| 11 Sep 2026
Seasonal divergence in phytoplankton community and size structure in contrasting fjord types in SW Greenland
Abstract. Arctic fjords are coastal ecosystems whose productivity is strongly impacted by glacial meltwater. Fjords affected by marine-terminating glaciers (MTGs) sustain highly productive, diatom-dominated blooms throughout the growing season, whereas fjords with land-terminating glaciers (LTGs) typically experience a diatom spring bloom followed by less productive, small-celled communities in summer. However, the biomass, community composition and size structure of these phytoplankton communities, particularly pico- and nano-sized components, remain underexplored. We investigated spring–summer contrasts in the phytoplankton of two adjacent SW Greenland fjords: Nuup Kangerlua, mainly impacted by MTGs, and Ameralik, impacted only by an LTG. Combining high-throughput imaging, pigment and metabarcoding approaches allowed us to resolve the functional (size, pigmentation) and taxonomic composition of the phytoplankton communities. Our analyses revealed seasonal divergence between the two fjords. In spring, both were dominated by colony-forming diatoms under nutrient-replete conditions. In summer, nanophytoplankton dominated both fjords. However, persistent subglacial upwelling sustained growth of non-colonial diatoms in the MTG fjord, whereas pico- and nano-sized chlorophytes and cryptophytes dominated the warmer, less saline, turbid and highly stratified surface waters of the LTG fjord. Increased importance of mixotrophy under these conditions suggests an efficient microbial loop, partly relying on regenerated nitrogen. In contrast, the inner MTG fjord was characterized by the harmful haptophyte Pseudohaptolina birgeri. Despite these compositional differences, phytoplankton biomass and primary production remained similar between fjords. Overall, our results suggest that continued glacial melt and retreat of MTGs onto land will alter phytoplankton community organization, with cascading effects on Arctic food webs and carbon cycling.
Review of Manuscript egusphere-2026-5308
Title: "Seasonal divergence in phytoplankton community and size structure in contrasting fjord types in SW Greenland" (Mikhno et al.)
Recommendation: Major revision
Line numbers refer to the line numbering in the preprint PDF.
1. Summary and general assessment
The authors compare the phytoplankton of two neighbouring SW Greenland fjords, Nuup Kangerlua (marine-terminating glaciers, MTG) and Ameralik (land-terminating glacier, LTG), in spring (May 2022) and summer (August 2021). They combine imaging flow cytometry (iFCM), FlowCAM, HPLC pigments and 18S rRNA metabarcoding. They conclude that spring is dominated by chain-forming diatoms in both fjords, that summer is dominated by nanophytoplankton, that small green algae and cryptophytes dominate the LTG fjord, and that mixotrophy is important there. They also report Pseudohaptolina birgeri in inner Nuup Kangerlua.
The multi-method dataset is valuable, the topic is relevant for Arctic change, and the paper is generally well written. However, the novelty is more limited than stated. The quantitative biomass data have methodological weaknesses. Several key conclusions (mixotrophy, role of nanophytoplankton in production, species-level findings, future projections) go beyond what the data show.
2. Novelty and literature check
The two fjords are already well studied. Meire et al. (2023, Nature Geoscience) used monthly sampling in 2016 at one station per fjord (GF10 and AM10), including 18S metabarcoding and flow cytometry, and reported diatom persistence in Nuup Kangerlua and picophytoplankton dominance in Ameralik. Stuart-Lee et al. (2023, Estuarine, Coastal and Shelf Science) reported, based on flow cytometry in 2019, much higher pico- and nanophytoplankton abundances in Ameralik by late summer. The main new elements are:
-the size-resolved carbon biomass from iFCM,
-the combination of methods,
-the high taxonomic detail for small flagellates, and
-the spatial coverage along full fjord transects.
The claim that "little information is as yet available" on smaller components (l. 482-484) should therefore be moderated. The cited literature is appropriate for the topic.
3. Major revision points
M1. Novelty and framing. The introduction states that the pico- and nanoplankton are "poorly resolved", but Meire et al. (2023) and Stuart-Lee et al. (2023) already addressed these fractions in the same fjords (by flow cytometry in both studies, and by 18S metabarcoding in Meire et al., 2023). Hypotheses (i) and (ii) (l. 72-76) largely restate those findings. Please state clearly what is new relative to these two studies (for example, size-resolved carbon biomass, full fjord transects and the high taxonomic resolution of small flagellates), and revise the statement in l. 482-484 accordingly.
M2. Study design, replication and statistics. Each glacier type is represented by one fjord, and spring (2022) and summer (2021) were sampled in different years, so season should be exchange with inter-annual variability. Only four stations were sampled in Ameralik, and two spring iFCM stations are missing (l. 135-136). The abstract states that biomass and primary production "remained similar" between fjords (l. 27-28), a test is missing and should be added for completeness (Section 3.1, l. 321-326). Spearman correlations shouldn’t be used (l. 220-223, Fig. 5b) because pool depths, stations and both seasons, so samples are not independent and temperature, salinity and PAR are confounded with season. No correction for multiple testing is applied. Please add formal fjord and season comparisons (for example, mixed models with station as a random effect), discuss pseudoreplication and the year effect explicitly e.g. "MTG vs LTG fjords".
M3. Reliability of the carbon biomass estimates. Please check the biomass estimates using C:Chl a ratios, pigment data or microscopy, or show them as relative values rather than absolute values. It looks like the iFCM carbon biomass is a lot higher than it should be. In the top 40 metres of water during the summer, the average amount of biomass is around 580 µg C L-1. At GF3 (see lines 341–343), the amount can reach up to 2.8 × 103 µg C L-1. However, the average summer chlorophyll a reading is only around 2.8 µg L⁻¹ (NK) and 1.9 µg L⁻¹ (AM), even at the DCM (see lines 293–294). This gives carbon:Chl a ratios of at least ~200–300, and higher outside the DCM, well above typical phytoplankton values. One possible reason for this is the biovolume approach (2D area × mean width; see lines 145–147), which has not been checked to see if it works for the cell shapes that are there. The conclusion that nanophytoplankton dominated the summer biomass and "most likely contributed substantially to primary production" (lines 632-633) depends on these values.
M4. Interpretation of the metabarcoding data The relative abundance of 18S rRNA gene reads is used to estimate community composition and is compared with biomass measurements. However, this approach does not account for differences in 18S gene copy numbers between taxa, and no quantitative control is reported. Therefore, the reported dominance of Mamiellophyceae (28%) and cryptophytes (21%) in Ameralik (lines 445–449) should be interpreted with caution and discussed alongside the biomass data, which show almost no picophytoplankton. Since Mamiellophyceae have relatively few 18S gene copies, their actual contribution may be underestimated, making the discrepancy with the biomass results even more pronounced. This also supports the concern raised in M3. Furthermore, the defined "phytoplankton" group (lines 211–216) includes taxa that can also be heterotrophic or mixotrophic, such as Chrysophyceae, Dictyochophyceae and Paraphysomonas (lines 455–456). The exclusion of Dinophyceae is explained only in the Supplement (lines 422–423) and should be clarified in the main text.
M5. Mixotrophy and microbial loop conclusions. The claims that mixotrophy is increased and that the microbial loop is efficient and depends on regenerated nitrogen are not sufficiently supported by the presented data. They are based mainly on the presence of genera known to include mixotrophic species and correlations between NH₄⁺ concentrations and picophytoplankton or small nanophytoplankton. However, these groups contribute very little to total biomass (lines 605–606). The study provides no direct evidence of grazing, bacterivory or nitrogen uptake. The authors also acknowledge that evidence for phago-mixotrophy in Micromonas is lacking (lines 535–538). Furthermore, mixotrophy cannot be assumed for an entire genus. In Teleaulax, Dinobryon and Pseudopedinella, it has been demonstrated only for certain species (e.g. T. amphioxeia and D. balticum) and can depend on environmental conditions. Therefore, it cannot be concluded from relative read abundances alone. The sources of NH₄⁺ (lines 558–560) also remain unclear. The authors should therefore present mixotrophy as a possible hypothesis rather than a demonstrated process. The claims should be removed from the abstract and hypotheses or supported by additional experimental data.
m1. Species-level inferences and novelty claims.The identification of Ostreococcus tauri and Pseudohaptolina birgeri is based on a 100% match of a short 18S V4 sequence. However, this marker may not reliably distinguish closely related species. For P. birgeri, no microscopy or cell counts are provided to confirm its presence. The authors should also clarify the geographic records reported by Schiffrine et al. (2024). Their main text does not specify whether P. birgeri has previously been recorded in Greenland waters. This is important because Ribeiro et al. (2020) reported the species in Baffin Bay and identical metabarcodes in Nares Strait, both close to Greenland. These records may contradict the claim of a possible „first report“ from Greenland fjords.
There is also an error in the species' historical distribution: P. birgeri was originally described from the Gulf of Finland (see also Ribeiro et al. 2020), not the Sea of Japan. The Sea of Japan bloom reported by Orlova et al. (2016) refers to the synonym P. sorokinii. This should be corrected.
m2. Inconsistent numbers and typographical errors. Several methodological and presentation issues need clarification and correction. The description of station numbers in the two fjord systems (lines 96–97) should be rewritten to make the sampling design clear.
The Si:N ratios (8:16 and 15:16) are presented in an unusual format, and the two spring values appear inconsistent. They should be converted to decimal values, with the corresponding depth ranges specified. The phosphate formula should also be corrected from PO₄⁺³ to PO₄³⁻.
There are several apparent figure and reference errors: the reference to Fig. 11 (line 400) and Fig. S3a for NOx (line 308) should be checked. Fig. S3 is also cited for Shannon diversity (line 425).
The statistical notation "** p ≤ 0.005" should be replaced with standard significance notation. The reference to dendroextras should also be checked, as the text cites dendextend.
The software versions and references need verification. In particular, the use of DADA2 v1.14.1 with R 4.4.2 and phyloseq 1.50.0 should be checked for compatibility, and the R 4.4.2 citation should be updated from R Core Team (2021). The authors should also correct the methods description: Cutadapt is a separate tool, not a DADA2 function, and ASVs are inferred using dada(), not learnErrors().
Finally, several minor language errors should be corrected, including "During both season", "where" instead of "were", and "compare to".
m3. Pigment analysis The pigment clusters (Fig. S11, l. 393-395) and EOF modes group unrelated taxa (green algae, dinoflagellates and cryptophytes in one cluster), which limits taxonomic interpretation. A standard approach such as CHEMTAX would allow comparison of pigment-based class contributions with iFCM carbon and read data or explain why it was not use. The interpretation of antheraxanthin as a light-stress signal (l. 389-391) is speculative. Detection limits should be given, because several pigments were "not detected" (l. 384, 389). Cophenetic correlation values should be reported in the main text.
m4. Causal statements and projections go beyond the data. Ice mélange, subglacial upwelling and light limitation are used as explanations, but no ice or plume data are presented, light attenuation (Kd) and euphotic depth are not derived from the PAR profiles (l. 105-106), and only one time point per season is available (acknowledged at l. 566-568). The abstract statement that continued melt "will alter" community organisation with "cascading effects" on food webs and carbon cycling is not tested here. Please rephrase it as a hypothesis and temper causal wording ("likely", "may").
m5. Title & references The title refers to "contrasting fjord types", although each fjord type is represented by a single fjord; "two contrasting fjords" would be more accurate. The reference list contains some errors: "D. Firsova, A." should be Firsova, A. D. (l. 720; also in the text, l. 595); Graßhoff is listed twice as author (l. 739); Balzano et al. (2012) cites the discussion paper instead of the final article (Biogeosciences, 9, 4553-4571, 2012; l. 693-695); and Redfield (1958) lacks the journal name (l. 875).
5. Overall evaluation
Scientific significance: good (2). Scientific quality: fair (3), with limitations in biomass quantification and statistical design. Presentation quality: good (2). The manuscript can become a useful contribution if novelty is stated more precisely, the biomass estimates are validated, and the mixotrophy and species-level conclusions are presented with appropriate caution.