Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

Rixen, Tim; Cowie, Greg; Gaye, Birgit; Goes, Joaquim; do Rosário Gomes, Helga; Hood, Raleigh R.; Lachkar, Zouhair; Schmidt, Henrike; Segschneider, Joachim; Singh, Arvind

doi:https://doi.org/10.5194/bg-17-6051-2020

Articles | Volume 17, issue 23

https://doi.org/10.5194/bg-17-6051-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Understanding the Indian Ocean system: past, present and future...

https://doi.org/10.5194/bg-17-6051-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 23

Reviews and syntheses

|

04 Dec 2020

Reviews and syntheses |

| 04 Dec 2020

Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

Tim Rixen, Greg Cowie, Birgit Gaye, Joaquim Goes, Helga do Rosário Gomes, Raleigh R. Hood, Zouhair Lachkar, Henrike Schmidt, Joachim Segschneider, and Arvind Singh

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Final revised paper (published on 04 Dec 2020)
Preprint (discussion started on 06 Apr 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review of Rixen et al', Anonymous Referee #1, 06 May 2020
- AC1: 'The authors point-by-point reply to the comments of reviewer #1', Tim Rixen, 26 Jun 2020
RC2: 'Review of Rixen et al.', Anonymous Referee #2, 18 May 2020
- AC3: 'The authors point-by-point reply to the comments of reviewer #2', Tim Rixen, 26 Jun 2020
RC3: 'Review of bg-2020-82 - Present past and future of the OMZ in the northern Indian Ocean', Anonymous Referee #3, 10 Jun 2020
- AC4: 'The authors point-by-point reply to the comments of reviewer #3', Tim Rixen, 26 Jun 2020
AC2: 'The authors point-by-point reply to the comments of reviewer #2', Tim Rixen, 26 Jun 2020

Peer-review completion

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

ED: Reconsider after major revisions (06 Jul 2020) by Viviane Menezes

AR by Tim Rixen on behalf of the Authors (01 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Sep 2020) by Viviane Menezes

RR by Annie Bourbonnais (05 Oct 2020)

Suggestions for revision or reasons for rejection

General comment:

In their revised manuscript, Rixen et al. addressed most of my concerns. The revised manuscript is greatly improved, and was completely re-structured. Although quite dense and not always concise, this manuscript will represents a great contribution to the special issue “Understanding the Indian Ocean system: past, present and future” in Biogeosciences. I recommend minor revisions after addressing the few comments below.
Overall, some confusion still persists regarding their O2 threshold used to define hypoxia. In their response, they explain that they considered 20 M as a upper threshold below which denitrification and anammox occurs, i.e., fixed nitrogen is transformed to N2. However, such a high O2 threshold is not supported by recent studies (including Dalsgaard et al. (2014) and Bristow et al. (2016)). For instance, Dalsgaard et al. (2014) reports O2 threshold in the nmol range for conversion of nitrate to N2. Other recent studies (e.g., Frey et al. 2020) report a much higher O2 threshold (up to 10 mol) for nitrate reduction to N2O, yet lower than the suggested value of 20 mol. Furthermore, later on (response to my comment, line 471 of the original manuscript), they admit using a O2 threshold of 6 M for denitrification in the PISCES model, which makes it even more confusing. I recommend the authors to lower their upper O2 threshold for denitrification (and hypoxia) to at most 10 M, which would be more in line with the value suggested by most recent studies as well as the value used in the PISCES model.
Additionally, sulfate reduction could theoretically only occur after the development of anoxic conditions, hence anoxic waters are not necessarily sulfidic (for instance, if nitrate concentrations are high, nitrate will first be used as the terminal electron acceptor during respiration). Defining anoxia as purely sulfidic conditions is consequently misleading.

Abstract:

Lines 20-22: This sentence is confusing. What do they mean by “… which includes negative feedback mechanisms reducing the oxygen consumption at decreasing oxygen concentrations.”? I assume denitrification/anammox and reduced respiration are the negative feedback mechanisms? I would change for: “which includes negative feedback mechanisms reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration).

Text:

Lines 123-135: I would consider a lower O2 threshold for microbial hypoxia of 10 M (see my comment above).

Lines 415-416: Why is the reliability of the older data set questioned?

Lines 575-578: I would cite Fassbender et al. (2018), which offers a concise reviews of mesoscale and sub-mesoscale circulation in the ocean.

Lines 618-623: I don’t think this mechanism is well represented in Figure 11. Export production seems to be equal in both panels (with and without eddies).

Technical corrections:

Line 254: change to “ in combination with the a strong inflow…”

Lines 924-925: This sentence needs to be revised: “from the lack of oxygen the tolerance to decreasing oxygen, critical concentrations…”

Figure 6: change to: “meridional overturning circulation in the Indian Ocean according to…”

Additional references:

Fassbender, A. J., Bourbonnais, A., Clayton, S., Gaube, P., Omand, M., Franks, P. J. S., ... & McGillicuddy Jr, D. (2018). Interpreting mosaics of ocean biogeochemistry. Eos, 99(10.1029).

Frey, C., Bange, H. W., Achterberg, E. P., Jayakumar, A., Löscher, C. R., Arévalo-Martínez, D. L., ... & Oleynik, S. (2020). Regulation of nitrous oxide production in low-oxygen waters off the coast of Peru. Biogeosciences, 17(8), 2263-2287.

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RR by Anonymous Referee #4 (20 Oct 2020)

Suggestions for revision or reasons for rejection

Rixen et al. assemble a comprehensive and useful overview of the state of knowledge of the OMZ in the northern Indian Ocean, specifically the Arabian Sea (AS) and Bay of Bengal (BoB). They address a wide range of topics related to biological and physical dynamics, paleo records, and model results. The authors addressed most of the 3 anonymous reviewers’ comments on the original manuscript. Overall, the revised manuscript is more comprehensible, however the revisions did not unify the different sections of the manuscript - the overall structure and content of each section remained the same, which results in repetition and disjointedness. Some examples: Sections 2.2 and 3.1 both discuss the link between OMZs and upwelling and parts could be combined; Sections 2.3 and 7.1 both discuss impacts of OMZ expansion and intensification on biology; Sections 2.1 and 4 both discuss the physical processes that ventilate the OMZ – from large scale circulation to mesoscale eddies – and these could be tied together somehow. Combining some of these sections and eliminating repetition would streamline this manuscript and help the reader understand the points the authors’ are trying to make.

Specific comments:

Lines 58-60. “The transition from anaerobic…hydrogen sulfide.” From the first part of this sentence I expected to read about the steps from anaerobic to aerobic. The second part of this sentence seems to be a non sequitur. Additionally, sulfate reduction can be coupled to the oxidation of multiple types of reduced carbon, not only methane.

Section 2.1. This entire section is confusing and needs work. The authors first state that both primary production and flux from the atmosphere supply oxygen to the surface (lines 171-172), but then make no mention of the role of primary productivity in affecting the gradient/flux between the OMZ and the surface (lines 194-205). Then it seems like the authors are arguing that vertical mixing ventilates the OMZ (lines 194-205), but then they discuss the importance of lateral mixing (lines 210-211).

Line 172. After “sources of dissolved oxygen”, insert “to the surface”.

Line 253. “Fig. 4c” should be Figs. 3a and 3b.

Lines 503 and 506, Fig. 7a. “deepening” and “OMZ depth” Do the authors mean “thickening” and “thickness”?

Line 540. “low oxygen consumption” Should this be “high oxygen consumption”?

Lines 622-623. “The consequence is an expansion of the volume of the OMZ.” Fig. 11 shows the opposite – eddies decrease the size of the OMZ. I understand that the effect of eddies can be to both increase and decrease the size of the OMZ but Fig. 11 only shows eddies decreasing the OMZ by increasing ventilation and the juxtaposition with the text is confusing.

Line 728. “Fig. 6a” should be 12b and 12c.

Section 7.1. There is no mention of the BoB.

Line 1291. “section 7.3” should be 7.2.

Figures 4 and 6. These figures do not add much information beyond what is stated in the text. I suggest removing them to help shorten this manuscript.

Figure 7a and b. Please provide maximum and minimum ranges for these mean values.

Figure 7b. “mean OMZ oxygen concentration” How is this value calculated? Volume-weighted average of water <20 micromolar oxygen?

Figure 8a and b. Same as with Fig. 7 – please provide ranges.

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ED: Publish subject to minor revisions (review by editor) (24 Oct 2020) by Viviane Menezes

AR by Tim Rixen on behalf of the Authors (07 Nov 2020) Author's response Manuscript

ED: Publish subject to technical corrections (19 Nov 2020) by Viviane Menezes

AR by Tim Rixen on behalf of the Authors (19 Nov 2020) Author's response Manuscript

Short summary

The northern Indian Ocean hosts an extensive oxygen minimum zone (OMZ), which intensified due to human-induced global changes. This includes the occurrence of anoxic events on the Indian shelf and affects benthic ecosystems and the pelagic ecosystem structure in the Arabian Sea. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features, reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.