Associate Editor discussion of reviewers’ comments and Authors’ responses Paper 2018-508. “Composition and cycling of dissolved organic matter from tropical peatlands of coastal Sarawak, Borneo, by Yongli Zhou et al.
I thank both reviewers for their thorough and constructive reviews of this Manuscript. They have identified points where the text requires clarification and in their judgement “Major revision” is required. I concur with this view and reiterate the need inter alia to address the use of Azide as preservative in freshwater/estuarine samples discussed in detail below.
The authors have engaged constructively with the reviewers’comment s but further work is required to the Level of a Major Revision addressing the reviewers (especially Reviewer #2) concerns. There is a specific technical issue of the use of Sodium Azide as a preservative of reactive DOM in the filtered water samples. This is a general practice based on NASA and COLOURS protocols codified in the REVAMP protocols ( G.H. Tilstone et al. 2002) which were developed and tested for the “Regional Validation of MERIS Chlorophyll products in North Sea coastal waters” .
The Azide ion itself has absorption in the UV and visible range. Tilstone etal. (2002) attribute 10% of the total absorption at 440 nm to the azide. This is not a fixed ratio but is determined by the relative amounts of azide and DOM and the UV absorption characteristic of the DOM at shorter wave lengths. The relative magnitude of this potential artefact will change at shorter wave lengths (in UV). Zhou et al. have addressed this potential weakness in their PARAFAC analysis by using the appropriate blanks. However, as noted by Referee #2 the problem may arise in the specific context of the determination of the spectral slope and the SUVA. While acknowledging these problems the author’s response has been to assert that they may be discounted relying in part on the more detailed spectral measurements on the azide blank solutions reported in the supplement to the associated paper by Martin et al. The spectrum of sodium azide in water has been reported in the literature (McDonald et al. J. Chem.Phys. 52(1): 1332-1340 (1970)). However, their rebuttal lacks any quantitative detail. The authors are reluctant to pursue this issue further as it introduces a complicated technical /methodological discussion which is only marginally related to their main conclusions about the distribution of terrigenous DOM.
Reviewer #2 also notes the possible role of heavy metals complexed with the DOM interacting with the azide ion to produce coloured species which change the absorption spectra of the solutions. These effects will not be compensated for by the use of azide blanks.
These technical issues somewhat weaken, but do not vitiate, the main conclusions. The problem ultimately arises from the use of a protocol under circumstances which are outside its demonstrated range of validity. These problems are exacerbated by the logistic constraints of working on small boats in remote locations where the alterative sample preservation protocol by refrigeration are not to hand. My suggested resolution of this problem is to publish the paper subject to a major revision as outlined in the Authors response to the review comments but include an additional supplement where there is quantitative discussion of the likely size of perturbations due to the azide blank including the relative magnitude of the errors which arise and a demonstration that these effects may be accounted for and do not alter the conclusions.
Phillip W. Ford

Dear Yongli Zhou,
Would you please address the minor revisions identified by Referee#1 in the attached report. Subject to this, your revised paper will be accepted for publication.