the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jan 2026
Trait-based mechanisms underpin regional hotspot of diatom-driven carbon export in an oligotrophic gyre
Abstract. The oligotrophic subtropical gyres, vast yet nutrient-poor, pose challenges to our understanding of efficient carbon sequestration. Here, we integrate taxonomic, sediment trap, and metagenomic analyses to investigate the mechanisms underlying regionally heterogeneous and efficient diatom-mediated carbon export in the western North Pacific Subtropical Gyre. We discovered that within a vertically stratified nutrient regime, diatom communities displayed clear niche partitioning: Navicula and Rhizosolenia were enriched in the nutrient-depleted surface mixed layer, while Nitzschia, Chaetoceros, and Thalassiosira tended to dominate the deep chlorophyll maximum – reflecting hydrographic control over community assembly. This trait-based community structuring directly influenced the composition and magnitude of diatom carbon export, with fluxes ranging from 10³ to 10⁵ cells m⁻² d⁻¹ and an estimated 0.13–194.85 μg C m⁻² d⁻¹. Total carbon export and export efficiency (carbon exported relative to production) was markedly enhanced at station affected by the Kuroshio (K2b), which was mainly driven by the large, carbon-rich Rhizosolenia, delineating a distinct regional hotspot. Critically, metagenomic analysis revealed a limited presence of bacteria genes encoding key carbohydrate-active enzymes capable of degrading diatom-derived fucose-containing sulfated polysaccharides (FCSP), indicating a key biochemical mechanism that may reduce organic matter remineralization and enhance flux preservation. Our findings establish a multi-process framework wherein hydrodynamic regimes select for export-prone diatom communities with specific functional traits (e.g., size, carbon content), and the biochemical resistance of their organic byproducts may synergistically promote efficient carbon export. This study deciphers the interacting controls on carbon sequestration heterogeneity in the oligotrophic ocean, with crucial implications for predicting the biological pump's response to global change.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2', Anonymous Referee #1, 28 Feb 2026
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RC2: 'Comment on egusphere-2026-2', Anonymous Referee #2, 02 Apr 2026
In their paper ‘Trait-based mechanisms underpin regional hotspot of diatom-driven carbon export in an oligotrophic gyre’ the authors seek to connect the vertical structure of diatom populations in the western North Pacific Subtropical Gyre to the magnitude and composition of export flux. The topic is really interesting as the drivers of export flux in the oligotrophic ocean remain open for discussion. The main strengths of the manuscript are the dataset and the ability to interface the vertical structure of the populations with flux observations. It is difficult to generate a dataset like this, despite the small number of stations. Unfortunately, the current claims in the manuscript are extremely strong and not directly supported by the data. However, part of this could be the presentation. The figures were difficult to connect to the main text because of the many small squares and the lack of direct connection between the visuals and the processes discussed in the main text. The manuscript really needs a set of clear figures which show the proposed mechanisms and connect to the oceanographic context of the problem.
I will now go line by line through the manuscript to hopefully aid the authors as they re-work the document:
Lines 26-27: It is unclear what is meant by ‘trait-based’. Are you claiming ‘depth’ is a trait? If so, I normally think about traits in connection to nutrient uptake etc.
Lines 32-36: This seems out of place with the rest of the abstract.
Lines 36-39: There needs to be a mechanism to make this convincing.
Lines 39-41: It is unclear how the study does this, and if it did, what the ‘interacting controls’ are.
Lines 72-87: There has been a lot of work out of the Hawaii Ocean Timeseries looking at export with a focus on Nitrogen Fixation. The authors really need to engage with these ideas I think.
Section 2.1 – Given there are only 5 stations, more care should be provided to explain why they were chosen at these sites.
Lines 123-126: Given the lack of citations, more care should be given to discussion of design.
Line 136: Nutrient fluxes etc. have strong importance to the plankton community. It is confusing why this manuscript was not leveraged more.
Lines 147-158: The description was confusing to read.
Lines 162-164: cite references
Lines 182-194: Is there any estimate as to the error bounds?
Lines 203-210: The methods, and what they conceptually mean needs to be explained clearly.
Fig 2: It is really hard to decipher this plot. You need smaller numbers of panels and representative profiles to make your points clear.
Fig 3: Again, this is very hard to decipher. The NMSDS plot is really busy and it is hard to see the conclusions being drawn. All plots have all stations and depths. The authors need to think carefully about presentation and the point they are trying to make. The remaining figures share these problems.
Section 3.3: Many things co-vary with depth. This section is confusing as it is unclear which data is used in the correlations. If all data is used unmixing the actual drivers requires careful consideration as many things vary with depth.
Lines 402-409: Do flux ratios match composition ratios?
Lines 412-417: This all needs to be explained more.
Lines 421-423: I do not believe that this claim is supported. The system is heterogeneous both in space and time. This paper really needs clear biophysical reasoning to work.
Lines 483-488: Is this claim derived from the data in this paper? If so, there is not enough data to claim this. However, I believe this is well established, starting with Karl et al 2012, the references within it and the bank of work which cites it.
Lines 493-495: I believe that the vertical structuring of diatom populations is well established.
Section 4.3: The authors need to define what they mean by ‘trait-based’.
Lines 509-511: It is unclear to me why nitrogen fixation makes its first appearance in the manuscript here. The standard argument for enhanced export in the NPSG comes from invoking DDAs.
Lines 547: Is it true that Rhizosolenia has a large carbon density or is this due to the giant cell size?
Line 562: The authors really need to work harder to connect their data to the mechanisms invoked.
Citation: https://doi.org/10.5194/egusphere-2026-2-RC2
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General Comments
This manuscript addresses an important question regarding diatom-driven carbon export in the western North Pacific Subtropical Gyre. However, the strength of the evidence does not match the ambition of the conclusions. The study relies on very limited spatial and temporal coverage, substantial methodological assumptions, and indirect inference to support claims of a “regional hotspot” and a trait-based mechanistic framework linking community composition, biochemical resistance, and export efficiency. Given the extremely small effective sample size—particularly the reliance on a single station (K2b) to define a “regional hotspot”—the conclusions appear overstated and insufficiently supported. In its current form, the study reads more as a preliminary case observation than as robust evidence for a regional-scale ecological pattern.
Specific Comments
1. Only five stations were sampled in summer, and sediment trap data were successfully obtained at just three stations (K2b, M22, K8a). Winter observations were limited to two stations and lacked sediment trap measurements entirely.
2. Carbon biomass was estimated using genus-level average carbon content, largely derived from literature compilations and biovolume-to-carbon conversions. Oligotrophic regions are typically dominated by small and fragile diatom taxa, whose cellular carbon content may deviate substantially from literature averages derived under different conditions.
3. Diatom counts were conducted after sedimentation concentration and 8 µm mesh filtration. The authors acknowledge that this procedure may underestimate large or fragile taxa such as Rhizosolenia. However, Rhizosolenia is precisely the dominant contributor to the proposed K2b “hotspot.”