Dear Prof. Dai,

Thank you very much for your point-by-point responses to the reviewers' comments. It appears that most of your responses are fairly reasonable. So we would appreciate it very much if you could submit the latest version of your manuscript to us. Once we get it, I will ask the two referees for further review.

Thank you again for your nice effort to improve your manuscript.

Kind regards,

Koji Suzuki
Editor

Dear Prof. Dai,

Thank you very much for revising your manuscript. The referees I previously asked to examine the first version of your manuscript kindly gave a number of useful comments and helpful suggestions on your revised manuscript (see below). So please read these carefully, and make a further revised manuscript with point-by-point responses to their comments.

Again thank you for your nice effort to improve your manuscript.

Kind regards,

Koji Suzuki
Editor

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Referee #1
Authors responded to all my original comments, but I feel that authors do not take into account main points of my comments.
- Uncertainties and difference between the CJE and PRE (similar issue was raised by reviewer #1): Authors re-estimated 13C value of organic carbon remineralized in the hypoxic zone to be -23.2 +/- 1.1, which was originally -21.8 +/- 0.6. Such a large change seems to be simply due to slight changes in end-member values. Authors may have to allow seasonal and spatial variations of end-members. Indeed, the large scattering and outlier evaluation in Figure 8 will result in much larger uncertainties in 13C value of OC remineralized. Authors should explain how the scattering and outlier are included in the uncertainty (+/- 1.1) of OC remineralized values calculated. In addition, I still do not see the visible difference between CJE and PRE in Figure 8 considering the scattering and natural variations. I think that authors try to obtain the accuracy that the method cannot provide.
- Use of Ca2+ as a conservative tracer: I am not convinced why authors use Ca, instead of salinity, as a conservative tracer although Ca is less conservative.
- Relative contribution of bottom sediment OM: Authors may compare the water column DO consumption rates relative to the bottom sediment DO consumption rates in order to show that the contribution of bottom sediments are negligible. Figure 4 shows stronger DO consumption in the bottom layer intuitively.
- Significant figures: Authors show that DO and DIC precisions are +/- 1 and 2 respectively. But, they say that “we kept one decimal place for DO/DIC…”. Authors should explain why they keep one decimal place although the values are meaningless.

Referee #2
The revisions made in the paper by Su et al. have substantially improved the quality of the paper which, to my opinion, is now ready for publication. You have answered the main points raised in my previous review, especially the one dealing with end-members which is crucial in quantifying the uncertainty of the message provided by the paper. I can see that you have changed the value and expanded the uncertainty of your proportion of terrigeneous OM in the mix generating hypoxia.
I particularly appreciated the long and detailed response letter which made the revisions easy to check and evaluate.
I have just a few technical points which should further improve the clarity of your paper:
- You use very often "pre" for predicted (by the mixing model) and "PRE" for Pearl River Estuary. It is confusing in the equations and Figures when endmembers are quoted (Eq. 7 page 10, or Figure caption and axis labels of Figure 7). In this Figure (7) especially, "PRE" and "pre" co-exist in the axis title which should be changed. You should choose one of them (e.g. Pearl River Estuary) and replace "pre" by "mod" for model.
- You use very often "PRE" for Pearl River Estuary and CJE for "ChangJiang Estuary". At least for the Figure captions, I would recommand to replace CJE by ChangJiang Estuary, as it is not so obvious for most readers and it would make the Figure easier to read.

Dear Prof. Dai,

It is a great pleasure now to accept your manuscript for publication in Biogeosciences. Thank you very much for responding to the reviewers' comments carefully.

Before it is published, please consider my editorial comments below:

P7, L10: are complicated
P7, L26: A number of the phytoplankton-centric hypoxia
P8, L29: during 17–18 July
P8, L30: between 13–16 July
P8, L31: between 19–27 July
P9, L18: Please indicate the maximum differences in temperature between the bottom and surface waters, because temperature can significantly affect the microbial activity.
P17, L21: than in the CJE (-8.8 ‰, -24.4±0.2 ‰) at S<0.4.
P17, L27: temperatures
P17, L28 in the PRE were
P28: Insert a space between θ and (°C).
P29: In references in Table 2, please use parentheses for the year published. For example, Hu et al. (2006).
P36, P37: In the regression equations of Figures 7b, 7e and 8, please remove the space immediately after the symbol “±”.

Thank you again for choosing the journal Biogeosciences.

Sincerely,

Koji Suzuki
Editor