the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jul 2026
Arctic phytoplankton bloom regimes are associated with distinct dissolved carbohydrate signatures and transparent exopolymer particle (TEP) formation
Abstract. Phytoplankton are major producers of marine dissolved organic matter (DOM), including carbohydrates that fuel microbial carbon cycling or contribute to the biological carbon pump through exopolymer particle aggregation. In the Arctic Ocean, climate-driven changes are expected to restructure phytoplankton communities, yet the consequences for the molecular composition of DOM and its fate within microbial and aggregation pathways remain unclear. Here, we examined potential linkages between phytoplankton community composition (18S-rRNA), dissolved organic carbon (DOC), dissolved combined carbohydrates (DCCHO) and transparent exopolymer particles (TEP) in the Fram Strait and the Kara Sea. An advanced spring bloom dominated by the haptophyte Phaeocystis pouchetii coincided with a DOM pool enriched in neutral and acidic carbohydrates and elevated TEP concentrations. In contrast, ice-edge diatom blooms dominated by Thalassiosira spp. were characterized by glucose-enriched DOM signatures and low TEP formation. Our findings indicate an association between specific phytoplankton taxa and exopolymeric carbohydrate composition, suggesting that shifts in phytoplankton biodiversity may alter carbon processing pathways.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4192', Anonymous Referee #1, 30 Jul 2026
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RC2: 'Comment on egusphere-2026-4192', Anonymous Referee #2, 24 Oct 2026
General comments
This manuscript examines phytoplankton communities, dissolved carbohydrates, and TEP in the Fram Strait and Kara Sea, combining biogeochemical measurements with 18S rRNA sequencing. The dataset is valuable, and the methods and figures are generally clear and sound.
My main concern is that the manuscript does not convincingly establish what is new compared with previous Arctic work on carbohydrates and extracellular polymeric substances (EPS). Much of the discussion is descriptive, while the proposed links between phytoplankton taxa, carbohydrate signatures, TEP formation, and carbon cycling are stronger than the evidence allows. Several analyses and a more critical engagement with the literature would be needed to support the central conclusions.
Major comments 1. Scientific novelty and previous Arctic studies
The manuscript needs to make a much clearer case for its scientific contribution. Carbohydrate and EPS production, their relationships with algal biomass, and microbial processing have already been investigated in Arctic environments (e.g., Aslam et al., 2012a, 2012b, 2016, 2018), among other studies. Although those studies focus largely on sea ice, they provide important context. The authors should in a first place acknowledge these studies and then go beyond this literature: what do the present observations actually add, and which results mainly confirm what is already known? This is, in my view, the most important weakness of the manuscript.
2. Phytoplankton biomass and dissolved carbohydrates
The relationships between chl-α and dissolved carbohydrate concentrations are not sufficiently examined. The multivariate analyses (Figs. 3B and 7) are useful, but direct correlations with total DCCHO and individual monosaccharides would help determine whether observed patterns reflect phytoplankton biomass, changes in carbohydrate composition, or other environmental controls. These analyses appear feasible with the existing dataset.
In addition, the use of neutral sugar yields to define bloom conditions raises questions about the independence of the subsequent comparisons of carbohydrate concentrations between bloom and non-bloom samples.
3. Taxonomic attribution, TEP formation, and carbon cycling
The proposed associations between carbohydrate signatures, Phaeocystis pouchetii, Thalassiosira spp., and TEP formation are interesting, but remain largely correlative. Taxonomic effects are difficult to distinguish from environmental variability and microbial processing, particularly since the Fram Strait and Kara Sea were sampled in different seasons and under different hydrographic conditions.
Moreover, the observed associations do not necessarily demonstrate a direct role of specific carbohydrates or phytoplankton communities in TEP formation. Similarly, the proposed implications for carbon export versus microbial recycling are not directly supported by the available data.
The authors should better acknowledge these limitations, compare their findings with previous taxon-specific studies, and distinguish observed relationships from proposed mechanisms. Where possible, the influence of regional and environmental factors should also be assessed.
4. Structure and focus
The manuscript is extremely long and very very descriptive than necessary, with repetitions in several occasions of regional, hydrographic, and bloom-related patterns. A shorter, more focused discussion organized around the research questions would make the scientific message easier to follow. Some secondary hydrographic details may in my opinion be moved to the Supplement. A combined Results and Discussion section might also help.
Minor comments
-The title would benefit from a more engaging and elegant formulation that better highlights the study’s scientific contribution. As currently phrased, the title lacks appeal and largely repeats findings that are already well established in the literature.
- The discussion of marine carbohydrates several sections rely on older references older references, particularly from the 1980s, 1990s, and 2000s. While these pioneering studies remain relevant, the authors should incorporate more recent literature on carbohydrate production, composition, and dynamics to better reflect current knowledge and place their findings in a broader scientific context.
Overall assessment and recommendation
The manuscript presents a valuable Arctic field dataset, but its scientific novelty remains insufficiently demonstrated. Previous studies have already examined carbohydrate and EPS production, composition, and microbial processing in Arctic sea-ice environments. While the present study addresses pelagic communities, the authors need to put in front the novelty of their work and its added value. The limited examination of chl-α and carbohydrate relationships, the overlap between taxonomic and regional or seasonal differences, and the largely correlative links proposed between carbohydrates, TEP, and carbon cycling merit in my opinion a better analysis.
For these reasons, I recommend rejection of the manuscript in its present form. However, if the Editor-in-Chief considers that a substantially improved manuscript could be produced by addressing the concerns raised in my review, I would have no objection for a new submission being considered.
References
Aslam et al. (2012a). Dissolved extracellular polymeric substance (dEPS) dynamics and bacterial growth during sea ice formation in an ice tank study. Polar Biology 35, 661–676. DOI: 10.1007/s00300-011-1112-0.
Aslam et al. (2012b). Production and characterization of the intra- and extracellular carbohydrates and polymeric substances (EPS) of three sea-ice diatom species, and evidence for a cryoprotective role for EPS. Journal of Phycology 48, 1494–1509. DOI: 10.1111/jpy.12004.
Aslam et al. (2016). Patterns and drivers of carbohydrate budgets in ice algal assemblages from first year Arctic sea ice. Limnology and Oceanography 61, 919–937. DOI: 10.1002/lno.10260.
Aslam et al. (2018). Identifying metabolic pathways for production of extracellular polymeric substances by the diatom Fragilariopsis cylindrus inhabiting sea ice. ISME Journal 12, 1237–1251. DOI: 10.1038/s41396-017-0039-z.
Citation: https://doi.org/10.5194/egusphere-2026-4192-RC2
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Summary:
The study “Arctic phytoplankton bloom regimes are associated with distinct dissolved carbohydrate signatures and transparent exopolymer particle (TEP) formation” by Hirschmann et al. documents dissolved organic carbon, nutrient and carbohydrate concentrations in addition to carbohydrate and phytoplankton community composition for three regions adjacent to the Arctic. Samples were collected during the spring and summer of 2021. The study also quantified TEP abundance using filtration onto a filter membrane and staining with alcian blue. In the results, the authors report a Phaeocystis and Pseudo-nitzschia-dominated community that correlated with the abundance of arabinose and rhamnose-containing dissolved polysaccharides as well as TEP abundance and a diatom (Thalassiosira) community that correlated with glucose-containing dissolved polysaccharides.
General comments:
I found the manuscript very thorough in how methods were documented and results were reported, which is evidenced in the few issues I am raising below. The figures are well designed and in particular the graphical abstract and the map in figure 1 are very illustrative. At times, following the test was complicated by the number of abbreviations and their inconsistent use. Most notably, I found that while a lot of detailed information was provided, as a reader it was difficult to connect this information logically. For me, the multiple levels of grouping of the samples (by location, by water mass, by apparent bloom state) and their interspersed use made drawing connections between data across different figures complicated. While the actual scientific questions and issues I have are few and only technical, I personally think that the overall message of the manuscript regarding 'bloom regimes' requires more context to be informative and convincing.
Specific comments:
In particular, the classification of bloom conditions requires additional support in my opinion. I am under the impression that the observational data on chlorophyll, nutrients, carbohydrates and community composition was used to deduce bloom conditions, i.e. whether a bloom had occurred or was occurring, but that this observational data was then also interpreted in the context of the deduced bloom status. For example, ll. 333-335: “Bloom conditions were defined as both chl-a concentrations > 1 µg L⁻¹ and NS yields > 5.9 %DOC, with the latter threshold indicating fresh DOM (Amon & Benner, 2003). Samples not meeting the criteria were classified as non-bloom samples. In both the FS and KS, bloom samples showed a twofold increase in DCCHO concentrations compared to non-bloom samples.” This logic appears circular, because if I am not mistaken, NS and DCCHO are both based on the same carbohydrate quantification. This manuscript could be a lot stronger with an orthogonal determination of bloom conditions.
Most prominently, this issue arises in section 4.1 “Regional and seasonal phytoplankton bloom dynamics”. This part of the discussion appeared to lean quite heavily on assumptions based on previous bloom dynamics, or if not then I did not understand where the information on the bloom dynamics originated from. In my opinion, interpreting the reported results in the context of bloom dynamics would highly benefit from some additional information on the time course of conditions in the sampled water masses. For example, if the authors could augment their observations with satellite or float data to interpret their observations in the context of ongoing processes, this section would stand to gain substantially in specificity. Without this context, I think it is difficult to compare the observations made on the two cruises beyond a purely descriptive standpoint.
Technical corrections:
For the graphical abstract, I would recommend following the Symbol Nomenclature for Glycans to depict the monosaccharides.
line 191: How long did the HCl hydrolysis run, 24 hours?
line: 224: Why did you decide to compare to standardized DCCHO mol%?
e.g. l. 435: PO₄-3 should be PO₄3-
lines 241ff and 453/454: Did you include any correction on the alpha-value for the many pairwise Spearman correlations?
Figure 4a: Please specify which box depicts which water layer.
Figure 6 – legend: “relative Abundance” should not be capitalized