the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Long-term impacts of mixotrophy on ocean carbon storage: insights from a 10,000-year global model simulation
Abstract. Mixotrophs — organisms that combine the use of light and inorganic resources with the ingestion of prey — have been shown in simulations to increase mean organism size and carbon export. These simulations have, however, been limited to decade-long timescales that are insufficient to investigate the impacts of mixotrophy on the ocean's long-term capacity for carbon storage. Here we explore these long-term impacts using a low-resolution ocean model that resolves important feedbacks between surface ecology and the ocean interior over multi-millennial periods. The model was compared in two configurations: one with a strict distinction between phytoplankton and zooplankton populations and one in which all populations were assumed to be capable of mixotrophy. Consistent with earlier studies, we found that increased carbon and nutrient export associated with mixotrophy was rapidly established within the first few years of the simulation and robust over long time scales. However, we also found that these increases were partially offset over longer time scales by a decline in dissolved inorganic carbon and nutrients entering the deep ocean via the sinking of surface waters. Over the 10,000 year duration of the simulations, we found that ecologically-driven changes in C export increased the oceanic C inventory by up to 626 Pg, and that this was partially offset by decline of 149 Pg in the preformed C inventory, leaving a net increase of up to 477 Pg C (1.3 %).
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RC1: 'Comment on egusphere-2025-3050', Anonymous Referee #1, 29 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3050/egusphere-2025-3050-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-3050-RC1 -
RC2: 'Reply on RC1', Anonymous Referee #2, 18 Sep 2025
This paper extends previous modeling work by Ward and Follows, in order to examine whether effects of mixotrophy on ocean biogeochemistry are robust over long timescales. Overall the work is a useful contribution to biogeochemical modeling and mixotrophy research, and the manuscript is clearly written. I have a few minor suggestions that may help clarify the results and their interpretation.
The details of the ecological model are summarized very briefly, because the model was described in a previous publication. However, there are some details that are important when trying to understand the model outcomes, which I think should be included in this manuscript. Specifically:
-How does export flux work? Is there a relationship between organism size and sinking rate? Is there sinking of detrital particles? Is there any particle aggregation?
-How does cellular stoichiometry work? Is stoichiometry fixed or flexible?
-How many potentially limiting nutrients are tracked and included when modeling growth rates (e.g., N, P, Fe)?
The authors focus on gradients of DIC and phosphate, and never discuss nitrate. Is there a reason for this? Do the phosphate and nitrate patterns tell the same story? Nitrogen limits phytoplankton growth more commonly than phosphorus, so it seems important to have some discussion of nitrate.
The authors interpret the effects of mixotrophy on phosphate in high latitude regions in terms of iron limitation. Specifically, their hypothesis is that mixotrophy alleviates iron limitation, leading to greater phosphate drawdown. This is an interesting idea that would benefit from some more exploration. Which regions of the ocean are iron-limited in this model? And is there more evidence from the model output, such as the growth limitation terms, that can shed light on whether mixotrophy alters iron limitation?
The Sensitivity to Tradeoffs section belongs in the results section, not the discussion.
Citation: https://doi.org/10.5194/egusphere-2025-3050-RC2 -
AC2: 'Reply on RC2', Marco Puglia, 07 Oct 2025
We thank the Reviewer for the clear and constructive comments and for recognizing the value of our long-term simulations. We intend to address all the points raised in order to improve the clarity of the manuscript.
We will also:
- clarify how the production and effects of sinking organic matter are represented in the model.
- add a short paragraph to the methods providing an overview of the basic structure of the model, including limiting nutrients and the representation of cellular stoichiometry.
- note that the EcoGEnIE does not currently include a representation of dissolved inorganic nitrogen. While this is not realistic, phosphate provides a reasonable representation of global macronutrient limitation.
- include some discussion of the modelled patterns of iron limitation.
- move the “Sensitivity to Tradeoffs” section from the Discussion to the Results.
Citation: https://doi.org/10.5194/egusphere-2025-3050-AC2
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AC2: 'Reply on RC2', Marco Puglia, 07 Oct 2025
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AC1: 'Reply on RC1', Marco Puglia, 07 Oct 2025
We thank the Reviewer for the thoughtful and constructive comments, as well as for recognizing the novelty of our approach and the clarity of the results. We intend to address all the points raised in order to improve the clarity of the manuscript.
We will also:
- add figures to the Appendix that demonstrate how the model state variables have achieved steady state without long-term trends.
- revise the description of carbon sequestration in the North Atlantic, explicitly mentioning the role of preformed nutrients.
- add a short paragraph to the methods providing an overview of the basic structure of the model.
- cite Ward et al. (2018) to explain how/why mixotrophs can have different stoichiometries.
- provide a more detailed explanation of how the present model variables (e.g. plankton size, carbon export, DIC and PO4) are defined.
- add ticks to the colour scales and clarify where the colours represent [Mixotrophic minus Two-Guild] vs log10[Mixotrophic divided by Two-Guild].
- add a figure to the Appendix showing baseline values of the variables in Figure 2.
- rewrite ambiguous sentences for clarity.
- revise our description of Figure 3 to emphasise the most relevant points.
- check all figures to ensure they show the correct values on the appropriate scales.
- clarify that all differences correspond to the ones found at the end of the 10K years.
- add an additional table showing differences in exported C and P.
Citation: https://doi.org/10.5194/egusphere-2025-3050-AC1
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RC2: 'Reply on RC1', Anonymous Referee #2, 18 Sep 2025
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