Dear Authors,

Thank you for your responses to the two referee reports.

While both reviewers recognized the significance and novelty of your research at the rarely studied Belo Monte hydropower complex, they also pointed out that your manuscript would require a thorough revision to improve the clarity of data presentation and interpretation as well as English writing. Based on these referee reports and my own reading, I thought I might need to reconsider your manuscript for publication in Biogeosciences after major revisions. Please also consider English editing before submitting the revised manuscript.

I would like to ask you to make all the changes easily identifiable in a marked-up manuscript and a point-by-point reply to the reviewers’ comments. I would also suggest you to specify the line numbers of the revised parts when you respond to the reviewer comments.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences

Dear Authors,

Thank you for submitting your revised manuscript that has carefully incorporated many comments and suggestions offered by the two reviewers.

Although your manuscript has improved in both data presentation and interpretation, I still have some lingering concerns about some technical details and important data presentation and descriptions, as detailed below. There are also many awkward sentences throughout the manuscript, so English editing by a native speaker would help you a lot enhance the clarity of the manuscript. For these reasons, I recommend a minor revision before the final decision.

I would like to ask you to make all the changes easily identifiable in a marked-up manuscript and a point-by-point reply to the editor comments. I would also suggest that you specify the line numbers of the revised parts, whenever your refer to them in your responses.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences


EC1 (editor comment): Both reviewers had suggested a thorough English editing (a reviewer comment and your response shown below). Despite your efforts, there are still many sentences that need to be checked by a native speaker. Just to name a few examples here (pay attention to the parts in “…”):
Abstract - The Belo Monte hydropower complex located in the Xingu River is “one of the largest in the world” in terms of energy production capacity, and “the largest operating” as a run-of-the-river (ROR) hydroelectric system…
The FCO2 (0.90 ± 0.47 and 1.08 ± 0.62 μmol m2 d-1 “to XR and IR respectively”)…
However, the associated intermediate reservoir may “overcome” these emissions due to altered riverine characteristics….
Introduction – …which “has” 5.1 Pg y-1 of carbon “terrestrially delivered” (Drake et al. 2018) and “???” about 2.1 Pg C annually “emitted” to the atmosphere (Raymond et al., 2013)…
Great debate emerged “from” the Belo Monte hydropower project…
The aim of this study is to “characterize” the CO2 emissions from the Belo Monte reservoirs in the first two years post-impounding by assessing the spatial and temporal variability of CO2 partial pressure (pCO2) and carbon dioxide fluxes (FCO2) in the XR and IR
Discussion - Although pCO2 and FCO2 are correlated (Rasera, et al., 2013), “in this study was observed” some specific examples where k produces different fluxes even when pCO2 was similar.
RC (reviewer comment; reviewer 2): 2. Language overall needs improvement. Too many commas used. Too many sentences that are confusing (many are mentioned in specific comments below).
AR (author response): The language and style of the whole text was revised and improved.

EC2: Please define ROR at its first use in the introduction (…run-of-the-river (ROR) hydropower systems have smaller reservoirs…).

EC3: As the two reviewers had commented earlier, specifying your research objective or hypotheses would help you transform the descriptive manuscript into a more focused one. There is still no clear goal statement in the abstract. Please consider specifying the major objectives (and hypotheses) in the following sentence: “We evaluated spatiotemporal variations of surface water CO2 partial pressure (pCO2), water-atmosphere CO2 fluxes (FCO2), and gas exchange coefficients (k600) in the XR and IR during the first two years after the impoundment of the Xingu River.”

EC4: Please specify what “altered riverine characteristics” are responsible for the observed change in CO2 emissions from IR (Abstract – “However, the associated intermediate reservoir may overcome these emissions due to “altered riverine characteristics”.). You could also make your point clearer in the concluding remark on ROR (“CO2 emissions from ROR reservoirs to the atmosphere are in the range of natural Amazon rivers.”), particularly in relation to your hypotheses. Do you mean that ROR does not alter CO2 emissions?

EC5: The technical detail questioned by the first reviewer (as shown below) is important, so please provide in Methods how you handled any dilution effect and checked the potential contamination by the remaining air in the evacuated vial. As the reviewer commented, typical vial evacuation procedures cannot remove the air inside 100%. Transferring a similar volume of air sample to the evacuated vial cannot guarantee that the vial has been removed of air 100%. You could easily test (and correct, if required) any potential left-over gas concentration by measuring CO2 concentration after filling the evacuated vial with the same volume of N2 gas as the typical volume of samples.
RC (reviewer 1): L150. How good was the evacuation? In my experience, it‟s very difficult to get a good vaccum, but probably 10% or more atmosphere will remain, which may dilute or contaminate your samples. Was this checked?
AR: We are confident in our sample storage methods, which our team has extensive experience with. A vacuum pump was used to create a vacuum, which was confirmed since the volume of gas pulled from the syringe into the vial was similar to the vial volume without the needing to manually depress the syringe‘s plunger. We have not added these details to the manuscript, as transferring gas to vials is a common method.

EC6: Spatial and temporal variations in pCO2 and FCO2 might be one of your key findings, but their descriptions are inconsistent or inaccurate, as sampled below. Please note that FCO2 in the low-flow period was distinctively higher in IR. And differences in pCO2 between XR and IR are not clear enough to say the difference definitely. Please double check and correct all the related descriptions throughout the manuscript.
Abstract – “Spatial heterogeneity was observed for pCO2 during both low and high water seasons while FCO2 showed significant spatial heterogeneity only during the high water period.”
Results - Significant difference in FCO2 was observed among environments sampled during high water season (F3:28= 7.94, R²= 0.43, p= 0.0089) while the low water season had relatively homogeneous FCO2 values (F3:17= 2.67, R²= 0.14, p= 0.08) (Fig.4 and Table 3).
Discussion - The increase in pCO2 during the high water season and can be related with the increased input of terrestrial organic and inorganic carbon into the rivers by surface run-off and subsurface flow of water…
During the low water season most of pCO2 and FCO2 decreased, especially in the river channel environment…

EC7: Adding to the previous comment on the key findings on spatiotemporal variations in pCO2 and FCO2, Fig 3 needs to be improved in presentation quality. It is a very big and busy figure and there are also formatting errors (e.g., units not put in parentheses) and lacking information about the labels (e.g., UD, DD?). Please think about how to present all the plots in a more efficient way, by e.g. putting “High water” and “Low water” labels on top of all the plots and sharing “Environment” category labels only once at the bottom of all the plots. If you cannot show all results on a page, splitting them into two sub-figures or using a horizontal page format could be an alternative idea. Plotting over the same range on the vertical axis would make it easier to compare the high- and low-flow results.(e.g., -1000 to 3500 for the plots c and d)

EC8: In section 4.2., your explanations about the higher FCO2 from IR during low water than in high water season were focused on the flooded OM as a potential source. What about the counteracting effect of phytoplankton uptake of CO2, which might have increased in the low water season, due to reduced turbidity and enhanced phyotoplankton uptake of CO2? Please refer to other studies that compared seasonal differences in reservoir primary production and CO2 emissions; and provide a more balanced discussion of potential contributing factors for seasonal variations in the source and sink of CO2 in reservoirs.

EC9: Please provide more details on the several same variables in Table 3. It is difficult to understand how the same variables of pCO2 and FCO2 differ across the rows.

Dear Authors,

Thank you for your efforts to improve the clarity of the manuscript. I am pleased to let you that your manuscript can be published subject to technical corrections of the following:

Line 17: please correct "for, XR and IR respectively) in IR" to "for XR and IR, respectively)"
Line 95: please use CO2 instead of “carbon dioxide”
Lines 98-99 (and throughout the manuscript): “FCO2” and “pCO2” have already been defined in the previous sentences.
Lines 423-424: The sentence (“ROR reservoirs did not appear to alter CO2 emissions compared to naturally flowing Amazonian clearwater rivers.”) is contradictory to your key conclusion, so you need to rewrite the sentence or could add certain conditions under which ROR reservoirs did not alter CO2 emissions substantially.
References with the author name “Brasil”: It appears that you have used some reports published by the government of “Brazil”. Please use English translation and indicate that the reports are written in Portuguese. If available, names of ministries or government institutes would be better the country name “Brazil”.
Figures: correct “u” in unit “umol” to the true micro.

Sincerely,

Ji-Hyung Park

Associate Editor, Biogeosciences