Towards constraining the drivers of variability and trends in subantarctic productivity

Traill, Christopher D.; Rohr, Tyler W.; Shadwick, Elizabeth H.; Buchanan, Pearse J.; Tagliabue, Alessandro; Bowie, Andrew R.

doi:10.5194/bg-23-6947-2026

Articles | Volume 23, issue 19

https://doi.org/10.5194/bg-23-6947-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-23-6947-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 19

Research article

|

06 Oct 2026

Research article |

| 06 Oct 2026

Towards constraining the drivers of variability and trends in subantarctic productivity

Christopher D. Traill, Tyler W. Rohr, Elizabeth H. Shadwick, Pearse J. Buchanan, Alessandro Tagliabue, and Andrew R. Bowie

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Final revised paper (published on 06 Oct 2026)
Preprint (discussion started on 23 Jan 2026)

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2026-44', Anonymous Referee #1, 18 Feb 2026

Please see attached pdf

Citation: https://doi.org/10.5194/egusphere-2026-44-RC1
- AC2: 'Reply to RC1', Christopher Traill, 18 Jul 2026
  
  Referee is thanked for their highly constructive feedback which has improved the quality and accessibility of the manuscript.
  Please find the detailed responses to major and minor comments in the attached response which include supporting figures, text and references.
  
  Citation: https://doi.org/10.5194/egusphere-2026-44-AC2
RC2:
'Review of 'Towards Constraining the Drivers of Variability and Trends in Subantarctic Productivity'', Anonymous Referee #2, 24 Jun 2026

The manuscript "Towards Constraining the Drivers of Variability and Trends in Subantarctic Productivity" by Traill et al. investigates the environmental drivers of net primary production in the SOTS region using a combination of remote-sensing-based NPP estimates, the intermediate-complexity ocean biogeochemical model PISCES, and CMIP6 model outputs.
The manuscript first examines discrepancies among satellite-based algorithms in identifying the drivers of high-NPP versus low-NPP years in the study area. It then explores the same question using an ocean biogeochemical model. The use of model output provides additional insight into the mechanisms driving NPP variability, including aspects of top-down control that cannot be captured by satellite algorithms alone. Finally, the discussion places these findings in the broader context of projected NPP trends from climate model simulations.
Overall, this is a well-written manuscript and a valuable contribution to the growing literature on the drivers of NPP variability and long-term trends in the Southern Ocean. The study fits well within the scope of Biogeosciences, and I enjoyed reading the manuscript and following its study design.
Suggested revisions
- Data availability and reproducibility
I checked the data availability and analysis code. With the exception of the PISCES-QUOTA-FE output, all datasets and code appear to be accessible. The PISCES-QUOTA-FE output is currently listed as restricted access on Zenodo. To ensure full reproducibility of the study, it would be important for this dataset to be made publicly available prior to publication.
- Placement of the CMIP6 results
While the methods and results are clearly presented, and the findings are effectively discussed in Sections 4.1 and 4.2, I found the placement of the CMIP6-related results somewhat unusual. Figures 8–11 are presented within Discussion Section 4.3 rather than in the Results section. This makes the narrative more difficult to follow and, in my view, disrupts the overall flow of the manuscript. I suggest moving the CMIP6 analyses and associated figures to the Results section and using Section 4.3 to synthesize the findings from the satellite products, the biogeochemical model, and the CMIP6 projections.

Citation: https://doi.org/10.5194/egusphere-2026-44-RC2
- AC1: 'Reply to RC2', Christopher Traill, 18 Jul 2026
  
  Referee 2 is thanked for their positive feedback and constructive suggestions for the manuscript. Model output is available on reasonable request to co-author Tagliabue. The CMIP6 results from Figure 11 have been moved to the results section, while Figure 10 has been moved to the appendix. Text is added to the Results section to integrate the CMIP6 results in the context of PISCES-QUOTA_FE results. Please find attached our detailed response to RC2 comments.
  
  Citation: https://doi.org/10.5194/egusphere-2026-44-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Submit a revised manuscript (01 Aug 2026) by Andrew Thurber

AR by Christopher Traill on behalf of the Authors (10 Aug 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 Aug 2026) by Andrew Thurber

RR by Anonymous Referee #1 (18 Aug 2026)

Suggestions for revision or reasons for rejection

I thank the authors for their work to address my comments, particularly the extra effort put into creating the heatmap figures. I found these useful, but I appreciate the point around losing the magnitude of differences with this approach. The comments have been addressed well, and the manuscript reads better as a result, particularly with the better integration of the CMIP6 section. This will be an important contribution to our understanding of changing productivity trends in the Subantarctic, as well as dealing with some of the issues in remote sensing/modelling of biophysical productivity drivers.

I have only a few minor comments left, otherwise I look forward to seeing the manuscript published.

Minor Comments:
Line 233: “Higher NPP for VGPM/eVGPM was associated with deeper MLDs through winter and shallower MLDs in spring (October-November), although not statistically significant”.
If the relationship is not statistically proven then it cannot be correct to say that higher NPP was associated with deeper MLDs, otherwise it is a contradiction. It would be more accurate to say that Higher NPP co-occurred with deeper MLD’s, but the direct association between the two was not statistically significant.

Line 315: “we found lower temperatures are unlikely to dominate reduced growth rates”. I’m not sure “dominate” is the correct word here, maybe “be the dominant driver of”.

Line 383: “In the highest NPP years, increased SSTs and more southern STF position”. Here, SST’s are plural and STF position is singular.

Line 389: “The highest differences in NPP between highest and lowest NPP year groups were in CESM2 and lowest differences in ACCESS-ESM1.5, CanESM5-CMOC, GFDL-ESM4 and CMCC-ESM2 models”. This sentence is quite confusing. It would be better to say, “The largest differences in NPP between highest and lowest NPP year groups were in CESM2 and the smallest differences in….”

Line 394: “during high NPP years (Figure 8b), Deeper”. Capitalisation

Figures: Biomass axes units should be mmol C m-2 ?

Line 574: “in the SOuthern Ocean”. Capitalisation

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ED: Publish subject to technical corrections (29 Aug 2026) by Andrew Thurber

AR by Christopher Traill on behalf of the Authors (01 Sep 2026) Author's response Manuscript

Short summary

Southern Ocean phytoplankton are a key part of the carbon cycle, yet year-to-year changes in ocean productivity are poorly understood. Using model simulations, this study shows how deeper mixing in the most productive years increases nutrient supply and changes the predator-prey relationship between phytoplankton and zooplankton. This helps explain why satellite productivity estimates disagree, and the reasons for why climate projections might be getting inaccurate estimates of future production.