Characterizing the origins of dissolved organic carbon in coastal seawater using stable carbon isotope and light absorption characteristics

Han, Heejun; Hwang, Jeomshik; Kim, Guebuem

doi:https://doi.org/10.5194/bg-18-1793-2021

Articles | Volume 18, issue 5

https://doi.org/10.5194/bg-18-1793-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-18-1793-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 5

Research article

|

16 Mar 2021

Research article |

| 16 Mar 2021

Characterizing the origins of dissolved organic carbon in coastal seawater using stable carbon isotope and light absorption characteristics

Heejun Han, Jeomshik Hwang, and Guebuem Kim

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Final revised paper (published on 16 Mar 2021)
Supplement to the final revised paper
Preprint (discussion started on 10 Aug 2020)
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Interactive discussion

Status: closed

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

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RC1: 'Review comments', Anonymous Referee #1, 29 Aug 2020
- AC1: 'Response to Anonymous Referee #1', Guebuem Kim, 03 Sep 2020
RC2: 'Review of bg-2020-727', Anonymous Referee #2, 06 Sep 2020
- AC2: 'Response to Anonymous Referee #2', Guebuem Kim, 11 Sep 2020

Peer-review completion

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

ED: Reconsider after major revisions (13 Oct 2020) by Minhan Dai

AR by Guebuem Kim on behalf of the Authors (26 Oct 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (28 Oct 2020) by Minhan Dai

RR by Anonymous Referee #1 (03 Nov 2020)

RR by Anonymous Referee #3 (13 Nov 2020)

Suggestions for revision or reasons for rejection

The work by Han et al. describes the DOM distributions in Sihwa Lake in spring 2017 and 2018. The multiple properties of DOM have been investigated e.g. the FDOM, the DO13C and so on. I generally agree with the comments by reviewer 1 and 2 in that this work provides a comprehensive view of the characteristics of coastal DOM with the involvement of multiple analysis on different aspects of DOM properties, and the authors have responded the comments in details. However, as pointed out by reviewer 2, the lack of clear logical argument gave me some difficulty to reach the conclusion. This was not quite improved in the revised version submitted. Thus I would suggest the authors to reorganize the discussion section for a more logical argument. The following comments may be of some help for this purpose.
1. The authors should more clearly summarize the new information this work brings in for our knowledge of DOM cycling in Sihwa Lake. This should be significantly beyond Kim and Kim’s work (2016) which studied the optical properties of DOM in Sihwa lake and found the contribution of the porewater to the DOM in water column. What is the major difference between Kim and Kim’s finding and the conclusion in this manuscript that sediment OM is the major source of DOM in Sihwa Lake?
The introduction section should more focus on the specific processes or new information this work identified. Progresses from other labs should be discussed in details in the introduction.
2. A more logical argument of the conclusion should consider the followings:
1) what is the excess DOM?
It is denoted in the manuscript as the part of DOC measured in 2017 above the mixing line of DOC in 2018. If my understanding is correct, this idea tries to evaluate the DOM influenced by conservative mixing of two endmembers (freshwater and seawater DOM) and the part of DOM added / removed during the mixing. However, the excess DOC denoted here is under the assumption that the DOC concentrations of freshwater endmembers remained constant between 2017 and 2018. Please provide enough evidence and argument to prove this assumption is reasonable. Plus, the excess DOM should be denoted at the first place it is mentioned except in the abstract.
2) Clarify the sources based on the properties of DOM
The origin of DOM in group 2 in the conclusion (and in the abstract) is more like a description of DOM properties which are characterized as lower DO13C, non-fluorescent, and low MW. The author should clarify what is the specific source and why the DOM from this source have these properties. Clarify the difference of terrestrial OM in group 2 from the source of DOM in the low salinity water.
3. Some minor comments as following:
1) whenever use “significant” in the text, please provide statistical analysis;
2) FDOMH mentioned in discussion was not defined in method section;
3) Change “uM” to “umolL-1”
4) Line 31: delete “reduced”.

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RR by Anonymous Referee #4 (17 Nov 2020)

Suggestions for revision or reasons for rejection

The authors used several chemical tracers including stable carbon isotope, optical properties, and ammonium to explore the source of dissolved organic carbon in coastal water of the Sihwa Lake in South Korea. This study demonstrates that the combination of multiple tracers could be a powerful approach for characterizing DOC in complex environmental settings such as a coastal system. Some of the conclusions are not well justified and more argumentation efforts are needed.

Specific comments:

1. Please provide background information on source and dynamics of NH4+ in coastal waters in the introduction section before using it as a tracer in the manuscript.
2. DO is measured in this study, but not used or discussed. It can be taken out if it does not add any useful information.
3. Please provide water residence time in spring VS. fall if possible.
4. The DOC input source may vary significantly with seasons, so it might not be appropriate to used the mixing line based on data from September of 2018 as the baseline to define the excess DOC for data from March of 2017. A proper assumption has to be made.
5. The authors suggested the marine sediments as the major source of DOC, based on carbon isotopic signature and correlation between DOC and NH4+, which seems to be rational. But a linear mixing line showing a decreasing trend of DOC with salinity was also observed. This usually occurs when terrestrial input is the major source of DOC. Please explain why marine sediments as the major source could result in such a DOC distribution pattern.
6. Line 290, please clarify how bottom sediment porewater was excluded as the source of DOC.
7. Line 299, changes in input sources could also result in SR variations.
8. Figure 2, please provide information on how grouping was defined in the caption.
9. Figure 5b, I suggest drawing a single regression line instead of two

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ED: Publish subject to minor revisions (review by editor) (02 Dec 2020) by Minhan Dai

AR by Guebuem Kim on behalf of the Authors (16 Dec 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (25 Jan 2021) by Minhan Dai

AR by Guebuem Kim on behalf of the Authors (06 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (08 Feb 2021) by Minhan Dai

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Short summary

The main source of excess DOC occurring in coastal seawater off an artificial lake, which is enclosed by a dike along the western coast of South Korea, was determined using a combination of various biogeochemical tools including DOC and nutrient concentrations, stable carbon isotope, and optical properties (absorbance and fluorescence) of dissolved organic matter in two different seasons (March 2017 and September 2018).