This study by González-Santana et al. attempts to assess the impact that the addition of sodium carbonate and sodium bicarbonate minerals to seawater mesocosms has on iron dynamics. The study presents new measurements of the different iron species over the course of the addition of alkalinity into the mesocosms. Unfortunately, the mesocosms do not look to have been setup to study this as a specific aim, which leads to several shortcomings in the research. As the authors point out the mesocosms were not carried out following trace-metal requirements, which will have substantially impacted their data. In general, the data itself looks very noisy, with no trends that would be statistically significant within the given dataset. Given the limitations in the field data, the authors could have conducted laboratory experiments or even simple biochemical modelling to compare with the data to see whether we would even expect to see any changes in the data given the limited impact that these alkalinity additions have on seawater chemistry.

The other major issue that I have with the study is that one of the conclusions is that the iron size fractions and phytoplankton are independent. This is clearly the case as this is not an iron-limited environment. If this study had been conducted in an iron-limited environment, then the results might have been very different.

While I am not opposed to publishing a null result (which I believe these to be), the authors could have compared the results to laboratory data or a biochemical model which would greatly strengthen this work.

11: OAE should be Ocean Alkalinity Enhancement. (Not alkalinization; alkalinization enhancement does not make sense).

15: replace levels with additions.

17: add a capital delta symbol before 2400.

20: delete “could” and “because of”.

21: add “due to” before OAE.

32: OAE should be Ocean Alkalinity Enhancement. (Not alkalinization; alkalinization enhancement does not make sense). Replace “both produce” with “lead to”.

37: replace “might have” with “has” and “consequences in” with “consequences for”.

38: replace “heightening” with “increasing”.

39: The hydrogen ion concentration is the pH (which you’ve already said increases!).

40: replace “hoisted” with “increased”.

45: Where are the proposed mineral dissolution products coming from? Are you suggesting the sediments, or biogenic materials in the water column, or something else entirely? Add “e.g.” before the list of potential minerals as more than these three are used.

46: “necessary” not “necessitated”.

47: “technique under” not “technique at”.

49: Replace “factor” with “variable of interest”.

64: delete “time”.

65: delete “the” before photooxidation and decreasing.

66: delete “in the”.

67: rephrase to “All of these factors can induce…”

71: “colloidal size fraction…”

75: replace “a significant higher” with “significantly higher”.

78: replace “by OAE” with “for OAE”.

82: The mass of Ca or Mg required to impact the amount of Mg and Ca in seawater makes this a negligible impact (~2.5 mM DIC, ~10 mM Ca, ~50 mM Mg). No one is proposing the scale of OAE that would impact Ca and Mg concentrations enough to impact iron.

87: Again, think about the mass balance and consider how much alkalinity enhancement you would have to do before impacting the concentrations of Ca and Mg! Even in these experiments with a doubling of the alkalinity (which is extreme!) this would only impact Ca by ~25% and Mg by ~5%.

89: “preceptive” is not what you mean here?

90: Stick to what you have done in this paragraph and simplify.

92: delete “previously”.

93: replace “used alkalinization methodology” with “added material” and “which” with “what”.

111: add capital delta symbol before 2400.

118: replace “Subsequently, 0.2 um filtered samples followed on” with “Aliquots were filtered (0.2 uM SartobranTM PES) for sFe, dFe and LFe.”.

120: delete the bracket after sampling and add “for” after “dark”.

121: replace “to” with “at”.

141: Define “TAC” here.

142: Do you mean +0 Fe additions?

150: Use this definition of TAC above, not here.

163: Add a space before units (do this throughout the paper, it is currently inconsistent).

194: Superscript “-1”.

196: I don’t think this is significant given the noise in the rest of the data.

211: % not ‰.

213: This sentence is unfinished.

216: replace “due to its” with “as it is”.

217: replace “uptake” with “to uptake”.

218: I don’t think the data conclusively shows that the cFe concentration was directly impacted by the addition of the sodium carbonates given the noise in the dataset.

236: This is unsurprising given that these experiments were conducted in an environment where iron is not limiting!

255: replace “previous to” with “before”.

258: replace “concerning D1500” with “in the D1500 mesocosm”.

263: replace “computed” with “calculated to be”.

269: The values in this paragraph don’t seem that similar. Be clearer and specifically compare values.

273: superscript “-1”.

277: “being the weaker ligands the most abundant” does not make sense.

282: remove “a” before “biological”.

288: replace “from” with “of”.

300: rephrase to “biological parameters. This suggest the alkalinity addition in the mesocosm experiments did not…”.

310: The sentence starting with “Where the…” needs to be rephrased.

312: “concentration” not “concentrations”.

313: I still don’t think this has been shown.

315: “change” instead of “present changes”.

316: “were added” not “addition” and “may” instead of “will shortly”.

318: This paragraph is discussion not conclusion as it is not related to what you did in these experiments.

327: delete “obtained”.

328: delete “i.e., did not alter the Fe cycle,” as it is just a repetition.

329: replace “was” with “were”.

330: delete “extensions” and reword the following two sentences.

Figures 2 and 3: Both figures need error bars.

The manuscript entitled “Ocean alkalinity enhancement using sodium carbonate salts does not impact Fe dynamics in a mesocosm experiment” by Gonzalez-Santana et al., is an interesting manuscript in which a mesocosm experiment to study the effect of Ocean Alkalinity Enhancement over the iron fractionation and other physicochemical and biological variables is evaluated. Although the authors indicate that 1) some contamination problems could have happened because the experiment was not conducted under stringten trace trace metal conditions according to the GEOTRACES protocol and 2) the main conclusion is that “The iron size fractionation, concentration and iron-binding ligands data obtained supports the fact that the addition of sodium salts in this mesocosm experiment did not lead to significant changes in the iron cycle, i.e., did not alter the Fe cycle, therefore phytoplankton was not affected by changes in this essential element”, in my humble opinión the present work deserves to be published after some minor changes. The purpose of the study (iron cycle under environmental chnaging conditions) is of great interest for the scientific community.

Minor comments:

Page 1, line 15. “consisting on the controlled variation of total…”

Page 1, line 19. The differences between TdFe and dFe should be explained.

Page 1, lines 22 and lines 28. In my humble opinión, these messages are contradictory. “There were variations in Fe size fractionation…” and …”this mesocosm experiment did not modify iron dynamics…”

Page 6, line 120. The symbol “)” after sampling should be deleted.

Page 9, line 195. The symbol “.” After mesocosm should be deleted.

Page 9, line 213. SOme information is missing after “Their variability….”

Page 10, line 130. Authors talk about sediment resuspension. Is the mesocosm open in the bottom to consider this possibility? Please explain.

Page 10, line 239. This argument would be enriched by including the following study:

• Cabanes, D.J.E., Norman, L., Santos-Echeandía, J., ... Laglera, L.M., Hassler, C.S., 2017. First evaluation of the role of salp fecal pellets on iron biogeochemistry. Frontiers in Marine Science, 2017, 3(JAN), 289.

Page 12, line284. This statement is only true for treatment ∆1500.

Page 12, line 301. Please change “….and  biological” by “…or biological”

Page 13, line 306. This statement is only true for treatment ∆1500.

Page 13, lines 316 and 317. Could this behaviour be associated to the buffering capacity of seawater?

Page 13, line 319. What does CDR mean? Please explain.

Page 13, line 321. Please insert a space before “Also….”.

Page 13, lines 320-321. comprison between the study carried put by Santana-Casiano et al., 2010 and the present study in which different salts that make the water more alkaline are added would be of great interest.