Acidification of the ocean due to the ongoing anthropogenic CO₂ emissions is affecting calcification and photosynthesis in the ocean. However, the effect of acidification on the community level is complex as different organisms might respond differently to this issue. Dorey et al. studied the effect of ocean acidification in natural rock pools, which is an interesting approach as the rock pools are isolated from the ocean during low tide forming a perfect mesocosm experiment with a natural benthic community. Conditions in the rock pools naturally vary greatly between day and night and between seasons, hence the community is expected to be fairly resilient to acidification, yet the experiments show a clear effect of the interventions.

My main concern about the paper is that it lacks a proper inorganic control. The assumption of the authors is that there is no equilibration of CO₂ added to the pools with atmospheric CO₂. This is a fair assumption in the open ocean, as it takes CO₂ several years to equilibrate with the atmospheric pressure but I’m not sure it holds in the experimental setup because of the large deviation of the pCO₂ in the treated pools from atmospheric values, fluctuations in pool temperature during the day (particularly in summer), and the large surface area and limited volume of the pools that are also exposed to the elements. Therefore, I think it is important to add a control experiment that will test if there is a change in seawater pCO₂ after a CO₂ enrichment at the study site in summer conditions. The control experiment can be done in a simple tub with similar dimensions to one of the pools, that is initially filled with artificial or filtered seawater and treated similar to the CO₂ enrichment experiment. It might also be useful to bubble N₂ into a second identical tub to stimulate low CO₂ and O₂ conditions and test the rate of penetration of these gases during a few hours.

The discussion about nocturnal dissolution should include more references about supersaturated CaCO3 dissolution in seawater – you can start with Milliman et al., DSR, 1999 and Subhas et al., GBC, 2022, and references therein.

Other comments:

Line 283: Data is plural.

Line 331: Is the total alkalinity concentration salinity normalised?

Line 334: day:night period should be close to 12:12 in September.

Lines 404-5: Not clear, do you refer to the absolute productivity or to the change in productivity due to the interventions?

 Table 2: It is not clear what do the numbers under “Treat” stand for. Is it the difference between the slopes of the linear regressions of all values from each treatment?

Lines 444-7: Seasonality in DIC is discussed in the text but not shown in Figure 3.

Lines 505-7: This is a key claim but it contradicts Figure 5. You should show how you came to it or remove this sentence.

Lines 585-9: The effect of pH on calcification is indirect through its effect on the carbonate ion concentration, and in any case is important at the site of calcification but not anywhere around the organism. Do the CCA calcify internally within the cells or externally? If internally, the calcification sites are not necessarily exposed to the ambient pH.

Lines 590-610: It is also likely that heterotrophic organisms graze some CaCO3 when eating the algae and dissolution occurs within their guts (hence independent of ambient saturation levels). Microbial respiration of organic matter attached to CaCO3 will have the same effect with dependence on saturation states.

Lines 649-652: These sentences don’t belong here, they were not discussed in the paper.

General comments:

The manuscript "Ocean acidification enhances primary productivity and nocturnal carbonate dissolution in intertidal rock pools" describes a novel and interesting experimental approach, manipulating the carbonate chemistry of intertidal pools to investigate the effects of ocean acidification on temperate intertidal communities. Overall, this is a well written and valuable paper that significantly contributes to the state of knowledge on the impacts of ocean acidification on marine ecosystems. The introduction addresses the current knowledge, the objectives and the hypothesis of the study. The methods are precise, and the results are well-presented. The discussion highlights the main findings and significance of the study. I can see no major issues with the manuscript, but have a number of suggestions which hopefully can improve the clarity of the paper.

My main comment concerns the general conclusion on the vulnerability of tide pool communities to ocean acidification. The experiment was carried out during emersion cycles only, and it seems important to me to discuss the potential response of tide pool communities during immersion cycles, as environmental conditions differ considerably. The tide pools studied here were submerged for nearly 12 hours a day. In future conditions of ocean acidification, communities would therefore be exposed to low pH for longer periods of time. I think this point is important to discuss.

Specific comments:

L.50: Please mention the IPCC scenario RCP8.5

L.164: “the relative area covered by each type of algae were estimated from aerial photographs”. Considering the important growth of green algae in some tide pools (especially in September) and the canopy it generates, how did you consider the surface of underlying coralline algae based on aerial pictures?

L.167:  I am a bit confused by figure 2. You state that the surface area ranged from 0.3 to 0.6 m², however it appears to be up to 0.7 m². Also, can you explain why the surface sometimes varies up to ± 0.2 m² among seasons with a similar volume? I suggest keeping figure 2 as supplementary material and only give the main characteristics of tide pools in the text.

L.168: “CCA: 30 to 71 % of the benthic cover”. These numbers differ from fig.2. Did you consider the average during the whole experiment? Please clarify it.

L.186: You state that you decreased the pH to 7.5 (same L.137), while L.197, the desired pH level was 7.6. Is it a difference between targeted pH and measured pH?

L.260: remove “(not NCP)”

L.334: day:night period in September should be around 12:12

L.394: I suggest writing “CO₂ addition increased O₂-derived NCP_lm by 20% on average over all seasons”. Same L.401

L.440: “≈10 mg O₂ L^-1” please give the exact value.

L.505: It does not seem reasonable to me to extrapolate the CCB calculation by considering the day:night duration. First of all, the emersion cycles are ~6-7h (2 cycles per day) and tide pools are therefore not exposed to day:night 10:14h in winter during emersion. In order to extrapolate, one should also consider the CCB during the immersion cycles, and then calculate the budget with a day:night photoperiod of 10:14. I suggest removing this sentence.

L.521: Tide pool communities are subjected to extremely variable environmental conditions for a few hours during emersion. Immersion cycles play an important role in the ability of species to colonize these upper-shore environments by renewing seawater in tide pools. Therefore, in a future scenario of ocean acidification, tide pools would be subjected to low pH conditions during emersion at night, but also during immersion cycles. This is likely to affect the ability of these communities to cope with ocean acidification, as they would be exposed to low pH conditions for longer periods of time. A few lines should be added about this in the discussion.

L.536: Confusing. I suggest writing “The community’s net primary production increased by 20% on average across all seasons, and was particularly visible in summer (+ 35%)”

Technical corrections:

L.38: Subscript for CO₂

L.205: add coma after “0 to 8 mg L^-1”