the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jan 2026
Mesopelagic fish responses to Pleistocene climatic variability in the Eastern Mediterranean and implications for the biological pump
Abstract. Mesopelagic fishes play a crucial role in the global carbon cycle through their diel vertical migration (DVM), but the impacts of neither natural nor anthropogenic climate change on DVM patterns are currently known. Studying the geological past can elucidate changes in DVM patterns under swelling climate pressure and allow estimating the impacts of the current climate crisis. We present a multi-proxy, ecosystem-level assessment of paleoenvironmental changes in the Eastern Mediterranean during the Middle Pleistocene (marine isotope stages MIS 23–18; 923–756 ka B.P.) and use the carbon and oxygen isotopic composition of fossil fish otoliths to assess the impacts of these changes on DVM and their possible implications on the biological pump. Temperature was the primary driver of ecosystem change during MIS 21 interglacial, whereas productivity became a dominant factor in MIS 19 interglacial. Responses of organisms throughout the water column varied. Our results indicate increased productivity across trophic levels during MIS 19, which affected foraminiferal biomasses, but did not inhibit fish DVM. In contrast, the early MIS 21 warming led to a reduction in DVM by mesopelagic fishes and consequently a drop in biological pump efficiency.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6151', Anonymous Referee #1, 16 Feb 2026
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RC2: 'Comment on egusphere-2025-6151', Anonymous Referee #2, 17 Feb 2026
The manuscript “Mesopelagic fish responses to Pleistocene climatic variability in the Eastern Mediterranean and implications for the biological pump” by Agiadi et al. represents a potentially interesting contribution to the understanding of biological pump variability in relation to Pleistocene climate changes, using otoliths as a proxy. The methodological approach is innovative and still rarely applied to this fossil group. However, the methodology section is, in its current form, difficult to follow and lacks clarity. Moreover the entire manuscripts is not enough focused on the central question it aims to address. I find a very dispersive methodologies and discussion that go far to the central question of the paper. The manuscript needs a substantial rewrite and restructuring
The manuscript entitled “Mesopelagic Fish Responses to Pleistocene Climatic Variability in the Eastern Mediterranean and Implications for the Biological Pump” by Agiadi et al. represents a potentially valuable contribution to our understanding of biological pump variability in relation to Pleistocene climate change, using otoliths as a proxy. The methodological approach is innovative and still rarely applied to this fossil group. However, in its current form, the Methods section is difficult to follow and lacks clarity. Moreover, the manuscript is insufficiently focused on the central research question it aims to address. Both the methodological description and the Discussion are overly dispersive and frequently deviate from the core objectives of the study.
In my opinion, the manuscript requires substantial rewriting and restructuring before it can be considered for publication.
Major comments
At line 655, the authors address a crucial point of the manuscript. They state that fish DVM is disrupted during MIS 21 based on a single data point that differs from the others within the same interglacial, where I instead observe a pattern very similar to that of the other interglacials in the investigated interval. Therefore, the generalized scheme presented in Fig. 7 does not appear to be fully supported by your data.
At line 621, the authors refer that the otoliths from MIS 21 derived from juvenile specimens. All the analized otoliths are from juvenile specimens? This point is crucial and should be clearly specified. For Ceratoscopelus, several data points are presented for the interglacial intervals, whereas only a single datum is available for the glacial interval, which makes any comparison statistically weak. During MIS 21, the isotope record of C. maderensis shows both heavier and lighter values, suggesting substantial variability that is not adequately discussed. Could this variability be related to differences in otolith growth stage? At present, the dataset appears insufficient to support the proposed interpretation. Similarly, the isotope values of Hygophum benoiti are largely concentrated within interglacial intervals, while the single datum from MIS 22 falls within the same range as the interglacial values. On this basis, the inference of a distinct glacial–interglacial signal seems insufficiently supported.
Moreover, there are aspects related to the references that I would like to clarify. In particular, several methodological concepts are referred to Agiadi et al. (2024), “Pelagic ecosystem responses to changes in seawater conditions during the Middle Pleistocene Transition in the Eastern Mediterranean” (DOI: https://doi.org/10.1101/2024.12.28.630586). However, this DOI redirects to a preprint version of the present manuscript submitted to Biogeosciences. This creates confusion, as essential methodological information appears to be delegated to a work that is effectively the same study or a very closely related preprint.
Moreover, at line 365 the manuscript cites Fuster-Alonso et al. (2024), which is listed in the references as a code repository developed for Agiadi et al., 2024. This latter work is a bioRxiv preprint addressing the same conceptual framework and methodological basis as the manuscript under review. The distinction between these contributions and their respective roles in the methodological framework is therefore unclear.
I apologize for insisting on this point, but I find the procedure confusing and potentially problematic from the perspective of transparency and reproducibility. Even if this approach is considered acceptable, I strongly recommend that the authors revise the methodology section to make it more explicit, self-contained, and comprehensible, without referring readers to external manuscripts or preprints for concepts that are essential to understanding the results and discussion of the present study.
Citation: https://doi.org/10.5194/egusphere-2025-6151-RC2
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General comments
This manuscript presents an impressive multiproxy dataset. The paper addresses a timely topic: how climate variability might affect diel vertical migration of mesopelagic fishes and potential implications for carbon export. The manuscript contains lots of information. The Discussion contains many ideas. the paper has clear potential.
However, in its current form the manuscript is structurally diffuse and not sufficiently focused on the central question it aims to address. The headline inference chain: warming → reduced DVM → reduced biological pump efficiency—is repeatedly asserted but not yet fully supported with a sufficiently tight mechanistic bridge and uncertainty treatment for a top-ranked journal. The Methods are also very lengthy, disproportionately long in the main text, and key assumptions are not stress-tested.
Major comments
For example, in the Introduction, it would be to define the core question and hypotheses/predictions concisely (what would you expect to observe in otolith isotopes if DVM weakens/strengthens? what would you expect in productivity/export proxies?). in the Methods, keep only what is essential for evaluating the central claims in the main text; move methodological background and secondary proxy details to Supplement where possible.
The results shold separate proxy reconstructions from derived interpretations; avoid mixing inference with description.
In the Discussion, focusing on one main thread would be more efficient. inferred vertical habitat/DVM → mechanisms → implications for export (cautiously) → alternative explanations → limitations.
I recommend a clear distinction between direct observations (otolith δ¹⁸O/δ¹³C, foraminiferal isotopes, TOC/productivity proxies) and derived quantities (lifetime-average depth; inferred DVM intensity; inferred pump efficiency) in the manuscript. And replace causal language, sucha as replacing “consequently reduced pump efficiency” with appropriately conditional phrasing unless you provide a quantitative bridge (“consistent with / may imply / compatible with”).
Also, please define precisely what you mean by “biological pump efficiency” in this context and which proxy observations are intended to constrain this important key character.
Maybe one explicit “prediction vs observation” table would be helpful. what patterns would support DVM weakening vs community reorganization vs changes in water-mass structure? Just a recommendation, but this is not necessary if the manuscript structure is improved significantly.
I recommend adding a dedicated subsection that contrasts your interpretation with other plausible mechanisms and relevant recent work, for example:
Salvatteci et al. (2022, Science) emphasize warming impacts on ecosystem and community organization and trophic structure. Britten & Sibert (2020, Nat. Commun.) on climate-related changes in pelagic communities. Lin et al. (2023, Science Advances) demonstrated long records showing warming effects on mesopelagic fish production/diversity and community reorganization.
4) Your lifetime-average depth model is plausible, but assumptions are rather strong and should be stress-tested. The lifetime-average depth reconstruction from otolith δ¹⁸O is an interesting approach, but it relies on strong assumptions that should be made transparent and, ideally, tested. My concerns are that the model depends on the assumed δ¹⁸O–depth relationship and the assigned ecological depth ranges of the foraminiferal calibrators used to fit the exponential curve. These assumptions are especially consequential in the Eastern Mediterranean, where stratification and water-mass structure can vary substantially over time.
Maybe adding a sensitivity analysis showing how inferred fish depths change under reasonable alternative parameterizations would help.
Otherwise, δ¹³C-based conclusions should be framed more cautiously. i.e., “consistent with” rather than diagnostic.
Other commetns:
Line 42. The statement that Hygophum benoiti and Ceratoscopelus maderensis are “two of the most common mesopelagic fishes worldwide” is definitely not correct. These taxa may be very common in the Mediterranean sediment otolith record (e.g., Lin et al. 2017, 2018; and related work on Mediterranean distributions), but global mesopelagic dominance patterns differ substantially by region (e.g., Schwarzhans 2013: Gulf of Guinea/Azores case study).