating the parameter space?

As an example of how the two points above might interact, I would particularly have liked to see whether a sensitivity analysis of the assumed N:P ratios within the H3 version of the model might influence the ability of this most complex structure to simultaneously reproduce P and N observations, as indicated by N* values (or maybe simply N:P ratios in the subsurface).

Additional more minor comments

Page 4, Line 20: this statement could perhaps be more specific given the different perturbations/versions of each of the models (H0-H3) now investigated.

Page 5, Line 27: ‘diazotrophs’ rather than ‘diazotrophics’

Page 7, Lines 5-14: I’m still not clear why this optimisation method couldn’t be performed for the N:P ratios for the phytoplankton and diazotrophs? (see also Page 7, Line 28)

Page 9, Line 25 onwards. Related to second major comment above, in the absence of any further analysis, some rationale for why this sensitivity analysis was only performed for H1 and only around the initially assumed values (rather than more fully exploring the parameter space), should be provided here.

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ED: Publish subject to minor revisions (review by editor) (19 Oct 2018) by Jack Middelburg

AR by Angela Kuhn on behalf of the Authors (02 Nov 2018) Author's response Manuscript

ED: Publish subject to technical corrections (05 Nov 2018) by Jack Middelburg

AR by Angela Kuhn on behalf of the Authors (09 Nov 2018) Author's response Manuscript

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Short summary

Recent studies demonstrate that marine N₂ fixation can be carried out without light. However, direct measurements of N₂ fixation in dark environments are relatively scarce. This study uses a model that represents biogeochemical cycles at a deep-ocean location in the Gulf of Aqaba (Red Sea). Different model versions are used to test assumptions about N₂ fixers. Relaxing light limitation for marine N₂ fixers improved the similarity between model results and observations of deep nitrate and oxygen.

Modelling the biogeochemical effects of heterotrophic and autotrophic N2 fixation in the Gulf of Aqaba (Israel), Red Sea

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Interactive discussion

Peer-review completion

Modelling the biogeochemical effects of heterotrophic and autotrophic N₂ fixation in the Gulf of Aqaba (Israel), Red Sea