This study investigated the ocean alkalinity enhancement (OAE) effect on fish organismal and community responses under laboratory and mesocosm settings. Overall, no negative effects of OAE were reported. Currently, OAE effects on marine biota remain poorly understood, preventing sustainable implementation of this carbon dioxide removal solution for climate change mitigation. Therefore, the data presented are important for advancing our understanding of the ecological risks associated with OAE. While the manuscript's overall quality is good and the results are highly novel, there are several minor comments listed below that could be addressed to further enhance the quality and clarity of the manuscript.

General comments:

The laboratory and mesocosm experiments differ in methodology. Why in the laboratory experiment only Ca OAE was done, contrary to the mesocosm? Why a range of TA was used for the mesocosm but not for the laboratory experiment? Why no replicated in the mesocosm?

This seems peculiar, as to get interchangeable results that could support experiments each other, the same methodology should be used. There is lack of explanation why the decision on using only a TA of 600 umol kg-1 has been used in the laboratory experiment and why this TA has been considered as the highest endpoint in both experiments.  Moreover, the lack of replicates in the mesocosm experiment reduced the power of statistical analysis. The authors should provide an explanation of why this was the case.

Another aspect that seems to be poorly introduced is the species-specific responses. The model species used was Atlantic herring, which as indicated by the authors has been proven resistant to pH fluctuations. It again seems peculiar why this particular species was used for the experiment. From the ecotoxicology point of view, it would make sense to test the OAE effect on the most sensitive species as all fish communities could be protected by regulating the application of OAE based on the most sensitive species results. The authors should address that.

Furthermore, in the mesocosm experiment the authors added silicate-based OAE to simulate different mineral addition scenarios. Most of the minerals regarded for usage in OAE would dissolve trace metals into the water and the trace metals aspect is regarded as potentially the most harmful. I wonder why this aspect hasn’t been studied by the authors.

Specific comments

Introduction:

Line 32-33: Ocean alkalinity enhancement does not only refer to rock weathering. There are different types of technologies that can increase alkalinity e.g. electrolysis technologies. Therefore this sentence has to be changed accordingly.

Line 33: Change ‘store Co2’ to ‘sequester CO2’.

Line 34: Again, storage seems like not the nest word to use for OAE CO2 removal mechanisms. Also, here the authors introduce a new terminology ‘negative emissions’. I found it confusing because above a term ‘ carbon dioxide removal’ has been used. It is not the same therefore authors should stick to carbon dioxide removal terminology.

Line 49-51: Add examples of how the physiological processes of fishes are sensitive to bicarbonate, CO2 and H+.

Line 58-62: The authors cite the effects of acidification of different physiological and behavioral responses of fishes. Add some more details about what were those effects, increased or decreased, beneficial vs. negative etc.

Line 62: This sentence sounds confusing. OAE is supposed to counteract ocean acidification and in this sentence, it reads like it is going to cause the same change in H+ concentrations as acidification. Correct accordingly.

Materials and methods:

Line 96: Add that the primary focus of the laboratory experiment was put on the Atlantic herring. In the mesocosm experiment, more species were studied.

Line 117: Specify what was the amount of pure Milli-Q water added to control tanks.

Figure 2 – it is hard to distinguish the Mineral type on plot b, the shapes are not distinct enough. Maybe make one of them empty. Use the same pH scale on graphs a and b so it is easier to compare.

Line 132: Were larvae only placed in the vials to clear their guts for 1.5 h? Is this a standard procedure? If yes, add a reference.

Line: 163: What are those nutrient concentrations based on? On local data? If yes, add a reference.

Line: 193-194: Were there different depths from which the primary producer's biomass was measured? The term ‘depth-integrated water samples’ is not clear. Please specify.

Line 199: Change to ‘For the laboratory experiment,’

Line 202: Change to ‘For the mesocosm experiment,’

Line 208: Were the data normally distributed or transformed for the nMDS analysis?

Line 210: Provide information on which R package has been used for the statistical analysis.

Line 212: Why only per capita sizes of fish were log 10 transformed? Provide an explanation.

Results:

Line 224: I would add that there was a larger spread in the OAE application on larvae survival than in the control (Fig. 3e).

Figure 4: The figure lack of color legend.

Line 264: The increase in biomass seems to be not only driven by cod but also herring according to Fig. S1. Update accordingly.

Discussion:

Line 289-291: So herring is overall resistant to ocean acidification. I wonder why this species has been used as a model organism. The results could be different for more sensitive species. Provide information on why this species has been used in the context of OAE. The information provided in Methods, line 96-98 does not mention the OAE aspect.

Following, some discussion about how more sensitive species might have responded differently to the OAE should be added here. Expand why such contrasting results were found between the current study and the other ones.

Line 294-302: Related to the above, the species-specific responses should be more pronounced. Expand this aspect.

Line 296: What does it mean: ‘Five species were investigated here’. Seems like the authors are referring to another study so should use the word ‘here’.

Line 303: Why tropical species would be more resistant to OAE, provide explanation and mechanisms.

Line 303 – 313: Authors should look for other than feeding reasons for the positive effect of OAE on biomass. Maybe those treatments were different in terms of environmental parameters (e.g. nutrients). Those data are not provided.

Line 337: The aspect of toxicity coming from trace metals dissolved from minerals should be addressed more here. The trace metals effect is potentially more stressful than solely changes in pH.