Review Roca-Marti et al. Sinking particle fluxes and biological carbon pump efficiency in the Labrador Sea during a Phaeocystis bloom decline

The paper describes and discusses the fate of a Phaeocystis bloom in the Labrador Sea in spring 2022, and impact on the biological carbon pump. The authors have a nice data set including bloom, late bloom and outlayer stations, and they have done a thorough job in methodological testing and discussing of premises on steady state for the models used, use of particles > 51 µm for the carbon estimates and stoichiometry. They also utilize a broader dataset to provide context to the observations, like remote sensing based primary production in the period before and during the field sampling to evaluate the efficiency of the biological carbon pump.  

I like the paper and work, I find the approach and methods useful and well described, and it fits well with the scope of BG. The dataset and concept of comparative testing of export within and outside a Phaeocystis bloom region is novel – including impressing many profiles for Th-based carbon export, and the data are elegantly used in combination with net primary production and quantification of both carbon and BSi to address the potential impact of diatom-associated export during this Phaeocystis dominated bloom. The title is appropriate, the abstract reflects the content well, and the references are approriate.

There are some structural issues on placement of figures and a chapter on earlier studies in the region that could be improved. A few places the methodology needs some improved clarification, but in general is this a well written paper, with clear ideas and methods. The assumptions are valid and clearly outlined, and the conclusions are substantial and sound and based on the results. See specific comments for more details.

This is obviously one of several papers from the expedition, and there are several references to publications in prep. These are hard to access, but may be in a more progressed state at the time of publication. A few places some more info could perhaps be provided. The figures are informative and well described in the Figure caption, and Tables provide valuable information on relevant numbers. A table including sampled depths could be useful (in Suppl.). The supplementary material is useful and works well as Suppl.

More specific comments:

Figures (2,) 3, 4 and 5 are placed in the Material and methods section, but are just briefly referred to here, while they are described and discussed more in detail in the results chapter. Suggest to move them there to ease the reading of the results sections (e.g Fig. 3 in 3.1, Fig. 4 in 3.2).

L116: Water was collected from six depths – which ones? A table in supplementary will do.

L119-120: type 1 water – please explain

Eq.1 and L142-143: perhaps my limited familiarity with these calculations, but unclear how at%xs refers to excess 13C atoms in either PC or DIC, when both PC and DIC are part of the equation

L151-163: the use of Chl a conc to calculate NPP is a bit confusing. Was this only for the central stations where NPP measurements were missing? Please clarify.

L156: Fluorometer profiles from the CTD – should this be fluorescence profiles?

L157-158: Since the Bertrand et al. paper is still in prep, it is hard to get the background for the non-corrected photochemical quenching related to a sub-chl max

L233: Similarly, is parts of method used for sampling marine snow catching deployments and processing referring to a Cisternas-Novoa et al. in prep. Please provide the most important information in this paper.

L375: Section 3.5 on the Comparison with other studies reads more like discussion than results to me. Suggest to move this to the Discussion section.

L418: 1 week – should be one week (spell out numbers <10)

L419: the Bertrand et al. in prep – can evt be a pers comm.?

L533-536: It is a valid and important point, but not new knowledge that NPP and export is separated in time by weeks – requiring a larger time scale to evaluate export efficiency. This is well done in the present paper, but I suggest acknowledging one or two of the earlier studies on this like Smetacek et al. 1984, and Kiørboe et al. 1996.

Smetacek V, Bodungen B, Knoppers B, Peinert R, Pollehne F, Stegmann P, Zeitzschel B. 1984. Seasonal stages characterizing the annual cycle of an inshore pelagic system. Rapp PV Reun Cons Int Explor Mer 183:126-135.

Kiørboe T, Hansen JLS, Alldredge AL, Jackson GA, Passow U, Dam HG, Drapeau DT, Waite A, Garcia CM. 1996. Sedimentation of phytoplankton during a diatom bloom: Rates and mechanisms. Journal of Marine Research 54:1123-1148.

This work provides high resolution measurements of sinking export fluxes (particulate organic carbon, POC; and biogenic silica, bSi) following a Phaeocystis bloom in the central Labrador Sea during a 2-week, using total and particulate ²³⁴Th measurements. This long process study observed the decline of a specifically large bloom in spring 2022.

This is a robust, comprehensive, high-resolution dataset presenting export and transfer efficiencies data obtained from a bloom of Phaeocystis in a relatively under sampled area. ²³⁴Th measurements are extremely detailed and results obtained are rigorous and the figures in the main text and in the supplementary material are very informative. The manuscript reads nicely, it is very well structured, with clear objectives and conclusions.

The results show a contrasting behaviour in relation to diatom blooms. Phaeocystis blooms are rarely examined in terms of the carbon export and sequestration during and the results prove the key influence of phytoplankton community structure on carbon export efficiency. The manuscript is worthy of publication following minor changes included in the attached pdf.