The authors presented a comprehensive mesocosm study in a mid-latitude fjord that employed two types of ocean alkalinity enhancement (OAE) techniques to study the changes in carbonate chemistry on metabolic rates in the experimental mesocosms. Different levels of OAE were tested and delays in phytoplankton bloom compared to control conditions were revealed. This study did not use the common OAE minerals (hydrated lime and olivine) directly but employed chemical additions that mimicked the outcome of applying these minerals hence other confounding factors such as trace metal release can be avoided, which is a clever design.

The manuscript is mostly well written but it can be verbose in places. First, here is a technical question that I hope the authors can address. In this study nutrient sample collection and processing, the authors used 0.45 µm filters for nutrient sample collection and then the samples were kept in the dark at ambient temperature until further processing (did you mean analysis)? See Line 165-166. Given the fact that nutrient stoichiometry is important in discussing metabolism in the Ca vs. Si based OAE schemes, this nutrient collection technique needs further clarification, and the authors should affirm that the pore size and sample preservation had not inadvertently altered nutrient concentrations. See below for a reference.

Reed, M.H., Strope, E.K., Cremona, F., Myers, J.A., Newell, S.E. and McCarthy, M.J., 2023. Effects of filtration timing and pore size on measured nutrient concentrations in environmental water samples. Limnology and Oceanography: Methods, 21,  1-12.

Below are some minor comments:

1. Be consistent with the descriptions of the duration of the experiment. 10-weeks (Line 92), 53 days (Line 127), and three-month (Line 369) were all used.
2. There are many places where the words “said”, “mentioned”, “aforementioned”, “particular”, “present” etc were used and in most cases these words are either unnecessary or confusing. Please remove or reword.
3. Throughout the context, while it is understandable that a calcium-based chemical alternation was made to the experimental system, using “calcium” appears a little misleading because both OAE approaches intend to increase concentrations of carbonate species in the water. Silicate weathering leads to an increase in carbonate ion concentration, and hydrated lime is essentially a direct base addition, not adding calcium per se. I would suggest that the authors to reconsider the term usage.
4. Line 44-45, improper punctuation.
5. Line 90, for an uncommon chemical/mineral, explain forsterite.
6. Line 115, what’s in this brine solution.
7. Line 162, provide more details on how pH was corrected and how the comparison looked like.
8. Line 178 vs. Line 190, clarify whether the “initials” were already fixed before the incubation.
9. Line 180, what’s “blackout”? Please use proper term/description.
10. Section 2.5, more details on Chl-a processing and analysis is needed. What’s the purpose of using the 200 µm mesh?
11. Line 239-244, the sentences read awkward and confusing. Please restructure and clarify, explain what’s the “controls” mean in the context of the experimental design.
12. Line 251-252, remove “significantly”, and did the experimental timing coincide with post bloom period in this fjord? If so, this needs to be mentioned in the method section.
13. Line 263, subtracted “from”?
14. Line 284, “slightly almost”, what does it mean?
15. Line 286-287, “little under” as “slightly below”?
16. Line 304-307, this sentence needs to be reworded as the current form is quite confusing.
17. Line 326-327, the sentence “Nonetheless …” is not clear.
18. Line 331, define “metabolic balance”
19. Line 343, “latter parameters” meaning?
20. Line 347-353, this paragraph appears fragmented and difficult to follow. Please revise.
21. Line 366-367, “persistent increase in pH and decrease in pCO2” needs proper context, it reads like these trends should correspond to the level of OAE, but not the duration of each experiment.
22. Line 372-374, “addition” with quotation marks, I’d make it more explicit that the experimental technique used surrogate of chemical mixers instead of direct mineral additions.
23. Line 409, this 1% fraction of DIC as CO2 (which should be aqueous CO2) is salinity and temperature-dependent, so some context is needed.
24. Line 415-421, this discussion needs to be placed in the context of the study region to make it the case.
25. Line 435, the ratio of observed Si and N uptake hinges upon the nutrient handling methods. Hence the ratio needs to be taken with a grain of salt.
26. Line 451, “in terms of”, meaning?
27. Line 455, clarify what the statement means.
28. Line 465, remove “herein”.
29. Line 467, the fact that Daphnia is a zooplankton needs to be mentioned here.
30. Line 484, “stronger” should be replaced with something like higher levels of chemical modification of seawater.

The supplemental materials could use more help with higher resolution figures.

This manuscript investigated the microbial responses to OAE approaches based on mesocosm experiments. The data presented are valuable in promoting the understanding of OAE impacts. My major suggestion for the authors is that, the discussion part could be expanded and polished a bit more, so that readers can get some take-home messages more easily. Right now, all figures are in the Results section, and they are all time-evolution of measured values. It is hard to extract key points from these figures. More in-depth analysis of these data would be helpful.

Another concern is that the manuscript seems quite colloquial. For example, Line 378-381, “The experiment was started under post-bloom conditions (Figure 2). Nutrient concentrations were low when the treatments were applied. Thus, an initial response in the microbial community production and respiration rates to the TA manipulation was likely concealed by the nutrient limitation. Actually, after a mixing event was simulated on day 26 and 28, a response could be discerned.”

Also, Line 386, “In the past, prior to the emergence of ocean acidification as a focal point in scientific inquiry…”, Line 459-460, “This difference between the controls was unexpected. This was probably a random response caused by the mesocosm effect…” etc. These do not sound like scientific languages, and I would recommend that the authors revise their expressions throughout the manuscript.

Some minor comments:

Figure 1, I don’t see the Fjord data but it is in the legend. I think the authors was hoping to add the legend to Figure 2, as Figure 2 has the data, but the legend does not have ‘Fjord’.

Line 358-360, “The latter are both being…” needs to be corrected.

This study explores non-equilibrated Ocean Alkalinity Enhancement (OAE) using silicate and calcium-based Total Alkalinity (TA) gradients (0 to 600 µmol · L-1) under natural conditions. The manipulation increased pH and decreased pCO2, impacting bloom formation after macro-nutrients were added. Overall, this study contributes to the current understanding of OAE field application. Here are my comments to help the authors refine this manuscript.

Major comments:

1. The authors proposed that the addition of TA higher than 150umol/L had delayed the bloom of phytoplankton, but the results may not be strong evidence for this argument. The GP-∆TA, Chl-a-∆TA, and CR-∆TA etc were analysed as shown in Figs.4,5,6 with a CI bar representing the Confidence Interval (CI). If I understand correctly, if CI bars overlap with the horizontal line (y=0), the GP or Chla and other parameters don’t have a significant linear relationship with the ∆TA on that day. Considering the CI bars in many of the subplots in Figs. 4,5,6 are large (especially during the bloom time), so this argument about delayed bloom may be overinterpreted. In addition, in Fig.4 A and C, the peak of the curve seemed to occur earlier in the Si 150 treatment than in the Si 0 treatment, which conflicts with the argument. I would suggest the authors reconsider this argument throughout the manuscript.
2. The design of macronutrient fertilization is a good way to understand the field application in other seasons, but there is limited information about why authors choose these certain macronutrient levels. Is it close to the real nutrient levels in different seasons? Please explain more details about the design in lines 251-260.
3. In discussion 4.1, I appreciate the authors trying to explain the reasons why a potential delayed bloom would occur. However, there is no sufficient information about local phytoplankton community composition in the experimental sites. Were there diatoms? Were there calcifying phytoplankton? In Fig.3, the peak of Chla was similar or even higher than the Fjord, is it possible some species benefited from the addition of TA? Considering the pH tolerance and CO2 utilization are species-specific, the information about local phytoplankton will be useful.

Minor comments:

1. Line 76 “iron, which is a co-limiting micronutrient”: please add a reference.
2. Line 130: five mesocosms?
3. Line 162: please add a reference about how you correct the pH.
4. Line 200: Where is the T in the equation?
5. Line 230: The unit of TA is umol/L in previous paragraphs, and the delta TA unit is uEq/L in table 1. Please double-check and explain uEq/L.
6. Line 236: “The difference between … was quite steep”, please rephrase this sentence.
7. Line 265: “In the silicate ones, …” should be “calcium ones”?
8. Line 268-270: It looks like the drawdown of N and P in Ca treatments was just as much as in Si treatments. Therefore, they were both N and P limited.
9. Line 274: What’s the grey line? Please explain the greyline, the dash lines, the phases I and II here.
10. Line 297: Please explain more about the top subplots in Fig. 4 and Fig. 5. Does the positive value mean the positive relationship between the parameter and delta TA?
11. Line 308-315: Please consider moving this paragraph to the method.
12. Line 358: Please explain more about what hydrated lime and forsterite are.
13. Line 374 -376: Please add references.
14. Line 399 “There was high variability in their tolerance to high pH”: what is the “high pH” range?
15. Line 415-419: Please add references.
16. Line 436: “In the latter, the community”, please state the specific phase or day.
17. Line 441-443: The relationship between diatom silicifying and Chl-a concentration is not clearly explained here. Please rephrase the sentence.