Overall General:

• Please introduce reserach questions and hypothesis that guide throught the text

Abstract

• Title is missing a preposition in the second line connecting the southern Greenland Ice Sheet
• L 18: Make sure readers not acquainted with glacial dynamics can follow your text, you could specify that the ablation season is connected to the boreal summer months
• L 18: Are bacteria only more active during that time or are the growing and colonising, the statement you give is ambiguous
• L19: ”the DOM may be exported”, was there any evidence that DOM is actively exported from the supraglacial ice, if so then you should use a stronger verb
• L24: What is meant here by supraglacial dark ice: biologically darkened ice? You should help readers not acquainted with all processes and particularities by taking them by the hand, especially for the abstract.

Introduction

• General point: I am missing a general research question and hypothesis deducted here. Please try to make a clearer statement on your scientific goal. It would be nice to also have this question(s) mentioned in the abstract
• L59: Since aromatic composition DOM was found in glacial run-off, not all of the degradation of allochthonous aromatic DOM can happen during the transport to glacier surfaces.
• General Comment Introduction: Indeed, glacial runoffs are known to show a “dual” source with both aromatic allochthonous and biolabile autochthonous DOM sources. Potentially the aromatic allochthonous fraction would be expected to be more susceptible to photodegradation, while the autochthonous fraction more susceptible to biodegradation. Did you find any evidence of production of Carboxyl-Rich Alicyclic Molecules (CRAM) from algal DOM in any other studies of glacial ecosystems (14-C young aromatics then), this could help to decipher that parts of the allochthonous aromatic DOM could in fact be autochthonous as well. Did you investigate 14-C ages in your samples? If not you can have a specific look on the polycondensed aromatic fraction in your FT samples to gain insight if a high number of black carbon-like, likely allochthonous molecular formulae exist in your samples, if so they could likely be highly susceptible to photodegradation.
• L70: Upon draining of DOM to the glacial bed where any studies performed that investigated how supraglacial DOM was transported in the glacial bed, were there any interactions and exchanges? I guess it is unlikely that supraglacial DOM behaves inert when passing englacial and subglacial systems. You could also review these sources.

Materials and Methods

• L83 please define “small” by estimate or accurate metrics
• L85 was the auger hole freshly produced? How long did it take for auger holes to refill?
• L90 times given are local time? Why was not timestamp given for the sampling of Q
• L90 it should already be clearer what this Q is without having to inspect Figure 1
• L93 last comma replaced by “and”
• L94 I wonder that SPE samples were stored in simple PC bottles, while DOC aliquots were stored in PTFE / glass bottles. The SPE samples will be considered more delicate than the DOC aliquot, could you elaborate on this. Did you conduct process blanks for SPE samples with FT analysis (also including the GF filtering procedure)?
• L96 replace “home” by something more clear (where)
• L96 Which analysis did filter retentates (surface debris) undergo, it came out of the blue in the M&Ms and results should be mentioned in the abstract and POM should be discussed in the introduction
• Figure 1: Minimap (A) has weak contrast, the box study area is not well readable, northing information is missing. Statement on the used basemap is missing; Map B is fairly too zoomed in, it doesn’t become clear what is shown (valley, slope, hill crest) maybe use the drone image underlain by additional geomorphologic information that make the sampling information clearer; The “field site” was not found by me on any of the maps; The categorisation of ice types shown in the legend does refer to illustration F only (?), how can you suspect this illustration to be true, is it clear that the depth of layers has roughly these dimensions- was ground trothing carried out by digging a snow / ice profile? Dimensions in panes C,D,E and F are missing.
• L 108: Calculations based on Stevens 2018: please elucidate more on this
• L118: Can you elucidate from the literature how Milli-Q extracts might influence the DOM yield compared to other extraction techniques
• L 120: “wwPTFE” = PTFE |Typo?
• L123: Usually such products are called water extracted organic matter (WEOM) “laboratory leachates” could also be column leachates asf.
• L128: The high temperature combustion technique doesn’t need to be described with this detail, it appears to be a standard
• L 136 – 137 see comment on L128
• L 141: see comment L 96
• L 149: please indicate what these supplementary methods are roughly about
• L160 and L163: Since Equation 1 and 2 are standard and published, citations will suffice instead of formulae
• L 165 f.: I highly recommend changing the naming of your “composition groups” to a less ambiguous naming. The current naming implies structure which cannot be determined by mass spectrometry. A less ambiguous nomenclature is presented by Merder et al. 2020 (https://dx.doi.org/10.1021/acs.analchem.9b05659) Table 2 in the Supplements, which is also co-authored by some of the Co-Authors here.
• L 171: This sentence can be deleted; I expect you wouldn’t act in the opposite way. If this is true on the other hand, is not proven by your sentence, you would need to argue this in the introduction justifying your selection of metrics by citations. Further metrics could be tested such as IDEG (Flerus et al., 2012) or ITERR (Medeiros et al., 2016) and the investigation of pcARO could also function as metric
• L 176/177: Can you present any insight into the actual biolability of purpurogallin as an empiric measure for your discussion. It doesn’t become clear to me what you imply here Example: An essential amino acid like tyrosine contains an aromatic ring structure and is widely considered biolabile: If the classification of biolability is closely connected to refractory characteristics of structures, this discussion capsizes when conditions of biological decomposition are not made clear or no empirical landmark on actual degradability is given.
• L 180 2.8 Statistics: Besides homoscedasticity did you test for normal distribution and were the sample sizes evenly distributed. With heteroscedasticity, non-normality and uneven samples sizes (which is the common case in geosciences) metric testing becomes less and less trustworthy. Please add normality testing, histograms and sample sizes to your supplements. In case of multiple violations of prerequisites for metric testing consider either multiple non-parametric testing e.g. with package “multcomp” (Bretz et al. 2011 ISBN 9781584885740) or Box-Cox transformation of data prior to ANOVA
• Figure 2 and section 3.1: Figure please add DEM data to the figure, here the viewer can just see blurry white with a large pixel size. I also wonder, what is the margin of error for your 9 days of travel time. The resolution of the orthophoto indicates that there are several potential travel passes that might occur for a single particle with certain likeliness
• L 230: I like how you follow these single formulae through your dataset, I imagine it might be interesting to produce a figure from this finding and also underlie it with some of your matching metrics. Since purpurogallin should absorb light, maybe you could also add some UV absorbance values if you still have some back-up sample to analyse- these might match. The current way of pure text and numbers presentation is making it hard to follow these exciting insights
• L 239: There are dozens of definitions on diversity, please specify which diversity you refer to
• L 249: Lettering indication should be self-explanatory please delete starting from “where values that have….”
• Figure 3: There are n= 17 samples and n= explanatory variables. The low ratio of sample to variable (1.21) suggests that the PCA model is not as selective as it could be. Please check your variables for collinearity and make sure to remove collinear variables. I am missing the Eigenvalue {Variance} / Component documentation, please add this to your supplementary data.
• Figure 4: The pane lettering in A-F is necessary but it would help the readership to also name the sample type above the pane. Since van Krev. plots are always prone to overplotting please consider scaling the point size to %RA. You could also specify in the plot what is % RA of the shown formulae versus the excluded formulae to specify not only diversity but also intensity
• Figure 4 vs Table 1: how do values in Table 1 correspond to Figure 4: Since you conducted a perfectly interesting subtraction technique for Figure 4 it would be nice to append a table 2 with the respective metrics (as indicated van Krev. are overplotted and usually not as insightful)
• L 276 there is a € instead of (E)

Discussion

• General: In the discussion you present a large number of metric data (x +- y % ) asf. Please try to limit these numbers to an absolute minimum and rather state the trends and significant differences from your results by rephrasing them in words. This will make it easier to follow the arguments. Also please try to discuss one thought in one paragraph only. It might be a good idea to enter subheadings above paragraphs to make clear which idea is discussed at the moment
• 4 Discussion: Instead of descriptive heading for 4.1 and 4.2 I would be very happy if you could include your research questions into the headings
• L 309: The whole section about the problem of lysis is too prominent in my eyes. You use very much space to discuss a potential artefact that you then rule out in the end did probably not happen at all or have no effect. Please shorten here. The readership should learn more about what the data tell you about potential processes here
• L338 how impactful can the two described degradation pathways of viral infection and fungal attack be. Undoubtly, they will have effect on the composition, but I would expect the effect to be much smaller. Also if I understand correctly, you identified the sample by the algal pigmentation visible as dark ice? So there was no tremendous viral induced loss of pigmentation
• L340: The NOSC metric could be used to maybe hint into the same direction.
• L344: A similar study was following DOM from source to sink in a closed alpine system. there is clear indication of photodeg Part 4.3 https://doi.org/10.5194/bg-20-3011-2023
• L345: Especially dark ice could also shield lower lying aromatics from sunlight and subsequent photodegradation by the low albedo of overlying aromatics. This is also shown for ocean darkening by various indicators https://doi.org/10.3389/fmars.2020.547829
• L 350 I would also suggest shielding of underlying aromatics
• L362 here it would of course have been nice if you had carried out some photodegradation experiments with your samples to track this pathway
• L365 here also lysis products might accumulate
• L369 treat EPS as plural pls.
• L 373, that is an important recommendation, but it is very big in the light what a single paper can achieve, that why it would be better to ask this in the form of questions and to mark knowledge gaps more precisely
• L382 but the, if the stream is not sharing a large amount of DOM composition and not showing mixing, how can you attribute it to be a connector of pools as happened in 3.3, then the outcome must be that pools are distinct and not connected by continuous flow, which can make sense with low flow velocities
• L382 double “.”
• L 377 f., the last paragraph comes without any reference to other scientific works, please try and also discuss these findings in the light of existing literature
• General question: how do you assess the different contributions of bio- and photodegradation in your sample set. I would like to see a clearer statement on which pools are to what extent affected by what

Conclusion

• General: I would prefer to also see a connection to research questions in this chapter
• L 390 what exactly do you mean by “microbial communities” this implies that micros where somewhat investigated more closely then presented here
• L 392: You state distinct composition differences; this is where you should say what exactly you found instead of suggesting more research
• L 394 and 395 The citations of Niwano and Müller,Keeler could also be moved to the introduction, this rather seems to be a motivation for your study than something relevant to the conclusions.

The paper by Doting et al. reveals the DOM molecular composition of a supraglacial micro catchment in the Greenland ice sheet surface with FT-ICR MS analysis. The manuscript is well-written and presents a unique dataset that holds significant value for the scientific community. However, the dataset utilized in the manuscript is solely limited to DOC concentrations and MS analysis and does not provide a comprehensive overview of environmental biogeochemistry in the studied site. Therefore, I would highly recommend to the authors that they may seek additional measurements to document environmental variabilities such as nutrients, isotopic compositions, or microbial diversity. If this is not possible for this manuscript, you should acknowledge this limitation by discussing the potential implications of the dataset’s scope and suggesting areas for future research. Additionally, I noticed that the manuscript suffers from complicated nomenclature, which implies different meanings for researchers from other disciplines. Therefore, I would suggest that the terms used in the manuscript be carefully reconsidered for alternatives (see below).  

Please see below for additional comments:

Line 36: if it provides protection from the UV, then it should reflect more and elevate the albedo rather than lowering it.

Line 91: Dark ice resembles ice that is not in contact with the sunlight, which is not the case. You may consider using a different term. Maybe brown ice or just surface ice.

Line 118: Leachate does not really define what you obtained from your samples. Leachate usually defines liquids that are drained through solids gravitationally or maybe via osmosis. What you did is more like an extraction rather than leaching because of 500 rpm shaking and centrifuging. Therefore, I would use a different term, such as surface debris extract. On the other hand, the component you define as the weathering crust is more like leachate because you let the auger hole fill with meltwater leachate and perform the sampling afterwards.

Section 3.3: PCA with dependent variables does not reveal the compositional differences between samples. It is not surprising that the CHON, CHO and CHOS point in different directions on the plot because they are basically the opposite representation of the same variable. The same applies to formula percentages of van Krevelen regions. PCA is usually for independent environmental variables to indicate how environmental conditions differ for the sampling sites. You should consider applying clustering methods on relative intensities for compositional differences between samples then you can add parameters on the ordination plot (e.g. NMDS + envfit in Vegan). You would possibly get a similar separation, but then you may know which cluster of samples is more similar to the other cluster compared to the remaining ones. Then, instead of PCA, you may simply use bar plot or box plot to visualize which parameter infer the bigger variation.

Figure 4: The figure caption is unclear, and I did not understand what the individual plots show. You should extend this figure by adding van Krevelen for all the formulas obtained from each sample. For example, 8403 formulas for lab leachate or 7540 formulas for dark ice. You could also involve RAs by symbol sizes.

Line 320: What do you mean by high particulate loading? What kind of particulate matter? You may explain more about what you observed with the microscope in the field.

Line 326: why preferentially? Are those more soluble?

Line 360: You should have check how molecular intensity of common molecular formulas change between samples. You have done this for several selected formulas, but you could extend this to all common formulas to see if there is indeed hydrological connectivity. Otherwise, number of common formulas do not necessarily indicate hydrological connection. You would have found common molecular formulas in any DOM samples.

The authors of “Molecular level characterization of supraglacial dissolved organic matter sources and exported pools the southern Greenland Ice Sheet” present a research project on DOM composition from various sources and hydrologic flow of material to the coast of southern Greenland. The goal of the work relies on molecular composition comparisons of samples collected in the field and leached in the lab. The work describes themes of DOM transformations on the Greenland ice sheet and during downstream transport but is organized in a way that is confusing. Some of those points are noted as important (for example in the introduction) and language focuses on assessing potential transformations of DOM from sample collections, but some connections between results and assessing these transformations of DOM are confusing throughout the text, and some parts may be missing (for example % biolabile values are included in the abstract, but not reported on in the main text). Maybe the introduction can be reorganized to narrow the scope/set the foundation for what’s to come a bit better, creating a clearer start, and the rest of the manuscript text follows with more clarifying text so that the messages are clear and continuous throughout. The language describing the importance of microbial transformations and photochemical transformations in the introduction is not paired well with the lack of microbial and photochemical data for this project. It reads like something is missing. The first two paragraphs of the introduction create a wide scope of the work, which is not supported by the rest of the text. Connecting those ideas across the introduction and results sections will be greatly strengthened and reduce confusion. All recommendations for revision can be achieved. The following comments are divided into two groups, major and minor revisions.

Major Revisions

1. The fluctuating use of the following words, compounds, molecular formulae, and composition, needs to be corrected for consistency and clarity. Parts of the introduction and results section use these terms interchangeably, yet they each have different meanings and may point to different measurements. It is not clear if the introduction is discussing composition measurements of DOM from various instruments or are they all FTICRMS? Are some of these studies measuring aliphatic compounds directly?

1. Introduction: Confusing themes, see some examples already stated in the first paragraph of this review. The first two paragraphs of the introduction seem like they should be the second and third paragraphs of the section and an opening paragraph should be added that sets the stage. The first sentence of the introduction focuses the reader on microbial blooms. Is that the most important thing to start with? Why? The same type of comment is true for the beginning of the second paragraph.

1. Some text in the abstract, introduction, and conclusion states more than what experiments were conducted and what was measured. Please clarify the language to be more specific and reduce confusion. State where you are speculating. Some text reads as though you monitored the transformations of DOM during downstream transport. This is a major source of confusion. Please clarify. Example in Line 74, “as it is transported” suggests that you followed a parcel of water and made collections during transport. Is that true? Were they grab samples along a gradient?

1. FTICRMS details in the methods are missing. What ionization technique and mode were used? If one type of ionization and mode were selected, why? Were your ions singly charged? If so, how did you confirm that? This is especially important when going from m/z values to masses in Da. What was the signal/noise threshold? How were molecular formulae assigned? These are important details to include in the main text. The supplementary section doesn’t provide enough details either and repeats some of what is included in the main text. This work would also benefit from reporting on their instrument performance from their DOM optimization standards, see Hawkes et al., 2020, especially since they are reporting on AImod, m/z, H/C, and O/C metrics from their samples. Are there instrument biases?

Reference: Hawkes, J.A. et al. 2020, An international laboratory comparison of dissolved organic matter composition by high resolution mass spectrometry: Are we getting the same answer? Limnology and Oceanography Methods, 2020, 18: 235-258.

Minor Revisions

Title: Typo. There is a word missing in between “pools” and “the”. Is the word “sources” necessary? Is “exported pools” a bit farfetched for this work? The work describes a supraglacial microcatchment, so perhaps “exported pools” is too wide a phrase.

Abstract: There is one sentence of results in this abstract and then the following sentence states how these findings have implications and importance for future work. Not enough information about the results of the work and why it is important is included in the abstract.

Introduction:

Lines 43-44: Not true. This has been characterized in Antarctica. Please be more specific.

Line 49: Define DOC.

Line 57: The use of the word “compounds”. Were compounds directly measured?

Line 61: Please provide definitions of allochthonous and autochthonous DOM pertaining to ice sheets.

Methods:

Line 82: Add a comma after “collection”

Line 94: Add the word “concentration” after DOC

Line 95: What is PC?

Line 96: What is “back in the home laboratory”?

Lines 83-96: Provide more information about the site names/sample names that correspond with the figure. See next comment about the confusion of the figure and caption for examples. Samples for FTICRMS were only collected at D and Q?

Figure 1: The lettering in panel B shows up as purple online and is very difficult to discern from the background. When the page is printed in black and white, it’s nearly impossible to read. Please change the color scheme to improve clarity. Is site Q on the other side of the stream? Is Q the stream itself? What is SI? The lettering scheme of panel B combined with the lettering scheme of the figure is confusing. My recommendation is to keep the panel lettering scheme and change the panel B lettering scheme of the sites to lowercase letters, numbers, or numerals. The letter “X” is shown in the legend for the sample site, but not used in the figure. What’s the difference between field site and sample site?

Line 107: Were these odd numbered hours? Please state that. Odd hours is not clear. How many time points were there between 7:00 and 21:00? n=?

Line 111: A full description of what?

Line 125: Please add the word “concentration” after “DOC”

Line 138 and throughout the text: Please check/correct nitrogen and carbon and N and C for consistency.

Line 141: What is “back in the home laboratory”?

Line 151: What is “Predator data station”? If it is a software system, please provide that information.

Line 165: Good use of the word “compositions”. Check the manuscript to reduce confusing when fluctuating among uses of “compounds”, “composition”, and “molecular formulae”.

Lines 171-173: This seems important, especially when the use of the term “biolabile” is used often and reported on in the abstract. Was this metric calculated? In

Line 179, it states that formulae classifications are informative for this study, but not all mentioned were reported on. Please clarify.

Line 173: Delete the word “above”.

Line 174: Either use “Notably,” or start the sentence with “These boundaries…”. Delete “It has to be noted that”

Results:

Figure 2: The purple font is difficult to read in the inset. Is the black triangle the north direction indicator? Is there a significance of the symbol used for Site D in the main figure? That box with the criss-cross in it? Can that be a circular data point instead? Consider using a black outline for the yellow starting point arrows. They cannot be read in the figure, in the caption, online, and in a printed copy. What is the elevation difference between Site D and the supraglacial stream?

Line 200: Add a comma after “TC”

Lines 200-205: Please report on the results from the leachate DOC concentrations.

Line 207: Provide a definition of what unique molecular formulae are and/or include it as assessment criteria in the methods section.

Lines 207-209: What does “no significant difference between sample types” mean? They all had the same what? And this suggests they were all the same in terms of number of formula assigned? What does that mean? How is it relevant? You can absolutely have the same number of formulae in a bundle of samples from the sample place or in different locations but completely different chemistry/chemistries. Please explain.

Line 215: The use of the word “compounds” is confusing. Did you measure compounds or composition, etc. (see major revision comment)? This also shows up in Lines 227. 230, 239, 258, 260, 261, 264, 323, 325, 328, and 382.

Line 231: This is a good example of the limitations of composition analysis but seems like a random point to make here. Why is this stated here? Discussion section instead? Make the ties to introduction sections that point to microbial and algal blooms?

Line 234: Same point for aglycone degradation product as the comment in line 231.

Line 236: What does “Aromaticity in dark ice was high” mean? Those values look low for aromatic nature (thresholds of AI are 0.5 and 0.67) but are greater than what was reported for the supraglacial stream. Please clarify and consider using more specific terms like “greater” instead of “high”. The word “high” is confusing. If it is helpful, put these thresholds in the methods section or further define them here.

Line 239: Molecular diversity is not the same as number of formulae. Please clarify.

Table 1: Add “(DOC)” after “carbon” and consider adding the identifier “determined by FTICRMS analysis” after “composition” in Line 247. Based on the text, it seems like the biolabile information is missing from the table. Are these results all from Sites D and Q? Define all acronyms in the Table caption, not just RA and #, or point to the methods section for that information.

Line 267: Add a comma after the word “aromatic”

Line 266: What’s the definition of “significantly different” DOM compositions? Is this based on calculations or chemical characterization?

Figure 3: Define “PC” in the caption. Point to the methods section for the acronyms and their definitions or provide them here. Are all these data from Sites D and Q?

Figure 4: Is panel A really all molecular formulae assigned in all samples in the dataset? This looks wrong. Consider moving the word “aliphatic” in panel A to a different location, maybe near the dotted line at H/C = 1.5 on the right-hand side? The chemical character information “aliphatic, HUP, etc.” should be included in the caption. Typo for panel E in the caption, change that symbol, which likely was an automatic revision, to (E). The note about the two CHONS formulae may confuse readers that these might be outliers? Are they? Were they a part of a homologous series?

Line 287: Be cautious here. The molecular backbone of DOM is CHO containing formulae, not CH, especially since this work doesn’t consider hydrocarbon molecular formulae. Classifying CHO as a heteroatom group is incorrect and confusing. Correct the text in the methods section to match.

Line 293: Molecular diversity is not exclusive to number of formulae. Please correct.

Discussion:

Lines 303-305: This sentence is repeated from the methods. Is it needed here? Please include any discussion of the comparisons across samples.

Line 308: Edit “DOC” to “DOM”

Line 310: Edit “concentrations or DOM composition” to “concentrations and/or DOM composition.”

Line 329-330: What does “preferentially leached” mean?

Line 341: Is this statement saying that one molecular formula accounts for 5.7-8.8 %RA of the dark ice DOM samples? How?

Line 360: Consider revising “…DOM pools, are hydrologically connected” to “DOM pools, may be hydrologically connected.” Measurements of hydrologic connectivity provide results to support what is written, but it is only speculated that that information may come from molecular composition information.

Lines 380-382: Confusing and contradictory to what was stated in the previous sentences. What is the main message here? What is the definition of upstream contributions? Just DOM? Debris? Microbes? Processes?

Line 382: Typo. Delete the extra period, “.”, after the parenthesis after “…0.05 %RA)

Conclusions:

Line 390: The part about microbial communities is confusing/incorrect as there were no biology measurements on microbial communities. Please revise.