Dear dr. Hopwood,

Thank you for the detailed relies to the comments of both reviewers. Based on your replies, I am confident that you will be able to revise the manuscript in such a way that it can be published in Biogeosciences. As you have seen, both reviewers were very critical about the clarity of the presentation of your findings. Please pay specific attention to this aspect. I look forward to seeing your revised manuscript.

best regards,
Caroline Slomp

Dear Dr. Hopwood,
Your manuscript has been re-assigned to me because Caroline Slomp, the associate editor to which your work has been initially assigned, is on leave from the editorial board.
I have read the revised version of your manuscript and the comments of one of the reviewers that have assessed the first version. The reviewer is satisfies with the revised version and has only minor comments that need to be addressed.
On my side, I have some comments on the present version of this manuscript that need to be considered before publication. First, the Introduction needs to be extended with a clear description of the added values of your work compared to present knowledge. In its present form, only lines 71-75 describe what will be done in this paper. I find that it is insufficient and I recommend that you extend with a section that clearly communicate to the readers what are the critical questions that will be addressed and answered in this manuscript. You mentioned that you have devised an experimental system to investigate the dynamics of iron in surface water under different physical, chemical conditions. It would help if you describe by a few lines the added knowledge in terms of understanding the Fe(II) dynamics in surface waters and the influence of the physics and biogeochemistry (which processes? )on it. Please have in mind when writing this section and revising your manuscript that Biogeosciences is an interdisciplinary journal where the interactions between the physics, biology and chemistry are the core scope (this is not marine chemistry).
Additionally, I recommend that you shorten the section “Materials and methods” and that you focus this section on what is relevant to Fe(II) collection and experiments. (sub sections 2.5 and 2.6 and part of section 2.4). Especially because a substantial part of this section is in another paper (in revision for BGD). For instance, Table 1 from Hopwood et al 2018 is similar to your Tables 1 and 2 and a large part of the text is in Hopwood et aI. (2018).
Please check the numbering of your sections: e.g. conclusions has to be section 5 and not a subsection of the discussion. There is no section 2 (we directly jump into section 2.1. I assume that this section must be related to “Materials and Methods”). There is no section 3.3
Minor:
In Table 2, it is mentioned that values are reported with mean value +std but in the table only pH is given with std (temperature is range, nutrient is one value).
Figure 1, legend: “three mesocosms”
Table 3: legend, the last sentence has no verb.
Table 4, legend: ND, not determined » not present in the table

Dear Dr. Hopwood,

I have read the revised version of your manuscript and I am pleased to accept it for publication in Biogeosciences. I would like that you check again the numbering of sections because in the version that I have there are still some issues. For instance, section 2, should be « Material and methods » and not « Mesocosm set up and sampling (MesoPat/MesoArc/Gran Canaria) ». This also concerns the other sections, so please check.
I thank you for having considered Biogeosciences for publication of your work.

Kind regards,

Marilaure Grégoire