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Review of the manuscript “Particulate organic matter in the Lena River...” by Ogneva et al.

The paper addresses a fundamental question of riverine fluxes of particulate organic carbon in still poorly studied permafrost regions, and its potential impact on surrounding marine environments, and as such it fits the scope and potentially can make a good addition to the journal.

Major critical comments are listed below.

• The main conclusions of the authors – that estimation of river OM discharge to the coastal zone cannot be based solely on the data of the main stem far from the deltaic region – is certainly useful for modelers, although not novel and generally agrees with large body of evidences collected for example, by Shirshov’s Institute of RAS in Arctic rivers and coastal zones (A.P. Lisitsyn’s marginal filters concept suggested more than 30 years ago).

• The Introduction is well written but it is way too general. Former studies on the POC were not discussed (Semiletov, Kutscher, E. Karlsson). As a result, specific objectives and novelty of this work are unlear; and no new nypothesis is proposed to be tested ( a degree of pOC lost in the deltaic zone or the age and origin of POC could be such hypotheses). In anyway, the authors should clearly position this work with respect to former studied of the Lena River to prove its real novelty.
• The Discussion is very much driven by postulated overwhelming role of phytoplankton in POC, d13C, D14C control in the main stem vs. delta. Without Chl a analysis, or any information on the phytoplankton, such a discussion is not substantiated and suggested explanations have low novelty and probably unwarranted. As a minimal research efforts, the authors could examine their TSM samples by SEM to show the presence of higher amount of diatoms in their deltaic samples vs main stem samples
• Examination of C/N ratio could aslo help a lot in distinguishing different sources of POM
• The discussion and data treatment (Fig 4) also ignore that part of POM may be represented by contemporary vegetation debris (i.e., lignin), especially from larch trees, dominating the Lena catchment

Specific comments:

L117-120 This might be true; however, di not the former works of Semiletov, Kutscher etc address the transformation of C between Zhigansk/Yakutsk and the delta?

L176-177 Provide some numbers on the magnitude of Delta14C between “old” and “modern” for non-experienced reader

L185-187 Neglecting the beginning of spring flood may underestimate sizable amount of riverine C, transported to the delta (which is not the case for the winter time). Justification ere is needed.

L197-198 Former studies already shown this; why additional efforts are needed?

L203-205 Unclear. If there is no difference in deltaic region (L197-198), why there should be any in the river main stem? More likely explanation is due to seasonal variations in C concentrations in the Arctic GRO dataset.

Fig. 2 is well presented. However, the data of former researchers, obtained at these transects (at least, the Yakutsk – Kusur one) should be also presented

Section 4.1.1 can be strongly shortened; the novelty of these findings is low. Summarize in one paragraph. Some relevant information can be shifted to the caption of Fig. S1.

L314-320 This is site description; re-arrange

L353 Present the numbers of velocities in thee regions

L420 There should be some data for the man stem

L439 d13C of POC?

L558-563 The novelty of the present study seems to be low

This paper reports majorly the measurement of TSM, POC, δ¹³C and ¹⁴C in the Lima River during 2019, a year that represented “lower-than-average” TSM exports and showcased a strong positive influence on phytoplankton growth. The paper highlights the importance of deltaic processes. Findings are potentially important because they inform how climate change may influence Arctic carbon fluxes to the ocean. However, the paper has a few areas that require improvement. The authors fail to provide a discharge time series for the year of data collection and ArcticGRO sampling period to put their findings in context. The paper can be improved if the following changes are made:  

1. In the introduction section, explain the importance of Lena River and why it is important to study it in 3 sub-sections (as mentioned in lines 180-183). Further, add statements about the research gap and focus of the current study.
2. Figure 1: Include the information on the sample number in the caption. Also, try to showcase three divisions of sample groups for easy understanding
3. Figure 2: Edit and add mean values of your results and ArcticGRO which you are discussing in Sections 3.2.2 to 3.2.4
4. Show the river discharge time series data relative to ArcticGRO. It is necessary to fully interpret these results.
5. Lines 301-302 mention that 2019 was a year of lower-than-average TSM export. Discuss the variation in TSM and POC on a large timescale and present a plot of temporal variation for better clarity.
6. Lines 309-310: Provide a figure or table comparing the POC variation with the published data
7. Also utilize discharge data to calculate the flux of TSM, and POC and compare it with previous reports.
8. Section 4.1.1 and 4.1.2: Again, it would be helpful to see the discharge time series for the ArcticGRO period of sampling vs other years such as 2019. These variations in POC% are hard to interpret without seeing the discharge time series. Further, it is often helpful to calculate the ratio of the coefficient of variation (CV) of your parameter (e.g., POC%) to the CV for Q; CVc/CVq to determine how much discharge is affecting the variation.
9. Minor comment: Check the mention of the figure numbers in the text. There is no figure 3c
10. Lines 420-421: Provide a reason for not analyzing δ¹³C of DIC. Additionally, you need to provide reasons why you considered that low δ¹³C of POC suggests a ¹³C depleted DIC pool with more references.
11. Line 420: δ¹³C of DIC was found to be negatively correlated with DOC concentration. Is this the observation of Brunet (2005)? If yes, please rewrite the sentence with the proper citation.
12. General comment. Do your data suggest any influence of lakes on your TSM and POC concentration? How do you rule them out?

The manuscript  presents and discusses the data on quantity and quality of suspended matter, including its organic fraction, in the Lena River main channel in its middle and lower sections, and in the deltaic section. The manuscript has logical structure, is clearly written, presents novel data and adds to the discussion on the fate of particulate organic matter at the interface between the river and the sea. Two major points arise from the endmember modeling of the POC isotopic signatures : an important contribution from phytoplancton in both riverine and deltaic sections, and a noticeable input from the Ice Complex deposits in the Lena Delta region. The manuscript text needs to be more focused on these two findings, and if the authors think I have omitted any other important aspects, these aspects also need to be clearly framed and put forward. Notably, the ‘Discussion’ section of the manuscript needs to be centered around these major findings ; it is vague and uninspiring in its present form. Certain sections of this Discussion section, i.e. subsections 4.1.1 and 4.1.2, are quite detached from hydrologic reality. The comparison with ArcticGRO data makes a large part of discussion, while the discussion is usually self-sufficient and mostly relies on the newly presented data. I would suggest a separate section named ‘Comparison with previously published datasets’ for this discussion rather than spreading them across different sections.

Overall, I recommend a moderate revision of this manuscript with re-review. The revision might affect, first and foremost, the Discusson section, in what is related to regional hydrology, and better framing the major conclusions. By-line comments are available in the attached pdf.