the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Jun 2026
From Phaeocystis to Chaetoceros: silicic acid as the primary driver of diatom bloom magnitude in an Arctic mesocosm experiment
Abstract. The phytoplankton spring bloom constitutes the majority of annual Arctic primary production. In the Eurasian Arctic it is usually composed of various diatoms and the haptophyte Phaeocystis; recently, Phaeocystis-dominated blooms seem to be expanding. Low silicic acid (Si) availability has been hypothesised to promote Phaeocystis blooms, though the key drivers of Phaeocystis and diatom competition are not yet well understood. To investigate how Si availability controls bloom succession and biogeochemical fluxes we conducted a 3-week mesocosm experiment in Kongsfjorden, Svalbard in May 2024 on the spring phytoplankton community. The initial phytoplankton community was composed predominantly of Phaeocystis and the diatom Chaetoceros. Eight 450-L mesocosms received an Si gradient (0–40 µM) with nitrogen and phosphorus additions every 48 hours and we tracked nutrient uptake rates, community composition, elemental cycling and export. The nutrient additions stimulated phytoplankton growth across all Si levels. Phase I (Days 0–14) was dominated by a Phaeocystis bloom, terminating synchronously due to nitrogen and phosphorus depletion despite variable Si availability. In phase II (Days 16–22) Chaetoceros emerged as the dominant species with clear Si gradient effects: Chaetoceros spp. sustained exponential growth (µ = 0.40 d-1) and achieved higher abundances in >3 µM Si treatments, while collapsing in Si-depleted mesocosms. Overall, particulate organic carbon export fluxes were low, but were strongly correlated with biogenic silica export fluxes, and increased exponentially in phase II of the experiment reflecting increasingly diatom-dominated sinking material. Our results demonstrate that Si availability does not govern initial Phaeocystis–diatom competition, but is the primary determinant of diatom bloom magnitude. The nutrient storage capacity of Chaetoceros confers resilience under transient depletion, while micrograzing and viral infection further mediate community successions. This study underscores the importance of integrating cell-specific and ecosystem-level measurements over extended timescales when predicting climate-driven shifts in Arctic primary production and carbon export.
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RC1: 'Comment on egusphere-2026-3186', Anonymous Referee #1, 12 Aug 2026
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AC1: 'Reply on RC1', Antonia U. Thielecke, 14 Oct 2026
We thank the reviewer for their time and consideration in reviewing this manuscript. While we appreciate their assessment of our thorough and multifaceted analysis we acknowledge that the manuscript required some restructuring and shortening to clarify the purpose of analyses and statistics and clearly present results and their interpretation
During revision we have streamlined the results and discussion sections to improve reading flow and comprehension.The results section was additionally rearranged to center the results presentation more around the comparison of Chaetoceros and Phaeocystis and we streamlined the remaining results sections to focus more on this key aspect of the paper. To achieve this we shortened especially results section 3.2 by removing the description of accumulation rates both from the text and from Figure 3. We also combined sections 3.4 and 3.5 to condense and clarify our results on nutrient use and uptake rates. In line with this we removed figure 13 due to redundancy with Figure 6. In Figure 12 we removed the small flagellate group for easier comparison between the key groups.We revised the applied statistics by replacing the use of Spearman correlation and linear models that were applied to phase averages, with full linear mixed models using log2 transformed Si additions. This new approach uses all sampling days and captures changes over time and effects of Si as well as the interaction of Si and time (Si x Day) in the same model, which reduces the number of statistical tests required. For improved visualisation we added the day-specific Si effects to each time series plot. To retain the phase-based concept used throughout the manuscript, phase-specific Si effects are derived from these full models. The change in the applied statistics changes individual model results, but key findings and patterns remain the same, retaining the overall story of the manuscript, while making the statistics more reliable and easier to follow. The visualisation of Si effects in the figures allows to reduce the description of model results in the text, contributing to making the manuscript shorter and easier to follow. We have also addressed all specific comments by the reviewer, to which we responded in detail in the supplementary (response to each specific comment in blue). We are confident that these changes have greatly improved the quality of the manuscript.
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AC1: 'Reply on RC1', Antonia U. Thielecke, 14 Oct 2026
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RC2: 'Comment on egusphere-2026-3186', Hua Xiang, 05 Sep 2026
Manuscript Title: From Phaeocystis to Chaetoceros: silicic acid as the primary driver of diatom bloom magnitude in an Arctic mesocosm experiment
Manuscript Number: egusphere-2026-3186
Reviewer: Hua Xiang
- General Assessment
Previous studies (e.g., Egge & Aksnes, 1992; Ardyna et al., 2020) often regarded low silicic acid concentrations as a prerequisite for Phaeocystis to outcompete diatoms. However, our study experimentally demonstrated the opposite: under nitrogen- and phosphorus-replete conditions, Phaeocystis still bloomed first even when the initial silicic acid concentration was extremely high (40 µM). This directly proves that silicic acid concentration does not determine who wins the initial race; rather, it is the depletion of nitrogen and phosphorus, not silicic acid, that triggers the collapse of the Phaeocystis bloom. However, despite the study's clear merits, several aspects require careful revision before the manuscript can be accepted for publication. I have organised my comments into Major and Minor categories below, with line numbers referring to the provided PDF.
- Major Comments
Comment 1 (lines 625‑630, line 140)
Issue: The authors repeatedly emphasise that “Phaeocystis bloom developed regardless of Si supply” (L625‑630) and use this to challenge the view that Si availability is a key determinant of Phaeocystis vs. diatom dominance. However, the authors themselves state (L140) that initial Si concentrations in the fjord water were already below detection limit at the time of mesocosm filling. This means that all mesocosms started under severely Si‑depleted conditions, which likely suppressed the initial recovery and division of Chaetoceros, indirectly favouring Phaeocystis. This is a legacy effect of pre‑experimental Si limitation, not a true “no effect” of Si. Consequently, the claim that Si does not govern initial competition is over‑stated and does not adequately account for initial conditions.
Comment 2 (lines 251‑260, Fig. 12)
Issue: The authors honestly acknowledge (L251‑L260) that because isotope enrichments exceeded the recommended 10% level, the measured C, N and Si uptake rates represent potential rather than in situ rates. This is a critical methodological constraint. However, in the results (especially Fig. 12, single‑cell uptake) and throughout the Discussion, these “potential” rates are treated as if they reflected actual competitive performance. For example, Fig. 12 shows higher Chaetoceros uptake on Day 6 than Phaeocystis, but this may simply reflect the maximum potential of Chaetoceros under enriched conditions, rather than its real performance under natural nutrient fluctuations. The current presentation risks over‑interpreting species‑specific differences in nutrient acquisition.
Comment 3 (lines 162‑164)
Issue: To prevent CO₂ limitation, the authors added NaHCO₃ and HCl to all mesocosms (L162‑L164). While this helped maintain pH, it artificially altered the dissolved inorganic carbon (DIC) pool and its carbon isotopic composition. The added HCO₃⁻ dilutes the ¹³C‑DIC label, potentially biasing the calculation of carbon fixation rates derived from ¹³C uptake. Furthermore, whether this manipulation affected the physiology of Phaeocystis (e.g., its preference for HCO₃⁻ utilisation) remains unknown.
Comment 4 (line 546, Table 2)
Issue: The text notes that “Phase II N:Si uptake ratios showed extreme treatment dependence (0.1‑15.7), with low values at high Si treatments (10‑40 µM: 0.2‑0.3)” (L546). This is striking because the classical Redfield N:Si ratio for diatoms is ~1. Values as low as 0.2‑0.3 under Si‑replete conditions suggest either a very low N demand, internal nitrate storage, or that an unmeasured N source (e.g., ammonium) contributed to growth. However, this key observation is not discussed in the relevant section (L660‑L680).
Minor Comments
- Line 75-80: The authors cite Ardyna et al. (2020) and Assmy et al. (2017) as evidence for the “low Si favours Phaeocystis” hypothesis, but they do not acknowledge alternative or complementary hypotheses that have been proposed to explain Phaeocystis dominance, such as: (1) differential grazing pressure (microzooplankton preferentially grazing diatoms), (2) colony formation as a defence mechanism, (3) temperature optima differences, or (4) light/shade adaptation. By presenting only the Si-limitation hypothesis, the authors set up a straw-man argument that their experiment then “challenges,” without giving due credit to the complexity of the existing literature.
- Line 232 The reference Liguori et al., 2020 is not present in the bibliography (only Bracher, Chen, etc. from 2020 are listed).
- Line 247: The parentheses contain an author’s internal note (“get details for stockholm mass spec”) that should never appear in a submitted manuscript.
- Lines 251–260: The Methods clearly state that because isotope enrichments exceeded recommended levels, the measured rates reflect potential rather than in situ uptake. However, in the Results (especially Fig. 12) and throughout the Discussion, the figures and text are labelled simply as “uptake rates,” without emphasising the “potential” qualifier. This risks over‑
- Line 630-640: While the authors later acknowledge that “Si limitation prior to the experiment” may have constrained Chaetoceros recovery (L860–865 in the Conclusion), this acknowledgment comes far too late and is not integrated into the main discussion of competition. The central conclusion—“Si availability does not govern initial Phaeocystis-diatom competition”—is stated repeatedly (L625–630, L858–860) without adequately weighing the counterargument that pre-existing Si depletion may have been the reason diatoms did not compete effectively at the start. This is a major logical gap.
- Line 630-640: This conclusion is based on literature showing similar optimal temperature ranges, but the experiment experienced a rapid temperature drop from ~2.3°C to -0.2°C around Day 15 (L390–392). The authors argue this drop did not affect productivity, but they provide no analysis of species-specific temperature sensitivity at sub-optimal temperatures. The literature cited (Hegseth et al., 2019; Simo-Matchim et al., 2017) may not address the effects of rapid cooling on division rates. Furthermore, temperature affects nutrient uptake kinetics (Lomas & Gilbert, 1999), so the interaction between temperature and nutrient limitation is complex and cannot be dismissed based on optimal growth ranges alone.
- Line 720-730: The authors acknowledge (L745) that “viral lysis rates or other pathogens were not assessed within the framework of our experiment,” yet they spend significant space (L720–745) constructing a narrative about viral and pathogen control that is entirely speculative. This is particularly problematic because the key evidence they cite (Kranzler et al., 2019) comes from culture experiments showing that Si limitation facilitates viral infection in diatoms—but they extrapolate this to a field community without direct measurements. The Discussion section would benefit from much more cautious phrasing.
- Line 820-830: The authors excluded large grazers (>200 µm) from the mesocosms (L132), yet they claim the dynamics were in “good agreement” with the fjord. Large grazers, especially calanoid copepods, can exert significant top-down control on Phaeocystis and diatoms (Nejstgaard et al., 2007). If these were excluded, the mesocosm community likely experienced reduced grazing pressure, potentially inflating Phaeocystis biomass relative to the natural fjord. The authors’ claim of “good agreement” is unsupported by quantitative data, and the logical implication is that the mesocosms may not accurately represent natural top-down regulation—a point that is critical for extrapolating the findings to the broader Arctic.
- Figure 4：The x‑axis shows Si treatments (0, 1, 2, 3, 5, 10, 20, 40 µM) at equally spaced intervals, even though the actual concentrations are non‑uniform (e.g., 0→1 vs. 10→20). This may visually distort the concentration‑response relationship
Figure 5. The caption does not indicate which day (0, 7, 14, or 22) corresponds to which panel
Tab2 should be formatted as a three-line table
Citation: https://doi.org/10.5194/egusphere-2026-3186-RC2 -
AC2: 'Reply on RC2', Antonia U. Thielecke, 14 Oct 2026
We thank Hua Xiang for the time and effort of reviewing our manuscript and appreciate the recognition of the value of our work. We carefully responded to the detailed comments in the supplementary (with our responses to the comments in blue). We believe that the implementation of these comments greatly improved our manuscript.
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Please see the attached Review Report in the Supplement.