Appraisal of the Work entitled: Negative effects of a zoanthid competitor limit coral calcification more than ocean acidification

This work seeks to test the effects of OA (i.e., decrease in pH, increase in pCO2) on the growth of a hard coral taxon (Porites), as well as on a soft-bodied cnidarians, Zoanthus. Changes in seawater chemistry were explored on the individual taxa and on the competitive interactions of the two. The overall impact of OA appears to have been weak on the coral taxon studied here, while the competitive interaction appeared to have taken a greater toll. Overall, the Zoanthus appeared to be a greater competitor than its coral counterpart. This finding is consistent with that of other studies regarding the competitive interaction between these two taxa, and when exposed to other stressors (e.g., increased water temperature).

Overall, the study appears to have been conducted appropriately, although, in my opinion, some aspects of it remain to be explained with greater transparency. If done successfully, a final review process should allow for the publication of this work.

Three main points deserve to be underscored:

1) The rationale for field monitoring is lacking, and the relative distribution of the field sites between themselves and with the laboratory should be made clear. Also any conclusions relative in situ interactions between these taxa from their collection sites are likely to be as speculative as from any other location where they may cooccur. This is simply the result of the venue under which this experiment was conducted (i.e., the lab).

2) Whether measures of calcification could be biased by changes in the mass of live tissue (in Porites) not in changes in skeletal mass per se. Also, whether changes in the mass of live tissue of Zoanthus could have occurred without changes in the number of its polyps. These alternatives need to be convincingly discussed.

3) The duration of the study is only two months (which should be expressed in days). As pointed out in the discussion, the impacts that were measured may only start to unfold after that period, without having taken their ecologically-relevant proportion (please see L339-342). As such, unless the authors can be convincing to keep their original focus, this work should be framed under the label of “initial impacts of OA and competitive interactions”. The notion that other (stronger impacts could unfold after a longer timeframe appears quite probable and potentially lead to different conclusions than the ones underscored in this work. The initial impacts of any phenomenon deserve attention, thus this is not a critique of the work itself, only of the way it is reported.

Abstract
The abstract is generally clear, despite that in some instances, it lacks some transparency (please see some examples below). As the authors must be aware, the abstract is the most read part of an article (after the title), thus all words should be chosen to have the maximum impact.

L22. What are ‘differential effects’?

L24. Is ‘exposure to’ necessary in the sentence? It seems to me that ‘competition’ is sufficiently clear.

L27-28. To me, ‘Responses to OA and the presence of the competitor varied by taxa and response variable.’ provides little information. The authors may want to consider going sooner into the details of their findings.

L32. Isn’t ‘photophysiology’ a general process? Perhaps explaining the exact outcome in the change of this process (e.g., photosynthetic rate) would be more informative to readers.

L33-34. Again, using ‘Our findings’ provide no useful information to readers but prevents the authors from using this word space to better elaborate on the exact nature of what they found.

L33-35. In my opinion, this sentence should be the first of the results of the abstract because it conveys the entire spectrum of the main and most relevant findings of the study.

Introduction

The introduction is relatively clear and concise. But, in my opinion, the author could provide improvements by clearly stating in the first or second paragraph the shortcoming in knowledge that they intend to fix. There is no point in letting readers guess beyond the second paragraph the purpose of the study. The study’s goal is somewhat stated at the end of the third and fourth paragraphs. It would be useful to be finessed this sooner in the introduction.

L43-45. Should this sentence be rephrased by stating that OA ‘may have’ (or equivalent wording) ‘dire consequences for marine habitat’? If I remember correctly, many of the findings of the reference stated for this statement (i.e., 4) dealt with laboratory work and models of future oceanic conditions, not actual in situ OA.

L52-54. The sentence ‘Conversely, the increase in dissolved CO2 due to OA can stimulate photosynthesis and potentially increase growth rates in non-calcifying, photosynthetic marine organisms [6,9]’ is an important statement to contextualize the present work. In my opinion, in a few words, it would be useful for readers that the mechanism(s) underpinning higher growth rate be disclosed, if they are known. This mechanism may allow for a better understanding of the results of the study, i.e., what accounts for higher growth under OA scenarios? On this point, are changes in growth the result of lowered pH or higher pCO2?

L82-84. This sentence’s structure could be improved upon. It seems a bit imprecise because it seems the authors are implying that are Porites competitors to corals.

L88-91. This statement (i.e., ‘Disparate organismal effects of a global stressor’) is central to the purpose of the study and thus it should be referenced.

Materials and Methods

In my opinion, the rationale for having conducted a field survey is lacking, chiefly given this experiment was conducted under laboratory conditions. This is easy to remedy. Furthermore, the localization of the field survey site, in relation to the laboratory is lacking. This information would be useful for readers to judge whether transportation stress could be an important issue, i.e., because of lengthy transportation. Last, why did the authors choose to measure several environmental parameters? This should be disclosed, with their choice to conduct field monitoring.

L94 (Benthic cover). Given the main finding of this study will be revealed from laboratory manipulations, it would be very useful for readers to know the exact purpose of the field monitoring. Consider: “Given xyz, we conducted a field monitoring …”. Furthermore, it seems to me that contrary to what is stated in this section, the study was not conducted in the field but in the lab. What/where exactly is ‘Bocas del Toro’? This is unclear to me. The authors should change this statement to provide transparency. E.g.: “samples of … were taken from …”. This wording would facilitate understanding of the exact context of the study. Should there be coordinates?

L104. Again, why want to evaluate the abundance of these species in situ? This should be clearly disclosed.

L104-105. Although I praise the authors for having undertaken field monitoring, at this point the purpose of this endeavour is still unclear. It should be clear why the authors monitored that area.

L105. What is the field site? Almirante Bay or Bocas del Toro? This is unclear, and coordinates should be provided for the field location.

L108-114. Should this methodological approach be referenced?

L121. What are these numbers (‘9.34906, -82.2583’). If coordinates, they should be first presented when mentioning the location of the field study.

L123. A reference for the light intensity and water conditions used herein should be presented.

L127-128. How do the authors know a ‘recovery’ occurred? Please provided this clarification in the text.

L133. How could readers be confident that an 8-week duration is sufficient for meaningful effects to unfold? Perhaps a key reference should be provided here.

L133-134. I would change the logic of presenting the treatment, from single effects (1 and 2) to competition (3), i.e., going from simple to more complex: Porites, Zooxanthids, Interaction.

L135-136. Why should this interaction be referred to as something else than ‘competition’?

L133. Was the statement ‘Porites with attached Zoanthus (CxZ)’ clearly explained in the sampling section? Some quantitative parameters should be provided, such as the maximum distance separating these species. Should the relative size of these two species be provided? It seems to me this should have been made crystal clear.

L134. What is the logic of choosing Zoanthus with Porites healthy coral tissue removed, instead of Zoanthus by themselves? And vice-versa for Porites?

L139-141. Should the choice of the pHs be disclosed?

L140-141. The order of the three treatments should be the same as presented earlier; this order (i.e., coral only, zoanthid only, coral and zoanthid) is, in my opinion, preferred.

L146-148. Do the authors mean that in each tank the three treatments were found? I could not understand it as written in the present format. This sentence is confusing, please rewrite it.

L149-151. Again, this sentence is confusing: were all the three treatments present in each tank? The phrasing ‘respective competition treatment’ gives the impression that all the replicates of actual competition treatments were found by themselves in separate tanks, which would not be an ideal choice. Please be clear regarding the grouping of treatments per tank. OK, I saw that the authors provided these measurements thereafter (L169-170). Thus, the authors should warn readers these data will be presented later in the text. Again, OK this data is presented also later (L195-201). Warning readers this data will be provided later would be useful.

L149-155. Should the resulting condition of seawater in each tank, as measured by the authors, be presented here. It is one thing to say what the experimental OA settings were, it is another to demonstrate the actual result of these settings in experimental seawater.

L165. Here, ‘shuffled’ is vague. Please provide more precision to this ‘shuffling’.

L182-183. The choice of pHs is explained. However, this explanation should have been presented when the choice of pHs was first presented.

L204-208. Could the weighing of porites be affected by the amount/mass of living tissue, which would not necessarily be affected by the amount/mass of calcified structure? In other words, could the mass of living tissues bias the measure of calcification?

L209-210. What is the rationale for having measured the Photosynthetic efficiency only at the end of the experiment? This information should be provided.

L220-222. Why weren’t Porites number of polyp counted as well? This is not logical to me.

L226-228. More information is needed here: what is the N for each combination of treatments? Here it would be useful to remind readers what were the dependent variables measured.

L228-229. As mentioned earlier, the notion of ‘multiple competition fragments’ (i.e., the exact treatment in each tank) is not clear to me. This should be rectified.

Results

Improvement in how the results of this experiment are presented is possible; please see details below. Also, reading the results made me realize that other parts of the manuscript (chiefly Material and Methods) were not as transparent as they could have been (detailed below but concerned with the Material and Methods).

L234-239. Benthic cover: should the overall estimated benthic cover (in % of the area [or transect]) by Porites and Zoanthus be presented? I now realize that in the Method section, the explanation of how the sites were distributed in space could be improved. E.g., “Five sites in A. Bay were used for sampling, with each site being of x m2 and separated by at least y m from other sites. These sites were chosen based on xyz parameters”. How these sites were chosen before sampling?

L241. In my opinion, “Environmental parameters” is giving the impression this data was measured in situ. A more explicit sub-title should be thought-of, i.e., representing conditions in the tanks. Perhaps: “Environmental parameters in the experimental tanks”.

L248-252. I now realize that I do not understand the method for measuring calcification, particularly when considering the treatment of competition. Changes in the mass of tissues of Porites and/or Zoanthus are poised to have affected the measured though to capture changes in calcification. In other words, more transparent explanations in the Material and Methods are necessary relative to this point.

L254-256. Could change in growth be affected by mass, and not by the number of polyps? In other words, could the mass of Zoanthus change despite having the same number of polyps? This point should be clarified.

L259-260. “There was a significant negative effect of pH and competition treatments on Porites photosynthetic efficiency, and no interactive effect of the two factors.” What is the meaning of these statistical results, i.e., what is the implication of not having an interaction? Will this be explained thereafter?

L262. Why use “ambient control” and not simply “control treatment”?

L263. Please include ‘the’ between ‘decreased’ and ‘photosynthetic’.

Discussion
The authors should present evidence that the two-month duration (i.e., precisely how many days?) was adequate to infer any meaningful conclusions on the interactions between the taxonomic groups studied here.

L271-277. In this paragraph, the authors could briefly reiterate their rationale for having expected an impact of OA on the species (and their competitive) interaction. This would help readers contextualize the results.

L285-287. Could the authors conclude that their result indicates that Zoanthus may be a superior competitor on the shallow coral reefs of Bocas del Toro? After all, this experiment was conducted under laboratory conditions. Wouldn’t as speculative to predict an effect in Bocas del Toro as in any other places where these species compete for space?

L302-304. Unless greater clarification is presented (please see above regarding how calcification has been measured herein), such a strong statement should be avoided. If changes in living tissue occurred, how could the authors prevent this from biassing the results of calcification? E.g., there could have been a change in the mass of living tissue, but not in calcification…?

L307-309. How this process may unfold should be presented. It is not clear at this point, and it relates to the point presented above. In other words, briefly present the conclusion of REF 36.

L339-342. This sentence points to the fact that the duration of the study may have been too short to derive conclusions of how this interaction may unfold in nature. I recommend that the authors include a paragraph to this effect, and rephrase their entire under the umbrella of short-term OA and competitive interactions.