Diapycnal dissolved organic matter supply into the upper Peruvian oxycline

Loginova, Alexandra N.; Thomsen, Sören; Dengler, Marcus; Lüdke, Jan; Engel, Anja

doi:https://doi.org/10.5194/bg-16-2033-2019

Articles | Volume 16, issue 9

https://doi.org/10.5194/bg-16-2033-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-16-2033-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 9

Research article

|

15 May 2019

Research article |

| 15 May 2019

Diapycnal dissolved organic matter supply into the upper Peruvian oxycline

Alexandra N. Loginova, Sören Thomsen, Marcus Dengler, Jan Lüdke, and Anja Engel

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Final revised paper (published on 15 May 2019)
Preprint (discussion started on 15 Jun 2018)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review of Loginova', Anonymous Referee #1, 02 Jul 2018
- AC1: 'Author Comment 1 (AC1) for Anonymous Review Comment 1 (RC1) on “Diapycnal dissolved organic matter supply into the Peruvian oxycline”.', Anja Engel, 10 Aug 2018
RC2: 'Chemical Oceanography, Biogeochemistry', Ronald Benner, 08 Jul 2018
- AC2: 'Author Comment 2 (AC2) for Review Comment 2 (RC2) by R. Benner on “Diapycnal dissolved organic matter supply into the Peruvian oxycline”.', Anja Engel, 10 Aug 2018

Peer-review completion

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

ED: Reconsider after major revisions (26 Aug 2018) by Gerhard Herndl

AR by Anja Engel on behalf of the Authors (28 Sep 2018) Author's response Manuscript

ED: Reconsider after major revisions (07 Oct 2018) by Gerhard Herndl

AR by Anja Engel on behalf of the Authors (05 Nov 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Nov 2018) by Gerhard Herndl

RR by Anonymous Referee #1 (29 Nov 2018)

Suggestions for revision or reasons for rejection

General Comments:

1. The Introduction needs to be better written. I was confused by the logical construction of the main points being made too many times. For example, the statement “changes in the remineralization rate of DOM might rather be linked to lack of bioavailable OM supply into the OMZ than to low-O2 conditions” was not preceded by any mention that DOM consumption rates vary in the water column. Instead, we were told that “Recent studies in the upwelling area and the corresponding OMZ off Chile… found bacterial activity of similar range to the oxygenated waters..” This finding does not tell us about DOM.

Another example of confusing text: “Assessing the possible effects of low-O2 conditions on the composition of DOM implies that the DOM is transported into the OMZ from the oxygenated waters.” I do not know what that means.

2. I am not a fan of averaging all of the measurements (as in Figure 4) and then evaluating the profiles of the mean concentrations for vertical fluxes. Clearly, the waters very near the coast are upwelling while those further offshore are highly stratified. In O2 (Fig 4a), for example, we see surface concentrations ranging from near zero at the coast to >200 uM offshore; the mean of O2 is neither useful nor insightful. I suggest evaluating the stratified region for diapyncal fluxes. Inclusion of the waters that are actually upwelling will bias the answer to the question posed. The authors agree with the when they state that “This spatial averaging is likely responsible for a lower near-surface diapycnal O2 flux compared to other eastern boundary upwelling systems”, so why do the averaging?

3. I’m unclear on why the authors offered in this paper two disconnected questions: i) how important is the diapycnal flux of DOM and ii) does the low O2 in the OMZ cause changes in the DOM composition?

Specific Comments:

Page/Line

3/20: the “L” is missing in the units for P.

3/22: “where” rather than “here”

5/1: remove “,” after “significant”. In general, the authors should look at every comma used and determined whether or not it is used correctly; there were too many poorly placed commas in the text. Do not use them where a pause is not intended.

7/11: I appreciate the author’s doubting the very high DOC values they report. DOC concentrations of 100-120 uM are not typically observed unless a river is nearby. Franz et al 2012 was cited as a paper similarly reporting high DOC off Peru, but that work was done in a mesocosm; I do not see where the authors reported high DOC in a natural setting. And the DOC data coming out of the Romankevich lab in the 80’s, another paper cited for high values off Peru, are also highly suspect. I wouldn’t cite that paper since the data are not reliable. I hope these high values were not included in calculating the diapycnal fluxes of DOC; the high values should be excluded.

7/20: “DCCHO represented from 1 to 25% of DOC”. I am confused about which DCCHO data to look at when considering that statement. In Table 1 I see the DCCHO in the depth range of 1-5 m sums to 1.7 umol/L (nS+S-N+S-H), while in Fig. 4d the DCCHO mean is closer to 10 umol/L. Which DCCHO data should I be looking at; the two sources (table vs figure) seem to disagree with one another.

Table 2: why are the numbers in bold type? I do not see the need for that. And why in the lower section is the upper most depth range “BML-38”, which reaches into the middle of the depth range just below it (24-52). Should it be “BML-24”, as in the upper section of the table?

8/21: remove ‘,’ after “to understand”

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RR by Ronald Benner (06 Dec 2018)

ED: Reconsider after major revisions (16 Jan 2019) by Gerhard Herndl

AR by Anja Engel on behalf of the Authors (25 Feb 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Mar 2019) by Gerhard Herndl

RR by Anonymous Referee #1 (01 Apr 2019)

ED: Publish subject to technical corrections (28 Apr 2019) by Gerhard Herndl

AR by Anja Engel on behalf of the Authors (02 May 2019) Author's response Manuscript

Short summary

High primary production in the Peruvian upwelling system is followed by rapid heterotrophic utilization of organic matter and supports the formation of one of the most intense oxygen minimum zones (OMZs) in the world. Here, we estimated vertical fluxes of oxygen and dissolved organic matter (DOM) from the surface to the OMZ. Our results suggest that DOM remineralization substantially reduces oxygen concentration in the upper water column and controls the shape of the upper oxycline.