Colored dissolved organic matter in shallow estuaries: relationships between  carbon sources and light attenuation

Oestreich, W. K.; Ganju, N. K.; Pohlman, J. W.; Suttles, S. E.

doi:https://doi.org/10.5194/bg-13-583-2016

Articles | Volume 13, issue 2

https://doi.org/10.5194/bg-13-583-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-13-583-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 2

Research article

|

02 Feb 2016

Research article |

| 02 Feb 2016

Colored dissolved organic matter in shallow estuaries: relationships between carbon sources and light attenuation

W. K. Oestreich, N. K. Ganju, J. W. Pohlman, and S. E. Suttles

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Final revised paper (published on 02 Feb 2016)
Supplement to the final revised paper
Preprint (discussion started on 18 May 2015)

Interactive discussion

Status: closed

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

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RC C2576: 'Review', Anonymous Referee #1, 04 Jun 2015
RC C3184: 'Review of "Colored dissolved organic matter in shallow estuaries"', Anonymous Referee #2, 26 Jun 2015
AC C4894: 'Response to Referee Comments', William Oestreich, 02 Sep 2015

Peer-review completion

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

ED: Reconsider after major revisions (10 Sep 2015) by Gerhard Herndl

AR by William Oestreich on behalf of the Authors (11 Sep 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Sep 2015) by Gerhard Herndl

RR by Anonymous Referee #1 (26 Oct 2015)

Suggestions for revision or reasons for rejection

The manuscript has been significantly improved, but still has some omissions and areas for improvement before publication.

On first reviewing the major revisions required meant that I missed some other significant issues with the paper.

Major comments:
1) Why is DOC data not reported? It is measured and is mentioned in the d13C methods and results. Indeed, it is a prerequisite measurement for the d13C mixing model.

It is unclear how DOC was quantified. Authors should add this to the methods independent from the d13C data.

Also, authors should present the DOC data, whether it behaves conservatively and how it corresponds to fDOM. i.e. is fDOM a good proxy for DOC in this system?

2) Once DOC is included, it becomes logical to include the DOC normalized color. Normally this would be reported as SUVA (mg-C L-1 divided by Decadic light absorption at 254 nm; e.g. Weishaar et al 2003). As the authors only have CDOM to 340 nm, then "carbon-normalized CDOM" can be reported and it can be noted that this is at 340nm.

The C-normalized CDOM provides an indicator of sample aromaticity (see Weishaar and many other studies). This provides more information about CDOM quality, processing and source that will be informative to the authors' discussions of DOM endmembers within the estuaries.

3) The authors do not discuss in situ processing of DOM and how this can modify d13C, slope or CDOM:fDOM. Photochemistry is the main sink for CDOM and should be mentioned in the intro (e.g. Mopper et al 2015 in Biogeochemistry of Marine DOM book). Photobleaching causes spectral slope to generally become steeper; SUVA to fall; d13C to become less depleted (e.g. Spencer et al 2009 JGR Biogeoscience Congo photodegradation). Therefore the samples in the current paper that are interpreted as being of Spartina origin, could also be photodegraded terrigenous DOM.

Minor comments (numbers = Page number; line number):
Document had no page numbers!
1; 28: give abbreviation for DOM here.
2;2: Define DOC here.
2; 15-24: Impact of CDOM on sea grass is important, but only one reason to study CDOM. The intro should be broadened. As the major chromophore in natural waters CDOM is important for photochemistry; for shading of biota from harmful UV light; for shading of phytoplankton from photosynthetic light; as a proxy for DOC quantity and quality. The document's reach, value and citation potential would be much expanded by referencing these other factors.
3;4: "seagrass viability" is only one motivation (see above).
3;5-6: give abbrev of DOC here
6;4: State "Naperian absorption coefficients" (i.e. with the 2.303 included) to distinguish from Decadic (without the 2.303 included).
7:3: How was DOC determined? Describe analytical blanks, precision etc.
Results:
Suggested improvement to results structure: Salinity; DOC; CDOM; slopes; FDOM; d13C...
Why are averages and st dev reported for values across very different samples across a large environmental gradient (i.e. strong salinity gradient)? It does not seem useful.
8:6: steeper not higher slopes (correct throughout). Also correct if lower is used instead of shallower.
9:6-8: Comparison to other literature should be moved to the discussion.
10:17: Mention whether the cited papers for slopes (Keith, Green & Blough) utilized the same slope ranges as you.
10:20: Right now this says low DOC concentrations. Yet DOC data is not presented. Once DOC is added, this can be kept.
10:29: Steep slopes also correspond to lower MW; greater photodegradation (see Helms et al 2008 and other references).
Page 12: Discuss impact of processing upon d13C. e.g. photodegradation (Spencer et al 2009) selectively mineralizes isotopically depleted DOC.
Page 13: Discuss how fDOM:CDOM might be impacted by processing in estuary.

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ED: Reconsider after major revisions (28 Oct 2015) by Gerhard Herndl

AR by William Oestreich on behalf of the Authors (08 Dec 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Dec 2015) by Gerhard Herndl

RR by Anonymous Referee #1 (13 Jan 2016)

ED: Publish as is (14 Jan 2016) by Gerhard Herndl

AR by William Oestreich on behalf of the Authors (19 Jan 2016)

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Short summary

Colored dissolved organic matter (CDOM) is a factor in determining penetration of light in estuaries. Important plant species growing in the beds of estuaries depend on such light penetration for survival. Previous studies have used CDOM fluorescence to approximate light absorption by CDOM but have found variable relationships between fluorescence and absorbance. This paper describes this variability in three east coast estuaries, and shows that this conversion is dependent on CDOM source.