Dear reviewer,

We would like to thank you for your comments, we have addressed them accordingly. We have revised large sections of the manuscript, including an improved separation of the results and discussion. We have also added statistical testing of the 2018 and 2020 coverage and thickness data, and included a species list as a supplementary table. Below are responses to the specific comments:

1. I would like to stress the importance of publication of data like the data this paper is based on, please publish the raw data together with the manuscript.

A species list is now available as a supplementary table.

1. I would welcome some thoughts in the introduction on why the aims under 1.1 are interesting to study? Do you expect differences between FOWF and standard OWF? Perhaps due to the floating and lack of tidal zonation?

Additional rationale for the study has been added.

1. I suggest the authors include these newer publications instead of the Vattenfall report. e.g. Bergman, M.J.N., Ubels, S.M., Duineveld, G.C.A., Meesters, E.W.G., 2015. Effects of a 5-year trawling ban on the local benthic community in a wind farm in the Dutch coastal zone. ICES J. Mar. Sci. 72, 962–972. https://doi.org/10.1093/icesjms/fsu193 And some of the papers in the special issue in Oceanography https://tos.org/oceanography/issue/volume-33-issue-4 And the long list of publications from Belgium, such as Degrear et al 2020 instead of the technical report Degraer 2019. Degraer, S., Carey, D., Coolen, J.W.P., Hutchison, Z., Kerckhof, F., Rumes, B., Vanaverbeke, J., 2020. Offshore Wind Farm Artificial Reefs Affect Ecosystem Structure and Functioning: A Synthesis. Oceanography 33, 48–57. https://doi.org/10.5670/oceanog.2020.405

References have been updated.

1. Please include a definition of all the EUNIS coding used in Fig 1, not only for the blue parts

The full EUNIS code names are now included in the figure description.

1. I am missing when (month + year please) the survey descibed in lines 65 - 75 was carried out and whether this survey was one of the two that were compared in lines 95 - 101.

Amended.

1. L58, the anchor chains are connected to the turbine foundation, which is connected to the turbine, the latter I consider to be the large machine on top of the structure, not the whole structure.

"Turbine" has been revised to state "Turbine Substructure", unless referring to the complete turbine.

1. L86 what do you mean by colonisation? The actual colonisation of the foundations by the species was not observed, was it? Perhaps you mean species presence? Of densities?

Updated to "initial coverage estimate".

1. L90 I dont understand. In the lines above you state that fauna was counted, but here you state that fauna was scored presence absence. Many Epifouling species can be easily quantified as percentage covered.

This section has been re-written to be clearer. The coverage of each individual taxa was initially attempted but proved problematic as it was noted that several different taxa (often soft- and hard- growth taxa) occupied the same space, with hard growth taxa such as barnacles and encrusting tube worms occupying spaces in-between anemones. This could generally not be observed but for when, for example, an ROV thruster shifted the anemones' tentacles so that the underlying fauna could be observed.

1. L95 - 101 which month of the year were these surveys carried out? And was it done using similar techniques as the other survey?

Amended. Yes, it was carried out using similar techniques, and this is now also included in the text.

1. The results only include summary data in Table 1. Please also publish the raw data, e.g. as online supplement, as these type of data are often hard to attain for others and it will increase the citations of your paper

A species list is now available as a supplementary table.

1. Fig 5 please explain the difference between 'M senile and spirobranchun' and 'Spirobranchus and M senile'. Is this a dominance difference?

Yes, the taxa mentioned first was the dominant taxa. This is now stated in the figure legend.

1. L126 What is biofilm? I know biofilm as microbial layers but I imagine one cannot see this on the footage. I cannot properly see this in Fig 5 as the photo has a very low resolution, but the brown matter between the anemones in the second photo from the left appears as dense aggregations of Jassa spp (aka Jassa silk) or other ampipods. See for example https ://doris.ffessm.fr/var/doris/storage/images/images/jassa_denombrement_my_02/127660 88-1-fre-FR/Jassa_denombrement_my_02.jpg

The text has been revised. The biofilm referred to is the brown veneer often present in the zonation below the Mytilus and Laminaria/Phaeophyceae.

1. L143 How did you test for significance of differences between the mooring lines? This is not described in the methods.

"Significant" has been replaced by "clear".

1. Figure 6 please include a size reference or a statement on the size of the chain links

Scale bars have been added to appropriate figures.

1. L170 I dont understand, the overall change in faunal thickness has decreased? What does this mean? Did you mean that there was a decrease in thickness between the 2 years? So the change is negative?

Yes, the thickness of growth has decreased for some of the structures. The section has been updated so to included statistical tests.

1. The discussion presents several results that are not in the results section. These should be moved to results and only discussed, not presented (again).

Amended.

1. L176 The starting statement is a result and should be presented there

Amended.

1. L190 need a reference for your statement that S. spinulosa habitats are often associated with high faunal biodiversity

A reference has been added.

1. L192 need a reference for 'which could further benefit commercial fish species' and please elaborate on the benefit, how does this benefit commercial species?

The sentence has been removed.

1. L195 you are introducing new results in the discussion, these should be included in the results section, together with fig 9. At what depth was this recorded and what was the size?

The section has been split up, and parts have been moved to the Results section. The size of the colony (~20 cm) and the depth (73.5 m) have been added to the text.

1. Fig 9 where in the picture is the D pertusum colony? This round shape in the top middle? Indeed an atypical and very closed growth form. Please include a size reference

Yes, the shape is atypical, but similarly shaped colonies have been observed on oil platforms. The colony is about 20 cm in diameter, this is now added to the text. There are two lasers, 10 cm apart in the figure, but they are unfortunately hard to see. A scale bar has been added to the figure.

1. L205 - 207 some of these are not stated in the results, this should go to results section.

Amended.

1. L225 I dont understand how a floating structure can have an intertidal zone. Since it is floating the water surface is always at the same position on the structure, is it not?

The structures are anchored to the seabed using suction anchors and are attached to these by heavy chains. This results in an intertidal zone. However, the intertidal zone is likely reduced as compared to a non-floating structure.

1. L229 I am confused about teh intertidal zones in this line. What is the difference between low intertidal and deep intertidal?

The sentence has been amended.

1. L233 a difference is either significant or not, it cannot be significant enough. Probably you meant large enough?

Amended.

1. L238 - 245 is a lot of information that should be in the results, new data is not expected in the discussion

The text has been revised and split up between the Results and Discussion sections.

1. L250-251 I disagree, figure 8 shows one large increase and multiple small decreases, most with a lot of variation, in all but 1 case I see zero is included in the mean +- SD interval, indicating that the differences are likely not significant, although the authors did not test for this. I suggest to state that variation is large but no clear pattern was observed, or actually test for differences between years and present the results

The section has been updated and statistical tests have been added.

1. L273 please publish the data with the paper as online supplement or place it in a open access repository. Available upon request is not ideal for long term availability.

A species list is now available as a supplementary table.

1. L37 Whomersley and Picken is not a publication of OWF

Amended.

1. L44 please define FOWF at first use

Amended.

1. L68 WROW should be WROV

Amended.

1. L72 what was the speed of the slower speed?

0.2 knots, the text has been revised.

1. L86 what is QC

Quality control, the text has been revised.

1. L87-89 what software did you use for the viewing and registration?

The section has been revised and additional information has been added.

1. L224 Whomersley and Picken is not about OWF

Amended.

1. L227 Wilhelmsson and Malm, 2008 is not in the North Sea.

Amended.

1. L290 Lengkeek is spelled wrong

Amended.

Dear reviewer,

We would like to thank you for your comments, we have addressed them accordingly. We revised large sections of the manuscript, including an improved separation of the results and discussion. We have also added statistical testing of the 2018 and 2020 coverage and thickness data and put more emphasis on the observations done within this study. Finally, we have included a species list as a supplementary table. Below are responses to the specific comments:

1. 26-34 This an important issue and it would strengthen the point if more citations could be added here especially those from other than the industry

The section has been revised and additional references have been added.

1. 74 – 75 please specify distance to the structure or give a least a range

Distances have been added to the text.

1. 68 misspelling of WROV

Amended.

1. 86 colonization refer to a process – shouldn´t it be more specific like “coverage” or “state of colonization?

Updated to "initial coverage estimate".

1. 93 please explain why nudibranchs and gastropods were excluded how did you overall selected your list and what was the rationale?

The text has been revised. It was eggs of nudibranchs and gastropods that were excluded.

1. 97-98 wasn´t the data secured such that the same person could compare and analyze the data later to insure more comparable data in 2018 and 2020?

Unfortunately it was not feasible to have the 2018 data re-evaluated by the structural inspectors present during the 2020 survey.

1. I would like to see the species list! I failed to find it in results or reference to an appendix!

A species list is now available as a supplementary table.

1. Same as above “different species of crustacean”!

A species list is now available as a supplementary table.

1. I would think the term “percent coverage” is better

Amended.

1. Although -tidal depth ranges are well-defined terms I find it a little bit odd to use these depth categories in relation to floating structures where indeed the organisms do not experience the consequences of tides (e.g. the Kelp is not exposed to diurnal desiccation, changing light levels etc.) I Suggest to use depth ranges in meters instead

The structures are anchored to the seabed using suction anchors and are attached to these by heavy chains. This results in an intertidal zone. However, intertidal zone is likely reduced as compared to a non-floating structure.

1. 130 Laminaria belong to the group of Phaeohyceae – should maybe be “Laminaria and other Phaeophyceae”

Amended.

1. 144 “no significant differences were noted on the mooring lines…” This is a statement that should certainly be substantiated with additional information. What parameter did you look at (e.g. communities, coverage, biomass, individual species, biodiversity indices…)? What was your data format? test used … etc.

"Significant" has been replaced by "clear".

1. 168-172. This part is not clear. The part about the coverage is clear but information on how you have distinguished between soft and hard fauna is missing (what species belongs to which groups) especially since the 2018 survey was performed by non-specialist. How was thickness determined from the underwater video film? Again bear in mind, that although this can easily be done by non-specialist, data is still subjective and the data quality could therefore be improved if the same person did both year (whether specialist or not). Legends on figure 7 an 8 missing information about the years and how changes is defied (2018 vs 2020?).

This section has been revised accordingly. New figures have been introduced and now include a T-test, conducted on the coverage and thickness data. Details on specific taxonomic groups with regards to hard and soft growth, respectively, are now included.

1. 175. Subheading should rather be “identification of species” as no non-indigenous species in fact was determined with certainty

Amended.

1. 176 – 178 should be under results!

Amended.

1. 193 - 197 This is a potential important finding, due to the threatened status of Desmophyllum, and I would recommend to contact external experts as you did with the Barnacles. A strong group I located just around the corner at Edinburg University.

Advised experts agree that the colony is likely D. pertusum, but due to the small size and uncharacteristic appearance a positive identification would require close up imagery of the calyx using a stills camera. The text has been amended.

1. 215-216. If diversity “is lacking” there is either 0 or 1 species present! Better to write whether there was fauna at all. Did you quantify species richness or diversity at all? In case this should be described under methods and results. Better to delete this sentence

The sentence has been revised.

1. 224 similar in what way?

In regard to species composition and distribution. This text has been revised.

1. What do you mean by “not significant enough” if no statistical analysis was performed write instead “differences was not clear”

Amended.

1. 235 Different I what way?

The text has been revised.

1. 239-241 How can you conclude the species succession follow the same trend as you have stated in the material and methods that only a few phyla was recorded by the non-specialists. Species succession implies that the relative contribution species to the community changes over time.

The section has been revised, and together with the new section regarding changes in taxonomic composition between 2018 and 2020. We do believe that we can say that Hywind FOWF seems to follow the same pattern as other offshore structures in the North Sea. However, the limited taxonomical resolution should of course be considered.

1. 251 the information about the uncertain due to lack of consistent methodology should (as mentioned in the above several times) should be flagged up front in MS and it also concerns the above comments on succession.

Information regarding inconsistencies between the years has been included in the Methodology section, as well as briefly in the Discussion section.