Trophic amplification of Southern Ocean plankton emerges from changing seasonality

Xue, Tianfei; Arteaga, Lionel A.; Pahlow, Markus; Frenger, Ivy

doi:10.5194/egusphere-2026-3722

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https://doi.org/10.5194/egusphere-2026-3722

Preprints

27 Jul 2026

| 27 Jul 2026

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Trophic amplification of Southern Ocean plankton emerges from changing seasonality

Tianfei Xue, Lionel A. Arteaga, Markus Pahlow, and Ivy Frenger

Abstract. Understanding how climate change will alter plankton dynamics in the Southern Ocean, a region of globally significant biogeochemical influence and strong seasonal variability, is critical for projecting the future of ocean ecosystems. Using a multi-model ensemble of 12 earth system models under the SSP5-8.5 climate-change scenario, we show that the seemingly stable Southern Ocean-wide plankton biomass masks opposing trends across three distinct biogeochemical zones. Plankton in the subpolar zone (40° S–60° S) remains near-stable owing to compensatory changes in bottom-up and top-down processes. In the subtropical zone (30° S–40° S) and seasonal ice zone (60° S–90° S), zooplankton decline and increase proportionally more than phytoplankton, constituting negative and positive trophic amplification, respectively. This trophic amplification is not uniformly distributed throughout the year but arises primarily from austral spring and summer. In the subtropical zone, shoaling-induced nutrient limitation reduces phytoplankton growth and concentration, causing a disproportionate decline in zooplankton grazing and biomass. In the seasonal ice zone, improved light availability from mixed-layer shoaling, sea-ice retreat, and warming enhance phytoplankton growth and surface concentration, driving disproportionately enhanced zooplankton grazing and biomass. In both zones, the seasonal trophic amplification signal is substantially stronger during the ecologically important spring and summer seasons than over the annual-mean, with potentially severe ecological consequences. Our results demonstrate that annual-mean projections could systematically underestimate both the magnitude and ecological severity of trophic amplification, and highlight the need for assessing seasonal and regional variations in plankton dynamics to better constrain future projections of Southern Ocean ecosystems.

Received: 24 Jun 2026 – Discussion started: 27 Jul 2026

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Tianfei Xue, Lionel A. Arteaga, Markus Pahlow, and Ivy Frenger

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RC1:
'Comment on egusphere-2026-3722', Anonymous Referee #1, 31 Aug 2026 reply
Does the paper address relevant scientific questions within the scope of BG? Yes

Does the paper present novel concepts, ideas, tools, or data? Yes

Are substantial conclusions reached? The conclusions are important, but could be better substantiated.

Are the scientific methods and assumptions valid and clearly outlined? Yes

Are the results sufficient to support the interpretations and conclusions? No, more support would strengthen their argument/interpretation.

Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? Not quite.

Do the authors give proper credit to related work and clearly indicate their own new/original contribution? They could cite some additional related work.

Does the title clearly reflect the contents of the paper? Yes

Does the abstract provide a concise and complete summary? Yes. However, L9-11 do not explain how that causes disproportionate response in zooplankton. And L12-13 are redundant with L7, suggest removing L7.

Is the overall presentation well structured and clear? Yes

Is the language fluent and precise? Yes

Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? Yes

Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? Yes

Are the number and quality of references appropriate? Yes

Is the amount and quality of supplementary material appropriate? Yes

General comments
This paper assesses projected changes in phytoplankton and zooplankton in three regions of the Southern Ocean as simulated by an ensemble of Earth system models (ESMs) under SSP5-8.5. The study finds three different patterns in the three regions, with trophic amplification in two of them. A new contribution of this study is examining if there is seasonal trophic amplification, instead of just a difference between a future time period (e.g. 10-30 yr mean) and the contemporary time period. The study also attempts to attribute the changes using a budget analysis for both phytoplankton and zooplankton. Though the results are supported by evidence, not all the interpretations of the results are substantiated. The key message is important and clearly described, highlighting seasonal changes and how they could be obscured by only examining annual means. It also prompts next steps for assessing downstream impacts of the seasonal trophic amplification.
The Results and Discussion often hypothesize how temperature, light, and nutrient availability caused the changes in plankton biomass, concentrations, and rates (e.g. L197, L280, L326). However, these terms are readily available from the ESMs on the ESGF server. A seasonal analysis of temperature, sea ice, downwelling radiation, light limitation, N limitation, and Fe limitation akin to the MLD analysis in Figures 4-6 would greatly strengthen the paper by attributing the mechanisms rather than hypothesizing on them.
The Arteaga et al. 2020 reference is not adequate for these statements because it examined Argo observational data, not model output.

Section 3.2.3 head reads “Spring and summer light availability drives positive trophic amplification in the SIZ,” but light availability is not directly examined.

Specific comments
Additional references
L24: Add also Petrik et al. 2020, Guibourd de Luzinais et al. 2023.

L34: Add also Asch 2015, Chen et al. 2025.

L56: Suggest adding Kearney et al. 2021 that has a detailed description of the BCG model components.

Methods need more detail
L53: Please list all the variables you used for this analysis and their CMOR names too.

Section 2.1: Add definition of years for "contemporary" and "future" analyses.

Table 2: Which years?

L144-147: I do not fully understand these methods. i) I don't understand why the normalization step is needed. ii) Is there only one budget per term (P, Z) from the multimodel mean, not one budget per term per model? iii) Seems like the averaging could lead to a mismatch of loss/gain terms, which then would be absorbed by the Other term.

Not clear where many of the numbers in the Results section come from
L206-207: How is the 67% more calculated? 10%/6%?

L233: 30% looks closer to 20%.

L252: Is 50% stronger the comparison between 16% vs. 10%?

L313-314: How is the >4x quantified? The relative change in Fig 6 c and d looks like 70% vs. 35% (~2x). Is it referring only to the Jan diff that is ~4x?

L317: “eleven times as much” Can you please give the two numbers for phytoplankton and zooplankton for verification of this quantity?

Imprecise language
L112-113: “Not capture fully” is imprecise wording. These could be captured, but ESM modelers choose not to. Omnivory occurs when >1 zooplankton type, which is represented in your ensemble. Slower turnover rates can be accomplished with differential temperature-dependence, mortality, and/or representation of life history stages.

L270: Is a deltaT of <0.3 mmolC/m2/d sharp? What is the context or criteria for calling this sharp?

L285: The MLD difference looks biggest to me (from the two lines) in Jul-Sep, but the bars are the smallest then.

L292: The statement “increasing loss terms offset the higher grazing intake across all seasons” is not accurate. Losses < Grazing Jun-Oct.

L295: You say there is no amplification window, but It seems that S, O, and D meet the criteria.

L322: “throughout most of the year” Just Aug-Jan, that's half not most.

L329: Elevated grazing only compensates Aug-Jan.

L330: “increase in zooplankton biomass” Losses seem to exceed grazing in Fig 6f (negative deltaT), but the change in biomass is positive Feb-May.

L401: “different … directions” All the shifts look earlier to me in Supp Fig C3.

L94-95: Could the difference between Argo and BGC-Argo be the time range? e.g. Argo has 10 years under cooler conditions and is missing 3 warmer years?

L165-166: I understand this in theory, but the figures do not meet my expectations given these explanations. e.g. Fig 4d,f: deltaZ is positive Jan-Jul, suggesting the difference between 2000s and 2090s is decreasing those months, but the relative change in those months is very large.

L196: Could the stability simply be that phytoplankton biomass has decreased proportionally with PP and GRAZ, such that u 🟰 PP/phy and g 🟰 GRAZ/phy are the same?

L248: October is also the biggest decrease in MLD in future compared to contemporary.

L260: I think this should say “decreasing grazing mortality.” dGrazing is lower Oct-Jun as indicated by the bars. dGrazing is negative in the amplified period, both in the budget (Fig 4c) and Dec-Apr in the g/g2000s comparison (Fig 4b), so it must be the decreased growth in (Fig 4b) that resulted in reduced PP.

Section 3.2.2: Could also mention that both phytoplankton and zooplankton show phenological shift earlier.

L285-286: How does one see that “From August to October, enhanced phytoplankton production driven by mixed-layer shoaling induced light improvement and rising temperatures leads to a subtle increase in phytoplankton biomass” in 5e? It does not show MLD or light. And deltaT is positive for May-Sep, but very negative for Oct.

L304: Could also mention a phenological shift earlier for both phytoplankton and zooplankton.

L327: Grazing can also be enhanced by warming. This goes for all regions.

Supp Fig B3: Point to your Supp figure with surface concentrations anytime you mention enhanced concentrations rather than integrated biomass in Results.

Results: How is surface irradiance projected to change in these 3 regions? Other studies (e.g. Krumhardt et al. 2022) have shown that some get cloudier and others less cloudy?

L350-353: You say your results emphasize the ZPC mechanism as if the other two were not important components of your results, but you didn’t assess ZGE or ZTL. Also, ZPC is GRAZ/PP right? Might be good to show in your 3 regions historically and over time. Especially since the ZGE, ZTL, and ZPC changes were quite variable around the S Ocean in the GFDL model.

L359: You can see if this is true of the heterotrophic processes by checking the temperature-dependence terms in each BGC model. Some examples in Kearney et al. 2021.

L360-361: Could you explain this “partially counteracted amplification” more? Are you only considering increased grazing with increased temperature (not increased metabolic demand and other losses)? e.g. Increased phytoplankton + Disproportionately higher grazing = Greater increase in zooplankton (reinforce positive amplification), whereas Decreased phytoplankton + Higher grazing = Weaker decrease in zooplankton (counteract negative amplification). This would only be true if the increased grazing on less phytoplankton biomass can meet the higher metabolic demands.

L372: It could also arise from biomass not accumulating in the zooplankton groups because their generation time is shorter than the zooplankton with complex ontogeny that actually live in this region (and thus biomass turnover rate is faster).

L377 and Fig C4: I would like a statistical test of this if possible. I notice some patterns. i) 2p2z-noFe outlier in STZ and SPZ. ii) 1p1z low range of STZ and high range of SPZ. iii) withFE lower than noFe in SPZ.

L383-384: How uncertain are the other physical changes, like sea ice, across the MME?

L400: The phenological shifts can also be seen in Figs 4-6.

Technical corrections
L29: The Asch et al. 2019 paper was specifically about fish spawning. This detail should be added.

Figure 1: Add a colorbar with units for chl. In the SIZ, the arrow is at the start of the seasonal cycle, which disagrees with the text saying, “austral spring and summer,” while the biomass is higher than the historic pattern in the last half of the cycle.

Table 2 and throughout: I’d suggest using mgC instead of mmolC for plankton biomasses, as I think plankton ecologists are more familiar with thinking in these units.

Figure 2: i) Please make this figure the whole page width and choose a different color for the purple line. It is too similar to the black. ii) I also suggest using the same y-axis limits for all left-hand phytoplankton and zooplankton, 0-180, and the same y-axis for all right-hand phytoplankton and zooplankton, 0-360. iii) Clarify if both the red and purple lines use the left y-axis labeled "obs".

L120: Can you add an estimate of how much zooplankton data is overlapping in both of these vs. how much is different? I assume <2012 is the same as COPEPOD and >2012 is new to Yang?

Eq 6: Technically, there is an efficiency constant on the GRAZ term such that not all GRAZ becomes zooplankton biomass.

Figure 4b, 5b, 6b: Add the one-to-one line to these plots. Also, how is one supposed to be able to distinguish April/Aug, Jan/June/July and Mar/May?

L273 and throughout: “predator-prey” is the common terminology, not “prey-predator.”

Figure A1: It would be nice to also see a second figure next to it that shows the # of months with data for each location.

Figure A2: Label (a)-(l) with each ESM so one does not have to go back and forth between the table and this figure to know which is which.

Figure B3: Suggest adding water column-integrated plots next to these to make comparison easier. Or add dashed lines to these.

L464/Figure B4: Just curious, why is the Fe time series so cyclical?

Figure B5: Would these changes be isolated to only the SIZ?

Figure C1: Why surface and not integrated here?

Figure C3: I do not see any blue bars. Is that correct?

References
Asch, R. G. (2015). Climate change and decadal shifts in the phenology of larval fishes in the California Current ecosystem. Proceedings of the National Academy of Sciences, 112(30), E4065-E4074.
Chen, K. S., Petrik, C. M., Asch, R. G., Thompson, A. R., & Auth, T. D. (2025). Continuing Long‐Term Shifts in Larval Fish Phenology in the Southern California Current Ecosystem Are Matched by Rapid Advances in the North. Global Change Biology, 31(3), e70141.
Guibourd de Luzinais, Vianney, Hubert Du Pontavice, Gabriel Reygondeau, Nicolas Barrier, Julia L. Blanchard, Virginie Bornarel, Matthias Büchner et al. "Trophic amplification: A model intercomparison of climate driven changes in marine food webs." PLoS One 18, no. 8 (2023): e0287570.
Kearney, K. A., Bograd, S. J., Drenkard, E., Gomez, F. A., Haltuch, M., Hermann, A. J., ... & Woodworth-Jefcoats, P. A. (2021). Using global-scale Earth system models for regional fisheries applications. Frontiers in Marine Science, 8, 622206.
Krumhardt, K. M., Long, M. C., Sylvester, Z. T., & Petrik, C. M. (2022). Climate drivers of Southern Ocean phytoplankton community composition and potential impacts on higher trophic levels. Frontiers in Marine Science, 9, 916140.
Petrik, C. M., Stock, C. A., Andersen, K. H., van Denderen, P. D., & Watson, J. R. (2020). Large pelagic fish are most sensitive to climate change despite pelagification of ocean food webs. Frontiers in Marine Science, 7, 588482.

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Citation: https://doi.org/10.5194/egusphere-2026-3722-RC1

Tianfei Xue, Lionel A. Arteaga, Markus Pahlow, and Ivy Frenger

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Short summary

Climate change alters the balance between ocean plants (phytoplankton) and the tiny animals eating them (zooplankton) in the Southern Ocean. Using 12 climate models, we show zooplankton respond more strongly to climate change than phytoplankton, but mainly in spring and summer. Near Antarctica zooplankton increase more than phytoplankton; in subtropical waters they decline more. Annual averages hide these seasonal extremes, underestimating ecological impacts on marine food webs.


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