Ignoring carbon emissions from thermokarst ponds results in overestimation of tundra net carbon uptake

Beckebanze, Lutz; Rehder, Zoé; Holl, David; Wille, Christian; Mirbach, Charlotta; Kutzbach, Lars

doi:https://doi.org/10.5194/bg-19-1225-2022

Articles | Volume 19, issue 4

https://doi.org/10.5194/bg-19-1225-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-19-1225-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 4

Research article

|

28 Feb 2022

Research article |

| 28 Feb 2022

Ignoring carbon emissions from thermokarst ponds results in overestimation of tundra net carbon uptake

Lutz Beckebanze, Zoé Rehder, David Holl, Christian Wille, Charlotta Mirbach, and Lars Kutzbach

Download

Final revised paper (published on 28 Feb 2022)
Preprint (discussion started on 12 Aug 2021)

Interactive discussion

Status: closed

RC1:
'Comment on bg-2021-212', Anonymous Referee #1, 28 Oct 2021

My review is attached as a pdf

Citation: https://doi.org/10.5194/bg-2021-212-RC1
- AC1: 'Reply on RC1', Lutz Beckebanze, 08 Dec 2021
  
  Dear reviewer,
  thank you for your work. Please see the attached document for our point-by-point reply.
  Best regards,
  Lutz Beckebanze and Zoé Rehder on behalf of the authors
  
  Citation: https://doi.org/10.5194/bg-2021-212-AC1
RC2:
'Comment on bg-2021-212', Anonymous Referee #2, 08 Nov 2021

The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2021-212/bg-2021-212-RC2-supplement.pdf

Citation: https://doi.org/10.5194/bg-2021-212-RC2
- AC2: 'Reply on RC2', Lutz Beckebanze, 08 Dec 2021
  
  Dear reviewer,
  thank you for your work. Please see the attached document for our point-by-point reply.
  Best regards,
  Lutz Beckebanze and Zoé Rehder on behalf of the authors
  
  Citation: https://doi.org/10.5194/bg-2021-212-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (11 Dec 2021) by Andreas Ibrom

AR by Lutz Beckebanze on behalf of the Authors (14 Dec 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (24 Dec 2021) by Andreas Ibrom

RR by Anonymous Referee #2 (09 Jan 2022)

RR by Anonymous Referee #1 (12 Jan 2022)

Suggestions for revision or reasons for rejection

Thank you for the revised manuscript. Nice job done. I think, however, that the manuscript still needs work to clarify the presentation. I found the text incoherent, in many cases. I suggest that the authors will put some effort to review the consistency of the used terminology (for instance, ponds and models) and the logic and order how the different modeling and upscaling steps appear in the text. I feel that the problem is the precision of description. These include also not so precise use of “CO2 flux” (shouldn’t it be net ecosystem exchange of CO2, or a component flux, with direction relative to the atmosphere). And the handling of the CH4 data raises questions. One reason for my stumbling through the text may be in the end of the introduction: “Due to the tower’s position, fluxes from the merged polygonal pond are the dominant source of the observed EC fluxes under easterly winds. The observed EC fluxes are dominated by semiterrestrial polygonal tundra with only a low influence from small thermokarst ponds from the other wind directions. We aim to deepen our understanding of carbon emissions from thermokarst ponds and constrain their impact on the landscape carbon balance. To this end, we (1) compare the water body and tundra fluxes focusing on temporal and spatial patterns, and we (2) investigate the influence of the merged polygonal pond on the landscape carbon balance.”

That paragraph should precisely describe the objectives and questions. I may be wrong, but should you state here something like that your tower was positioned so that you can assess fluxes from the merged polygonal pond (large enough to get a pure ‘thermokarst pond’ signal) and polygon tundra composed of small ponds and terrestrial tundra (centers and rims?). Then, the objectives are, 1) use footprint model and 2) model net ecosystem CO2 exchange using the footprint weights of terrestrial tundra and thermokarst ponds (polygon ponds?), 3) examine temporal and spatial patterns of NEE & CH4 from terrestrial and aquatic tundra/land cover types, and 4) investigate the influence of the small thermokarst ponds (merged and polygon centers?) on the landscape NEE of CO2 (and CH4?) over the xxx period.

Line numbering would help pointing the comments, but here just some examples from the text
Usually the mat&met and results are written in past tense. Check some published papers for a reference. A language edit maybe.

p. 4. Fig. 1 add somewhere that there are larger thermokarst lakes in the map (not included in the study)

p. 5. Add what’s the area of upscaling somewhere in the mat&met

p. 6. There are some illogic in describing the models and missing equation references
Explanation of the different grey shades is good in the figure legend, but no need in the main text.
Number the models according to their appearance in the text. i.e. NEE (1), ….

p. 7. in addition to the R2, RMSE and mean absolute error would be great, because the R2 is not informative how close the absolute values are.
Would it make any more sense to explain the extraction of pond values before the gap-filling? And be precise if question is about the merged pond or polygon ponds or something else
Why it is worth mentioning that others compared between different vegetation types and here between tundra and water?

p. 8. At hand? How the others gap-fil CH4 fluxes? Can’t you use the data even without gap-filling if enough data? Give a rationale why these wind directions were chosen for the comparison, now pretty random in the text.

p. 9. When comparing, variation is better to be expressed as the ratio of the standard deviation to the mean. It is coefficient of variation.
In the permutation test section: Should there be something else than “one of the two groups” in the next sentence? pairs? “In this test, we randomly assign each 30-min flux to one of two groups and calculate both groups’ median and their differences.”

p. 10. About the shore sections. “These peaks did not correlate with any of the four land-cover classes” is unclear
How about the high mean values in the sector 300-360? Don’t neglect if 120 deg gets attention.

Fig. 6. is really nice

p.14. Comparison with the other pond studies in the area is a bit confusing. Methodological differences are possible but note also whether those were same or different ponds and how large the samples were.
Maybe specify that with emissions you mean NEE of CO2 (direction to the atmosphere)

Fig A3. Lake? Do you mean the merged pond?

Hide

ED: Publish subject to minor revisions (review by editor) (14 Jan 2022) by Andreas Ibrom

AR by Lutz Beckebanze on behalf of the Authors (28 Jan 2022) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (29 Jan 2022) by Andreas Ibrom

AR by Lutz Beckebanze on behalf of the Authors (29 Jan 2022) Manuscript

Short summary

Arctic permafrost landscapes feature many water bodies. In contrast to the terrestrial parts of the landscape, the water bodies release carbon to the atmosphere. We compare carbon dioxide and methane fluxes from small water bodies to the surrounding tundra and find not accounting for the carbon dioxide emissions leads to an overestimation of the tundra uptake by 11 %. Consequently, changes in hydrology and water body distribution may substantially impact the overall carbon budget of the Arctic.