General Comments:

The authors have used satellite observations to identify two unusual suspected coccolithophore blooms off the Shetland Islands and in the Clyde Sea. They have presented reanalysis model data and satellite observations to try and attribute the drivers of each bloom. I feel that overall the manuscript lacks detail and I encourage the authors to take more care with the analysis of data, preparation of the figures and formation of the conclusions. I also require further justification the satellite ocean color and chlorophyll data is appropriate to use for this coastal, shallow water study (i.e. does turbidity/ DOM impact the measurements), especially as it outlines in the methods that it ‘masks coccoliths’. I would recommend the manuscript undergoes major revisions.

Specific Comments:

I encourage the authors to provide more evidence to allow the reader to ‘see’ how the 2021 summer is different to a ‘normal’ year, particularly in terms of ocean color, SST and currents. I feel more detail is needed when the methodologies are described, including more mention of uncertainties, errors, assumptions and model performance. I appreciate that the authors lack in situ data but I feel they could delve deeper into the data and reanalysis output that they do have. I also would encourage the authors to make their figures more digestible for the reader e.g. including latitude and longitude, marking key events such as the storms and presenting observational data where possible. The reanalysis model also appears to consistently underpredict the chlorophyll concentration and barely shows a seasonal cycle in most years including 2021. I encourage the authors to discuss this and to make it clear what value the reanalysis brings to the paper. I have provided further detailed comments below.  

Technical corrections:

Line 15-35: Personally, I would start with the background on coccolithophores and their importance in the introduction. The first paragraph could be merged with the last paragraph of the introduction.

Line 52: ‘and so includes coccolithophores’ amend to ‘which could represent coccolithophores’ as it is a general PFT that doesn’t have specific parameterisations for coccolithophores but is parameterised to produce calcite.

Line 54: with the sites being close to the coast and in relatively shallow waters does this disrupt the chlorophyll estimates (e.g. from turbidity/DOM)?

Line 55: extra bracket

Line 58: add more detail about the OLCI instrument as it’s not clear to the reader

Line 59: If the estimation process masks for coccoliths is it appropriate to use it in this study?

Line 71: More detail should be provided regarding the parcels simulations. What timestep was used? Were the simulations the same length (i.e. how many days)? What depth were the particles released (or was it surface only?)? Currently, there is no way for this work to be replicated as there are scant details provided.

Figure 1: It would be helpful for the reader to see what a normal ‘June’ looked like for the satellite data to allow for comparison and for the reader to evaluate how ‘unusual’ these blooms are.

Figure 1: the bathymetry map requires a colourbar

Line 80: is the weather magazine reference meant to have a date in place of b?

Line 82: we hypothesise?

Line 90: missing a bracket

Figure 2: why is chlorophyll concentration not presented alongside the ocean colour data? Is it because the measurements are not robust in such shallow coastal areas? Also, for Figure 2b there is a plotting error with a black triangle projected from the top right of the figure down to the bottom-mid left. I would find it instructive to see where particles were advected to at a similar time period during a ‘normal’ year otherwise it’s hard to confirm that these current pathways are unusual.

Section 4.1: First introduce why you have switched to discussing diatoms when the paper is focused on coccolithophore blooms.

Section 4.1.2: I’m not clear why this section focuses on 1983 as well. I don’t think this has been introduced to the reader yet and very little data, besides the Table, is presented for 1983 so I’m not sure how much it adds to the paper.

Physical environment section: all of the sentences are quite vague, perhaps some quantitative numbers (i.e temperatures) could be added in line?

Figure 4: I find it confusing that the observations are blue dots for 2021, the black line is the NWS 2021 mean and then the dashed blue line is the mean of the 1998-2020 observations. I first thought that the blue dashed line was the 2021 reanalysis. Perhaps pick a different color for either the line or the observations or add a legend to make it more digestible.

Figure 4: no graphs have any latitude or longitude axis which makes it hard to pin where 55.27N -5.11 E is. Why has a specific location been chosen rather than an average over the extent of the bloom?

Line 126: ‘Table 1(a) includes statistics for 1983, the previous year that such a bloom was seen in the Clyde Sea.’ – how was it ‘seen’? 1983 precedes satellite chlorophyll and there is no reference provided for the bloom.

Line 174: even with low rainfall in April it would surprise me if the level of nutrients would limit diatom growth enough that a population wouldn’t pick up in May when nutrients became more available post-mixing events. The P1 diatom observations do also increase again in May (which is hard to tell with the figures on different y-scales) and I suspect the chl concentration was greater than the ~0.5 mg/m3 shown for the P2 observations. There is a lack of substantive evidence in this paragraph to support the claims, which I appreciate is due to a lack of data availability but the authors still need to be careful with the conclusions they reach.

Figure 4: The P2 chlorophyll concentration from observations for 2021 is consistently below the mean observations from early April onwards – how does this line up with the ocean color data that is posited to be showing a coccolithophore bloom? It seems from the P2 magenta reanalysis lines that there have been much larger blooms in May previously but these are not mentioned. Did these blooms show up in ocean colour data?

Figure 4: it would be helpful if you could mark the storm timings on the figures. Could the SST observations that fed into the 2021 reanalysis be added to the figure?

Abstract: Having now reviewed the manuscript I am not sure about the cold temperatures restricting the bloom i.e. ‘We hypothesize that the cold restricted the usual spring bloom of diatoms. A restricted spring bloom would mean higher than usual concentrations of nutrients in the summer.’ The authors discuss how storm events and colder SSTs indicate mixing. My understanding is that satellite ocean colour can only ‘see’ the top ~10m or so. Therefore, is it not the dilution of the phytoplankton populations across a deeper mixed layer that has led to low chl concentrations in May?

review by D.G. Bowers

I enjoyed reading this paper. Coccolith blooms are one of the easiest things to identify in ocean colour images of the north-west European shelf in summer and they are a key part of the pump that transfers carbon from the atmosphere to the ocean floor. It is therefore important to study them and understand how they work.

This paper considers two unusual coccolith blooms in the summer of 2021, observed from space, and discusses how they might have been formed. One is to the east of Shetland, the other in the Clyde Sea. I am convinced that these are coccolith blooms but less convinced by the explanations offered (especially for the Clyde bloom). It's a bit tentative.

The main tool that the authors have for seeing what happened at sea level is a reanalysis. I understand that this is a run of a physical-biological model, corrected by reference to the available data. The chlorophyll concentration from this reanalysis for the Clyde Sea and for the group (nanoplankton) that includes coccoliths is shown as the green line in figure 4b. Have I got that right? But that DOES NOT show a bloom in June, right? So the best guess at what is going on at sea level is that there is no bloom, even though the satellites clearly see one. Am I right in thinking that is the nub of the problem? So maybe the point to make is that the reanalysis is not working?

Maybe that is the take home message from this paper. It looks like you can explain the Shetland bloom by advection of phytoplankton from outside but the Clyde bloom remains a bit of a mystery. No harm in that. In fact it is useful to know that we don't understand things properly.

So, I think my main suggestion if you agree with the above is not to be frightened of saying that these blooms are not properly understood yet. The one in the Clyde Sea is observed to happen but we cannot explain why. There are also a couple of small points:

line 59 you say that the estimation process masks clouds,sunglint and coccoliths. Isn't it a bit worrying that the tool you are using to observe coccoliths might mask them out?

The bathymetry map of figure 2c could do with a key.

line 88 tidal mixing restricted to mostly near-surface waters. I don't understand that. Aren't the tides here weak? Do you mean wind is mixing near surface waters?

Not sure I understand figure 2 very well. Is it telling us that the Shetland bloom is consistent with the phytoplankton being advected in from elsewhere but the Clyde Sea ons is not?