|The authors have made some improvements on the paper, but I think they could still make an extra effort to bring this work to the standards of Biogeoscience and that the paper does justice to the enormous effort in acquiring these data.|
I still think it is useless and unjustified to use the parameterization of Cole and Caraco (1998) developed for lakes to compute fluxes in a river. This just adds to confusion and distraction to the paper. On the other hand it would be extremely useful to compute the gas transfer velocity from slope and flow using parameterization for rivers as used by Liu et al. (2022). This approach is applicable to all rivers (large and small) even those with low flow and low slope as Ket river. Liu et al. (2022) explain in detail how to make the computations, this can be achieved without too much effort with some GIS modelling. The authors could then use the gas transfer velocity from the chamber measurements to validate the modelled values.
The use of multiple gas transfer velocity values is confusing, and this is worsened by the fact that the legend of figures and Table that are not explicit. Table 1 and Figure 3 report FCO2 and FCH4 and because the legend is very short it is not possible to know just by reading the table/figure what these values correspond to. Are these the chamber measurements (for FCO2) or the computed values ? If these are the computed values how were the computations made ? With the constant Kt of 4.46 m d-1 "representative for large lowland rivers" (L207), the gas transfer velocity computed from wind with Cole and Caraco, or using an average K derived from chambers ?
There is (in my opinion) very little gain in computing fluxes with all these different methods. On the contrary this is just a distraction and source of confusion for the readers.
I still think it would be useful to extract Strahler order and plot the data as function of Strahler order rather than bundling all of the data into “tributaries”. This is quite useful approach, check figure 2 of Butman & Raymond (2011, https://www.nature.com/articles/ngeo1294). Given the enormous amount of work to acquire the data, the authors might want to spend a couple of hours extracting Strahler order that can be computed with DEM data and GIS tools such as Quantum GIS (freeware).
I still think it is superfluous to present in table 2 the correlations of fluvial CO2 and CH4 with lithology. There is no established link between lithology and CH4. Lithology affects HCO3- content but not CO2. The fluvial CO2 content depends mainly on respiration not lithology. The respiration that leads to CO2 in streams occurs in soils and/or in-stream. This is why Lauerwald et al. (2015, doi:10.1002/
2014GB004941) modelled CO2 in rivers using catchment net primary production and not lithology. This is why CO2 in rivers correlates with DOM rather than Ca2+.
L 76 : The authors have not scrutinized the literature sufficiently. There have been direct measurements of CO2 in “Northern Eurasia”. Please refer to the work of Castro-Morales et al. (2022) in the Ambolikha River in northeast Siberia, published in December 2021. It is not the job of reviewers to make the literature overview for the authors. This paper should be relevant for the discussion as it also reports diurnal variations.
L81 : I still do not understand what you mean by “high latitude regions are important”. This statement is so vague it could mean anything.
L83: So, what ? In which form will this carbon be released ? Will this carbon release impact atmospheric CO2 or CH4 ? If the organic carbon is released in extremely refractory form and is not converted (or converted super slowly) by microbial activity into CH4 or CO2, then this will have no impact on atmospheric carbon content.
I suggest that the authors make their data-set publically available on publication either as a supplement or an entry in a data-repository (zenodo or equivalent).
Butman D., P.A.Raymond (2011) Significant efflux of carbon dioxide from streams and rivers in the United States. Nat. Geosci. 4, 839–842, https://doi.org/10.1038/NGEO1294
Castro-Morales, K., Canning, A., Kortzinger, A., Gockede, M., Kusel, K., Overholt, W. A., Wichard, T., Redlich, S., Arzberger, S., Kolle, O., and Zimov, N.: Effects of reversal of water flow in an Arctic stream on fluvial emissions of CO2 and CH4, Journal of Geophysical Research: Biogeosciences, 127, e2021JG006485, 10.1029/2021JG006485, 2022.
Lauerwald, R., G. G. Laruelle, J. Hartmann, P. Ciais, and P. A. G. Regnier (2015), Spatial patterns in CO2 evasion from the global river network, Global Biogeochem. Cycles, 29, 534–554, doi:10.1002/2014GB004941.