|The new version of the paper of Catherine Davis and coauthors on “Vertical distribution of planktic foraminifera through an Oxygen Minimum Zone: how assemblages and test morphology reflect oxygen concentrations” reads better than the 1st version, and I agree to most of the points made in the reply to the reviewer’s comments and the revised manuscript. However, there is one issue that may need rethinking, and some minor points they want to be addressed before final publication of the paper.|
I understand that new data on the molecular genetics of G. hexagonus are not easy to get on short notice. I also understand that Davis and coauthors may have the feeling that G. hexagonus is a normal component of Atlantic assemblages of modern planktic foraminifers. However, I know the specimens presented by Smart et al. (2018) by heart; the specimens really look like G. hexagonus, but they are not numerous. Therefore, we could not get data on the genotypes. Also, the G. hexagonus found in sediment assemblages are not numerous, and may have been transported into the Atlantic Ocean by currents (if these are G. hexagonus at all; I did not have a look at them). Therefore, I would suggest to be careful with stating that G. hexagonus is present in all modern oceans; this would need proof of an entire life cycle of G. hexagonus, including reproduction, in Atlantic water masses. Having said all this, Section “4.2 Globorotaloides hexagonus as an OMZ Indicator Species” reads better than before. The authors may still add that "final proof of the presence of G. hexagonus in the Atlantic may be provided by molecular genetics", which I would regard as a good way to solve the issue.
Line 44: Breitburg et al., 2108 should possibly read 2018
Line 196: “extremely oxygen depleted OMZ” is not a term defined below in this paper. I would suggest to drop “extremely”. Along the same lines, the paper would benefit from dropping a number of adjectives in places, which would make reading more scientific.
Line 211: change “transitional (nets sampling between these two extremes)“ to “between these two concentrations”.
Line 213: I would suggest to change “the densest population“ to „highest standing stock”, which is possibly more correct in a scientific way. In general, densities may be changed for standing stock throughout the paper.
Lines 244-252: I would suggest to add the numbers of empty tests to the Table 1.
Line 273: better change “interaction” to “relationship”; interaction insinuates determination, which is possibly not the case.
Line 287: “Globorotaloides hexagonus tests were light for their size“, relates to something that is not mentioned. “Light” compared to what?
Line 299: Please briefly describe what is meant by “compactness” and “aspect ratio”.
Line 337-339: „It is more likely that cytoplasm-bearing tests of T. sacculifer found below the
photic zone are a consequence of their very high abundance in the surface ocean and
reflected premature mortality and/or the retention of cytoplasm following reproduction.“ Did these specimens still have their spines? If yes, they may not have reproduced.
Line 358: Predation may be another reason to be taken into account.
Line 427 of the 1st version of manuscript: Sorry, this should have read Buchwald. Lines 441-442 in the new version of manuscript does still contain the same transposed digits: “Buchwald et al., 2105”, which may be changed to 2015.
Lines 456-464 presents a repetition of statements presented above, and may be rewritten or removed.
Final paragraph, lines 465-480: Another explanation for larger test sizes may be that G. hexagonus continuously grew larger under less optimal environmental conditions, i.e., lack of oxygen, and only reproduced with a delay when the environmental conditions had improved to support survival of the offspring (see Mojtahid et al., 2015 for G. ruber) and Schiebel and Hemleben (2017) for a more general explanation of the phenomenon.
Figure 1: O2 concentration larger than 7.5 ml L-2 are not presented in the map, and may be cut from the scale. By doing so, the relevant part of the scale may be more detailed.
Figure 9: The few red and yellow markers are not easy to see in the paper copy of the figure, and I would suggest to use darker color. Also, the green and blue markers would benefit from darker color.