A global hotspot for dissolved organic carbon in hypermaritime watersheds of coastal British Columbia

Oliver, Allison A.; Tank, Suzanne E.; Giesbrecht, Ian; Korver, Maartje C.; Floyd, William C.; Sanborn, Paul; Bulmer, Chuck; Lertzman, Ken P.

doi:https://doi.org/10.5194/bg-14-3743-2017

Articles | Volume 14, issue 15

https://doi.org/10.5194/bg-14-3743-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-14-3743-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 15

Research article

|

15 Aug 2017

Research article |

| 15 Aug 2017

A global hotspot for dissolved organic carbon in hypermaritime watersheds of coastal British Columbia

Allison A. Oliver, Suzanne E. Tank, Ian Giesbrecht, Maartje C. Korver, William C. Floyd, Paul Sanborn, Chuck Bulmer, and Ken P. Lertzman

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Final revised paper (published on 15 Aug 2017)
Supplement to the final revised paper
Preprint (discussion started on 19 Jan 2017)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Reviewer comment', Anonymous Referee #1, 15 Feb 2017
- AC1: 'Response to comments by Referee 1', Allison Oliver, 18 May 2017
RC2: 'Review: Globally significant yields of dissolved organic carbon from small watersheds of the Pacific coastal temperate rainforest. Oliver et al.', Anonymous Referee #2, 09 Mar 2017
- AC2: 'Response to comments by Referee', Allison Oliver, 18 May 2017
RC3: 'Review of Oliver et al.', Anonymous Referee #3, 11 Apr 2017
- AC3: 'Response to comments by Referee 3', Allison Oliver, 18 May 2017

Peer-review completion

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

ED: Reconsider after major revisions (19 May 2017) by Steven Bouillon

AR by Svenja Lange on behalf of the Authors (24 May 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (29 May 2017) by Steven Bouillon

RR by Anonymous Referee #1 (14 Jun 2017)

Suggestions for revision or reasons for rejection

General comments
In the revised manuscript, the authors have made some efforts to respond to all the comments of reviewers. It is undeniable that this study is of interest due to the location and the study sites, their high DOC yields, and the overall large dataset on DOM composition including isotopic data and optical measurements. However, even in its revised version, I consider that the current MS is not suitable for publication and must deserve more intensive work in the interpretation of the seasonal variations and the identification of the drivers leading to seasonal changes in stream DOM concentration and composition in this part of the globe.
Thus, the discussion about seasonal changes in DOM concentration and composition (sections 4.2 and 4.3, could be merged) is limited to a comparison between wet and dry periods. However, there are significant and gradual changes occurring both during wet and dry periods. For example, DOC concentrations decrease in all catchment during the wet period of the water year 2014-2015 and these decreases are linked with clear changes in DOM composition (increasing d13C-DOC, increasing SUVA and decreasing Sr). Such fluctuations contrast markedly with those reported during the previous dry period that is characterized by gradual increasing DOC concentrations, decreasing SUVA, Sr and d13C-DOC values. The authors refer well to several studies that have investigated the DOC flushing process in other small catchments, but it has to be noted that DOC flushing is a generic term and several flushing mechanisms have been reported (Boyer et al., 1996; Sanderman et al., 2009; Lambert et al., 2013 and also Pacific et al., 2010 DOI 10.1007/s10533-009-9401-1). How the authors can better constrain DOC export in their catchments with all their dataset on DOM composition? Are these fluctuations related to (1) changes in DOM sources mobilized along the soil profile (topsoil versus subsoil, e.g. Sanderman et al., 2009), (2) changes in the production mechanisms of terrestrial DOM (e.g. Lambert et al., 2013), (3) depletion of a DOM pool depleted in 13C during the wet period (e.g. Boyer et al., 1996), (4) increasing in-stream production during the dry period…? Fluorescence data (Freshness Index, FI values and PARAFAC model) are poorly included in this part of the discussion while they can provide critical information on DOM sources and dynamic. Is there some relationships between d13C-DOC and other proxies of DOM composition that could support one of these explanations? A more robust and convincing discussion on these aspects would represent a major improvement for the MS.

Lines 373-375: From my perspective, I see clear variation in d13C-DOC. Considering for example the catchment 703 (but it is also applicable for the others catchments), there is a clear and significant decrease from the dry period to the beginning of the wet period in 2014, then d13C-DOC increase along the wet period. This pattern is less obvious for the water year 2015-2016, but we can also observe a gradual increase in d13C-DOC from August 2015 to march 2016. Seasonal changes is also supported by the fact that the authors reported positive relationships between d13C-DOC, discharge and temperature (line 539). There is something here that should be investigated more deeper.
Line 482: this is partly true for the upland catchment studied by Boyer et al. (1996) (note that drier period in their catchment = winter period), but there is different “forms” of DOC flushing (See Sanderman 2009; Lambert 2013; Pacific et al., 2010 DOI 10.1007/s10533-009-9401-1).
Line 501: “highly terrestrial” is quite odd. “mainly terrestrial”?
Line 519: Please clarify “wider range of DOM from soil material”.
Line 529: Caution should be taken when interpreting relationships based on the relative contribution of PARARAC components as when one goes up, another goes down. For example, the increase in C6 suggested by the authors is not clear in Figure 7 (very low increase observed at only one date of the dry period). It will be more relevant to use maximal fluorescence intensity of components to investigate absolute increase or decrease.
Lines 539-542: these suggestions cannot be made simply based on the relationships observed between d13C-DOC and discharge and temperature. Moreover, changes in d13C-DOC are significant (> 1 ‰).
Lines 542-543 : Higher freshness index values do not necessary imply higher biodegradability.
Lines 546-565: this is a long discussion on an aspect of DOM cycling that has not been investigated in the study (the authors have no estimation on the biodegradability of DOM in their catchments).
Lines 578-590: is there some correlation between the relative repartition of Hemist and Folic Histosols and stream DOM concentration and composition? As data seems to be available (supplements S1.2), this would represent a strong argument to support this hypothesis.
Lines 591-604: additional figures for illustrating the different links between lakes/wetlands and stream DOM will be great here.
Reference Lambert et al. (2013) is still missing.

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ED: Publish subject to minor revisions (Editor review) (15 Jun 2017) by Steven Bouillon

AR by Anna Wenzel on behalf of the Authors (03 Jul 2017) Author's response Manuscript

ED: Publish as is (10 Jul 2017) by Steven Bouillon

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Short summary

Rivers draining small watersheds of the outer coastal Pacific temperate rainforest export some of the highest yields of dissolved organic carbon (DOC) in the world directly to the ocean. This DOC is largely derived from soils and terrestrial plants. Rainfall, temperature, and watershed characteristics such as wetlands and lakes are important controls on DOC export. This region may be significant for carbon export and linking terrestrial carbon to marine ecosystems.