the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Aug 2026
Reviews and Syntheses: Carbon and Nitrogen Stable Isotope Analysis of Antarctic Macroalgae: a 35-Year Record for Environmental Change
Abstract. Stable isotope analysis for Antarctic macroalgae has received minimal research interest outside of trophic ecology. However, carbon (δ13C) and nitrogen (δ15N) isotope analysis provides insights into nutrient uptake, productivity and response to environmental change. This review collates all available CN, δ15N and δ13C data available for macroalgae from the Antarctic continent. Fieldwork was also carried out in late 2023 to the northern West Antarctic Peninsula to collect specimens for stable isotope analysis. Prior to the 2023 survey, only 29 publications were found spanning a 35-year period from 1987−2022 with a strong bias towards data collection in the South Shetland Islands. A bias was found towards the Rhodophyta and Phaeophyta phyla, with H. grandifolius and Desmarestia sp. being the most studied species. This review highlights Antarctic seaweeds had a lower average CN compared to the global average, likely driven by the non-nitrogen limiting environment. Both δ15N and δ13C varied between species as well as spatially, δ13C recorded a wider range (~25 ‰) than for δ15N (~16 ‰). Spatial variation is assumed to be driven by site specific differences as well as latitudinal changes with light availability. δ13C was more negative for the Rhodophyta phylum, with some species plotting below –30 ‰, however no significant difference was found between Rhodophyta and Phaeophyta species for δ13C and all recorded average δ13C values indicative of both passive CO2 uptake and carbon concentration mechanism use. When plotted temporally, δ13C showed variation with sea ice extent across the 35-year record. Within the last decade, both sea ice extent and macroalgae δ13C became more variable and suggesting an unexplored link between primary producer response and sea ice instability. Significantly more research is required for stable isotope analysis of Antarctic seaweeds. Variability in both δ15N and δ13C will impact trophic food web studies, both spatially and temporally. There is a lack of data for sites outside of the Antarctic Peninsula as well as ~100 species currently having no data available. This review also highlights the potential for macroalgae to be an additional proxy for monitoring primary producer response to ongoing climatological changes for the Southern Ocean.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-4443', Anonymous Referee #1, 01 Sep 2026
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CC1: 'Authors Reply to RC1', Darren R. Gröcke, 10 Sep 2026
We thank the anonymous reviewer for their comments, which we address below in bold.
Reviewer 1 – 1st September 2026 comments
This is a review of Alldred & Gröcke’s synthesis review of macroalgal stable isotope values. They add some new data collected opportunistically from a tourist voyage. The actual question the authors are trying to answer are not entirely clear, so it is difficult to evaluate if their mission was accomplished. There are very few novel ideas in this manuscript.Yes, this is a review /synthesis paper, with the primary aim to collate all available δ13C and δ15N data for Antarctic macroalgae and then to include our own macroalgae data generated from individual blades from locations often not visiting by scientists. New and novel ideas would be more appropriate for a manuscript publishing experimental results, which this is not. We would also like to highlight that the new sample set (and subsequent data) was collected as part of my PhD project which used the cruise tourist company Viking. This is a novel approach to collecting and researching the environment in the Antarctic. We will consider restructuring the introduction to make clearer the aims of the review and reasoning behind presenting our new data that improves the spatial gaps identified around the Gerlache Strait region away from research bases.
Several so-called analyses were simply map plots where it is left to the reader to guess at latitudinal trends. Any inferences between sea ice cover and stable isotope values of macroalgae are vastly overshadowed by the huge variability of the isotope values.
Variability is inevitable in marine ecosystem. This compilation of data from the literature highlights this. The 35-year record is not intended to be conclusive but identifies the potential to link environmental parameters with the benthic ecosystem. The purpose of this review/synthesis is to identify where future research is needed, this temporal link to sea ice, using data that has been collected at different time points and locations will, inevitably, have high variability. Therefore, we are tentative in our discussion and conclusions, since a full statistical approach would not be appropriate whereby the data has been collected and presented differently between each publication. Further highlighting the need for future macroalgae research.
I was excited when I was asked to review this manuscript and, unfortunately, I was sorely disappointed. There are severe problems with the statistical analyses done (data are not independent replicates) and in some cases, there are no analyses done at all (latitudinal trends, relationships to sea ice cover). It feels like the authors had an opportunity to collect some macroalgal samples for stable isotope analysis and were now looking for a framework to publish them. There is very little novel information, although more data is always a good thing. The many grammar mistakes made it very difficult for me to read this extremely long manuscript. The authors also do not seem to have a strong foundation in stable isotope analyses given the inaccurate language used many times. A really useful source for this is Sharp’s Principles of Stable Isotope Geochemistry, especially Table 2.1.
Yes, there are some grammatical mistakes, but this is just a consequence of being human and not perfect or a machine. Reviews are often long manuscripts and so they need careful attention and time to read it carefully. One of us (Gröcke) is also offended by the personal reference to reviewer stating the “authors do not seem to have a strong foundation in stable isotope analyses”: especially considering Gröcke has been publishing in stable isotope geochemistry for 30 years and has managed and operated a stable isotope facility since 2003. A quick Google search by the reviewer and they would have discovered this. We will correct this mistakes in the revision. We will explore statistical analyses further but considering the lack of consistency between samples and individual datasets, i.e., the purpose for why they were collected and the lack of n, any statistical results will have considerable errors and assumptions. The use of cruise liners (e.g., Viking, Hurtigruten) to collect macroalgal samples is indeed opportunistic: they select sites with first, the tourists in mind, followed by the scientific crew they allow to be on the ship. However, considering the lack of data from these regions in the Antarctic it is now opening doors to understand more about the region – than just where the national research stations are based. In addition, the number of trips and samples that the PhD student (Alldred) has been able to collect from unsampled areas in the Western Antarctic Peninsula is significant: many of these samples have also been deposited in UK museums for future research and archiving, such as the National History Museum (London) and the World Museum (Liverpool). So yes, sites where opportunistic as sometimes weather played a role in what could be collected, sites where there was limited seaweed, and changing sites at the last-minute limited sample site replication.
If this manuscript is being considered for publication at all – and I caution against it – then the manuscript should be shortened at least by half and clear and solid analyses should be performed. There is much redundant/repeated information and lengthy portions of the manuscript deal with detailed descriptions of isotope ranges etc. Are those really useful?
Reducing the length of the manuscript will be decided by the handling editor. We shared this version with editors before submitting to determine if it was suitable, and they encouraged our submission of the manuscript “as is”. We do feel, however that as a review paper there is a need for a longer introduction than a typical paper. In contradiction to this, the reviewer states they would like additional papers cited, concepts explained more, and ironically write, “A review should not just state what was published, but what the findings were”. If we were to do this then the manuscript would become even larger!
Lastly, the authors seem very dismissive of the efforts of other publications, which I found disturbing.
We are apologetic if the reviewer considered this in our paper. This was definitely not our intent. It is a review paper and some of the previous research (including ours for reasons explained above) do have shortcomings and they should be stated. We will carefully re-read the manuscript and constructively rewrite sections that may be too critical. We do highlight that we also struggled with species diversity: a well discussed issue with limited field site choice, limited working time at the sites within the expedition cruise industry.
Following are some specific comments.
The Abstract could be more informative. It states “insights into nutrient uptake, productivity and response to environmental change.” Of that, only environmental change (ice cover) is further mentioned in the abstract. More detail on “temporal and spatial variability” could be given. The abstract lacks actual results.Our intention was to give an overview of the review since, as pointed out, it is a long paper. We want to focus on what we see as the most important/useful outcome being the temporal link to sea ice variability that should be explored further. We will look to re-write the abstract, but we are constrained by the word limit.
Introduction
39 – Macroalgal (not algae) (adjective) – this is a problem throughout the manuscriptMinor grammatical error – we can change to macroalgal – will check throughout.
and 41: 7.22 x106km2 and C/year – decide on a format of superscript or not
Corrected.
43 – macroalgae is plural – macroalgae contribute (no s).
Corrected.
47 – comma after Southern Ocean
Corrected.
59 – how is sea ice related if not indirectly through light, which is then mentioned separately. Clarify this
Removed sentence to make this clearer.
65 – add Amsler et al. 2023 as an important recent paper that addresses this issue:
Amsler, C.D., Amsler, M.O., Klein, A.G., Galloway, A.W., Iken, K., McClintock, J.B., Heiser, S., Lowe, A.T., Schram, J.B. and Whippo, R., 2023. Strong correlations of sea ice cover with macroalgal cover along the Antarctic Peninsula: Ramifications for present and future benthic communities. Elem Sci Anth, 11(1), p.00020.Amsler et al., 2023 is cited on Line 66 following this sentence, but we can also cite it for that previous sentence too.
70 – Iken et al. 2023 does not address macroalgal cover but macroalgal stable isotope values. Change this to Amsler et al. 2023
Corrected.
82 – add some relevant references to the ecology of P. decipiens, such as (and others):
Wiencke, C. and Tom Dieck, I., 1989. Temperature requirements for growth and temperature tolerance of macroalgae endemic to the Antarctic region. Marine Ecology Progress Series, pp.189-197. Becker, S., Walter, B. and Bischof, K., 2009. Freezing tolerance and photosynthetic performance of polar seaweeds at low temperatures.We disagree. P. decipiens is a minor component in this compilation and review and thus should not be discussed in more detail than others.
90 – cite some of the original early literature for this well-studied system, for example:
Klöser, H., Ferreyra, G., Schloss, I., Mercuri, G., Laturnus, F. and Curtosi, A., 1993. Seasonal variation of algal growth conditions in sheltered Antarctic bays: the example of Potter Cove (King George Island, South Shetlands). Journal of Marine Systems, 4(4), pp.289-301.We missed this paper, we can add this to the introduction alongside many early literature that has already been cited.
91 – What is SSI?
South Shetland Islands – stated in Line 64 “South Shetland Islands (SSI)”. SSI is used hereafter.
118 – data is plural (were) – this is used incorrectly throughout the entire manuscript
Corrected.
121 – the publication by Weykam is also biochemically important, providing C:N ratio:
Weykam, G., Gómez, I., Wiencke, C., Iken, K. and Klöser, H., 1996. Photosynthetic characteristics and C:N ratios of macroalgae from King George Island (Antarctica). Journal of Experimental Marine Biology and Ecology, 204(1-2), pp.1-22.Yes, we agree – hence why it is cited throughout the manuscript and included in Figure 2.
123 – is a figure on the increase of published information really useful for anything? Delete
Figures showing publication output across time are common for a review paper, it clearly shows the increase in publication output as well as the type of biogeochemical data collected. Figure 2 allows for a visual representation of publication output, rather than a lengthy description that would add to an already “lengthy manuscript”. We find this useful and would keep in the revised manuscript.
129 – In addition to Iken et al. 2023, which provides data from a few species from a large spatial range, also see Whippo et al. 2024 from that same survey, which provides isotope and CN data from a larger number of species from individual locations. This paper is also missing from Figure 2. Whippo, R., Iken, K., Amsler, C.D., Lowe, A.T., Schram, J.B., Klein, A.G., Heiser, S., Amsler, M.O., McClintock, J.B. and Galloway, A.W., 2024. Fatty acid profiles and stable isotope composition of Antarctic macroalgae: a baseline for a combined biomarker approach in food web studies. Polar Biology, 47(4), pp.367-386.
Whippo et al., 2024 used the field data collected by Iken et al., 2023 – this has been cited and included in the compilation. It would be wrong to include that data again. In addition, mentioned Oswalt et al., 2025 also use the macroalgae dataset from Iken et al., 2023, and hence not included in this database.
136 – surveyed for what? Paragraph started out with macroalgal biochemical data and isotopes, but some of the surveys mentioned here are not about macroalgae at all
The papers cited here, whilst not focusing on macroalgae, did publish either CN, δ13C or δ15N data for seaweeds from King George Island and therefore, they have been included in the data file. We highlight throughout the review that many of the papers cited are trophic ecology publications that have also generated some macroalgae biogeochemical data. These are included in the datafile, and their data incorporated into the figures and analysis.
Section 1.1. lists studies that provide data and spatial coverage but doesn’t really talk about biochemistry of macroalgae. A review should not just state what was published, but what the findings were
The review is structured to introduce the general topic and further sections go on to discuss the output of these publications. If their findings are introduced in Section 1.1 it would cause confusion and repetition if it was again brought into the later sections of the review paper. And of course it would make the manuscript even longer!
167 – this makes it sound as if these references are about Antarctic macroalgae, that > 60% of Antarctic Rhodophyta lack CCM, which they are not
Reworded to state that 60% of Rhodophyta so far analysed from the Antarctic record δ13C values indicating that they do not posses a CCM. δ13C values below −30 ‰ are indicative of passive CO2 uptake and not CCM use (see Figure 3).
169 – specify that this is about d13C values
This paragraph only discusses the δ13C isotopic system. δ13C is regularly mentioned throughout the paragraph but we will re-read and specify it accordingly where needed.
170-179 – is there an analysis that does not confound depth with species identity? If saying that there is a depth-related pattern, the same species across a depth gradient needs to be analyzed, as different species can have different causes for their isotope values than depth.
We are unclear why the reviewer has highlighted this in an introductory section. This paragraph gives an overview for the δ13C mechanism within a macroalgae context. The papers referenced here are widely cited as relating δ13C to irradiance and, therefore, depth. This sentence draws attention to experimental and field studies that have linked δ13C to irradiance and depth for several species. We are only reporting what the literature states.
182-184 – can you give examples of these factors influencing d15N values? This is very vague
The following sentences in this paragraph discuss various influences on δ15N within macroalgae. We introduce the impact of seabird guano on elevating δ15N values (Line 188) and the seasonal variation in δ15N is discussed from Line 195. Lines 182–184 introduce the background to seaweed δ15N analysis at the start of this paragraph.
180, 203-213 – explain how phytoplankton dynamics relate to macroalgal d15N. Make better connections, the entire section seems to talk about phytoplankton blooms and not macroalgae
Phytoplankton blooms cause rapid uptake of nitrogen from the available nitrogen pool. This can cause enrichment in the remaining nitrogen pool and thus seaweed, may record elevated δ15N values due to the biological pump transferring carbon and nitrogen to sedimentary deposits. This is a seasonal phenomenon and a relevant, potential factor on macroalgae δ15N values. We will revisit this paragraph and consider rewording it to make it clearer to the reader.
216 – nutrient limited environments
Changed.
226 – add the Weykam et al. 1996 paper here
Added.
235 – macroalgae were
Corrected.
248 – Was there a specific tissue portion that was consistently sampled?
Yes – can be introduced to state blade material is used.
287 – when reporting a range, use the capital delta sign, Δ
This paragraph relates to CN not δ13C or δ15N and so this is not the correct notation for CN. Δ is not used, that we are aware, for a range between values, but the discrimination between two isotopic products (e.g., Δ13CA-B = δ13CA – δ13CB): please see Sharp’s equation 2.16 and subsequent text.
289 – green and red algal CN is the same, cannot argue one being lower than the other. Look at the SD! Also, make sure you use the correct noun in the sentences. The CN values of Phaeophyta were higher, not the Phaeophyta.
Thanks. Amended this grammatical error to clarify no significant difference was evident between the phyla.
288-290 – The numbers reported for replicates do not match what the authors report as their collections, except for Chlorophyta (n=35). Rhodophyta should be n=53 not n=52 unless one plant was not measured. n=600 for Phaeophyta is incorrect, it should be n=20. What I understand from the methods, the authors collected multiple samples along the blades of Himanthothallus. It seems they treated each of those sub-samples as individual (independent) samples, which is not correct. They could take an average value per plant and then use it in the calculation of the overall mean per group. This is especially important when using the data for statistical analyses. Again, the data as they use them are not independent and are not appropriate for the statistical analyses they employ. Obviously, this also applies to the other isotope metrics.
We will double check these numbers again, to make sure n = the number of individual macroalgae specimens and not number of analyses. Our analytical approach was to take incremental sub-samples along the blade to determine if any seasonal pattern is recorded in the biogeochemical data. Although not specimen replicates the data we present highlights the individual variation withing single blades. We can highlight and amend this, but what the review paper shows it that depending on how or where past papers sampled their macroalgae this may also contribute to the significant variation recorded.
295 – why is (15.8 ‰) in parentheses?
Corrected.
305 – what does “most elevated phylum” mean?
Amended to clarify δ13C values were less negative for Rhodophyta compared to Chlorophyta and Phaeophyta.
310 – the authors say “significantly enriched”, this needs to be specified as enriched in 13C. When referring to the ratio (delta value), “enriched” is an incorrect term.
Grammatical error – has been corrected.
308 and 311 – misspelling of “utricularis”
Corrected.
304 – The authors say ”δ13C values ranged between –34.7 ‰ (H. grandifolius) and –9.8 ‰ (Iridaea sp.) for this study“ and then in Line 312 they say “H. grandifolius was the most negative (–23.4 ± 4.6 ‰, n=589)”. I don’t understand why there are two extremely different values being cited for H. grandifolius.
First is the range for all analyses and species, whereas the second relates to the average for that species. We will amend this average, since the reviewer suggested above to average each blade.
328-330 – this information does not seem relevant here. This could maybe be added to the introduction. Is there any solid reason to doubt the identification skills of these other researchers? And size is not a good reason to doubt identification accuracy. For example, the large brown algae Desmarestia anceps and D. menziesii are easily confused, while the small green alga Lambia antarctica cannot be confused with anything. If the large species are so easily identified, why did the authors not separate Desmarestia sp? To be honest, this should probably Desmarestia spp unless they can confirm they only have one of the species, and in that case, they should be able to say which one.
Many of the papers included in this compilation state “unidentified species” or just macroalgae and do not even go down to genus level. It is a common issue, and one we also encountered in many isotope-ecology papers. We even highlight such limitations in our own dataset.
335 – The authors say “Desmarestiales are key primary producers, along with H. grandifolius (> 70 data entries),…” It seems the authors are unaware that H. grandifolius is a member of the Desmaretiales. It makes me wonder how familiar the authors actually are with the macroalgal flora of the Southern Ocean.
This is a minor mistake and does not justify the tone used here by the reviewer. This error stemmed from a mistake between Desmarestia sp. versus Desmarestiales. We will correct accordingly.
339 – The authors should note that there are not that many Chlorophyta species in the Southern Ocean. Naturally, there will be fewer datapoints. Also, if they identify this as such a problem, why did they not try to collect more species and individuals? They only collected one species (Monostroma sp.). Continuing, it seems it would suffice to say that data of individuals that were not identified at least to the genus level were omitted. The authors take a very dismissive tone about other investigations, which may have their limitations but there is no need to downgrade their work.
As explained previously in this response and in the manuscript, field site choice did hamper our dataset. Also, such cruises do not permit SCUBA diving opportunities for health and safety reasons. Therefore, the species biodiversity of the dataset is somewhat limited, we can emphasise this more when revising the manuscript. In addition, the PhD is mainly focused on H. grandifolius since it is a species known to have blades that contain the record of multiple seasons. We highlight in the review that future work should consider focussing more on Chlorophyta. But, with so few analyses compared to the other phyla it is a shortcoming that should be noted in the review.
351 – why is “Coraline algae “ capitalized (here and elsewhere)? Also, not misspelling of coralline
Corrected.
365 – The authors need to adjust the treatment of their H. grandifolius data as pointed out above. These are not replicate measures, they are pseudoreplicates and not appropriate for this type of analysis as used by the authors.
Corrected.
369 – “regardless of study, site or depth” – was there an actual analysis done to test this?
Reworded.
387 – Nitrogen should not be capitalized
Corrected.
392 – revise the wording of “kelp-like H. grandifolius” It is not a kelp and calling it kelp-like can be confusing. What aspect of it is kelp like? Maybe the morphology but internal cell structure and physiology are not – hence, the comparison to actual kelps in terms of storage is flawed.
It is a large brown macroalgae that, like kelp, is the dominant seaweed in the Southern Ocean as kelp is in the Arctic. Comparisons have been made as, isotopically, there is more data regarding kelps and so this was deemed an appropriate comparison. We have removed kelp-like from the manuscript.
406 – “significantly higher nitrogen content for Rhodophyta” – be specific. It was only higher than Phaeophyta, not Chlorophyta
Corrected.
452 – add one or both of the recent Amsler et al. 2023 papers that suggest exactly this. These authors are not the first ones to suggest this. They already cite the Amsler et al. 2023 paper in Elementa but they are missing the following paper: Amsler, C.D., Amsler, M.O., Heiser, S., McClintock, J.B., Iken, K., Galloway, A.W. and Klein, A.G., 2024. Vertical distribution of brown and red macroalgae along the central Western Antarctic Peninsula. Botanica Marina, 67(1), pp.1-10.
Paper now included, thank you for drawing out attention to this paper which we missed.
490 – no, Chlorophyta have less negative (not more positive) d13C values. None of these values are positive.
Apologies – corrected.
505 – the same issue as noted for Line 335
Changed to Desmarestia sp. and H. grandifolius.
509 – please provide a reference for this statement
The database compiled notes many Chlorophyta are listed as “Unidentified” or not to species level. We will amend accordingly.
518 – please provide examples for the misidentifications that have occurred from Chlorophyta
It is in the database where it states, “Unidentified Species”. We can only go on what the papers report. We will change the wording to make it clear that species was “unidentified” and not “misidentified”.
523 – please add ”anomalously low nitrate δ15N values” as this statement would otherwise refer to macroalgae and the cited study does not investigate macroalgae
Corrected.
532-534 – Are those data anywhere? And what does “depleting trend” mean? If not, please provide a reference for where the data can be found.
The papers cited include this data for various Laminariales, both Bebb et al., 2023 and Dyer et al., 2019 conducted δ13C and δ15N analysis of brown seaweeds to investigate internal variation. There is very limited research on this aspect of macroalgae, especially within the Southern Ocean. A more thorough analysis of the incremental sampling and biogeochemical results and interpretation are intended for another manuscript that just focuses on this. This review was focused on what is available through time, whereas the other planned manuscript will look at seasonal/sub-seasonal interpretations.
547 – probably more important than cruise ship wastewater is the vicinity of coastal penguin rookeries, which should at least be mentioned.
As discussed, δ15N values weren’t that positive and sub-tidal specimens didn’t really reflect any influence of penguin guano. This is surprising to us, but again was not the focus of our study.
587 – change to “the future response”
Corrected.
604/05 – provide a reference for this statement
This sentence was meant in terms of depth gradients and inter-tidal environments have less negative δ13C as species rely on CCMs. We will clarify this paragraph and include references.
610 – the authors are clearly not familiar with the isotope language. “depleted δ13C values” do not exist – a ratio cannot be depleted.
Corrected.
618 – awkward phrasing “species that’s blades”, replace with which or whose
Corrected.
619 – 15 ‰ difference in which isotope?
The paragraph is clearly about δ13C. Section 3.4.2 δ13C – a biological proxy for primary production in macroalgae. We will reword to include δ13C in case the reader has forgotten.
625-628 – interesting how the authors berate other studies form not providing information on the exact thallus structure when they do not seem to provide this information for their own data either.
Our H. grandifolius data does provide incremental sub-sample data for this species, however due to the length of the manuscript this has been omitted to keep to a more succinct structure. We will highlight this and indicate that a more detailed paper on our H. grandifolius data from December 2024 and January–February 2026 should be published soon. We also note that this review paper aims to highlight areas for further research, this lack of data on morphological biogeochemical differences is, we believe, an important issue that requires additional research: as already indicated by Bebb et al., 2023 and Dyer et al., 2019.
635 – is comparing isotope values of different groups for individual sites useful? It has been well established that groups can differ. The author themselves brought up reasons for why that is the case. Also, Fig. 8-10 are too coarse to be able to meaningfully compare specific sites across the three figures. Any spatial trends cannot be gleaned from these figures. What is the hypothesis? If there is a latitudinal trend? If so, use a regression analysis. Just plotting points on a map and squinting at it is not an appropriate analysis. Much of the text in this paragraph (3.4.3) is redundant description of ranges and can be eliminated.
These spatial figures show data collected from a range of species, depths and years therefore we decided against statistical analyses as there are too many environmental and collecting variables in this compilation: thus, we favoured the production of a visual map to show variation but also to show spatial nature of the dataset. Upon revision we will look to reduce the spatial discussion section of the manuscript but highlight that the spatial coverage is biased towards research stations. Although broad latitudinal trends may be captured with a dedicated research program (e.g., Iken et al., 2023; Cardona et al., 2019) the additional sampling at IAATO cruise sites could fill in spatial gaps. We would also like to stress the spatial variation has wider implications for trophic ecological studies and there is considerable variation even between sites that are close together. We will amend accordingly with a revision.
637 – Fig. 8 does not indicate the location of Yankee Harbour
Upon revision this section would be cut down, if still required Yankee Harbour can be added to the map. It is an island in the South Shetland Islands commonly visited by IAATO cruise vessels with a large Gentoo penguin colony.
644 – Not “Nitrogen” but “Nitrogen isotope ratios”
Corrected.
656-659 – the references for this statement do not seem to refer to Antarctic macroalgae; this should be made clear as growth rates may act under different constraints in Antarctic macroalgae. For example, the following statement (659-660) is likely only true if nitrogen is limiting.
Unfortunately, there are no such references for Antarctic macroalgae that correlate growth rates, nutrient turnover and biogeochemical data. We will clarify these references upon revision and also include it as another direction for future experimental work.
666 – see comment above. If a latitudinal trend is to be tested, use a regression analysis.
We have conducted a test of the biogeochemical data versus latitude and will present in a revised version: these graphs/figures will be in the supplementary file and only the results of the regression present in the paper.
668 – Fig. 5 does not show locations
Corrected.
674 – you cannot talk about a “strength” of a relationship without running an actual analysis
We will revise this – but we will also state our reservations regarding this due to the many variables of the review dataset previously discussed and highlighted.
698 – it’s unclear what all this has to do with any observed temporal trends in isotope values; this is about potential future expansions in habitat
A revised manuscript would have a separate future work and wider implications section to improve clarification and structure.
717 – again, instead of a solid analysis, the authors just offer an image and leave it to the reader to take away a vague sentiment of variability. There is high variability in isotope values but the authors claim that these values track variation in sea ice cover. You should consider similar approaches as in Amsler et al. 2023, who did an actual analysis of algal cover in relation to sea ice concentrations. A similar approach could be developed for macroalgal stable isotopes. Also, the work of Iken et al. 2023 investigating latitudinal trends in stable isotope values should be included in this discussion.
We propose a tentative, unexplored biogeochemical link to historical sea-ice cover and subsequent isotopic values. These studies included are largely trophic ecological studies that were not intended for this purpose. Therefore, variability is to be expected but the variation observed here would benefit from future investigation. We do agree, a similar approach could be developed for macroalgae isotopes as with Amsler et al. (2023), however, this was beyond the scope of the PhD since it was relying on “opportunistic” cruise sites. We have just recently had discussions with sea-ice experts at the UK Antarctic Science Conference held at Durham and so plan to make some amendments to the sea-ice discussion. Notwithstanding, our tentative link between macroalgae δ13C and sea-ice remains.
Citation: https://doi.org/10.5194/egusphere-2026-4443-CC1 -
AC1: 'CC1 Reply to R1 (reuploaded as AC)', Freya Alldred, 06 Oct 2026
We thank the anonymous reviewer for their comments, which we address below in bold.
Reviewer 1 – 1st September 2026 comments
This is a review of Alldred & Gröcke’s synthesis review of macroalgal stable isotope values. They add some new data collected opportunistically from a tourist voyage. The actual question the authors are trying to answer are not entirely clear, so it is difficult to evaluate if their mission was accomplished. There are very few novel ideas in this manuscript.Yes, this is a review /synthesis paper, with the primary aim to collate all available δ13C and δ15N data for Antarctic macroalgae and then to include our own macroalgae data generated from individual blades from locations often not visiting by scientists. New and novel ideas would be more appropriate for a manuscript publishing experimental results, which this is not. We would also like to highlight that the new sample set (and subsequent data) was collected as part of my PhD project which used the cruise tourist company Viking. This is a novel approach to collecting and researching the environment in the Antarctic. We will consider restructuring the introduction to make clearer the aims of the review and reasoning behind presenting our new data that improves the spatial gaps identified around the Gerlache Strait region away from research bases.
Several so-called analyses were simply map plots where it is left to the reader to guess at latitudinal trends. Any inferences between sea ice cover and stable isotope values of macroalgae are vastly overshadowed by the huge variability of the isotope values.
Variability is inevitable in marine ecosystem. This compilation of data from the literature highlights this. The 35-year record is not intended to be conclusive but identifies the potential to link environmental parameters with the benthic ecosystem. The purpose of this review/synthesis is to identify where future research is needed, this temporal link to sea ice, using data that has been collected at different time points and locations will, inevitably, have high variability. Therefore, we are tentative in our discussion and conclusions, since a full statistical approach would not be appropriate whereby the data has been collected and presented differently between each publication. Further highlighting the need for future macroalgae research.
I was excited when I was asked to review this manuscript and, unfortunately, I was sorely disappointed. There are severe problems with the statistical analyses done (data are not independent replicates) and in some cases, there are no analyses done at all (latitudinal trends, relationships to sea ice cover). It feels like the authors had an opportunity to collect some macroalgal samples for stable isotope analysis and were now looking for a framework to publish them. There is very little novel information, although more data is always a good thing. The many grammar mistakes made it very difficult for me to read this extremely long manuscript. The authors also do not seem to have a strong foundation in stable isotope analyses given the inaccurate language used many times. A really useful source for this is Sharp’s Principles of Stable Isotope Geochemistry, especially Table 2.1.
Yes, there are some grammatical mistakes, but this is just a consequence of being human and not perfect or a machine. Reviews are often long manuscripts and so they need careful attention and time to read it carefully. One of us (Gröcke) is also offended by the personal reference to reviewer stating the “authors do not seem to have a strong foundation in stable isotope analyses”: especially considering Gröcke has been publishing in stable isotope geochemistry for 30 years and has managed and operated a stable isotope facility since 2003. A quick Google search by the reviewer and they would have discovered this. We will correct this mistakes in the revision. We will explore statistical analyses further but considering the lack of consistency between samples and individual datasets, i.e., the purpose for why they were collected and the lack of n, any statistical results will have considerable errors and assumptions. The use of cruise liners (e.g., Viking, Hurtigruten) to collect macroalgal samples is indeed opportunistic: they select sites with first, the tourists in mind, followed by the scientific crew they allow to be on the ship. However, considering the lack of data from these regions in the Antarctic it is now opening doors to understand more about the region – than just where the national research stations are based. In addition, the number of trips and samples that the PhD student (Alldred) has been able to collect from unsampled areas in the Western Antarctic Peninsula is significant: many of these samples have also been deposited in UK museums for future research and archiving, such as the National History Museum (London) and the World Museum (Liverpool). So yes, sites where opportunistic as sometimes weather played a role in what could be collected, sites where there was limited seaweed, and changing sites at the last-minute limited sample site replication.
If this manuscript is being considered for publication at all – and I caution against it – then the manuscript should be shortened at least by half and clear and solid analyses should be performed. There is much redundant/repeated information and lengthy portions of the manuscript deal with detailed descriptions of isotope ranges etc. Are those really useful?
Reducing the length of the manuscript will be decided by the handling editor. We shared this version with editors before submitting to determine if it was suitable, and they encouraged our submission of the manuscript “as is”. We do feel, however that as a review paper there is a need for a longer introduction than a typical paper. In contradiction to this, the reviewer states they would like additional papers cited, concepts explained more, and ironically write, “A review should not just state what was published, but what the findings were”. If we were to do this then the manuscript would become even larger!
Lastly, the authors seem very dismissive of the efforts of other publications, which I found disturbing.
We are apologetic if the reviewer considered this in our paper. This was definitely not our intent. It is a review paper and some of the previous research (including ours for reasons explained above) do have shortcomings and they should be stated. We will carefully re-read the manuscript and constructively rewrite sections that may be too critical. We do highlight that we also struggled with species diversity: a well discussed issue with limited field site choice, limited working time at the sites within the expedition cruise industry.
Following are some specific comments.
The Abstract could be more informative. It states “insights into nutrient uptake, productivity and response to environmental change.” Of that, only environmental change (ice cover) is further mentioned in the abstract. More detail on “temporal and spatial variability” could be given. The abstract lacks actual results.Our intention was to give an overview of the review since, as pointed out, it is a long paper. We want to focus on what we see as the most important/useful outcome being the temporal link to sea ice variability that should be explored further. We will look to re-write the abstract, but we are constrained by the word limit.
Introduction
39 – Macroalgal (not algae) (adjective) – this is a problem throughout the manuscriptMinor grammatical error – we can change to macroalgal – will check throughout.
and 41: 7.22 x106km2 and C/year – decide on a format of superscript or not
Corrected.
43 – macroalgae is plural – macroalgae contribute (no s).
Corrected.
47 – comma after Southern Ocean
Corrected.
59 – how is sea ice related if not indirectly through light, which is then mentioned separately. Clarify this
Removed sentence to make this clearer.
65 – add Amsler et al. 2023 as an important recent paper that addresses this issue:
Amsler, C.D., Amsler, M.O., Klein, A.G., Galloway, A.W., Iken, K., McClintock, J.B., Heiser, S., Lowe, A.T., Schram, J.B. and Whippo, R., 2023. Strong correlations of sea ice cover with macroalgal cover along the Antarctic Peninsula: Ramifications for present and future benthic communities. Elem Sci Anth, 11(1), p.00020.Amsler et al., 2023 is cited on Line 66 following this sentence, but we can also cite it for that previous sentence too.
70 – Iken et al. 2023 does not address macroalgal cover but macroalgal stable isotope values. Change this to Amsler et al. 2023
Corrected.
82 – add some relevant references to the ecology of P. decipiens, such as (and others):
Wiencke, C. and Tom Dieck, I., 1989. Temperature requirements for growth and temperature tolerance of macroalgae endemic to the Antarctic region. Marine Ecology Progress Series, pp.189-197. Becker, S., Walter, B. and Bischof, K., 2009. Freezing tolerance and photosynthetic performance of polar seaweeds at low temperatures.We disagree. P. decipiens is a minor component in this compilation and review and thus should not be discussed in more detail than others.
90 – cite some of the original early literature for this well-studied system, for example:
Klöser, H., Ferreyra, G., Schloss, I., Mercuri, G., Laturnus, F. and Curtosi, A., 1993. Seasonal variation of algal growth conditions in sheltered Antarctic bays: the example of Potter Cove (King George Island, South Shetlands). Journal of Marine Systems, 4(4), pp.289-301.We missed this paper, we can add this to the introduction alongside many early literature that has already been cited.
91 – What is SSI?
South Shetland Islands – stated in Line 64 “South Shetland Islands (SSI)”. SSI is used hereafter.
118 – data is plural (were) – this is used incorrectly throughout the entire manuscript
Corrected.
121 – the publication by Weykam is also biochemically important, providing C:N ratio:
Weykam, G., Gómez, I., Wiencke, C., Iken, K. and Klöser, H., 1996. Photosynthetic characteristics and C:N ratios of macroalgae from King George Island (Antarctica). Journal of Experimental Marine Biology and Ecology, 204(1-2), pp.1-22.Yes, we agree – hence why it is cited throughout the manuscript and included in Figure 2.
123 – is a figure on the increase of published information really useful for anything? Delete
Figures showing publication output across time are common for a review paper, it clearly shows the increase in publication output as well as the type of biogeochemical data collected. Figure 2 allows for a visual representation of publication output, rather than a lengthy description that would add to an already “lengthy manuscript”. We find this useful and would keep in the revised manuscript.
129 – In addition to Iken et al. 2023, which provides data from a few species from a large spatial range, also see Whippo et al. 2024 from that same survey, which provides isotope and CN data from a larger number of species from individual locations. This paper is also missing from Figure 2. Whippo, R., Iken, K., Amsler, C.D., Lowe, A.T., Schram, J.B., Klein, A.G., Heiser, S., Amsler, M.O., McClintock, J.B. and Galloway, A.W., 2024. Fatty acid profiles and stable isotope composition of Antarctic macroalgae: a baseline for a combined biomarker approach in food web studies. Polar Biology, 47(4), pp.367-386.
Whippo et al., 2024 used the field data collected by Iken et al., 2023 – this has been cited and included in the compilation. It would be wrong to include that data again. In addition, mentioned Oswalt et al., 2025 also use the macroalgae dataset from Iken et al., 2023, and hence not included in this database.
136 – surveyed for what? Paragraph started out with macroalgal biochemical data and isotopes, but some of the surveys mentioned here are not about macroalgae at all
The papers cited here, whilst not focusing on macroalgae, did publish either CN, δ13C or δ15N data for seaweeds from King George Island and therefore, they have been included in the data file. We highlight throughout the review that many of the papers cited are trophic ecology publications that have also generated some macroalgae biogeochemical data. These are included in the datafile, and their data incorporated into the figures and analysis.
Section 1.1. lists studies that provide data and spatial coverage but doesn’t really talk about biochemistry of macroalgae. A review should not just state what was published, but what the findings were
The review is structured to introduce the general topic and further sections go on to discuss the output of these publications. If their findings are introduced in Section 1.1 it would cause confusion and repetition if it was again brought into the later sections of the review paper. And of course it would make the manuscript even longer!
167 – this makes it sound as if these references are about Antarctic macroalgae, that > 60% of Antarctic Rhodophyta lack CCM, which they are not
Reworded to state that 60% of Rhodophyta so far analysed from the Antarctic record δ13C values indicating that they do not posses a CCM. δ13C values below −30 ‰ are indicative of passive CO2 uptake and not CCM use (see Figure 3).
169 – specify that this is about d13C values
This paragraph only discusses the δ13C isotopic system. δ13C is regularly mentioned throughout the paragraph but we will re-read and specify it accordingly where needed.
170-179 – is there an analysis that does not confound depth with species identity? If saying that there is a depth-related pattern, the same species across a depth gradient needs to be analyzed, as different species can have different causes for their isotope values than depth.
We are unclear why the reviewer has highlighted this in an introductory section. This paragraph gives an overview for the δ13C mechanism within a macroalgae context. The papers referenced here are widely cited as relating δ13C to irradiance and, therefore, depth. This sentence draws attention to experimental and field studies that have linked δ13C to irradiance and depth for several species. We are only reporting what the literature states.
182-184 – can you give examples of these factors influencing d15N values? This is very vague
The following sentences in this paragraph discuss various influences on δ15N within macroalgae. We introduce the impact of seabird guano on elevating δ15N values (Line 188) and the seasonal variation in δ15N is discussed from Line 195. Lines 182–184 introduce the background to seaweed δ15N analysis at the start of this paragraph.
180, 203-213 – explain how phytoplankton dynamics relate to macroalgal d15N. Make better connections, the entire section seems to talk about phytoplankton blooms and not macroalgae
Phytoplankton blooms cause rapid uptake of nitrogen from the available nitrogen pool. This can cause enrichment in the remaining nitrogen pool and thus seaweed, may record elevated δ15N values due to the biological pump transferring carbon and nitrogen to sedimentary deposits. This is a seasonal phenomenon and a relevant, potential factor on macroalgae δ15N values. We will revisit this paragraph and consider rewording it to make it clearer to the reader.
216 – nutrient limited environments
Changed.
226 – add the Weykam et al. 1996 paper here
Added.
235 – macroalgae were
Corrected.
248 – Was there a specific tissue portion that was consistently sampled?
Yes – can be introduced to state blade material is used.
287 – when reporting a range, use the capital delta sign, Δ
This paragraph relates to CN not δ13C or δ15N and so this is not the correct notation for CN. Δ is not used, that we are aware, for a range between values, but the discrimination between two isotopic products (e.g., Δ13CA-B = δ13CA – δ13CB): please see Sharp’s equation 2.16 and subsequent text.
289 – green and red algal CN is the same, cannot argue one being lower than the other. Look at the SD! Also, make sure you use the correct noun in the sentences. The CN values of Phaeophyta were higher, not the Phaeophyta.
Thanks. Amended this grammatical error to clarify no significant difference was evident between the phyla.
288-290 – The numbers reported for replicates do not match what the authors report as their collections, except for Chlorophyta (n=35). Rhodophyta should be n=53 not n=52 unless one plant was not measured. n=600 for Phaeophyta is incorrect, it should be n=20. What I understand from the methods, the authors collected multiple samples along the blades of Himanthothallus. It seems they treated each of those sub-samples as individual (independent) samples, which is not correct. They could take an average value per plant and then use it in the calculation of the overall mean per group. This is especially important when using the data for statistical analyses. Again, the data as they use them are not independent and are not appropriate for the statistical analyses they employ. Obviously, this also applies to the other isotope metrics.
We will double check these numbers again, to make sure n = the number of individual macroalgae specimens and not number of analyses. Our analytical approach was to take incremental sub-samples along the blade to determine if any seasonal pattern is recorded in the biogeochemical data. Although not specimen replicates the data we present highlights the individual variation withing single blades. We can highlight and amend this, but what the review paper shows it that depending on how or where past papers sampled their macroalgae this may also contribute to the significant variation recorded.
295 – why is (15.8 ‰) in parentheses?
Corrected.
305 – what does “most elevated phylum” mean?
Amended to clarify δ13C values were less negative for Rhodophyta compared to Chlorophyta and Phaeophyta.
310 – the authors say “significantly enriched”, this needs to be specified as enriched in 13C. When referring to the ratio (delta value), “enriched” is an incorrect term.
Grammatical error – has been corrected.
308 and 311 – misspelling of “utricularis”
Corrected.
304 – The authors say ”δ13C values ranged between –34.7 ‰ (H. grandifolius) and –9.8 ‰ (Iridaea sp.) for this study“ and then in Line 312 they say “H. grandifolius was the most negative (–23.4 ± 4.6 ‰, n=589)”. I don’t understand why there are two extremely different values being cited for H. grandifolius.
First is the range for all analyses and species, whereas the second relates to the average for that species. We will amend this average, since the reviewer suggested above to average each blade.
328-330 – this information does not seem relevant here. This could maybe be added to the introduction. Is there any solid reason to doubt the identification skills of these other researchers? And size is not a good reason to doubt identification accuracy. For example, the large brown algae Desmarestia anceps and D. menziesii are easily confused, while the small green alga Lambia antarctica cannot be confused with anything. If the large species are so easily identified, why did the authors not separate Desmarestia sp? To be honest, this should probably Desmarestia spp unless they can confirm they only have one of the species, and in that case, they should be able to say which one.
Many of the papers included in this compilation state “unidentified species” or just macroalgae and do not even go down to genus level. It is a common issue, and one we also encountered in many isotope-ecology papers. We even highlight such limitations in our own dataset.
335 – The authors say “Desmarestiales are key primary producers, along with H. grandifolius (> 70 data entries),…” It seems the authors are unaware that H. grandifolius is a member of the Desmaretiales. It makes me wonder how familiar the authors actually are with the macroalgal flora of the Southern Ocean.
This is a minor mistake and does not justify the tone used here by the reviewer. This error stemmed from a mistake between Desmarestia sp. versus Desmarestiales. We will correct accordingly.
339 – The authors should note that there are not that many Chlorophyta species in the Southern Ocean. Naturally, there will be fewer datapoints. Also, if they identify this as such a problem, why did they not try to collect more species and individuals? They only collected one species (Monostroma sp.). Continuing, it seems it would suffice to say that data of individuals that were not identified at least to the genus level were omitted. The authors take a very dismissive tone about other investigations, which may have their limitations but there is no need to downgrade their work.
As explained previously in this response and in the manuscript, field site choice did hamper our dataset. Also, such cruises do not permit SCUBA diving opportunities for health and safety reasons. Therefore, the species biodiversity of the dataset is somewhat limited, we can emphasise this more when revising the manuscript. In addition, the PhD is mainly focused on H. grandifolius since it is a species known to have blades that contain the record of multiple seasons. We highlight in the review that future work should consider focussing more on Chlorophyta. But, with so few analyses compared to the other phyla it is a shortcoming that should be noted in the review.
351 – why is “Coraline algae “ capitalized (here and elsewhere)? Also, not misspelling of coralline
Corrected.
365 – The authors need to adjust the treatment of their H. grandifolius data as pointed out above. These are not replicate measures, they are pseudoreplicates and not appropriate for this type of analysis as used by the authors.
Corrected.
369 – “regardless of study, site or depth” – was there an actual analysis done to test this?
Reworded.
387 – Nitrogen should not be capitalized
Corrected.
392 – revise the wording of “kelp-like H. grandifolius” It is not a kelp and calling it kelp-like can be confusing. What aspect of it is kelp like? Maybe the morphology but internal cell structure and physiology are not – hence, the comparison to actual kelps in terms of storage is flawed.
It is a large brown macroalgae that, like kelp, is the dominant seaweed in the Southern Ocean as kelp is in the Arctic. Comparisons have been made as, isotopically, there is more data regarding kelps and so this was deemed an appropriate comparison. We have removed kelp-like from the manuscript.
406 – “significantly higher nitrogen content for Rhodophyta” – be specific. It was only higher than Phaeophyta, not Chlorophyta
Corrected.
452 – add one or both of the recent Amsler et al. 2023 papers that suggest exactly this. These authors are not the first ones to suggest this. They already cite the Amsler et al. 2023 paper in Elementa but they are missing the following paper: Amsler, C.D., Amsler, M.O., Heiser, S., McClintock, J.B., Iken, K., Galloway, A.W. and Klein, A.G., 2024. Vertical distribution of brown and red macroalgae along the central Western Antarctic Peninsula. Botanica Marina, 67(1), pp.1-10.
Paper now included, thank you for drawing out attention to this paper which we missed.
490 – no, Chlorophyta have less negative (not more positive) d13C values. None of these values are positive.
Apologies – corrected.
505 – the same issue as noted for Line 335
Changed to Desmarestia sp. and H. grandifolius.
509 – please provide a reference for this statement
The database compiled notes many Chlorophyta are listed as “Unidentified” or not to species level. We will amend accordingly.
518 – please provide examples for the misidentifications that have occurred from Chlorophyta
It is in the database where it states, “Unidentified Species”. We can only go on what the papers report. We will change the wording to make it clear that species was “unidentified” and not “misidentified”.
523 – please add ”anomalously low nitrate δ15N values” as this statement would otherwise refer to macroalgae and the cited study does not investigate macroalgae
Corrected.
532-534 – Are those data anywhere? And what does “depleting trend” mean? If not, please provide a reference for where the data can be found.
The papers cited include this data for various Laminariales, both Bebb et al., 2023 and Dyer et al., 2019 conducted δ13C and δ15N analysis of brown seaweeds to investigate internal variation. There is very limited research on this aspect of macroalgae, especially within the Southern Ocean. A more thorough analysis of the incremental sampling and biogeochemical results and interpretation are intended for another manuscript that just focuses on this. This review was focused on what is available through time, whereas the other planned manuscript will look at seasonal/sub-seasonal interpretations.
547 – probably more important than cruise ship wastewater is the vicinity of coastal penguin rookeries, which should at least be mentioned.
As discussed, δ15N values weren’t that positive and sub-tidal specimens didn’t really reflect any influence of penguin guano. This is surprising to us, but again was not the focus of our study.
587 – change to “the future response”
Corrected.
604/05 – provide a reference for this statement
This sentence was meant in terms of depth gradients and inter-tidal environments have less negative δ13C as species rely on CCMs. We will clarify this paragraph and include references.
610 – the authors are clearly not familiar with the isotope language. “depleted δ13C values” do not exist – a ratio cannot be depleted.
Corrected.
618 – awkward phrasing “species that’s blades”, replace with which or whose
Corrected.
619 – 15 ‰ difference in which isotope?
The paragraph is clearly about δ13C. Section 3.4.2 δ13C – a biological proxy for primary production in macroalgae. We will reword to include δ13C in case the reader has forgotten.
625-628 – interesting how the authors berate other studies form not providing information on the exact thallus structure when they do not seem to provide this information for their own data either.
Our H. grandifolius data does provide incremental sub-sample data for this species, however due to the length of the manuscript this has been omitted to keep to a more succinct structure. We will highlight this and indicate that a more detailed paper on our H. grandifolius data from December 2024 and January–February 2026 should be published soon. We also note that this review paper aims to highlight areas for further research, this lack of data on morphological biogeochemical differences is, we believe, an important issue that requires additional research: as already indicated by Bebb et al., 2023 and Dyer et al., 2019.
635 – is comparing isotope values of different groups for individual sites useful? It has been well established that groups can differ. The author themselves brought up reasons for why that is the case. Also, Fig. 8-10 are too coarse to be able to meaningfully compare specific sites across the three figures. Any spatial trends cannot be gleaned from these figures. What is the hypothesis? If there is a latitudinal trend? If so, use a regression analysis. Just plotting points on a map and squinting at it is not an appropriate analysis. Much of the text in this paragraph (3.4.3) is redundant description of ranges and can be eliminated.
These spatial figures show data collected from a range of species, depths and years therefore we decided against statistical analyses as there are too many environmental and collecting variables in this compilation: thus, we favoured the production of a visual map to show variation but also to show spatial nature of the dataset. Upon revision we will look to reduce the spatial discussion section of the manuscript but highlight that the spatial coverage is biased towards research stations. Although broad latitudinal trends may be captured with a dedicated research program (e.g., Iken et al., 2023; Cardona et al., 2019) the additional sampling at IAATO cruise sites could fill in spatial gaps. We would also like to stress the spatial variation has wider implications for trophic ecological studies and there is considerable variation even between sites that are close together. We will amend accordingly with a revision.
637 – Fig. 8 does not indicate the location of Yankee Harbour
Upon revision this section would be cut down, if still required Yankee Harbour can be added to the map. It is an island in the South Shetland Islands commonly visited by IAATO cruise vessels with a large Gentoo penguin colony.
644 – Not “Nitrogen” but “Nitrogen isotope ratios”
Corrected.
656-659 – the references for this statement do not seem to refer to Antarctic macroalgae; this should be made clear as growth rates may act under different constraints in Antarctic macroalgae. For example, the following statement (659-660) is likely only true if nitrogen is limiting.
Unfortunately, there are no such references for Antarctic macroalgae that correlate growth rates, nutrient turnover and biogeochemical data. We will clarify these references upon revision and also include it as another direction for future experimental work.
666 – see comment above. If a latitudinal trend is to be tested, use a regression analysis.
We have conducted a test of the biogeochemical data versus latitude and will present in a revised version: these graphs/figures will be in the supplementary file and only the results of the regression present in the paper.
668 – Fig. 5 does not show locations
Corrected.
674 – you cannot talk about a “strength” of a relationship without running an actual analysis
We will revise this – but we will also state our reservations regarding this due to the many variables of the review dataset previously discussed and highlighted.
698 – it’s unclear what all this has to do with any observed temporal trends in isotope values; this is about potential future expansions in habitat
A revised manuscript would have a separate future work and wider implications section to improve clarification and structure.
717 – again, instead of a solid analysis, the authors just offer an image and leave it to the reader to take away a vague sentiment of variability. There is high variability in isotope values but the authors claim that these values track variation in sea ice cover. You should consider similar approaches as in Amsler et al. 2023, who did an actual analysis of algal cover in relation to sea ice concentrations. A similar approach could be developed for macroalgal stable isotopes. Also, the work of Iken et al. 2023 investigating latitudinal trends in stable isotope values should be included in this discussion.
We propose a tentative, unexplored biogeochemical link to historical sea-ice cover and subsequent isotopic values. These studies included are largely trophic ecological studies that were not intended for this purpose. Therefore, variability is to be expected but the variation observed here would benefit from future investigation. We do agree, a similar approach could be developed for macroalgae isotopes as with Amsler et al. (2023), however, this was beyond the scope of the PhD since it was relying on “opportunistic” cruise sites. We have just recently had discussions with sea-ice experts at the UK Antarctic Science Conference held at Durham and so plan to make some amendments to the sea-ice discussion. Notwithstanding, our tentative link between macroalgae δ13C and sea-ice remains.
Citation: https://doi.org/10.5194/egusphere-2026-4443-AC1
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CC1: 'Authors Reply to RC1', Darren R. Gröcke, 10 Sep 2026
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RC2: 'Comment on egusphere-2026-4443', Anonymous Referee #2, 17 Sep 2026
I read with interest the manuscript by Alldred and Gröcke regarding carbon and nitrogen stable isotope patterns, as well as C:N ratios, in Antarctic macroalgae, utilizing both existing database information and new data collected during a tourist cruise along the Antarctic Peninsula.
After an initial reading, I realized that this manuscript offers an interesting opportunity to revisit a relevant yet understudied topic concerning a group of organisms that underpin coastal biogeochemical processes across Antarctica; furthermore, it provides information that could help us understand the fate of these communities in the context of climate change in the region.
However, the manuscript shows various shortcomings that make it difficult to read (for instance, it is overly long and often redundant), and, also, I believe the rationale for combining existing data with new data was not clearly articulated. The editor-in-chief will likely need to define the type of research article into which this study fits.
Another aspect that remains unclear to me is that, while the study is purely descriptive regarding general trends and data groupings, the attempts to establish causality, or at least link the findings to environmental variability, are not well resolved. An effort is made to relate the data to sea-ice variability; however, a major issue arises here: much of this variability cannot be explained without specific data sources and the local effects of sea ice. A review of the literature and the authors' own survey reveals that most of sampling for isotopic data was conducted from spring onwards—a period when the impact of sea ice is significantly reduced.
I would like to detail a few aspects of the manuscript that, in my opinion, could be revised. The manuscript is quite long, so I do not address every aspect in my assessment; I believe these points will be complemented by the comments from the other referees.
Title:
I do not believe this title accurately reflects the manuscript's content and focus. It is clearly not a synthesis, nor does it shed light on environmental impacts. While "review" is somewhat closer, there are areas where the literature review or meta-analysis was less than exhaustive.
Abstract:
Lines 16–18: I believe the authors are attempting to highlight the significance of their survey as a “before and after” in the knowledge of the biogeochemistry of marine macroalgae, however, previous studies have already made significant progress in our understanding of stable isotope variability beyond the scope of trophic ecology. It is also striking that they note existing knowledge is heavily biased toward the WAP, given that their own survey was also conducted in that same region.
Lines 27–31: I believe that establishing a relationship between sea ice variability and δ13C is highly speculative, especially considering the limited statistical analysis performed.
Lines 30–36: What is certainly clear from this manuscript is that much more research is needed on this topic, and that a study of this nature serves as a starting point for further exploring the implications of the environmental changes we are witnessing.
Introduction:
I find it overly long; the opening paragraphs attempt to contextualize macroalgae and their role in Antarctic systems. This has already been well summarized in previous reviews, which should be cited (e.g., Antarctic Seaweeds, Springer, 2020), rather than attempting to cover it with single citations.
Line 98: I believe the introduction should begin here.
Lines 119–120: It might be better to say "40 years." I also think the section dedicated to the history of biogeochemical studies on Antarctic macroalgae should be better organized, following a chronological sequence. In many cases, the text jumps back and forth between current and older studies. On the other hand, I do not think paragraphs 148–156 add much to this description of the evolution of these studies.
Lines 120-121: It is worth reiterating here that broad generalizations, such as describing the work as "primarily focused on taxonomy, distribution, and life cycles of species", can be misleading; a significant portion of the studies actually addressed the distribution and ecology of these organisms, aspects not fully captured by the cited references. Therefore, I suggest that in such instances, authors cite a comprehensive overview from other authors (e.g., by Amsler, Wiencke, or Gómez) that details the history of macroalgal research, rather than listing individual papers that fail to adequately illustrate the point. By the way, Himantothallus belong to the order Desmarestiales.
Lines 180–213. This section focuses on the δ15N signature, including numerous references to phytoplankton but very few to macroalgae. This is likely due to the limited number of available studies; however, I noted that one of the few papers reporting δ15N values in Antarctic macroalgae (Fernandez et al., 2024) is not mentioned in this subsection.
Lines 214–232. This section presents findings regarding C/N ratios in a somewhat confusing manner, jumping between various factors of variability. Furthermore, regarding the relationship with carbon metabolism and macroalgal structure, I suggest reviewing some citations that cover this topic in greater depth. In general, I agree with the authors that there is limited information on the morpho-functional aspects of C/N ratios and isotopes, but some does exist (see: https://link.springer.com/chapter/10.1007/978-3-642-28451-9_2).
Lines 130–131: It is unclear here which biogeochemical data the authors are referring to. It would be advisable to specify this, as the term is very broad.
Lines 133–135: Not only for this reason, but also because most seaweed species along the WAP can be found and accessed. Regarding the warming of this region, I do not believe the cited papers are the most relevant for supporting such a broad assertion.
Lines 235–247: Sampling. To establish sampling parameters, it would be relevant for the authors to report the conditions under which the algae were collected—for example, in the case of intertidal algae, whether collection occurred at low tide and up to which zone (supralittoral?); and for subtidal algae, why the sampling started at 5 m. A large number of species grow between 0 and 5 meters in Antarctica, particularly species that regrow rapidly once the ice disappears in spring. Finally, upon reviewing the metadata and the text in lines 279–286, it is unclear to me how many intertidal species were collected. Apparently, there were five, whereas Himanthothallus grandifolius was the only subtidal species. I realized that the Desmarestia specimens were not identified to the species level; this is unusual, given that the four common species in this region are morphologically very distinct. Considering that authors indicated in the introduction, the number of species analyzed (7) seems very low to me. In this regard, I would specifically like to comment on the authors' statement in lines 285–286: "The data generated from this survey provides more biogeochemical data than the 29 papers included in our literature analysis combined." I believe the tone should be more measured, especially considering that, collectively, the previous studies cover a much larger number of species than those collected during this expedition. Clearly, having more analyses is not the same as analyzing more species.
Regarding the statistical analysis of the data, I believe it is essential to specify and detail the methods used, as parametric (e.g., ANOVA) and non-parametric (Kruskal-Wallis, Dunn) methods are mentioned throughout the Results and Discussion section. Furthermore, it would be very useful to include the statistical results in supplementary tables.
Results and Discussion: I found the structure of this section somewhat confusing, as it incorporates explanatory elements regarding sampling and data summaries.
Lines 287–313: As I understood, the data collected during the field campaign and the database are described separately; however, the database summary includes references to and comparisons between both types of results. It might be beneficial to condense this section by merging the descriptions and highlighting only the similarities and differences.
Lines 328–345: I do not understand this part. What is its relevance to the results? The authors appear to be conflating the dominance of macroalgal groups with the ease of sampling them. In general, intertidal algae in Antarctica are far easier to collect than subtidal algae! The reality is simply that there are far fewer species of green algae in Antarctica compared to brown or red algae. Perhaps these clarifications should have been made in the methodology section. Moreover, Phaeophyta (brown algae) is not recognized as a phylum; this should be changed to Phaeophyceae, as brown algae belong to the phylum Ochrophyta.
Lines 385-398: This analysis lacks structure. There are multiple reasons why algae collected from different sites and under different conditions vary in their C and N content. This will inevitably be reflected in the patterns reported in both the databases and the data from the current study. For instance, a topic insufficiently analyzed in this paper are the endogenous factors, such as growth patterns and the accumulation of carbon-rich anti-stress compounds. The paper would benefit from a more structured analysis rather than simply listing various potential factors in a scattered manner.
Lines 458-459: Snow algae are not macroalgae!
Lines 466-467: Do these averages include snow algae?
Lines 431-447: These types of explanations are not yet fully developed. For example, Cystosphaera is unrelated to the Desmarestiales. Furthermore, the multiple factors potentially explaining the variability are merged, yet none points to what actually might have influenced the data.
Lines 591-594: I think this discussion could be complemented by findings showing that Antarctic macroalgae, especially Phaeophyceae, utilize β-carboxylation as an anaplerotic pathway to support growth under low-light and low-temperature conditions. There is abundant literature on this subject.
Lines 605-607: However, depth is highly significant for C acquisition and carbon balance. Moreover, in the paper by Zenteno et al., the observed changes in δ13C with depth were due to the fact that the different sampled species originated from different depths; the isotopic signature did not vary with depth within a single species.
Line 615: Yes, the authors are right; however, some grey literature exists regarding internal processes in Antarctic macroalgae. Please revise the links:
https://epic.awi.de/id/eprint/26416/1/BerPolarforsch1997238.pdf (Page 70)
https://www.scielo.cl/pdf/rchnat/v74n2/art04.pdf
Geospatial variation. Here, the authors could summarize their trends more concisely rather than outlining explanations regarding environmental gradients in this region that are not based on hard data. Although the paper incorporates this into its framework, it does not provide consistent information on environmental variability (past, present, or future).
Lines 685-692: Not always. In many polar algae, carbon demands for growth intensify during periods of low light in winter or late winter/spring.
The discussion might benefit from the concept of carbon balance, which is strongly determined by available daylight. Algae collected in spring/summer were likely undergoing active growth, resulting in increased respiration.
Lines 745-746: ..and in Antarctic waters?
Lines 759-762: I agree; however, a more detailed examination of older papers could help explain some of the patterns observed here.
Minor queries:
Although I am not a native speaker, I believe the text requires revision to correct spelling and other errors.
Fig. 3. This seems interesting for defining thresholds. Why wasn't a similar approach taken for δ15N?
Reference list. This section needs improvement. It lacks a standard format, for example, regarding the citation style for books and book chapters, format of journal names, misspelled author names, etc.
Citation: https://doi.org/10.5194/egusphere-2026-4443-RC2 -
CC2: 'Reply on RC2', Darren R. Gröcke, 01 Oct 2026
We thank the anonymous reviewer for their comments, which we will address below.
Reviewer 2 – 17th September 2026 comments
I read with interest the manuscript by Alldred and Gröcke regarding carbon and nitrogen stable isotope patterns, as well as C:N ratios, in Antarctic macroalgae, utilizing both existing database information and new data collected during a tourist cruise along the Antarctic Peninsula.
After an initial reading, I realized that this manuscript offers an interesting opportunity to revisit a relevant yet understudied topic concerning a group of organisms that underpin coastal biogeochemical processes across Antarctica; furthermore, it provides information that could help us understand the fate of these communities in the context of climate change in the region.
However, the manuscript shows various shortcomings that make it difficult to read (for instance, it is overly long and often redundant), and, also, I believe the rationale for combining existing data with new data was not clearly articulated. The editor-in-chief will likely need to define the type of research article into which this study fits.
This submission was previously shared with several senior Biogeosciences editors who agreed it would be a solid submission to the journal and that the review and data analysis was appropriate. Hence why it is submitted and considered a Review and Synthesis. We would argue that this data is worth publishing considering the scarcity of seaweed isotopic data in the Antarctic, the lack of spatial sites and that the co-operation between tourist cruises and scientific parties can work together to assess a greater area of the polar environment: and not just at research stations! There are of course some pit-falls in this approach which will be discussed in a revised version and considered when analysing the data.
Another aspect that remains unclear to me is that, while the study is purely descriptive regarding general trends and data groupings, the attempts to establish causality, or at least link the findings to environmental variability, are not well resolved. An effort is made to relate the data to sea-ice variability; however, a major issue arises here: much of this variability cannot be explained without specific data sources and the local effects of sea ice. A review of the literature and the authors' own survey reveals that most of sampling for isotopic data was conducted from spring onwards—a period when the impact of sea ice is significantly reduced.
I would like to detail a few aspects of the manuscript that, in my opinion, could be revised. The manuscript is quite long, so I do not address every aspect in my assessment; I believe these points will be complemented by the comments from the other referees.
Title:
I do not believe this title accurately reflects the manuscript's content and focus. It is clearly not a synthesis, nor does it shed light on environmental impacts. While "review" is somewhat closer, there are areas where the literature review or meta-analysis was less than exhaustive.
Reviews and Syntheses is required by the journal Biogeosciences for all review manuscripts. Upon prior discussion with senior editors regarding how best to frame this manuscript it was decided that it would be labelled a review manuscript.
Abstract:
Lines 16–18: I believe the authors are attempting to highlight the significance of their survey as a “before and after” in the knowledge of the biogeochemistry of marine macroalgae, however, previous studies have already made significant progress in our understanding of stable isotope variability beyond the scope of trophic ecology. It is also striking that they note existing knowledge is heavily biased toward the WAP, given that their own survey was also conducted in that same region.
We acknowledge that our own survey is also limited to the peninsula, we will revise the manuscript to draw further attention to this. We do, however, provide new data for sites with no prior data in the Gerlache Strait region that has largely been ignored. We acknowledge the significant contribution of prior studies and hope that this collation of the available data is of interest and/or generates further research. We will revise the abstract to highlight the work that has previously been done.
Lines 27–31: I believe that establishing a relationship between sea ice variability and δ13C is highly speculative, especially considering the limited statistical analysis performed.
Statistical analyses are difficult to undertake for this record, due to the limited data collected over multiple decades and different geospatial areas. Although our ideas are speculative, we propose a potential relationship that would require further dedicated research. Since 2016 sea ice has shown a marked decline, what this means for the benthic community is uncertain and isotope analysis of macroalgae could be one avenue for assessing primary productivity. We will reword our discussion of the potential link to sea ice and highlight the speculative nature of this assessment, but we argue it is something worth pursuing.
Lines 30–36: What is certainly clear from this manuscript is that much more research is needed on this topic, and that a study of this nature serves as a starting point for further exploring the implications of the environmental changes we are witnessing.
We will revise the abstract to emphasise this point, the main issue with this dataset is the sporadic and limited spatial distribution of the data. We hope the outcome from this work is a renewed interest in macroalgae and its importance in the wider Southern Ocean ecosystem.
Introduction:
I find it overly long; the opening paragraphs attempt to contextualize macroalgae and their role in Antarctic systems. This has already been well summarized in previous reviews, which should be cited (e.g., Antarctic Seaweeds, Springer, 2020), rather than attempting to cover it with single citations.
We can condense the introduction further; this was also brought up by reviewer 1, but both reviewers do ask for further explanation on various topics – thus, probably not reducing the length of the paper overall. It should be noted however that review papers are generally not short!
Line 98: I believe the introduction should begin here.
We would argue that some of the points prior to Line 98 are necessary and provide an overview of the broader ecosystem. We will reconsider this upon revision.
Lines 119–120: It might be better to say "40 years." I also think the section dedicated to the history of biogeochemical studies on Antarctic macroalgae should be better organized, following a chronological sequence. In many cases, the text jumps back and forth between current and older studies. On the other hand, I do not think paragraphs 148–156 add much to this description of the evolution of these studies.
Noted – can be corrected to 40-years. The start of this paragraph was intended to give a brief timeline of publications (Lines 120–132), before going into further detail with trends over the last 40-years in terms of research focus and their geospatial coverage. We can consider shortening the second paragraph, but we feel it is important to explain that there are few original publications and that many subsequent publications use the previous data.
Lines 120-121: It is worth reiterating here that broad generalizations, such as describing the work as "primarily focused on taxonomy, distribution, and life cycles of species", can be misleading; a significant portion of the studies actually addressed the distribution and ecology of these organisms, aspects not fully captured by the cited references. Therefore, I suggest that in such instances, authors cite a comprehensive overview from other authors (e.g., by Amsler, Wiencke, or Gómez) that details the history of macroalgal research, rather than listing individual papers that fail to adequately illustrate the point. By the way, Himantothallus belong to the order Desmarestiales.
This typo has been amended to Desmaresitia sp. and H. grandifolius.
Lines 180–213. This section focuses on the δ15N signature, including numerous references to phytoplankton but very few to macroalgae. This is likely due to the limited number of available studies; however, I noted that one of the few papers reporting δ15N values in Antarctic macroalgae (Fernandez et al., 2024) is not mentioned in this subsection.
Since macroalgae reflect the marine δ15N signature we felt it necessary to provide background into the Southern Ocean δ15N system that is influenced by the spring phytoplankton bloom. There is little information regarding isotopic fractionation by Antarctic macroalgae species, nor are assimilation rates quantified. We can restructure this paragraph to make clearer the link between the Southern Ocean δ15N signature, variation due to phytoplankton blooms and subsequent influence on macroalgal δ15N values. Fernandez et al (2024) is cited throughout, but focusses on carbon uptake strategies on Antarctic macroalgae, even though they present both δ13C and δ15N.
Lines 214–232. This section presents findings regarding C/N ratios in a somewhat confusing manner, jumping between various factors of variability. Furthermore, regarding the relationship with carbon metabolism and macroalgal structure, I suggest reviewing some citations that cover this topic in greater depth. In general, I agree with the authors that there is limited information on the morpho-functional aspects of C/N ratios and isotopes, but some does exist (see: https://link.springer.com/chapter/10.1007/978-3-642-28451-9_2).
We thank reviewer 2 for the addition of this reference and will consider restructuring this paragraph to have an improved flow. We wanted this paragraph to be brief as the focus of this manuscript is primarily the δ13C and δ15N data, but CN is widely reported alongside stable isotopes and so necessary to include.
Lines 130–131: It is unclear here which biogeochemical data the authors are referring to. It would be advisable to specify this, as the term is very broad.
Both δ13C and δ15N data is reported by Iken et al. (2023), although the focus is more towards the carbon. This can be amended to clarify the meaning.
Lines 133–135: Not only for this reason, but also because most seaweed species along the WAP can be found and accessed. Regarding the warming of this region, I do not believe the cited papers are the most relevant for supporting such a broad assertion.
We do discuss accessibility of the WAP, something that also impacts our own dataset as well as the greater biodiversity found here which makes it understandable that most studies focus on this region. We can reword to make this point clearer and add further references supporting the statements.
Lines 235–247: Sampling. To establish sampling parameters, it would be relevant for the authors to report the conditions under which the algae were collected—for example, in the case of intertidal algae, whether collection occurred at low tide and up to which zone (supralittoral?); and for subtidal algae, why the sampling started at 5 m. A large number of species grow between 0 and 5 meters in Antarctica, particularly species that regrow rapidly once the ice disappears in spring. Finally, upon reviewing the metadata and the text in lines 279–286, it is unclear to me how many intertidal species were collected. Apparently, there were five, whereas Himanthothallus grandifolius was the only subtidal species. I realized that the Desmarestia specimens were not identified to the species level; this is unusual, given that the four common species in this region are morphologically very distinct. Considering that authors indicated in the introduction, the number of species analyzed (7) seems very low to me. In this regard, I would specifically like to comment on the authors' statement in lines 285–286: "The data generated from this survey provides more biogeochemical data than the 29 papers included in our literature analysis combined." I believe the tone should be more measured, especially considering that, collectively, the previous studies cover a much larger number of species than those collected during this expedition. Clearly, having more analyses is not the same as analyzing more species.
The dataset was collected as an opportunistic field season in collaboration with Viking, an expedition cruise operator. Therefore, limited sampling time and constraints on site selection hindered sampling and only the seven species mentioned could be targeted. Sampling occurred at low tide at IAATO landing sites. The species H. grandifolius was initially targeted as a separate project related to assessing intra-specimen variability in blades that do not shed annually, hence the high number of analyses, but that detailed dataset in intended for discussion in a more detailed blade-by-blade analysis. A second field season successfully collected more of this species and a separate publication for this is in progress. We will restructure this section to clarify the sampling protocol and our meaning. Desmarestia sp. was opportunistically collected, and further identification was not possible at the time.
Regarding the statistical analysis of the data, I believe it is essential to specify and detail the methods used, as parametric (e.g., ANOVA) and non-parametric (Kruskal-Wallis, Dunn) methods are mentioned throughout the Results and Discussion section. Furthermore, it would be very useful to include the statistical results in supplementary tables.
All statistical tests are described in the results section, especially with regards to Figure 4. The difference in data collection between publications and decades make it statistical analysis somewhat difficult and subjective. We can revisit the dataset and determine whether further analyses are appropriate here.
Results and Discussion: I found the structure of this section somewhat confusing, as it incorporates explanatory elements regarding sampling and data summaries.
Lines 287–313: As I understood, the data collected during the field campaign and the database are described separately; however, the database summary includes references to and comparisons between both types of results. It might be beneficial to condense this section by merging the descriptions and highlighting only the similarities and differences.
We will can reconsider how the datasets are incorporated into the manuscript, and condensing this section.
Lines 328–345: I do not understand this part. What is its relevance to the results? The authors appear to be conflating the dominance of macroalgal groups with the ease of sampling them. In general, intertidal algae in Antarctica are far easier to collect than subtidal algae! The reality is simply that there are far fewer species of green algae in Antarctica compared to brown or red algae. Perhaps these clarifications should have been made in the methodology section. Moreover, Phaeophyta (brown algae) is not recognized as a phylum; this should be changed to Phaeophyceae, as brown algae belong to the phylum Ochrophyta.
Upon reflection we agree it is overly wordy and unnecessary to the wider manuscript. We can change Phaeophyta to Ochrophyta, this term has changed between many publications, and we agree the consensus is now Ochrophyta.
Lines 385-398: This analysis lacks structure. There are multiple reasons why algae collected from different sites and under different conditions vary in their C and N content. This will inevitably be reflected in the patterns reported in both the databases and the data from the current study. For instance, a topic insufficiently analyzed in this paper are the endogenous factors, such as growth patterns and the accumulation of carbon-rich anti-stress compounds. The paper would benefit from a more structured analysis rather than simply listing various potential factors in a scattered manner.
We will consider restructuring the Results & Discussion into a more succinct style, we aimed to separate the CN, δ15N and δ13C datasets as well as our own data with the wider database. With this being a review with new data it was difficult to determine how best to merge the two together.
Lines 458-459: Snow algae are not macroalgae!
We agree. We will reword this section to focus just on Chlorophyta but highlight some publications have wrongly included Snow Algae in their datasets.
Lines 466-467: Do these averages include snow algae?
No
Lines 431-447: These types of explanations are not yet fully developed. For example, Cystosphaera is unrelated to the Desmarestiales. Furthermore, the multiple factors potentially explaining the variability are merged, yet none points to what actually might have influenced the data.
We can rework the CN section to focus more on influences of the data. Lines 437 to 447, however, focus on the incomplete sample set and with the low species diversity of the dataset we felt it more appropriate to discuss the phyla differences and context within the global CN average. We can incorporate further ecophysiological discussion points to this section.
Lines 591-594: I think this discussion could be complemented by findings showing that Antarctic macroalgae, especially Phaeophyceae, utilize β-carboxylation as an anaplerotic pathway to support growth under low-light and low-temperature conditions. There is abundant literature on this subject.
In our research collating the Antarctic biogeochemical literature we did not come across these terms; we will follow up on this and include this literature where required. There is very little data available for Antarctic seaweed species under low-light conditions. Primarily data was collected from November to February, this is an issue that we can highlight in the manuscript.
Lines 605-607: However, depth is highly significant for C acquisition and carbon balance. Moreover, in the paper by Zenteno et al., the observed changes in δ13C with depth were due to the fact that the different sampled species originated from different depths; the isotopic signature did not vary with depth within a single species.
Thank you for the clarification we will reword this section to make this clearer.
Line 615: Yes, the authors are right; however, some grey literature exists regarding internal processes in Antarctic macroalgae. Please revise the links:
https://epic.awi.de/id/eprint/26416/1/BerPolarforsch1997238.pdf (Page 70)
https://www.scielo.cl/pdf/rchnat/v74n2/art04.pdf
We would like to thank Reviewer 2 for these publications and will review and include them where necessary in the discussion. We are currently working on another publication discussion seasonal C content and photosynthetic activity in relation to daylength for H. grandifolius stable isotope variation across the thallus.
Geospatial variation. Here, the authors could summarize their trends more concisely rather than outlining explanations regarding environmental gradients in this region that are not based on hard data. Although the paper incorporates this into its framework, it does not provide consistent information on environmental variability (past, present, or future).
In line with both Reviewer 1 and 2’s comment we will amend the spatial discussion significantly upon revision to condense this section considerably. Difficulties arise considering different sites were targeted in different years, depths and not all species were analysed from all sites. Therefore, statistical analysis of the spatial trends need to be carefully considered regarding this variability in the data collection. We consider the impact of climate change on Antarctic seaweed presented in the dataset as minimal. Although this may be inaccurate there is limited information collected on climate/environmental parameters at the time of those collections in those specific areas. Hence, this is why we compare the data generally to the sea ice record that in part reflects climate change.
Lines 685-692: Not always. In many polar algae, carbon demands for growth intensify during periods of low light in winter or late winter/spring.
Thank you for the clarification we have amended this discussion point and found the above publications useful in this regard.
The discussion might benefit from the concept of carbon balance, which is strongly determined by available daylight. Algae collected in spring/summer were likely undergoing active growth, resulting in increased respiration.
Where possible we will revaluate the dataset to consider carbon balance and aim to correspond this with time of collection. We will, of course, reframe the discussion to bring up carbon balance where appropriate and would fit with the wider CN discussion.
Lines 745-746: ..and in Antarctic waters?
Amended to clarify the high N content of polar seaweeds is a result of the non-nitrogen limiting environment of the Southern Ocean.
Lines 759-762: I agree; however, a more detailed examination of older papers could help explain some of the patterns observed here.
We will incorporate the above publications where necessary into the introduction and discussion sections, we hope to condense the review where possible under both Reviewer 1 and 2’s suggestions that the manuscript is too long.
Minor queries:
Although I am not a native speaker, I believe the text requires revision to correct spelling and other errors.
We will revise the manuscript and improve grammatical errors carefully upon resubmission
Fig. 3. This seems interesting for defining thresholds. Why wasn't a similar approach taken for δ15N?
An initial draft for Figure 3 did include a schematic for δ15N values for nitrogen sources to the Southern Ocean, but these are wide ranging in their values and specific to individual studies and sites. Carbon, however, is strongly linked to the photosynthetic mechanism and lends itself to a simplistic schematic such as Figure 3. We could include the nitrogen schematic but again this would increase the length of the manuscript!
Reference list. This section needs improvement. It lacks a standard format, for example, regarding the citation style for books and book chapters, format of journal names, misspelled author names, etc.
The reference manager Mendeley was used for referencing; this was set to the style for Biogeosciences. We will check all references to ensure they are in the correct style.
Citation: https://doi.org/10.5194/egusphere-2026-4443-CC2 -
AC2: 'CC1 Reply to R2 (reuploaded as AC)', Freya Alldred, 06 Oct 2026
We thank the anonymous reviewer for their comments, which we will address below.
Reviewer 2 – 17th September 2026 comments
I read with interest the manuscript by Alldred and Gröcke regarding carbon and nitrogen stable isotope patterns, as well as C:N ratios, in Antarctic macroalgae, utilizing both existing database information and new data collected during a tourist cruise along the Antarctic Peninsula.
After an initial reading, I realized that this manuscript offers an interesting opportunity to revisit a relevant yet understudied topic concerning a group of organisms that underpin coastal biogeochemical processes across Antarctica; furthermore, it provides information that could help us understand the fate of these communities in the context of climate change in the region.
However, the manuscript shows various shortcomings that make it difficult to read (for instance, it is overly long and often redundant), and, also, I believe the rationale for combining existing data with new data was not clearly articulated. The editor-in-chief will likely need to define the type of research article into which this study fits.
This submission was previously shared with several senior Biogeosciences editors who agreed it would be a solid submission to the journal and that the review and data analysis was appropriate. Hence why it is submitted and considered a Review and Synthesis. We would argue that this data is worth publishing considering the scarcity of seaweed isotopic data in the Antarctic, the lack of spatial sites and that the co-operation between tourist cruises and scientific parties can work together to assess a greater area of the polar environment: and not just at research stations! There are of course some pit-falls in this approach which will be discussed in a revised version and considered when analysing the data.
Another aspect that remains unclear to me is that, while the study is purely descriptive regarding general trends and data groupings, the attempts to establish causality, or at least link the findings to environmental variability, are not well resolved. An effort is made to relate the data to sea-ice variability; however, a major issue arises here: much of this variability cannot be explained without specific data sources and the local effects of sea ice. A review of the literature and the authors' own survey reveals that most of sampling for isotopic data was conducted from spring onwards—a period when the impact of sea ice is significantly reduced.
I would like to detail a few aspects of the manuscript that, in my opinion, could be revised. The manuscript is quite long, so I do not address every aspect in my assessment; I believe these points will be complemented by the comments from the other referees.
Title:
I do not believe this title accurately reflects the manuscript's content and focus. It is clearly not a synthesis, nor does it shed light on environmental impacts. While "review" is somewhat closer, there are areas where the literature review or meta-analysis was less than exhaustive.
Reviews and Syntheses is required by the journal Biogeosciences for all review manuscripts. Upon prior discussion with senior editors regarding how best to frame this manuscript it was decided that it would be labelled a review manuscript.
Abstract:
Lines 16–18: I believe the authors are attempting to highlight the significance of their survey as a “before and after” in the knowledge of the biogeochemistry of marine macroalgae, however, previous studies have already made significant progress in our understanding of stable isotope variability beyond the scope of trophic ecology. It is also striking that they note existing knowledge is heavily biased toward the WAP, given that their own survey was also conducted in that same region.
We acknowledge that our own survey is also limited to the peninsula, we will revise the manuscript to draw further attention to this. We do, however, provide new data for sites with no prior data in the Gerlache Strait region that has largely been ignored. We acknowledge the significant contribution of prior studies and hope that this collation of the available data is of interest and/or generates further research. We will revise the abstract to highlight the work that has previously been done.
Lines 27–31: I believe that establishing a relationship between sea ice variability and δ13C is highly speculative, especially considering the limited statistical analysis performed.
Statistical analyses are difficult to undertake for this record, due to the limited data collected over multiple decades and different geospatial areas. Although our ideas are speculative, we propose a potential relationship that would require further dedicated research. Since 2016 sea ice has shown a marked decline, what this means for the benthic community is uncertain and isotope analysis of macroalgae could be one avenue for assessing primary productivity. We will reword our discussion of the potential link to sea ice and highlight the speculative nature of this assessment, but we argue it is something worth pursuing.
Lines 30–36: What is certainly clear from this manuscript is that much more research is needed on this topic, and that a study of this nature serves as a starting point for further exploring the implications of the environmental changes we are witnessing.
We will revise the abstract to emphasise this point, the main issue with this dataset is the sporadic and limited spatial distribution of the data. We hope the outcome from this work is a renewed interest in macroalgae and its importance in the wider Southern Ocean ecosystem.
Introduction:
I find it overly long; the opening paragraphs attempt to contextualize macroalgae and their role in Antarctic systems. This has already been well summarized in previous reviews, which should be cited (e.g., Antarctic Seaweeds, Springer, 2020), rather than attempting to cover it with single citations.
We can condense the introduction further; this was also brought up by reviewer 1, but both reviewers do ask for further explanation on various topics – thus, probably not reducing the length of the paper overall. It should be noted however that review papers are generally not short!
Line 98: I believe the introduction should begin here.
We would argue that some of the points prior to Line 98 are necessary and provide an overview of the broader ecosystem. We will reconsider this upon revision.
Lines 119–120: It might be better to say "40 years." I also think the section dedicated to the history of biogeochemical studies on Antarctic macroalgae should be better organized, following a chronological sequence. In many cases, the text jumps back and forth between current and older studies. On the other hand, I do not think paragraphs 148–156 add much to this description of the evolution of these studies.
Noted – can be corrected to 40-years. The start of this paragraph was intended to give a brief timeline of publications (Lines 120–132), before going into further detail with trends over the last 40-years in terms of research focus and their geospatial coverage. We can consider shortening the second paragraph, but we feel it is important to explain that there are few original publications and that many subsequent publications use the previous data.
Lines 120-121: It is worth reiterating here that broad generalizations, such as describing the work as "primarily focused on taxonomy, distribution, and life cycles of species", can be misleading; a significant portion of the studies actually addressed the distribution and ecology of these organisms, aspects not fully captured by the cited references. Therefore, I suggest that in such instances, authors cite a comprehensive overview from other authors (e.g., by Amsler, Wiencke, or Gómez) that details the history of macroalgal research, rather than listing individual papers that fail to adequately illustrate the point. By the way, Himantothallus belong to the order Desmarestiales.
This typo has been amended to Desmaresitia sp. and H. grandifolius.
Lines 180–213. This section focuses on the δ15N signature, including numerous references to phytoplankton but very few to macroalgae. This is likely due to the limited number of available studies; however, I noted that one of the few papers reporting δ15N values in Antarctic macroalgae (Fernandez et al., 2024) is not mentioned in this subsection.
Since macroalgae reflect the marine δ15N signature we felt it necessary to provide background into the Southern Ocean δ15N system that is influenced by the spring phytoplankton bloom. There is little information regarding isotopic fractionation by Antarctic macroalgae species, nor are assimilation rates quantified. We can restructure this paragraph to make clearer the link between the Southern Ocean δ15N signature, variation due to phytoplankton blooms and subsequent influence on macroalgal δ15N values. Fernandez et al (2024) is cited throughout, but focusses on carbon uptake strategies on Antarctic macroalgae, even though they present both δ13C and δ15N.
Lines 214–232. This section presents findings regarding C/N ratios in a somewhat confusing manner, jumping between various factors of variability. Furthermore, regarding the relationship with carbon metabolism and macroalgal structure, I suggest reviewing some citations that cover this topic in greater depth. In general, I agree with the authors that there is limited information on the morpho-functional aspects of C/N ratios and isotopes, but some does exist (see: https://link.springer.com/chapter/10.1007/978-3-642-28451-9_2).
We thank reviewer 2 for the addition of this reference and will consider restructuring this paragraph to have an improved flow. We wanted this paragraph to be brief as the focus of this manuscript is primarily the δ13C and δ15N data, but CN is widely reported alongside stable isotopes and so necessary to include.
Lines 130–131: It is unclear here which biogeochemical data the authors are referring to. It would be advisable to specify this, as the term is very broad.
Both δ13C and δ15N data is reported by Iken et al. (2023), although the focus is more towards the carbon. This can be amended to clarify the meaning.
Lines 133–135: Not only for this reason, but also because most seaweed species along the WAP can be found and accessed. Regarding the warming of this region, I do not believe the cited papers are the most relevant for supporting such a broad assertion.
We do discuss accessibility of the WAP, something that also impacts our own dataset as well as the greater biodiversity found here which makes it understandable that most studies focus on this region. We can reword to make this point clearer and add further references supporting the statements.
Lines 235–247: Sampling. To establish sampling parameters, it would be relevant for the authors to report the conditions under which the algae were collected—for example, in the case of intertidal algae, whether collection occurred at low tide and up to which zone (supralittoral?); and for subtidal algae, why the sampling started at 5 m. A large number of species grow between 0 and 5 meters in Antarctica, particularly species that regrow rapidly once the ice disappears in spring. Finally, upon reviewing the metadata and the text in lines 279–286, it is unclear to me how many intertidal species were collected. Apparently, there were five, whereas Himanthothallus grandifolius was the only subtidal species. I realized that the Desmarestia specimens were not identified to the species level; this is unusual, given that the four common species in this region are morphologically very distinct. Considering that authors indicated in the introduction, the number of species analyzed (7) seems very low to me. In this regard, I would specifically like to comment on the authors' statement in lines 285–286: "The data generated from this survey provides more biogeochemical data than the 29 papers included in our literature analysis combined." I believe the tone should be more measured, especially considering that, collectively, the previous studies cover a much larger number of species than those collected during this expedition. Clearly, having more analyses is not the same as analyzing more species.
The dataset was collected as an opportunistic field season in collaboration with Viking, an expedition cruise operator. Therefore, limited sampling time and constraints on site selection hindered sampling and only the seven species mentioned could be targeted. Sampling occurred at low tide at IAATO landing sites. The species H. grandifolius was initially targeted as a separate project related to assessing intra-specimen variability in blades that do not shed annually, hence the high number of analyses, but that detailed dataset in intended for discussion in a more detailed blade-by-blade analysis. A second field season successfully collected more of this species and a separate publication for this is in progress. We will restructure this section to clarify the sampling protocol and our meaning. Desmarestia sp. was opportunistically collected, and further identification was not possible at the time.
Regarding the statistical analysis of the data, I believe it is essential to specify and detail the methods used, as parametric (e.g., ANOVA) and non-parametric (Kruskal-Wallis, Dunn) methods are mentioned throughout the Results and Discussion section. Furthermore, it would be very useful to include the statistical results in supplementary tables.
All statistical tests are described in the results section, especially with regards to Figure 4. The difference in data collection between publications and decades make it statistical analysis somewhat difficult and subjective. We can revisit the dataset and determine whether further analyses are appropriate here.
Results and Discussion: I found the structure of this section somewhat confusing, as it incorporates explanatory elements regarding sampling and data summaries.
Lines 287–313: As I understood, the data collected during the field campaign and the database are described separately; however, the database summary includes references to and comparisons between both types of results. It might be beneficial to condense this section by merging the descriptions and highlighting only the similarities and differences.
We will can reconsider how the datasets are incorporated into the manuscript, and condensing this section.
Lines 328–345: I do not understand this part. What is its relevance to the results? The authors appear to be conflating the dominance of macroalgal groups with the ease of sampling them. In general, intertidal algae in Antarctica are far easier to collect than subtidal algae! The reality is simply that there are far fewer species of green algae in Antarctica compared to brown or red algae. Perhaps these clarifications should have been made in the methodology section. Moreover, Phaeophyta (brown algae) is not recognized as a phylum; this should be changed to Phaeophyceae, as brown algae belong to the phylum Ochrophyta.
Upon reflection we agree it is overly wordy and unnecessary to the wider manuscript. We can change Phaeophyta to Ochrophyta, this term has changed between many publications, and we agree the consensus is now Ochrophyta.
Lines 385-398: This analysis lacks structure. There are multiple reasons why algae collected from different sites and under different conditions vary in their C and N content. This will inevitably be reflected in the patterns reported in both the databases and the data from the current study. For instance, a topic insufficiently analyzed in this paper are the endogenous factors, such as growth patterns and the accumulation of carbon-rich anti-stress compounds. The paper would benefit from a more structured analysis rather than simply listing various potential factors in a scattered manner.
We will consider restructuring the Results & Discussion into a more succinct style, we aimed to separate the CN, δ15N and δ13C datasets as well as our own data with the wider database. With this being a review with new data it was difficult to determine how best to merge the two together.
Lines 458-459: Snow algae are not macroalgae!
We agree. We will reword this section to focus just on Chlorophyta but highlight some publications have wrongly included Snow Algae in their datasets.
Lines 466-467: Do these averages include snow algae?
No
Lines 431-447: These types of explanations are not yet fully developed. For example, Cystosphaera is unrelated to the Desmarestiales. Furthermore, the multiple factors potentially explaining the variability are merged, yet none points to what actually might have influenced the data.
We can rework the CN section to focus more on influences of the data. Lines 437 to 447, however, focus on the incomplete sample set and with the low species diversity of the dataset we felt it more appropriate to discuss the phyla differences and context within the global CN average. We can incorporate further ecophysiological discussion points to this section.
Lines 591-594: I think this discussion could be complemented by findings showing that Antarctic macroalgae, especially Phaeophyceae, utilize β-carboxylation as an anaplerotic pathway to support growth under low-light and low-temperature conditions. There is abundant literature on this subject.
In our research collating the Antarctic biogeochemical literature we did not come across these terms; we will follow up on this and include this literature where required. There is very little data available for Antarctic seaweed species under low-light conditions. Primarily data was collected from November to February, this is an issue that we can highlight in the manuscript.
Lines 605-607: However, depth is highly significant for C acquisition and carbon balance. Moreover, in the paper by Zenteno et al., the observed changes in δ13C with depth were due to the fact that the different sampled species originated from different depths; the isotopic signature did not vary with depth within a single species.
Thank you for the clarification we will reword this section to make this clearer.
Line 615: Yes, the authors are right; however, some grey literature exists regarding internal processes in Antarctic macroalgae. Please revise the links:
https://epic.awi.de/id/eprint/26416/1/BerPolarforsch1997238.pdf (Page 70)
https://www.scielo.cl/pdf/rchnat/v74n2/art04.pdf
We would like to thank Reviewer 2 for these publications and will review and include them where necessary in the discussion. We are currently working on another publication discussion seasonal C content and photosynthetic activity in relation to daylength for H. grandifolius stable isotope variation across the thallus.
Geospatial variation. Here, the authors could summarize their trends more concisely rather than outlining explanations regarding environmental gradients in this region that are not based on hard data. Although the paper incorporates this into its framework, it does not provide consistent information on environmental variability (past, present, or future).
In line with both Reviewer 1 and 2’s comment we will amend the spatial discussion significantly upon revision to condense this section considerably. Difficulties arise considering different sites were targeted in different years, depths and not all species were analysed from all sites. Therefore, statistical analysis of the spatial trends need to be carefully considered regarding this variability in the data collection. We consider the impact of climate change on Antarctic seaweed presented in the dataset as minimal. Although this may be inaccurate there is limited information collected on climate/environmental parameters at the time of those collections in those specific areas. Hence, this is why we compare the data generally to the sea ice record that in part reflects climate change.
Lines 685-692: Not always. In many polar algae, carbon demands for growth intensify during periods of low light in winter or late winter/spring.
Thank you for the clarification we have amended this discussion point and found the above publications useful in this regard.
The discussion might benefit from the concept of carbon balance, which is strongly determined by available daylight. Algae collected in spring/summer were likely undergoing active growth, resulting in increased respiration.
Where possible we will revaluate the dataset to consider carbon balance and aim to correspond this with time of collection. We will, of course, reframe the discussion to bring up carbon balance where appropriate and would fit with the wider CN discussion.
Lines 745-746: ..and in Antarctic waters?
Amended to clarify the high N content of polar seaweeds is a result of the non-nitrogen limiting environment of the Southern Ocean.
Lines 759-762: I agree; however, a more detailed examination of older papers could help explain some of the patterns observed here.
We will incorporate the above publications where necessary into the introduction and discussion sections, we hope to condense the review where possible under both Reviewer 1 and 2’s suggestions that the manuscript is too long.
Minor queries:
Although I am not a native speaker, I believe the text requires revision to correct spelling and other errors.
We will revise the manuscript and improve grammatical errors carefully upon resubmission
Fig. 3. This seems interesting for defining thresholds. Why wasn't a similar approach taken for δ15N?
An initial draft for Figure 3 did include a schematic for δ15N values for nitrogen sources to the Southern Ocean, but these are wide ranging in their values and specific to individual studies and sites. Carbon, however, is strongly linked to the photosynthetic mechanism and lends itself to a simplistic schematic such as Figure 3. We could include the nitrogen schematic but again this would increase the length of the manuscript!
Reference list. This section needs improvement. It lacks a standard format, for example, regarding the citation style for books and book chapters, format of journal names, misspelled author names, etc.
The reference manager Mendeley was used for referencing; this was set to the style for Biogeosciences. We will check all references to ensure they are in the correct style.
Citation: https://doi.org/10.5194/egusphere-2026-4443-AC2
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CC2: 'Reply on RC2', Darren R. Gröcke, 01 Oct 2026
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This is a review of Alldred & Gröcke’s synthesis review of macroalgal stable isotope values. They add some new data collected opportunistically from a tourist voyage. The actual question the authors are trying to answer are not entirely clear, so it is difficult to evaluate if their mission was accomplished. There are very few novel ideas in this manuscript. Several so-called analyses were simply map plots where it is left to the reader to guess at latitudinal trends. Any inferences between sea ice cover and stable isotope values of macroalgae are vastly overshadowed by the huge variability of the isotope values. I was excited when I was asked to review this manuscript and, unfortunately, I was sorely disappointed. There are severe problems with the statistical analyses done (data are not independent replicates) and in some cases, there are no analyses done at all (latitudinal trends, relationships to sea ice cover). It feels like the authors had an opportunity to collect some macroalgal samples for stable isotope analysis and were now looking for a framework to publish them. There is very little novel information, although more data is always a good thing. The many grammar mistakes made it very difficult for me to read this extremely long manuscript. The authors also do not seem to have a strong foundation in stable isotope analyses given the inaccurate language used many times. A really useful source for this is Sharp’s Principles of Stable Isotope Geochemistry, especially Table 2.1.
If this manuscript is being considered for publication at all – and I caution against it – then the manuscript should be shortened at least by half and clear and solid analyses should be performed. There is much redundant/repeated information and lengthy portions of the manuscript deal with detailed descriptions of isotope ranges etc. Are those really useful?
Lastly, the authors seem very dismissive of the efforts of other publications, which I found disturbing.
Following are some specific comments.
The Abstract could be more informative. It states “insights into nutrient uptake, productivity and response to environmental change.” Of that, only environmental change (ice cover) is further mentioned in the abstract. More detail on “temporal and spatial variability” could be given. The abstract lacks actual results.
Introduction
39: Macroalgal (not algae) (adjective) – this is a problem throughout the manuscript
43: macroalgae is plural – macroalgae contribute (no s).
47: comma after Southern Ocean
59 – how is sea ice related if not indirectly through light, which is then mentioned separately. Clarify this
65 – add Amsler et al. 2023 as an important recent paper that addresses this issue:
Amsler, C.D., Amsler, M.O., Klein, A.G., Galloway, A.W., Iken, K., McClintock, J.B., Heiser, S., Lowe, A.T., Schram, J.B. and Whippo, R., 2023. Strong correlations of sea ice cover with macroalgal cover along the Antarctic Peninsula: Ramifications for present and future benthic communities. Elem Sci Anth, 11(1), p.00020.
70 - Iken et al. 2023 does not address macroalgal cover but macroalgal stable isotope values. Change this to Amsler et al. 2023
82 – add some relevant references to the ecology of P. decipiens, such as (and others):
Wiencke, C. and Tom Dieck, I., 1989. Temperature requirements for growth and temperature tolerance of macroalgae endemic to the Antarctic region. Marine Ecology Progress Series, pp.189-197.
Becker, S., Walter, B. and Bischof, K., 2009. Freezing tolerance and photosynthetic performance of polar seaweeds at low temperatures.
90 ff – cite some of the original early literature for this well-studied system, for example:
Klöser, H., Ferreyra, G., Schloss, I., Mercuri, G., Laturnus, F. and Curtosi, A., 1993. Seasonal variation of algal growth conditions in sheltered Antarctic bays: the example of Potter Cove (King George Island, South Shetlands). Journal of Marine Systems, 4(4), pp.289-301.
91 – What is SSI?
118 – data is plural (were) – this is used incorrectly throughout the entire manuscript
121 – the publication by Weykam is also biochemically important, providing C:N ratio:
Weykam, G., Gómez, I., Wiencke, C., Iken, K. and Klöser, H., 1996. Photosynthetic characteristics and C:N ratios of macroalgae from King George Island (Antarctica). Journal of Experimental Marine Biology and Ecology, 204(1-2), pp.1-22.
123 – is a figure on the increase of published information really useful for anything? Delete
129 – In addition to Iken et al. 2023, which provides data from a few species from a large spatial range, also see Whippo et al. 2024 from that same survey, which provides isotope and CN data from a larger number of species from individual locations. This paper is also missing from Figure 2
Whippo, R., Iken, K., Amsler, C.D., Lowe, A.T., Schram, J.B., Klein, A.G., Heiser, S., Amsler, M.O., McClintock, J.B. and Galloway, A.W., 2024. Fatty acid profiles and stable isotope composition of Antarctic macroalgae: a baseline for a combined biomarker approach in food web studies. Polar Biology, 47(4), pp.367-386.
136 – surveyed for what? Paragraph started out with macroalgal biochemical data and isotopes, but some of the surveys mentioned here are not about macroalgae at all
Section 1.1. lists studies that provide data and spatial coverage but doesn’t really talk about biochemistry of macroalgae. A review should not just state what was published, but what the findings were
167 – this makes it sound as if these references are about Antarctic macroalgae, that > 60% of Antarctic Rhodophyta lack CCM, which they are not
169 – specify that this is about d13C values
170-179 – is there an analysis that does not confound depth with species identity? If saying that there is a depth-related pattern, the same species across a depth gradient needs to be analyzed, as different species can have different causes for their isotope values than depth.
182-184 – can you give examples of these factors influencing d15N values? This is very vague
180ff, 203-213 – explain how phytoplankton dynamics relate to macroalgal d15N. Make better connections, the entire section seems to talk about phytoplankton blooms and not macroalgae
216 – nutrient limited environments
226 – add the Weykam et al. 1996 paper here
235 – macroalgae were
248 – Was there a specific tissue portion that was consistently sampled?
287 – when reporting a range, use the capital delta sign, Δ
289 – green and red algal CN is the same, cannot argue one being lower than the other. Look at the SD! Also, make sure you use the correct noun in the sentences. The CN values of Phaeophyta were higher, not the Phaeophyta.
288-290 – The numbers reported for replicates do not match what the authors report as their collections, except for Chlorophyta (n=35). Rhodophyta should be n=53 not n=52 unless one plant was not measured. n=600 for Phaeophyta is incorrect, it should be n=20. What I understand from the methods, the authors collected multiple samples along the blades of Himanthothallus. It seems they treated each of those sub-samples as individual (independent) samples, which is not correct. They could take an average value per plant and then use it in the calculation of the overall mean per group. This is especially important when using the data for statistical analyses. Again, the data as they use them are not independent and are not appropriate for the statistical analyses they employ. Obviously, this also applies to the other isotope metrics.
295 – why is (15.8 ‰) in parentheses?
305 – what does “most elevated phylum” mean?
310 – the authors say “significantly enriched”, this needs to be specified as enriched in 13C. When referring to the ratio (delta value), “enriched” is an incorrect term.
308 and 311 – misspelling of “utricularis”
304 – The authors say ” δ13C values ranged between –34.7 ‰ (H. grandifolius) and –9.8 ‰ (Iridaea sp.) for this study “ and then in Line 312 they say “H. grandifolius was the most negative (–23.4 ± 4.6 ‰, n=589)”. I don’t understand why there are two extremely different values being cited for H. grandifolius.
328-330 – this information does not seem relevant here. This could maybe be added to the introduction. Is there any solid reason to doubt the identification skills of these other researchers? And size is not a good reason to doubt identification accuracy. For example, the large brown algae Desmarestia anceps and D. menziesii are easily confused, while the small green alga Lambia antarctica cannot be confused with anything. If the large species are so easily identified, why did the authors not separate Desmarestia sp? To be honest, this should probably Desmarestia spp unless they can confirm they only have one of the species, and in that case, they should be able to say which one.
335 – The authors say “Desmarestiales are key primary producers, along with H. grandifolius (> 70 data entries),…” It seems the authors are unaware that H. grandifolius is a member of the Desmaretiales. It makes me wonder how familiar the authors actually are with the macroalgal flora of the Southern Ocean.
339 – The authors should note that there are not that many Chlorophyta species in the Southern Ocean. Naturally, there will be fewer datapoints. Also, if they identify this as such a problem, why did they not try to collect more species and individuals? They only collected one species (Monostroma sp.). Continuing, it seems it would suffice to say that data of individuals that were not identified at least to the genus level were omitted. The authors take a very dismissive tone about other investigations, which may have their limitations but there is no need to downgrade their work.
351 – why is “Coraline algae “ capitalized (here and elsewhere)? Also, not misspelling of coralline
365 – The authors need to adjust the treatment of their H. grandifolius data as pointed out above. These are not replicate measures, they are pseudoreplicates and not appropriate for this type of analysis as used by the authors.
369- “regardless of study, site or depth” – was there an actual analysis done to test this?
387 – Nitrogen should not be capitalized
392 – revise the wording of “kelp-like H. grandifolius” It is not a kelp and calling it kelp-like can be confusing. What aspect of it is kelp like? Maybe the morphology but internal cell structure and physiology are not – hence, the comparison to actual kelps in terms of storage is flawed.
406 – “significantly higher nitrogen content for Rhodophyta” – be specific. It was only higher than Phaeophyta, not Chlorophyta
452 – add one or both of the recent Amsler et al. 2023 papers that suggest exactly this. These authors are not the first ones to suggest this. They already cite the Amsler et al. 2023 paper in Elementa but they are missing the following paper:
Amsler, C.D., Amsler, M.O., Heiser, S., McClintock, J.B., Iken, K., Galloway, A.W. and Klein, A.G., 2024. Vertical distribution of brown and red macroalgae along the central Western Antarctic Peninsula. Botanica Marina, 67(1), pp.1-10.
490 – no, Chlorophyta have less negative (not more positive) d13C values. None of these values are positive.
505 – the same issue as noted for Line 335
509 – please provide a reference for this statement
518 – please provide examples for the misidentifications that have occurred from Chlorophyta
523 – please add ”anomalously low nitrate δ15N values” as this statement would otherwise refer to macroalgae and the cited study does not investigate macroalgae
532-534 – Are those data anywhere? And what does “depleting trend” mean? If not, please provide a reference for where the data can be found.
547 – probably more important than cruise ship wastewater is the vicinity of coastal penguin rookeries, which should at least be mentioned.
587 – change to “the future response”
604/05 – provide a reference for this statement
610 – the authors are clearly not familiar with the isotope language. “depleted δ13C values” do not exist – a ratio cannot be depleted.
618 – awkward phrasing “species that’s blades”, replace with which or whose
619 - 15 ‰ difference in which isotope?
625-628 – interesting how the authors berate other studies form not providing information on the exact thallus structure when they do not seem to provide this information for their own data either.
635 – is comparing isotope values of different groups for individual sites useful? It has been well established that groups can differ. The author themselves brought up reasons for why that is the case. Also, Fig. 8-10 are too coarse to be able to meaningfully compare specific sites across the three figures. Any spatial trends cannot be gleaned from these figures. What is the hypothesis? If there is a latitudinal trend? If so, use a regression analysis. Just plotting points on a map and squinting at it is not an appropriate analysis. Much of the text in this paragraph (3.4.3) is redundant description of ranges and can be eliminated.
637 – Fig. 8 does not indicate the location of Yankee Harbour
644 – Not “Nitrogen” but “Nitrogen isotope ratios”
656-659 - the references for this statement do not seem to refer to Antarctic macroalgae; this should be made clear as growth rates may act under different constraints in Antarctic macroalgae. For example, the following statement (659-660) is likely only true if nitrogen is limiting.
666 – see comment above. If a latitudinal trend is to be tested, use a regression analysis.
668 – Fig. 5 does not show locations
674 – you cannot talk about a “strength” of a relationship without running an actual analysis
698 ff – it’s unclear what all this has to do with any observed temporal trends in isotope values; this is about potential future expansions in habitat
717 ff – again, instead of a solid analysis, the authors just offer an image and leave it to the reader to take away a vague sentiment of variability. There is high variability in isotope values but the authors claim that these values track variation in sea ice cover. You should consider similar approaches as in Amsler et al. 2023, who did an actual analysis of algal cover in relation to sea ice concentrations. A similar approach could be developed for macroalgal stable isotopes. Also, the work of Iken et al. 2023 investigating latitudinal trends in stable isotope values should be included in this discussion.