Response to Anonymous Referee #1

We thank the reviewer for the positive assessment of the dataset, the spatial coverage, and the integration of greenhouse-gas, hydrochemical, and colloidal measurements. We agree that the manuscript will benefit from a clearer treatment of colloidal dissolved organic carbon, more cautious interpretation of concentration stability, explicit limitations of dialysis-based fractionation, moderated novelty claims, and a more restrained use of PCA.

Comment 1 - Colloidal DOC dynamics

The manuscript discusses colloidal transport extensively, especially in Sections 3.6 and 4.2.2, yet the review of colloidal DOC dynamics is insufficient. The authors mainly cite studies related to Fe/Al colloids and trace metals. The discussion would benefit from engagement with broader colloid and DOM literature from Arctic and boreal systems, and from treating DOC colloids both operationally and mechanistically.

Response: We agree. The original text focused too strongly on metal-bearing Fe-Al colloids and did not sufficiently explain what the measured colloidal DOC fraction represents. We will explicitly define colloidal DOC as an operational 1 kDa-0.45 um fraction and discuss it mechanistically as a mixture of humic supramolecular assemblies, high-molecular-weight organic macromolecules, microbial/plant-derived polymers, and organic coatings or complexes on mineral surfaces. We will also add broader Arctic and boreal DOM references, including Amon et al. (2012), Mann et al. (2012), Guéguen et al. (2006), Fellman et al. (2010), and Novak et al. (2022), in addition to the existing colloid/trace-metal references.

Comment 2 - Concentration stability versus compositional and process stability

The manuscript’s central conceptual conclusion is that DOC/DIC pools are buffered whereas pCO2 remains dynamically regulated. This is interesting. On the other hand, stable DOC concentration does not necessarily imply buffered carbon cycling because DOC composition may vary substantially, DOC lability may shift, and residence time effects are unknown. Concentration stability, compositional stability, and process stability need to be distinguished.

Response: We fully agree. Our conclusion should be limited to concentration-level patterns of bulk DOC and DIC, not to stability of DOC composition, lability, residence time, or overall carbon cycling. We will replace several uses of “buffered dissolved carbon pools” by “stable bulk DOC and DIC concentrations” or “concentration-level buffering”. We will add an explicit caveat stating that our dataset does not resolve DOC molecular composition, biodegradability, photoreactivity, or water residence-time effects.

Comment 3 - Novelty and framing as an integration advance

While the combined dataset is valuable, several components have already been investigated individually in the Lena system. The actual novelty lies more in integrating GHG measurements with colloidal geochemistry, obtaining high-resolution longitudinal observations during post-freshet recession, and linking CO₂ evasion with trace-element partitioning. The manuscript would be stronger if the authors framed the work as a synthesis/integration advance rather than a fundamentally new conceptual discovery. In addition, the related claims (e.g., first assessment, first evaluation, first colloidal transport, etc.) should be moderated.

Response: We agree. We will moderate claims of “first” assessments and reframe the manuscript as an integrated longitudinal study that combines high-resolution greenhouse-gas observations, solute chemistry, and operational colloidal partitioning under the same post-freshet hydrological conditions. This framing better reflects the actual advance and acknowledges previous Lena River work on individual components.

Comment 4 - Operational limitations of dialysis-based colloid separation

The manuscript heavily emphasizes colloidal controls on trace elements and DOC. However, the colloidal dataset is somewhat limited, relying on one operational cutoff. Are there changes in microbial activity, colloidal aggregation, and metal adsorption/desorption during dialysis over 3-5 days? Although the authors state concentrations changed <10%, this does not guarantee preservation of original colloidal structure. The operational limitations of dialysis-based colloid separation should be discussed a bit.

Response: We agree. The dialysis data provide robust operational partitioning between <1 kDa and 1 kDa-0.45 um fractions, but they do not preserve or identify the exact in situ colloidal structure. We will explicitly state that dialysis can potentially influence aggregation, adsorption/desorption, and microbial processing during the 3-5 day equilibration. The <10% change in <0.45 um concentrations indicates bulk chemical stability during dialysis, but not necessarily full preservation of the original size distribution or colloid morphology. Note that ideally, dialysis bags should be deployed directly in situ within river or lake waters (e.g., Vasyukova et al., 2010; Pokrovsky et al., 2012; Mikhaleiko et al., 2026a), as this minimizes potential alterations of colloidal structure during sample handling and incubation. However, such deployment was not compatible with the logistics of a continuous ship-based survey covering 1500 km of the Lena River, where prolonged equilibration times (3–5 days) at individual locations were not possible.

 Comment 5 - Direct colloidal characterization and future work

Is there any colloidal characterization in the present study or planned research to better strengthen conclusions regarding Fe-Al-organic colloids? For example, particle size distribution, zeta potential, Fe speciation, organic ligand composition, and DOM molecular composition.

Response: In the present study, direct characterization of colloid size distributions, surface charge, Fe speciation, organic ligand composition, or DOM molecular composition was not performed. The inference of Fe-Al-organic colloidal carriers is based on operational dialysis, strong co-variation of trace elements with Al, Fe, and DOC, and previous Lena River studies that directly characterized Fe-bearing particles and colloids. We will make this limitation explicit and propose specific future measurements.

We agree that a multidisciplinary characterization of colloidal material, including in situ spectroscopic techniques, would greatly improve understanding of the structure of organic matter–trace metal complexes and the composition of organic ligands. Examples of such approaches have been provided in previous studies by our group, including nuclear magnetic resonance characterization of Al–DOM associations in Central Siberian rivers (Pourpoint et al., 2017) and Fourier-transform ion cyclotron resonance mass spectrometry analysis of DOM molecular composition in western Siberian rivers (Kurek et al., 2025). However, applying these approaches to Lena River colloids was beyond the scope and budget of the present study, although such analyses are clearly desirable for future work.

A major technical limitation is the need to concentrate large volumes of river water to obtain sufficient colloidal material for resolving molecular and structural organization, because most microscopic and spectroscopic methods have relatively low sensitivity at natural riverine concentrations. Such pre-concentration can itself introduce artefacts, including aggregation, restructuring, coagulation, and conformational changes of organic–mineral colloids. Consequently, although advanced structural analyses would be highly valuable, the resulting material may not fully preserve the original colloidal state present under natural river conditions.

Kurek, M.R., Pokrovsky, O.S., Krickov, I.V., Lim, A.G., Korets, M.A., McKenna, A.M., Spencer, R.G.M.: Assessing the molecular-level controls of dissolved organic matter cycling in Western Siberia Lowland rivers, J. Geophys. Res.: Biogeosciences,  30(4), Art No e2024JG008537, DOI:10.1029/2024JG008537, 2025.

Pourpoint F., Templier J., Anquetil C., et al. : Probing the aluminum complexation by Siberian riverine organic matter using solid-state DNP-NMR, Chemical Geology, 452, 1-8, 2017.

  Comment 6 - PCA variance explained and moderation of dominant controls

In the PCA interpretation, the first two components explain only 23% and 13% of variance. As such, >60% of variability remains unexplained. Statements regarding dominant controls should be moderated.

Comment 7 - Interpretation of low methane concentrations

The low CH₄ values are interesting. Are they related to floodplain connectivity, oxidation, or sediment processes?

Response: We agree that the low and spatially uniform CH₄ signal deserves more interpretation. We will add a short discussion emphasizing that the post-freshet recession period likely reduced connectivity with anoxic floodplain habitats relative to peak flood, while high oxygen concentrations and turbulent main-channel transport likely favored methane oxidation and atmospheric loss. Limited exchange with anoxic sediments in the deep, fast-flowing main stem may also contribute. The floodplain lake sample with elevated pCO₂ and CH₄ supports this interpretation, because hydrologically disconnected or weakly connected floodplain waters can retain stronger methanogenic signatures.

Comment 8 - Terminology: “Low Lena” versus “Lower Lena”

Low Lena may read Lower Lena or use consistently?

Response: We agree. We will use “Lower Lena River” consistently when referring to the studied river sector and “lower Lena reaches” only in descriptive lower-case usage. This also aligns with the manuscript title.

1. Independent pCO₂ control measurements and their discrepancies

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  2. Potential causes of tributary-scale anomalies

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3. PCA presentation and confidence of interpretation

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4. Citations for the proposed seasonal CO₂ mechanisms

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  5. DOC/DIC concentration versus composition, lability, and speciation

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We sincerely thank Reviewer 3 for the positive evaluation of our manuscript and for the careful line-by-line reading. We are pleased that the reviewer recognizes the value of the dataset and its contribution to understanding carbon emissions, colloidal transport, and trace-element mobility in a large Arctic river system during the post-freshet period. We also appreciate that the comments are primarily editorial and aimed at improving clarity, consistency, and precision.

In the revised manuscript, we will carefully address all line-specific suggestions. In particular, we will standardize geographical terminology and spelling throughout the paper, using Lower Lena consistently where appropriate, and we will harmonize terms such as mainstem/main stem, numerical en dashes, and tense usage in the Discussion. We will also correct ambiguous or inappropriate wording, including replacing “carbon speciation” with a more accurate term such as “carbon forms” or “carbon pools,” and we will correct typographical errors such as “kkk” in the gas-transfer section. We will also provide the supplier and matrix description for the SLRS-6 certified reference material.

We agree that the figures require several clarifications. Figure 1 will be revised to label Yakutsk, Kyusyur, Samoilov Island, and the sampled tributaries consistently with the text, so that the spatial discontinuities discussed later can be more easily interpreted. The blue and red lines in Fig. 1B will be clearly identified, the Fig. 6 caption will be corrected to match the dataset actually used in the analysis, and the y-axis range in Fig. 8 will be adjusted so that all error bars are fully visible. We will also inspect the apparent outliers in Fig. 2. If they correspond to valid samples, they will be retained and briefly explained; if any reflect analytical or transcription errors, they will be corrected or removed.

We also agree that some material currently placed in the Results section is interpretative or comparative and would be better located in the Discussion. During revision, we will move or shorten such passages, particularly where broader comparisons or mechanistic interpretations are presented. Finally, we will refine the colloid-related wording by clearly defining the operational fraction as dissolved material <0.45 µm with colloids >1 kDa, avoiding ambiguous expressions such as “total dissolved concentration,” and moderating statements about colloidal stabilization, Al-rich particles, fulvic/humic ligands, and permafrost effects. The revised text will be more cautious, emphasizing that our conclusions apply to the studied Lower Lena reach and sampled hydrological period rather than to all Arctic rivers universally.