Dear Authors,

Thank you for your detailed responses to two referee reports and a short comment.

Two reviewers and Dr. Akhand raised a number of critical issues on your manuscript. I also agree to their primary concerns; above all the lack of clear study objectives and arguments unsupported by data (primarily those regarding the roles of phytoplankton productivity and OC mineralization in the Hooghly and exogenous C inputs in the Sundarbans). As reviewer 2 mentioned, major differences in measured parameters appear to derive simply from the dominant influence of fresh inputs to the Hooghly estuary. However, many of your arguments have been based on the assumption of anthropogenic impacts on the Hooghly C dynamics, without providing detailed descriptions and data on anthropogenic sources of C.

Given the large number of critical issues to address, a thorough revision beyond major revisions would be required to reconsider your manuscript for publication in Biogeosciences. The revised manuscript will be sent to the reviewers to make sure that you have adequately addressed all the raised issues and minor technical corrections. Regarding your assumption on the Hooghly as an anthropogenically modified system, I wondered if your summary of published water quality data (as shown in the supplement) would provide sufficient data supporting your arguments on the dominant role of C sources of anthropogenic origin driving the reported C dynamics in the Hooghly estuary. Please compile (and discuss) more concentration and isotope data on C species of anthropogenic origin, from your own surveys or the literature, to relate them to your interpretations.

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’ and my own comments to facilitate the second round of review. It would be a good idea to indicate line numbers of the revised manuscript when you respond to the reviewer comments.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences

Dear Authors,

Thank you for revising your manuscript in response to two referee reports and a short comment.

One of the two reviewers involved in the first round of review provided a positive evaluation of your revised manuscript, as shown below. I also think that your revision has been successful in addressing many critical issues raised during the first round of review. However, the same reviewer also indicated some remaining concerns about many editorial errors in the revised manuscript. I have also found many technical errors and unclear sentences that should be fixed before the final acceptance of your manuscript.

I would like to ask you to take a careful look at details and fix "numerous" editorial errors and weird English expressions including those I list below, just as a "small" selection of examples. Please make all the changes easily identifiable in the final marked-up version of your manuscript so that I can do a quick check-up before the final decision.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences



While this paper doesn’t present much that is new or novel, it is a solid paper. The authors have done a good job addressing the reviewer concerns.
The authors have done a good job addressing the concerns of the reviewers. I would recommend that the authors have a very close look over the paper as many small mistakes in both language and cited data still persist. For example line 83 -check figure 5 in Bouillon et all - the discrepancy is not 72%.


- Title: In my view, one of the primary contributions of your study would be adding post-monsoon data sets on the carbon biogeochemistry of the Hooghly-Sundarbans system. Therefore, I would suggest that “the Hooghly-Sundarbans estuarine system” remain somewhere in the title, like “The postmonsoon carbon biogeochemistry of “Hooghly-Sundarbans” estuaries under different levels of anthropogenic impacts”
- Comparison with literature data of stable carbon isotopes: Please cite more recent papers and provide updated ranges of stable C isotope ratios in both natural (Marwick et al 2015. The age of river-transported carbon: A global perspective, Global Biogeochem. Cycles, 29: 122–137) and sewage (Jin et al. 2018. Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater. Biogeosciences 15: 6349–6369.) sources of DOC and POC.
- L 33- : The long sentence can reformulated to make the point clear, like “A mixing model suggested a combination of processes including freshwater intrusion, carbonate precipitation and dissolution…”
- L 34: Please remove “in the”.
- L 36: “The signatures…” is a difficult sentence to understand and what are those signatures?
- L 42: “evidence for only C3 plants was noticed” – Do you mean “the contribution of C3 plants was dominant in δ13CPOC in the Sundarbans”?
- L 45: Do you mean “higher by ~ 1291 μatm”? Or the average is 1291 μatm?
- L 45: “surface run-off” – This hydrological term might be missing what you want to describe - “ terrestrial (terrigenous) exports of carbon”?
- L 49-51: Please correct “are” to “is” and think about revising the entire sentence to get to the main point: “anthropogenically influenced estuary was dominant as CO2 source ”
- L 66 and thereafter: Please correct spacing errors (0.53 Pg) throughout the manuscript.
- L 91: Please change “C export fluxes” to “C exports”.
- L 114: Do you mean that “ estuaries in the southern India have been the focus of the study of estuarine biogeochemical cycles in India”?
- L 117-119: Please do not repeat the same expression. You have a similar goal statement in L 142-145.
- L 142-145: Please state the study objective more clearly, like “The primary objective of this study was to compare…”, in case you move the goal statement in L 117-119.
- L 212: Please specify sampling location number as appeared on the site map.
- L 240: Which instrument did you use to measure d13C in POC?
- L 267: You need to first define AOU (the departure from atmospheric equilibrium). Isn’t your equation different from the more common one (O2 eq – O2 measured).
- L 324: Please rewrite “Spatially, in the Hooghly, maximum…”.
- L 361: Here you are distinguishing “zones” based on salinity. Would “regimes” better mean your point?
- L 368: Please rewrite this and many other similar cases. - DIC concentrations “observed in this study” were “higher than” that reported by Samanta et al. (2015)
- L 373-377: Please rewrite this long and vague sentence.
- L 386: Abbreviations like “CP” and “CD” are not used frequently, but difficult to identify there meanings in scattered sentences.
- L 391: PP and CO2 OG?
- L 392: advocated, for what?
- L 421: Please rewrite this and many other similar cases – “study that measures rates of these processes”
- L 425: play “a” significant role
- L 441-445: This type of information would better fit into a relevant section in Results.
- L 465 and many other cases: “higher that the range (226.9 ± 26.2 to 324 ± 27μM) reported by…”
- L 475: ΔC?
- L 491: Do you mean “significant DOC formation mediated by primary productivity”?
- L 494: You cannot exclude the possibility that significant mineralization of DOC may be compensated by phytoplankton CO2 uptake in the eutrophic estuary. Please consider this in explaining the weak relationship.
- L 528: Here you can go into more detail about sewage-derived pollution by citing Samanta et al.
- L 554: Primary signal for surface runoff mediated POC addition?
- L 562: Check grammatical error.
- L 676: The goal of “conclusions” is not to summarize major findings. Please combine sentences into a coherent paragraph to conclude your article by addressing the hypotheses you proposed in the introduction.
- Table 1: Pay attention to numerous errors including those of spacing between number and units.
- Tables 2 & 3: Provide units below the presented parameters.
- Figs. 2-6 lack the legend.

Dear Authors,

Thank you for your thorough revision. I am pleased to let you know that your manuscript can be published after some technical corrections as noted below.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences

Please correct the following (and corresponding parts in the main text):

- Line 32: "DO: 91 - 104%"
- L 37-38: Please specify " removal of DIC" by what process? Did you mean "through mineralization of mangrove-derived organic C"?
- L 38: Did you mean "The concentration of DOC in the Hooghly was ~ 40% higher than in the Sundarbans"?
- L 39: What "biogeochemical processes"?
- L 45: Please specify "where" or "by what processes" following "modifications".
- L 46: Did you mean "decomposition" or "degradation" by "respiration"?