the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jul 2025
Response of phytoplankton communities to the onset of the 2020 summer marine heatwave in the Drake Passage and Antarctic Peninsula
Abstract. Extreme warming events are increasingly more intense and frequent in the global ocean. These events are predicted to drive profound and widespread effects on marine ecosystems, yet their impact on phytoplankton, the base of the marine food web, are still largely unknown. Our understanding of the impact of these phenomena in marine ecosystems is particularly poor in the remote and logistically challenging Southern Ocean. During summer 2020, the research vessel Hespérides sampled the water column of the Drake Passage and northern Antarctic Peninsula before (early January) and during the early phase (late January-early February) of a marine heat wave, that resulted in sea surface temperature anomalies of up to +3 °C. Here, we take advantage of this exceptional opportunity to document the effects of an extreme warming event on the nutrient and phytoplankton (diatom and coccolithophores) distributions across the main zonal systems of the Southern Ocean. Overall, our results indicate that biogeographical variability of diatom and coccolithophore assemblages, the two dominant phytoplankton group in the Southern Ocean, mirrored the physical and chemical properties of the water masses delineated by the Southern Ocean fronts. Analysis of a suite of satellite-derived oceanographic parameters revealed that development and persistence of the 2020 marine heat wave were closely tied to mesoscale anticyclonic eddy dynamics. The increase in sea surface temperatures during the onset of the marine heat wave was associated with a remarkable increase in diatom abundance, that reached bloom concentrations, and a shift in the diatom assemblages towards an increase in the relative abundance of the small diatom Fragilariopsis cylindrus/nana in the southern Drake Passage. In turn, coccolithophore abundance decreased north of the polar front during the warm water event, most likely due to a remarkable decrease of nitrate by approximately one order of magnitude lower than average summer concentrations. We speculate that these unusually low nitrate levels were the result of either the advection of nitrate poor waters from lower latitudes by an anticyclonic eddy and/or nutrient consumption by substantial development of soft-tissue phytoplankton biomass. Overall, our results reinforce the notion that a warmer Southern Ocean will favour an increase of small phytoplankton cells in the southern Drake Passage and northern Antarctic Peninsula with unpredictable consequences in the marine-food web and biogeochemical cycles that need to be urgently quantified and parametrized.
- Preprint
(6336 KB) - Metadata XML
-
Supplement
(2641 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-2892', Anonymous Referee #1, 25 Jul 2025
Manuscript Title: Response of phytoplankton communities to the onset of the 2020 summer marine heatwave in the Drake Passage and Antarctic Peninsula
General Comments
This manuscript provides important insight into phytoplankton community responses to the 2020 summer marine heatwave in a climatically sensitive region of the Southern Ocean. The study combines in situ hydrographic, pigment, and microscopic data with satellite observations to document compositional changes in phytoplankton, notably a shift to smaller phytoplankton.
This is a timely and relevant contribution, especially in light of the increasing frequency and intensity of marine heatwaves due to climate change. However, while the manuscript presents a valuable dataset and is generally well-written, the current version lacks a strong analytical framework, remains descriptive in tone, and would benefit from a more focused hypothesis and clearer ecological interpretations. Integration of pigment and microscopy data is also limited. I believe the paper can be significantly improved with revisions and recommend publication after addressing the comments below.
Abstract
- Add specific quantitative outcomes (e.g., % changes in phytoplankton groups, temperature anomalies etc).
- Clarify the main ecological implication of observed shifts - how do they influence productivity or carbon export?
Introduction
The introduction provides a good overview of the context surrounding marine heatwaves and their ecological importance, particularly in polar systems.
- The authors should clarify the rationale behind combining satellite data with in situ pigment and taxonomic data and articulate the research question or hypothesis more clearly. This would provide stronger direction for the reader and better frame the significance of the study's findings.
Methods
While the general methodological framework is outlined, important details are missing or need clarification.
- The pigment analysis section should specify which marker pigments were used to identify different phytoplankton groups, along with a reference to the methodology.
- Similarly, the taxonomic identification protocols need further elaboration: how were small flagellates and cryptophytes distinguished under the microscope? What criteria or taxonomic references were followed?
- Additionally, the sampling design lacks clarity. The number of stations, depth profiles, replication strategy, and temporal frequency should be explicitly stated. A table summarizing each station, its coordinates, and associated environmental and biological parameters would be highly informative.
- The statistical treatment of the data is underdeveloped. At present, community-environment relationships are not quantitatively analyzed. Application of multivariate techniques such as Principal Component Analysis (PCA), or Canonical Correspondence Analysis (CCA) would help strengthen the interpretation of community structure and its drivers.
Results
- The authors describe shifts in phytoplankton composition and pigment concentrations across stations but do not substantiate these changes with statistical comparisons. Including statistical testing e.g., ANOVA, regression, or clustering would support the observed trends and improve scientific rigor.
- Figures could be greatly improved. Stacked bar charts illustrating group-level phytoplankton abundance by station or region would provide a clearer visualization of community shifts. Overlaying pigment concentrations on satellite SST or chlorophyll maps would also help to connect in situ and remote observations.
- Integrate pigment and microscopy-based data to corroborate group dominance.
Discussion
- The discussion is informative but lacks depth in its ecological interpretations. The authors should explore the mechanisms underlying the observed shift from diatoms to cryptophytes and flagellates. What biological or ecological traits allow cryptophytes to flourish during warm, stratified, possibly low-nutrient conditions? Discussion of motility, mixotrophy, or cell size could offer explanations.
- The authors should also contextualize their findings within the broader literature. Have similar shifts been reported during other MHWs in polar or temperate systems? Drawing comparisons with other studies would strengthen the generalizability of the results.
- Importantly, the biogeochemical implications of these community changes are not discussed. How might a shift toward flagellates affect carbon export efficiency, nutrient recycling, or trophic transfer? Finally, the discussion should acknowledge the limitations of the current study, including the absence of nutrient data, limited temporal coverage, and potential sampling biases.
Conclusion
The need for continued monitoring and integration of ecological and biogeochemical data in a changing Southern Ocean should be included.
Figures and Tables
- Several figures are of low resolution and should be revised to meet publication standards.
- A figure showing phytoplankton community composition by station, using bar plots or maps, would add clarity.
- A table including station metadata, environmental variables, and phytoplankton abundances would greatly assist readers in interpreting spatial patterns.
- If zooplankton data were collected during the cruise, these should be reported and briefly discussed, as they could influence phytoplankton standing stocks through grazing.
Language
The manuscript is well-written overall, with only a few grammatical errors.
Citation: https://doi.org/10.5194/egusphere-2025-2892-RC1 -
RC2: 'Comment on egusphere-2025-2892', Anonymous Referee #2, 18 Aug 2025
Review to the manuscript entitled “Response of phytoplankton communities to the onset of the 2020 summer marine heatwave in the Drake Passage and Antarctic Peninsula” by Andrés S. Rigual-Hernández et al., submitted to EGUsphere
General comments:
I read this contribution by Andrés S. Rigual-Hernández et al. with great interest, as it addresses the important question of species-specific phytoplankton responses to marine heatwaves (MHWs) in the Southern Ocean — a region as climatically relevant as it is sensitive to climate change. The study is based on an exceptionally valuable in-situ dataset collected along the Drake Passage, one of the most critical ocean gateways on the planet and a notoriously challenging region to sample. The transect cuts across key physical, chemical, and biological gradients and fronts that structure the Southern Ocean, providing an opportunity to examine how climate forcing — in this case warming and stratification — impacts nutrient availability and phytoplankton communities across boundaries between Subantarctic, Polar, and Antarctic shelf-influenced waters. The focus on diatoms and coccolithophores, two functional groups central to food webs and biogeochemical cycles, further highlights the importance of this work.
The authors assembled a comprehensive multiparametric dataset, combining in-situ, satellite-derived, and reanalysis products, which enables exploration of the drivers of productivity and species composition in relation to hydrographic and air–sea exchange processes. I particularly value the sampling design, which captured conditions both before and during the MHW, allowing for an assessment of potential community shifts linked to warming. The manuscript is well written, and the study has notable potential to advance our understanding of diatom and coccolithophore responses to warming in the Southern Ocean, not only along the Drake Passage but also in adjacent areas such as the Northern Antarctic Peninsula.
That said, I have a few comments and suggestions for strengthening the manuscript:
Baseline context: The paper would benefit from a clearer discussion of baseline conditions in the study region, situating results within the existing literature. Explicitly distinguishing which findings align with or diverge from established knowledge would sharpen the argument for local MHW impacts.
Integration of results: Rather than structuring the discussion separately as “nutrient distributions” and “phytoplankton abundances/species distribution,” I recommend integrating these aspects. Both are interdependent and currently discussed across subsections, which leads to some redundancy and confusion. An integrated analysis would better highlight the mechanistic links the authors aim to establish.
Hypothesis clarity: In some instances, the proposed mechanisms could be more fully elaborated to guide the reader through the reasoning. A schematic diagram could be very effective in summarizing the hypothesized processes.
Sampling depth limitation: The exclusive use of surface (5 m) samples should be acknowledged more explicitly as a limitation. While surface-based studies are common, especially in remote regions such as the Drake Passage, phytoplankton communities below the surface mixed layer may respond differently, and this should be discussed in terms of implications for the study’s conclusions.
Warm-pool mechanism: The argument that temperature anomalies arose from warm pools advected southward by anticyclonic eddies is compelling. However, if this mechanism is correct, why is there no evidence of concomitant advection of coccolithophore-enriched waters in the northbound compared to the southbound transit? This apparent discrepancy requires clarification.
Specific comments:
L25: replace “are” by “is”.
L35: when you say: mirrored the physical and chemical properties” is not fully clear whether you refer to “background” conditions or in relation to the MHW.
L39-40: (..) abundance reaching bloom concentrations (…)”
L45-46: Wouldn’t surface waters advected from lower latitudes be relatively enriched in coccolithophores, even in the presence of a small diatom bloom?
L64-66: I would reframe to: “little information exists about the effects of marine heatwaves on phytoplankton in the Southern Ocean (…)
L83-84: could you expand on the reasons for this ecological shift in response to sea ice decline?
L97-99: is the reduced mineral ballast availability the main issue about this compositional shift? Or is it also – if not mostly – about the lesser carbon sequestration by smaller-sized phytoplankton (i.e., lower carbon content)?
L109: delete “of” before “temperature”.
L109-110: are referring to temperature as a key factor affecting the physiology of the cells, or by controlling the dynamics of the mixed layer depth?
Figure 1: You mention 51 stations for the nutrient and phytoplankton data selected for this study, but I counted only 48 in the figure. Perhaps consider changing the color of the black labels for improving its visualization.
L352 – Results: several parts of this section read like “discussion of results”, especially sub-section 3.1, which leads to some repetition along the ms. Already the in the first sentence, you start by saying that MHWs and anticyclonic eddies are related, the latter reinforcing the first, before presenting the results. I would leave this kind of interpretation for thew discussion.
L353-354: please keep just one title for sub-section 3.1; I would recommend choosing “Satellite-derived and model data”.
Figure 3 – I see a station number 17 which is not in represented in Figure 1; and a station number 12 in Figure 1 which is not in represented in Figure 3. Please indicate to which parameters correspond the colors in Figure 3a. There is no easy way in representing the data but I find this figure somewhat challenging to related with Fig. 1 as well as in terms of time. Please consider representing the the two transits along the Drake Passage in a way that facilitates visualizing the changes in the region over time (i.e., before and during the MHW) and space (oriented from north to south). I would recommend using latitudes instead of the stations numbering to have a clearer persective on the meridional extent of the observed changes.
L460-461: By only addressing the mean fluorescence and satellite Chl-a instead of the spatial variation pattern you might be losing relevant information for discussing the imoacts of the eddy-entrapped MHWs on surface productivity. For example, the northbound transit appears to have higher fluorescence and lower satellite Chl-a compared to the southbound. These differences may hold relevant information which is not discussed in the paper.
Figures 4 and 5 – I would add a curve representing the temperature anomaly at the top of the plots, to facilitate the visualization of how phytoplankton responded before and during the MHW.
Line 501: add “in” after “concentrations”.
Line 510-511: not clear, please refrase.
Line 516: add “community” after “diatom”.
L543: Discussion: I would not start with the characterization of the MHW; I think the discussion would be clearer if you would start with an integrated discussion of the region’s “background conditions” (i.e., the southbound transit), including both biological and chemical (nutrients) data. This section would provide a contribution to the existing knowledge about this remote region, in comparison to previous studies. Then, in a second section, you could discuss the mechanisms behind the onset of the MHW, and discuss its associated effects on chemistry and biology.
L551: When you say “This patern is characteristic of the Drake Passage”, it is not clear whether you are just referring to eddy formation or to MHWs being trapped in such eddies in the region.
Current sections 4.1, 4.2 and 4.3 read a bit disconnected and the titles don’t fully capture their content. It is not clear what is “background” and what is “anomaly-driven”.
L643: delete “in the”
L770: “seem” instead of “seems”.
L779: reframe to: “algal biomass accumulation between early January and early February during almost did not change”
L800-803: why would the drop in nitrates be so “damaging” for coccolithophores and not for small diatoms?
L802-812: I am not fully convinced by these culture-based arguments to explain to reduced abundance of coccolithohores, foremost of E. huxleyi, during the warming event…
L813-816: As much valuable as this contribution is, I would recommend more moderation in doing such far-reaching speculations given that this is data from one single expedition, and only the surface part of the photic zone of a rather dynamic ocean region.
L836: delete “of the”.
Citation: https://doi.org/10.5194/egusphere-2025-2892-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 318 | 39 | 12 | 369 | 22 | 10 | 20 |
- HTML: 318
- PDF: 39
- XML: 12
- Total: 369
- Supplement: 22
- BibTeX: 10
- EndNote: 20
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1