Impact of peatlands on carbon dioxide (CO<sub>2</sub>) emissions from the Rajang River and Estuary, Malaysia

Müller-Dum, Denise; Warneke, Thorsten; Rixen, Tim; Müller, Moritz; Baum, Antje; Christodoulou, Aliki; Oakes, Joanne; Eyre, Bradley D.; Notholt, Justus

doi:https://doi.org/10.5194/bg-16-17-2019

Articles | Volume 16, issue 1

https://doi.org/10.5194/bg-16-17-2019

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

Special issue:

Biogeochemical processes in highly dynamic peat-draining rivers...

https://doi.org/10.5194/bg-16-17-2019

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

Articles | Volume 16, issue 1

Research article

|

08 Jan 2019

Research article |

| 08 Jan 2019

Impact of peatlands on carbon dioxide (CO₂) emissions from the Rajang River and Estuary, Malaysia

Denise Müller-Dum, Thorsten Warneke, Tim Rixen, Moritz Müller, Antje Baum, Aliki Christodoulou, Joanne Oakes, Bradley D. Eyre, and Justus Notholt

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Final revised paper (published on 08 Jan 2019)
Supplement to the final revised paper
Preprint (discussion started on 06 Sep 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', Anonymous Referee #1, 03 Oct 2018
- AC1: 'Response to Referee #1', Denise Müller-Dum, 09 Nov 2018
RC2: 'Referee comments', Anonymous Referee #2, 30 Oct 2018
- AC2: 'Response to Referee #2', Denise Müller-Dum, 10 Nov 2018

Peer-review completion

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

ED: Reconsider after major revisions (19 Nov 2018) by Steven Bouillon

AR by Denise Müller-Dum on behalf of the Authors (26 Nov 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Nov 2018) by Steven Bouillon

RR by Anonymous Referee #2 (05 Dec 2018)

Suggestions for revision or reasons for rejection

I think the authors have done a good job at addressing the comments that I had on the earlier version of the manuscript titled “Impact of peatlands on carbon dioxide (CO2) emissions from the Rajang River and Estuary, Malaysia”. The main questions that I raised on the previous version was concerning 1) how the CO2 emissions were estimated 2) How the 13C-DIC data were interpreted, and 3) if it is really correct to call it seasonality when “only” two campaigns at different seasons were conducted.
In the revised version I think the authors have handled these concerns in a satisfactory way by, 1) More clearly and detailed describe how the emissions were calculated, by including the different k parameterizations get an emission range and use this range as an uncertainty estimate, and finally but not last discuss the methods and results in a more transparent and open way highlighting the shortcomings. 2) Included a more in-depth analysis of the 13C-DIC data (I appreciate the keeling plot inclusion) and discussed its outcome much more in detail. 3) Highlighted the seasonality issue, that the study is based on “just” two campaigns representing snapshots from two different seasons.

Although the authors have done a good job in revising the manuscript I still I have some minor issues that needs to be solved prior to a publication. This should however be easy fixed by the authors and after that the manuscript should be a great contribution to the research field.

Minor comments:
P3, L19, remove “However”, is not needed.
P10, L10-15, decrease the number of digits on the k estimates. Because these are really estimates, which the authors also discuss the uncertainty of. So one decimal is enough.
P11, L21, Should both rivers on this row be named Rajang? Is the Huang et al study from the same river? In such case is should be highlighted and expressed differently.
P11, L29, remove tracking on comma sign
P12, L15, add starting parenthesis for the Rozanski ref
P12, L24, is really equilibrium with the atmosphere at +1 per mille?
P13, L16, yes, but pCO2 is partly at least controlling pH. Keep track on the “hen and the egg”.
P13, L17, the difference in 13C-DIC is also a function of the different pH, so not solely a function of different C sources, although maybe related.
P15, L16, yes, but you have now also added the new 13C-DIC interpretation paragraph on P12 where you discuss the methanogenisis and its potential source. I find this statement a bit redundant in that context.

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ED: Publish subject to minor revisions (review by editor) (11 Dec 2018) by Steven Bouillon

Dear Denise and co-authors,
Thank you for the revised version of your manuscript and the detailed author replies. As you can see, the revised ms has been re-evaluated by one of the original referees, who feels that you have adequately addressed their suggestions and concerns. I generally agree with this, but do have a few concerns on the Keeling plot approach now added to the interpretation of the d13C-DIC data. This is an elegant approach but all too often used for settings in which it is not applicable – keep in mind that the approach essentially stems on a mass balance for a reservoir to which a certain quantity is added. Since we are looking here at a concentration gradient across a salinity gradient, the slopes of a Keeling plot in this case offer not new information on the sources of the DIC. Such mixing plots can be useful to find a d13C value when there is a net input of C to an existing C pool in a given system with clear boundaries. Hence, plotting the data collected along the salinity gradient is not informative – it basically returns the weighted average d13C value of all data points. You currently only use the approach in quantitative terms for the freshwater data, which is less problematic but it nevertheless still assumes that the data can be interpreted as representing data from a gradual addition of DIC to an initial DIC pool – which would require some discussion to convince the reader that this is applicable.
Furthermore, on p14, line 19 and further, you mention “the y-intercept of the Keeling plot for freshwater samples suggests that the initial freswhater source has a d13C-DIC of -18.6 ‰” – this is not really consistent with how a Keeling plot should be interpreted; the intercept represents the d13C value of the DIC added to the systems’ initial DIC pool.
I would recommend to reflect on the applicability of the Keeling plot approach to your data – and rework this section of the manuscript.
One further suggestion:
-p16, L8-17: this is a somewhat circular line of reasoning: pH and pCO2 are intrinscically linked - see comment by Referee #2 (“the hen and the egg”).
[page and line numbers refer to the track changes version]

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AR by Denise Müller-Dum on behalf of the Authors (12 Dec 2018) Author's response Manuscript

ED: Publish as is (14 Dec 2018) by Steven Bouillon

AR by Denise Müller-Dum on behalf of the Authors (14 Dec 2018) Manuscript

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Short summary

Southeast Asian peat-draining rivers are potentially strong sources of carbon to the atmosphere due to the large amounts of organic carbon stored in those ecosystems. We present the first assessment of CO₂ emissions from the Rajang River, the largest peat-draining river in Malaysia. The peatlands’ influence on the CO₂ emissions from the Rajang River was smaller than expected, probably due to their proximity to the coast. Therefore, the Rajang was only a moderate source of CO₂ to the atmosphere.