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: open (until 02 Oct 2026)
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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
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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
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CC1: 'Authors Reply to RC1', Darren R. Gröcke, 10 Sep 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.