Dear Dr Robinson

Thank you for responding to reviewer comments. I am happy to invite you to submit a revised manuscript, giving attention to reviewer as detailed in your response.

Regards,

Clare Woulds

Dear Dr Robinson

Thank you for your revised manuscript. I would like to request further revision in order to avoid an emphasis on conclusions which do not seem fully supported.

Despite your response to reviewer comments, I am not comfortable with the emphasis of this study being on showing that C addition enhances bioremediation. I think that viewing the study through that lens is leading you to make statements that overstretch and are not fully supported by the results. The results presented are that C addition increases both N2 fixation and NH4 flux from the sediment, and it just seems rather contrived to say that these indicate enhanced bioremediation (even if the study were run for longer, and if holothurian biomass were measured, and increased – none of which were actually the case here). I take the argument that the concept of retaining N within the sediment is an alternative form of remediation, but surely NH4+ production does not achieve this (the ammonium would presumably be free to escape and cause eutrophication), and neither does adding N to the system through nitrogen fixation. I therefore think that the conclusion that carbon addition ‘may result in greater retention of nitrogen within the system…’ is a bit of a stretch, and masks the fact that amendment with C actually leads to N being added to the system.

Nonetheless, the introduction states that C addition is being practised in these new types of system, and that the resultant N cycling is not well understood. I think the manuscript would be much stronger if simply driven by an aim to understand the N cycling changes in response to the C addition practice. Then the full implications of the results can be properly acknowledged, and its impact on bioremediation can be discussed in a fully critical manner, i.e. there could be increased N retention, but there is probably also N addition, and also increased ammonium fluxes, and it is not clear that either of those things is entirely desirable.

Line 572: Please clarify the following statement ‘For example, Castine et al. (2012) found that denitrification and anammox only removed 2.5 % of total nitrogen inputs to settlement ponds in intensive shrimp and barramundi farms’. How was the rest of the N removed, if not by a permanent removal mechanism? What ‘counts’ as ‘removed’ (stored in the sediment?).

I would be glad to receive another revised manuscript with these comments addressed.

Yours sincerely,

Clare Woulds

Dear Dr Robinson

Many thanks for the additional revisions and your accompanying response. I am now happy to recommend publication.

Regards,

Clare