Dear authors,

many thanks for responding to the referee comments. As you can see, the referee recommendations range from minor revisions to rejection. Overall, I decided that major revisions are necessary before the manuscript can be considered for publication. There are 2 major comments in particular that I am concerned about and that have been raised by multiple referees: 1) The novelty of the study and 2) the suitability of the study site as a natural laboratory. From your response(s) regarding point 1) I read that while some of the sites have been investigated in the peer-reviewed literature, your study provides new data from these sites plus additional sites that have not been investigated yet. Likewise from your response(s) regarding point 2) I read that you e.g. see some analogues to conditions described in future projections. In both cases I would advice you to make these points much clearer in the text, better link to existing studies that were conducted at the same sites (even though with other data) and give these other studies appropriate credit and be more precise about where you see the analogues and under what conditions/scenarios these analogues may emerge.

I would also like to note that I intend to send the revised manuscript back to the same referees for their evaluation.

Best regards
Peter Landschützer

Dear authors,

I have now studied your revised manuscript with track changes and went back to the referee comments and your response as well as my own recommendation. To recap: The majority of referees considered the natural analogy comparison problematic and urged the authors to substantially revise the manuscript in this respect. Likewise, a second major concern was raised related the novelty of the study as some results on the sites are already published. In response to these referee concerns I further explicitly asked in my decision letter for you to substantially revise (i.e. major revisions) the manuscript to clarify these points. From my reading of your response to the referee comments and the final revised paper (following my editorial comment) I dont believe you have taken the comment into serious considerations. This is also fairly easy visible by the minor changes in the track-changed version of the manuscript. This has further been recognised by the referee comments I received from referee#2 who has reviewed the revised manuscript. Unfortunately, there were some technical issues, hence referee#2 has sent the comments to me electronically and only used the online system for the recommendation. Please find the comments at the end of this letter. I dont think it was necessary to change your position regarding the natural analogy assumption, however, some concerns from round 1 have not been (fully) addressed. While I read e.g. that the authors added some points on the limitations (lines 252-262), I dont believe this qualifies as the discussion the referees were looking for. E.g. in his referee letter, referee#3 is concerned about the "extreme variability" at these sites and how this effects the natural analogy assumption (in particular, the referee called the the usefulness as natural laboratory limited). In the response and the revised manuscript, however, I barely see this addressed (with the exception of the link to the tidal influence). I was also hoping that the authors would use this occasions to not simply cite the studies that have previously explored these and other sites, but also clearly discuss the new findings in comparison to previous studies (also at different sites). The referees clearly seem to be concerned about the natural analogy assumption and they refer back to previous literature and studies that do show that such an analogy can be problematic (see also the comments from referee#2 on the revised manuscript below). Again, I dont expect the authors to change their position that the site is a useful natural laboratory, but I expect the authors, in response to the referee concerns, to provide clear and solid evidence and a substantial discussion also in comparison to previous studies.

Based on my reading and the referee comments I decided that my position has not changed since round 1, i.e. the authors need to conduct major corrections to the manuscript. All referees welcome the description of the site and the system (although I would welcome not only citing previous studies, but also highlighting how the new data advance the knowledge in comparison to these previous studies). In particular - and I cant stress this enough - I want the authors to further address the referee concerns about the natural analogies, e.g. the variability or "noise" as mentioned by the referees (referee#3 in the first round and referee#2 below) and have a fair and substantial discussion on the limitations also in light of other studies conducted (also at different sites).

Best regards
Peter Landschützer

In case you

comments by referee#2:

The Authors corrected most of the mistakes made in the original version and added some details which were lost. This help to better understand the work done. However, the authors did not consider reviewing their position regarding the ability of this new system to become a laboratory for understanding the effect of climate change on organisms. Personally, I find this position unusual. I perfectly know that some of the so called natural analogue found so far are far to be perfect, and that is the problem. Most of conclusions from some of the studies published, although always important to advance in science, make actually only noise.. ,because the environmental conditions are not what we expect for the future and responses only confusing. Looking to the description of your system I see:
- the lack of CO2 seeps. So the ms must be around submarine groundwater influence and the word CO2 seeps should not be mentioned because they do not exist, or this study do not show if they are in the near- or offshore water.
- the quality of the groundwater discharge makes conditions too variable in pH (yes as most of the natural analogues), too altered (in bicarbonate, At, calcium, Mg, likely other metals etc), too limited in space (meters from the beach with a tide of 2 m!!). Ok, no problem, nice and useful but I continue to think that it is not an exceptional spot to study OA, not great to study how life has persisted through past eras (what a pretention!). It could help to understand the early life on Earth from the Precambrian..?????? Could allow to disentangled adaptation and evolution…??? Please, do not speculate.
- “the accurate chemical characterization” is a two date sampling on the seawater carbonate chemistry only. No mention about other true chemicals and metals discharged. IPCC does not include pollutants in its scenarios.
- the sampling was made, I insist, in the intertidal zone (i.e., The intertidal zone is the area where the ocean meets the land between high and low tides) when considering a tide of 2 m as at the study site. The effect of discharged water, so the low pH area, is close to the beach and immediately dissipate 50 m far, even less depending the study site, from the shore. The lat & long scale does not help to appreciate the real extension of the affected area but having a look in google earth it is clear that for instance Playa del Faro (the biggest area of this study) is a 80m long.
- This ms is only about the seawater conditions found. There is no data about what could be the biota living and affected by the gradient in the beach, which is an area affected by the tide and the local strong waves. It is not enough to cite an in press paper (not cited in the literature list). All discussions on that part are purely speculations.
- Fig. 5 is not really important and good for a book. How brackish groundwater is released and reach the sea is an already known mechanism.
- Fig. 6 is honestly inacceptable.
- Fig. B1. I do not understand what exactly means. Why min pH was used? Why the data are connected by a line although sampling is not continuous? Clearly, tide has a direct effect of the groundwater release.

Dear authors,

many thanks for submitting a revised manuscript. I appreciate the time and effort you have put into addressing the referees comments and I am happy to say that the manuscript is now ready for publication after a few remaining technical corrections (suggested by referee#2) have been addressed.

Bets regards
Peter Landschützer