Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

Thushara, Venugopal; Vinayachandran, Puthenveettil Narayana Menon; Matthews, Adrian J.; Webber, Benjamin G. M.; Queste, Bastien Y.

doi:https://doi.org/10.5194/bg-16-1447-2019

Articles | Volume 16, issue 7

https://doi.org/10.5194/bg-16-1447-2019

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

https://doi.org/10.5194/bg-16-1447-2019

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

Articles | Volume 16, issue 7

Research article

|

09 Apr 2019

Research article |

| 09 Apr 2019

Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal

Venugopal Thushara, Puthenveettil Narayana Menon Vinayachandran, Adrian J. Matthews, Benjamin G. M. Webber, and Bastien Y. Queste

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Final revised paper (published on 09 Apr 2019)
Preprint (discussion started on 12 Jul 2018)

Interactive discussion

Status: closed

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

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RC1: 'Review of “Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal” by Thushara et al.', Anonymous Referee #1, 13 Aug 2018
- AC1: 'Interactive comment on “Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal” by Venugopal Thushara et al.', PN Vinayachandran, 24 Oct 2018
RC2: 'Review of ‘Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal’ by Thushara et al.', Emmanuel Boss, 03 Oct 2018
- AC2: 'Interactive comment on “Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal” by Venugopal Thushara et al.', PN Vinayachandran, 24 Oct 2018

Peer-review completion

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

ED: Reconsider after major revisions (09 Nov 2018) by Ciavatta Stefano

AR by P N Vinayachandran on behalf of the Authors (15 Dec 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Dec 2018) by Ciavatta Stefano

RR by Emmanuel Boss (19 Dec 2018)

Suggestions for revision or reasons for rejection

Review of: “Vertical distribution of chlorophyll in dynamically distinct regions of
the southern Bay of Bengal” by Thushara et al.

Reviewer: Emmanuel Boss, UMaine

I read the revised manuscript. It is extremely descriptive (describing in chronological order observed features). The authors do not take full advantage of the sensors on the glider (the backscattering is not analyzed) nor are they taking full advantage of the model (which can provide them with model-consistent explanation to phenomena). The author can easily address photo-acclimation by using the chl/bbp ratio (after appropriate ‘background’ is removed from bbp). Bbp will also help in the interpretation of mixing events and depth integrated inventories.

The paper is still full of ‘bottom-up’ statements such as: “However, the subsurface layers
still possess enough nutrients to support phytoplankton growth” – it is self-evident that where you find [chl] near the surface there are sufficient nutrient and light, even when [chl] are very low. As Laws (2105) suggest, even in what some considered to be oceanic ‘deserts’ cell divide daily meaning they have sufficient nutrients and light. Try to be more nuanced in your attempt to explain the observed.

One way to think about the ocean plankton is as being very close to steady state with perturbation enhancing/reducing phytoplankton growth-rates. When growth-rates are enhanced, phytoplankton accumulation occurs, observed as an increase in biomass. When growth-rates are decreased, loss processes have the upper hand and biomass decreases. It is almost always result of small differences between loss and growth processes. The model you are using (within its assumptions) can help you diagnose this. A physical phenomena is often the cause of such small decoupling. I do not want to you to espouse a view that you do not believe in, but want you to think about the difference between accumulation rate (which you observe) and growth-rate (which you can estimate from a productivity model – but cannot calculate from your observations directly).

Some other comments:
Your MLD criterion is clearly not a good mixing indicator (chlorophyll is stratified within it in some cases). Why not use a more stringent one?

The NPQ correction scheme does not work well as we still observe a strong diel periodicity in the chlorophyll time/depth series. If you see diel periodicity in bbp this will indicate that this periodicity is likely due to cell cycle and not NPQ.

Provide uncertainties in the chlorophyll values you report (since you did not seem to calibrate them against environmental samples). Have you attempted to match them up with remote sensing at the surface?

Are you using the exact same model parameters as those of Dunne (there are many to tune with)? If yes, say so. If not, provide those you use.
I suggested minor revisions as well as not to re-review a revised manuscript as I don't want to impose my views on the authors. They should be free to publish the paper they want to publish.

Dear authors: the purpose of my review is to help you get the paper out there. If you feel my comments are ‘off the mark’ feel free to contact me and if convinced I will be more than happy to change them.

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RR by Dr. Abhisek Chatterjee (11 Jan 2019)

ED: Publish subject to minor revisions (review by editor) (14 Jan 2019) by Ciavatta Stefano

AR by P N Vinayachandran on behalf of the Authors (07 Feb 2019) Author's response Manuscript

ED: Publish as is (22 Feb 2019) by Ciavatta Stefano

AR by P N Vinayachandran on behalf of the Authors (05 Mar 2019) Manuscript

Short summary

Chlorophyll distribution in the ocean remains to be explored in detail, despite its climatic significance. Here, we document the vertical structure of chlorophyll in the Bay of Bengal using observations and a model. The shape of chlorophyll profiles, characterized by prominent deep chlorophyll maxima, varies in dynamically different regions, controlled by the monsoonal forcings. The present study provides new insights into the vertical distribution of chlorophyll, rarely observed by satellites.