Publisher’s note: the content of this comment was adjusted on 8 January 2026 after approval of the OS co-editors-in-chief since some formulations were inappropriate.

REVIEW GUINALDO & NEUKERMANS 2025

The authors present a brief descriptive study of the 2023 marine heat waves in the Northeast Atlantic, with two case studies in the Celtic and Barents seas. I found the study to be a decent documentation of this event and I acknowledge the quality of the analysis.

However, the form of the study. not specific enough or NOT supported by results ? It’s a paper which does not produce any data, new methods or concepts and does not study any process. This is purely descriptive, which would be fine in principle, if the authors would stick to interpreting their results. (acidification = no analysis, Atlantification = not even defined, poleward expansion = not quantified at all although the authors conclude about a spatial shift.... , ecological niche/hotspots = not defined + not quantified, emergence = not defined/quantified, top-down controls = not analyzed, bottom-up controls = not analyzed, for example, there are interpretations of the role of light without any analysis of light/PAR; or the role of atmospheric conditions with no analysis of atmospheric conditions, sea-ice and ocean currents are mentioned many times also without any result shown).

. What is least acceptable is that, in most cases, the claims are not backed up by results. Furthermore, on top of those concepts that are discussed without any analysis, many aspects of the methods are incomplete (see point-by-point comments below: undefined/unprecise concepts: climate variability, Atlantification, optimal bloom development zones ?, trailing/leading regions ? ; incomplete references; missing methods: extent of the bloom ?, leading edge ?, thermal range of coccos?). Many assertions are (most of the time true) but baldly claimed, not even backed up by any references. Due to this bad referencing, the authors presume the reader is an expert (otherwise it’s a very complicated read through although it is written quite succinctly) and oblige the reader to look back-and-forth for the needed information throughout the manuscript (many times introduced way after being used: e.g. upper thermal range, never explained).

I also raise a few formal points, i.e. some wording so as not to overstate -sell- the overall importance of E. Huxleyi in the carbon cycle and also in the trophic chain. There is no evidence that they contribute significantly to the BCP beyond the “ballast effect”,  as they contribute to a small (~10%) share of the total biomass and the total BCP, release CO2 during calcification, and they are generally avoided by grazers.

I recommend the authors to take the time to fix this study and to re-submit later on. A great deal of work is needed before authors can provide analysis to back up all their assertions. An easier alternative would of course be to clarify, or indicate the source of their unsupported results, and/or (re)move them into the discussions.

DETAILED COMMENTS:

METHODS:

Extent of the bloom: I find it not straightforward to understand how the mean/max of the extent of the bloom is derived. Do I understand properly that the bloom start/end is calculated following Hopkins et al. 2015 (with you own custom criterions) pixel by pixel with daily images ? Right ? So the mean/max you are displaying are temporal ? Of daily maps or aggregated monthly maps ? I doubt daily maps have enough coverage to derive a bloom extent, at least in the BS. Also, In which time windows (I guess not the whole year, only the bloom period I guess) ? Maybe try to be more specific in the A1.3.

INTRODUCTION

Lines 25-26 – I thought thin strain has been renamed “Gephyrocapsa huxleyi”. Please double-check and eventually fix it. The authors acknowledged the relative small contribution of coccos to the global NPP Line 23 (although I would appreciate some references here). Is the 1-10% contribution is for actually E. huxleyi or for coccolithophores in general? Verify. Same for PIC, orders of magnitudes compared to other carbonated-shell species would help rather than using non-quantitative adjectives such as “important” or “significant” (there are repetitions by the way).

Line 36 & 37 & 60 : cocco or Ehux ? Throughout the manuscript the authors use one or the other inter-changeably. Please stick to one wording consistently.

Line 69: space (and coma?) missing:  “(Guinaldo et al., 2025)on top”

Line 72-81: I am a bit puzzled by this paragraph and the use of the term “climate variability” although I see what the authors mean as they refer to Sando et al. 2010. However, the authors of this study in 2010, made sure to re-define this term as they used a pretty narrow definition where climate is limited to ocean heat transport. In ocean modelling, we use climate for atmospheric conditions/forcings for instance. It seems that the authors here are making the confusion (or just are not being specific enough in the wording, maybe replace “climate variability” by precisely what you actually mean OR define it) between atmospheric conditions, climate variability and ocean heat transport. It seems that the authors are trying to explain that warming in winter is a remote effect (through advection – ocean heat flux) whereas in summer, it’s a local effect (local atmospheric warming). Could you re-phrase this paragraph?

Line 84: thermal range of coccos, which is ? there is a tendency to build sentences like this one with “while” in middle connection two informations which are not related, quite confusing to read and energy-demanding to reconstruct. Maybe you mean something like :

“ In the CS, oceanic conditions remained favorable for coccolithophores until mid-June … while … the second half of summer was marked by less favorable conditions. “

Furthermore, I do not understand how temperatures exceeding upper thermal range can be a favourable condition for growth. If I try to the brain gym, I go in the first paragraph (or Fig. 2b) and I infer SST is about 17.5degC in the CS. What is the upper thermal range definition ? It is not define unless I missed the obvious. Help the reader, repeat numbers, define concepts, and repeat references to figures (Fig. 2a,b), etc.

Lines 99: “The primary limiting factor for blooms in the BS is the photosynthetically active radiation (PAR), which drives bloom onset and decline.”

That is a bold statement without any reference. Maybe true in general, but we know that Ehux occupy a niche that is not only driven by light. It is for example shown that viral lysis can terminate such blooms. Plus, this is interpretation because your findings do not back up this result.

Additionally, you start the paragraph with “The PIC dynamics reflected these environmental conditions” and finishes with “This bloom dynamic correlated with the environmental

Conditions” . vague statement, never quantitative or specific. No description of environmental description is given in this paragraph so the reader has to remind perfectly the previous section. Try to be specific, the earlier the better. What bloom dynamic (peak? Bloom duration? )

Line 102: “potentially reflecting the multi-annual North Atlantic variability…”

Another interpretation that should go in the discussion and that is not supported by results here. Speculation.

Line 109: “linked to increased atlantification” where /how do you document Atlantification ? By the position of the polar front ? Say it here. This article is for experts only. You say it later lines 115-116.

Line 116: “Interannual variability in the position of the polar front is accompanied by shifts in PIC maxima,” another vague and non-demonstrated statement. Is there a cause-consequence demonstration that the position of the polar front influence PIC max ? I don’t see it at least.

Line 122: First reference of methods here as section A1. Which should actually be A1.3. If you want to use methods as supplementary, you need to be irreproachable. You are not. References to other methods sections are not proper. All methods should be cited like here in order: study cite when you first introduce BS and CS, Satellite data. No MHV definition. Btw, MLD is no satellite data.

Line 145: double bracket ((. The studied area is the Arctic ? I thought it was the North Atlantic? Barents Sea could be both as a frontal area but you need, again, to be consistent. Sea-ice melt induced stratification does not “facilitate the accumulation of nutrients”. It just stratifies. And then “These processes likely contributed to the unprecedented bloom intensities observed in recent years.” … A purely speculative paragraph in the result sections…

Line 152: “Here, the bloom period remains limited by PAR availability” Where is the demonstration ?

Line 157: So now “The establishment of these temperatures was locally modulated by climate variability” so  what is meant here ? Climate variability is ocean heat transport or atmospheric warming. If the former, it’s not local, it’s remote. If you mean atmospheric warming, then you have revise entirely the introduction and better frame/define.

Line 160: Coccos or E. Huxleyi ?

Line 161: un-defined term: “optimal bloom development zones”

Line 162: “in trailing/leading regions” not defined or referenced to methods. Definition in methods is incomplete.

Line 164: Okay, but you do not investigate any ocean currents… am I right ? Why ?

Line 166: Impact the surface area ? What does that mean ? it changes the surface area of the Barents Sea ?

Line 175: and the fact that coccolith sheds light when they shed, i.e. when the bloom is dying…

Line 176: or modelling them?

Line 178: Now another concept: ecological niche. First time. Not defined, not characterized.

Line 180-182: analysis on atmospheric winds ? Stroms ? Air temperature ?  PAR?

Line 190 : This lesson is hard to take by a study which does not produce any data or study any process.

Line 195: “This study reaffirms the poleward expansion of temperate phytoplankton communities” where is documented this poleward expansion ? You did not bring any analysis that support that or add to the previous literature. Same for “highlights the emergence of new ecological hotspots in high-latitude regions”. Have you conducted a time-of-emergence analysis ? How did you show an emergence ? How do you support it ?

Line 198: “These shifts, while globally evident, impact regional biogeochemical cycles and food web dynamics.” I do not understand the sense of this sentence, what is evident at global scale ? And How does this oppose regional bgc cycles and food web dynamics.

Line 199: “Predatory species” … un-related. Coccos are not an important food source, neither a big share of the phyto biomass.

Line 204: No reference for arctic acidification ?

Line 207 : It is not counter-intuitive, sea-ice is melting away (even in winter) and there is more ocean-atm interactions/forcing. Plus there is a compensation effect with the outflowing freshwaters in the Fram strait. So this is both a buoyancy and mechanical effect.

Line 209: wrong placement of citation. Sallée et al. 2021 is about MLD and stratification ONLY.

Line 209: speculations. How is that calcicfying species is a critical carbon sequester ? I though calcification produces CO2 ? The carbonate pump of course is responsible for a small share of the BCP. But why this is important them ? Through What process ? What is sequestration ? Do you define it ? it’s not trivial at all.

Line 212: “the Barents Sea’s historical increases in bloom intensity may reflect enhanced nutrient inputs, favorable light conditions, and prolonged ice-free seasons driven by Arctic warming.” Coccos do not need much nutrients, check literature. Are coccos in the Arctic Waters ? I though they were staying south of the polar front. What connection with sea-ice then ? For the BS, how can you discard grazing pressure ? Viral lysis ? other losses ? Which driver is more important ? How do you choose what is important ? Also for CS, I mean, is there less light in the CS ? Less nutrients? Do you provide support for any claim ?

Line 215: “Tipping point”

Line 216: First appearance of modelling…

Line 242: trailing edge / leading edge of what ? the blooms or the North Atlantic… ?

Line 245: provide ETOPO version.

Line 278: So the upper thermal range is here, and is 16degC ? How those criterions have been decided ? Is it arbitrary ?

The manuscript «Exceptional 2023 marine heat wave reshapes North Atlantic coccolithophore blooms» assesses the impact of the 2023 big North Atlantic marine heatwave on blooms of a key coccolithophore species in terms of bloom intensity, extent, and phenology in both the Celtic Sea and the Barents Sea. The study finds a decline in bloom intensity and extent in the Celtic Sea, while the Barents Sea experiences record-breaking bloom expansion, which, according to the authors, is likely due to ongoing Atlantification and sea ice retreat. The authors discuss the implications for the carbon cycle and marine food webs, emphasizing the importance of continued monitoring in the context of current climate change.

I found the study to be scientifically relevant as it addresses the response of fundamental marine organisms to extreme climate events. Especially, as coccolithophore plays a role on carbon sequestration and on climate regulation through the production of DMS. However, I found this article to be chiefly descriptive, and to not contribute any new data or new methods. I think it is reviewing and commenting concepts that are not all provided by their results. It does not directly examined the mechanisms underlying the observed bloom changes, but rather relies on other studies. The study frequently references broad concepts such as Atlantification, ecological niches, or acidification yet lacking of specific information (nor even concise definitions) or supporting analyses. These terms are (look like) used as buzzwords without being their effects quantified, and with insufficient evidence nor support by the article results. I also believe the paper is not written for non-specialists, as the referencing is quite incomplete and some key concepts are not explained at all.

Though I think the use of satellite data is justified and the variables selected are appropriately used, some methodological definitions such as how bloom extent and leading/trailing edges are defined and calculated, or what is the thermal range for E. huxleyi blooms, are missing.

Finally, I think that authors overstates the role attributed to the coccolithophore species in the carbon cycle. Authors suggest a major contribution to the biological carbon pump, but not providing sufficient evidence for that.

All in all, I think that the structure of this paper need to be revised in order to provide justification for the observed changes in the coccolithophore blooms, and including analyses of their potential drivers as, by now, it is unclear which novelties are specifically provided by their own results.

Specific comments (I just provided few comments as I think the manuscript needs a thorough revision)

Lines 16 and 21: Here, which conditions are specifically referred to.

Lines 35 to 40: It is not clear if author claim that coccolithophore are affected by or can resist acidification and warming.

Line 43 and 52 (where it is redundantly written): Are authors claiming that long-term warming and internal variability the drivers of MHWs?

Line 47: What “hazards arising from different sources..”?

Line 55: In June, but for how long?

Lines 99 and 103: If these are the main drivers of the coccolithophore blooms duration and extension, then what are (quantification) the role of MHWs?

Lines 118 to 120: I think this is a vague sentence without referenced.

Line 178: Then, why 2023 is different to other years?

Line 195 to 215: This paragraph seems to discuss other studies but not including properly the results of the present manuscript.