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: open (until 09 Sep 2026)
- RC1: 'Comment on egusphere-2026-4192', Anonymous Referee #1, 30 Jul 2026 reply
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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