the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2026
Spatial patterns in phytoplankton community composition and their influence on carbon export on the northwest Weddell Sea continental shelf
Abstract. Typically, diatoms, haptophytes, and cryptophytes dominate phytoplankton biomass and abundance in the Southern Ocean, determining bloom dynamics and carbon transfer and respond to physical factors such as sea ice concentration and mixing. However, phytoplankton community composition remains understudied in many regions of the Southern Ocean. Here, we present pigment-based phytoplankton community-composition data from a summer cruise to the northwest Weddell Sea continental shelf. To our knowledge, these are the only pigment-based community composition data from this key region for deep-water formation and carbon transport. Taken together, the relative biomass of the three dominant phytoplankton groups (39.7% diatoms, 30.2% haptophytes, and 14.9% cryptophytes) explain the majority (R2 = 0.37 – 0.82) of carbon export to 150 m. These findings highlight the need to consider frequently overlooked phytoplankton types, alongside diatoms, in Southern Ocean models of carbon export due to the key, but differing, roles these groups play in production-export dynamics.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Ocean Science.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 06 Aug 2026)
- RC1: 'Comment on egusphere-2026-3148', Anonymous Referee #1, 08 Jul 2026 reply
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- 1
Overall recommendation: Major Revision
This manuscript presents a valuable observational dataset from the northwestern Weddell Sea continental shelf, a region where in situ observations remain logistically challenging and scientifically important. The combination of phytoplankton pigment analyses with sediment trap measurements addresses a timely and relevant question concerning the links between phytoplankton community composition and biological carbon export in the Southern Ocean. The dataset itself is valuable, the manuscript is generally well organized, and the analytical procedures appear appropriate.
However, I believe that the manuscript, in its current form, substantially overstates both its novelty and the strength of the conclusions that can be supported by the available evidence. My concerns therefore relate primarily to the interpretation of the results and their positioning within existing literature, rather than to the quality of the observations themselves.
Overall, I believe the manuscript has the potential to become an important contribution, but substantial revision is necessary before it can be considered for publication.
Major comments
1. Positioning of the study within existing literature
The manuscript repeatedly states (including in the Abstract, Discussion and Conclusions) that this dataset represents "one of the only, if not the only, HPLC dataset" and one of the only phytoplankton community composition datasets available from the Weddell Sea continental shelf. I do not believe these statements accurately represent the current state of knowledge.
Over the last decade, several studies have investigated phytoplankton community composition using HPLC/CHEMTAX approaches across the Antarctic Peninsula and adjacent Weddell Sea sectors, providing information on phytoplankton functional groups, environmental controls and regional ecological variability. While these studies differ in spatial coverage and scientific objectives, they represent important regional context and should be considered when positioning the present contribution.
Importantly, acknowledging this previous work does not reduce the novelty of the present study. Instead, it allows the authors to more accurately define what is genuinely novel here, namely the coupling of pigment-derived phytoplankton community composition with sediment trap observations from this particular continental shelf environment.
I therefore encourage the authors to revise statements regarding the uniqueness of the dataset and to distinguish more clearly between the scarcity of observations from this specific shelf region and the broader availability of HPLC/CHEMTAX observations across the Antarctic Peninsula-Weddell Sea sector.
Examples include, but are not limited to, previous HPLC/CHEMTAX studies from the Antarctic Peninsula and adjacent Weddell Sea (e.g., Mendes et al., 2012, http://dx.doi.org/10.1016/j.dsr.2012.03.002; Mendes et al., 2018, https://doi.org/10.1016/j.dsr2.2017.12.003), recent regional studies investigating the role of phytoplankton community composition in biogeochemical processes and carbon cycling (e.g., Senger et al., 2026, https://doi.org/10.3354/meps15088), and additional regional investigations from other research groups addressing pigment-based community reconstruction and Southern Ocean biogeochemistry.
2. The relationship between phytoplankton community composition and carbon export is not sufficiently demonstrated
The Title, Abstract and Conclusions all emphasize that phytoplankton community composition influences particulate organic carbon export. While this represents an important scientific hypothesis, I do not believe the analyses presented fully demonstrate this relationship. The manuscript convincingly characterizes the spatial variability of phytoplankton communities and presents valuable sediment trap observations. However, these two components remain largely independent lines of evidence rather than being quantitatively linked through robust evidence. In addition, the manuscript would benefit from a more explicit discussion of the potential temporal and spatial mismatch between the phytoplankton community sampled in the water column and the integrated particulate flux collected by the sediment traps.
The application of the PCCratio following Kramer et al. (2025) is an interesting exploratory exercise. However, in the present manuscript this relationship is evaluated using only four sediment trap deployments. Consequently, statistical significance is not achieved (p > 0.05), and the available evidence does not support statements referring to a "strong predictive capacity" or broad conclusions regarding the influence of phytoplankton community composition on carbon export. Throughout the manuscript, the distinction between exploratory evidence, statistical association, ecological interpretation and predictive capability should be made much clearer.
3. The Discussion and Conclusions extend beyond the strength of the evidence
Several sections of the Discussion and Conclusions extrapolate from a single cruise and a limited number of sediment trap observations to broader implications for Southern Ocean biogeochemistry and Earth System Models. Although these implications are scientifically interesting and certainly worthy of discussion, they should be presented as hypotheses or future research directions rather than as conclusions demonstrated by the present study. Overall, I recommend moderating several statements throughout the Discussion and Conclusions so that the level of inference remains proportional to the strength of the available evidence.
4. Interpretation of the haptophyte signal as Phaeocystis
Throughout the manuscript, the pigment-derived haptophyte signal is frequently interpreted as representing Phaeocystis. While this is certainly a plausible interpretation, I encourage the authors to discuss this result more cautiously. Unlike other sectors of the Southern Ocean (e.g., the Ross Sea), where large Phaeocystis blooms are a characteristic feature of the seasonal phytoplankton succession, the northwestern Weddell Sea and Antarctic Peninsula are generally characterized by more heterogeneous phytoplankton assemblages in which diatoms, small flagellates (e.g., cryptophytes) and athecate dinoflagellates frequently coexist, and Phaeocystis generally represents a less dominant component of the phytoplankton community than in regions such as the Ross Sea..
In addition, several regional studies have demonstrated that pigment-based discrimination between Phaeocystis and small Type B athecate dinoflagellates (e.g., Gymnodinium spp.) is not always straightforward (e.g. Mendes et al. 2018). These groups may exhibit sufficiently similar pigment signatures to complicate their discrimination using standard CHEMTAX parameterizations. Previous regional CHEMTAX applications (e.g. Mendes et al. 2018) have shown that improved discrimination can be achieved by refining the initial pigment ratio matrices using diagnostic accessory pigments characteristic of the dinoflagellates. Consequently, the attribution of the haptophyte pigment signal exclusively to Phaeocystis may depend on the specific CHEMTAX parameterization adopted and should therefore be discussed explicitly as a potential source of uncertainty. A brief discussion of the initial pigment ratio matrix adopted and its suitability for this particular region would therefore strengthen the manuscript. This issue is particularly relevant because the manuscript's ecological interpretation relies heavily on the inferred relative contribution of haptophytes. Unless supported by independent taxonomic observations (e.g., microscopy or molecular analyses), I recommend moderating interpretations that directly equate the haptophyte pigment signal with Phaeocystis throughout the manuscript.
5. Additional justification for the PCCratio
The application of the PCCratio proposed by Kramer et al. (2025) is potentially valuable. However, the manuscript would benefit from a clearer explanation of the ecological rationale underlying this metric, particularly the justification for placing dinoflagellates (together with Synechococcus) in the denominator. Furthermore, while this metric represents an interesting framework for exploring relationships between phytoplankton community composition and carbon export, its application to the present dataset should be interpreted cautiously given the limited number of observations available. Finally, I recommend avoiding direct comparisons, suggesting that the present relationship is stronger than that reported by Kramer et al. (2025), since the substantially different sample sizes and associated uncertainties make such comparisons difficult to interpret.
Minor comments
Throughout the manuscript, terms such as "association", "influence", "control", "prediction" and "predictive capacity" should be used more consistently, as they imply different levels of inference.
Statements regarding the uniqueness of the dataset should be revised following a more comprehensive review of the regional literature.
Several conclusions should be moderated to better reflect the statistical evidence presented.
The Conclusions would benefit from emphasizing that this study provides valuable observations and generates promising hypotheses regarding links between phytoplankton community composition and carbon export, while recognizing that additional observations are required to establish robust predictive relationships.
Final assessment
In summary, I believe this manuscript contains a valuable dataset from an important and under-sampled region of the Southern Ocean and has the potential to make a meaningful contribution to our understanding of phytoplankton community composition and carbon export. However, its scientific impact would be substantially strengthened by (i) a more complete and balanced review of the regional literature, (ii) a clearer distinction between observations and inference, (iii) a more cautious interpretation of pigment-derived taxonomic assignments, and (iv) conclusions that remain fully consistent with the strength of the evidence presented. With these revisions, I believe the manuscript would provide a more robust and balanced contribution to Southern Ocean literature.