This paper develops a model to examine how macroalgae cultivation and harvesting affects mCDR efficacy and marine biogeochemistry. The paper examines three hypotheses, (1) the extent to which macroalgae cultivation can add to mCDR, (2) how nutrient uptake and light limitation affects lower trophic levels, and (3) the consequences of dumping macroalgae biomass on the biogeochemistry of deep ocean waters.

This investigation is done by combining information from NEMO and ERA5 and modifying MEDUSA and using parameters from the literature. A number of assumptions are made to model parameters.

The paper adequately addresses hypothesis (1) see Figure 3, where the authors show that total PgC /yr changes with model parameters. However I am not convinced that the paper address (2); although the paper suggests that phytoplankton  NPP was reduced by macroalgae, I think this is an over-interpretation of the model results. I am not convinced that slow-growing macroalgae will greatly affect phytoplankton NPP. The model shows that phytoplankton NPP is reduced by macroalgae, but this is after ignoring herbivory and trophic interactions. Finally, (3) there is little discussion regarding biogeochemical impacts and given the model, this hypotheses/section is not needed.

Nevertheless, the paper is well written and logically structured, however the authors need to explore the literature regarding seaweed cultivation, especially with regards to differences between cold-water and warm-water species.

Some specific comments:

31-34 Provide some citations that support the concept that POC and DOC are released from macroaglae.

34-37 If macroalgae release POC and DOC as a fraction of NPP, then cultivated macroalgae can also be expected to release POC and DOC during production. I agree that harvest will remove most of the OC, however simply stating that no macroalgae carbon would be sequestered opposed the statement made in the previous statement.

123  Why was depth set between 5 to 10 m? In East Asia, species such as Undaria pinnatifida are cultivated on ropes deployed at depths of 1 m. Some types of Saccharina are also cultivated at 1 m depths. For some examples, see Choi et al. 2025.Journal of Marine Science and Engineering 13; Sato et al. 2023. Frontiers in Marine Science 10; Hwang et al. 2018. Algae 33

130 Macrocystis, Saccharina, Sargassum, and Eucheuma all have different methods of cultivation. Note that Sargassum is not commonly cultivated and Eucheuma is cultivated in shallow waters.

Table 1 

Growth rates of these four taxa are not the same; the same can be said for many of the parameters listed in this table. For example, even within taxa, the C content can vary between 0.2 to 0.4 mg / mg (Sato et al. 2025. Phycological Research). 

What exactly is non-harvest loss? Is this the DOC and POC released during production? See Pain et al. 2021 Journal of Phycology 57; Canvin et al. 2024 Journal of Applied Phycology 36; Zhong et al. 2024 Marine Environmental Research 202; Neves et al. 2025 Science of the Total Environment 982; 

Do the temperatures used in the simulation reflect the location where each taxa are expected to be cultivated? 

Do the temperatures used in the simulation, which was run for 20 years, reflect the slow increase in water temperature and how this influenced the increase in herbivory due to herbivorous fish? See Verges et al. 2022 Scientific Reports 12; Barrientos et al. 2022 Frontiers in Marine Ecosystem Ecology 9

When are the nutrients provided in the model?  I assume it is a pulse, but the nutrient regime is not clear.

How is growth calculated? What is the initial biomass (inoculation) ?

221-223 “frequent harvest”: Generally, for most Saccharina species harvesting occurs during a set period of time and there is really only “one” harvest. For example, if a long-line of kelps are harvested, this can only occur once. Note that harvesting will only occur during the end of the cultivation season for most Saccharina and Sargassum species, such as S. horneri or S. fusiforme. In other words, harvesting is periodic and does not occur daily over the entire growing season.

3.1 Seaweed production and harvesting: This might be an ignorant question, but what happens to the unharvested seaweeds in the model? OR is all seaweed harvested?

230 Please provide details on how air-sea CO2 flux is estimated.

Figure 4 & 5 & 6 This is a very over-optimistic perspective of macroalgae cultivation. Why do the regions that have productive seaweed farms (i.e., East Asia, South East Asia) poorly resolved? Can we expect that seaweed cultivation can occur around Antarctica? These figures are a phytoplankton-centric perspective of seaweed cultivation.

307-315 Although these models are great are providing some thought into what may happen if we could potentially cultivate macroalgae at scales covering the entire ocean, I doubt that it is feasible and vastly overestimates the true potential of macroalgae aquaculture. If the model is conditioned on feasible cultivation areas, would is the potential sequestration rate? 

Publisher’s note: a supplement was added to this comment on 4 March 2026.

Introduction Summary:  The Additionality and Baseline Crisis 

The manuscript employs the NEMO-MEDUSA global biogeochemical model to simulate the efficacy of macroalgae-based Marine Carbon Dioxide Removal (mCDR). The authors conclude that a global cultivation strategy could yield a 27% "net" sequestration efficacy. However, the fundamental scientific contribution of the paper is undermined by its treatment of the "Baseline."

A rigorous mCDR assessment must quantify Additionality: the carbon removed above and beyond what the natural ocean would have sequestered in the absence of intervention. In this study, the authors report a 27% efficacy while simultaneously observing a 50% suppression of natural phytoplankton communities. This represents a massive "Ecological Opportunity Cost." If the seaweed merely replaces the sequestration function of a natural biological pump, the reported efficacy is a "Gross" figure that misleadingly suggests a climate benefit where there may be a net-zero or even negative carbon result.

II. Detailed Technical Critique 1. The Thermodynamic Metric Error: NPP vs. NEP

A central technical deficiency throughout the manuscript is the conflation of Net Primary Production (NPP) with the drivers of air-sea CO2​ flux.

• The Conceptual Flaw: NPP represents the carbon fixed by the seaweed itself. However, the atmosphere is blind to NPP; it only "sees" the Net Ecosystem Production (NEP), defined as NEP=GPP−Rcommunity​.
• The Respiration Penalty: Large-scale seaweed farms are not isolated carbon sinks; they are complex ecosystems that support high rates of community respiration, including organic matter exudation and microbial decomposition.
• The Modeling Gap: If the NEMO-MEDUSA implementation uses NPP-driven drawdown to calculate the pCO2​ gradient, it inherently ignores the carbon "leakage" back into the DIC pool from the farm’s own respiratory community. The authors must explicitly state whether their flux calculations are driven by a true NEP deficit. Failure to account for the respiration of the farm's heterotrophic community results in a significant overestimation of the "hole" created in surface DIC.

2. Kinetic Decoupling: The "Slow Tap" vs. The "Leaky Bucket"

The authors attribute the 27% efficacy to physical transport—the "Leaky Bucket" model (consistent with Heane et al., 2023 and Ho et al.). This assumes that the carbon deficit sinks into the deep ocean before it can be "filled" by the atmosphere.

• The "Slow Tap" Argument: I contend that the primary rate-limiter is the Kinetic Lag of the gas transfer velocity (k). Chemical re-equilibration of the mixed layer with the atmosphere has a characteristic timescale (τ) of roughly one year.
• Seasonal "Expiration": Seaweed growth is a transient, seasonal event. As established in recent research regarding non-equilibrium NEP deficits (e.g., Ito et al., 2025), the biological drawdown happens within a 3–4 month window.
• The Physics of Failure: By the time the atmosphere begins to respond to the seasonal pCO2​ drop, the bloom has ended, the mixed layer has deepened, or the "deficit" has physically reset. The 27% efficacy is not a result of "losing" carbon to the deep; it is a result of the atmosphere never having enough time to pay the "carbon debt" created by the seaweed. The authors must provide a sensitivity analysis of the equilibration timescale versus the bloom duration to justify their efficacy figures.

3. Ecological Substitution: The Borum & Sand-Jensen Zero-Sum Constraint

The model treats macroalgae as an "additive" removal tool, which contradicts established ecological principles of competitive exclusion.

• The Constant NPP Principle: Following Borum & Sand-Jensen (1996), total system NPP in marine environments is remarkably constant. When macroalgae are introduced, they shade and outcompete natural phytoplankton for light and nutrients.
• The Iron Fertilization Fallacy: The authors note a 74% growth reduction without iron supplementation. However, if iron is added, natural phytoplankton (which have higher surface-area-to-volume ratios) often respond faster, blooming and shading the seaweed.
• The Substitution Penalty: If the model replaces a natural phytoplankton community—which may have had its own sequestration efficacy—with a seaweed farm, the Net mCDR is only the difference between the two. The authors must present a "Net-Net" analysis: (SystemFluxSeaweed​−SystemFluxBaseline​). Reporting 27% efficacy while ignoring the 50% loss of natural workers is scientifically indefensible.

4. The Carbonate Counter-Pump: The "Bach Critique"

The manuscript treats seaweed as a "soft-tissue-only" pump, ignoring the inorganic carbon cycle.

• Epibiont Calcification: Seaweed fronds are colonised by calcifying epibionts (bryozoans, tube worms). As highlighted by Lennard Bach, calcification releases CO2​ (Ca²⁺+2HCO₃⁻→CaCO₃+CO₂+H₂O).

The Ballast Crisis: Displacing calcifying phytoplankton (coccolithophores) removes the mineral ballast that allows organic carbon to sink rapidly. Without this "ballast," organic seaweed fragments remineralize in the upper ocean, returning CO2​ to the atmosphere much faster. The authors must incorporate an Alkalinity Budget to prove that the seaweed-dominated system actually lowers pCO2​ more than the coccolithophore-dominated baseline it replaces.

Summmary verdict  and actions required

Overall Verdict: This paper sits in a difficult middle ground. While it utilizes a sophisticated model (NEMO-MEDUSA) and expands the geographical scope of macroalgae mCDR beyond the typical EEZ boundaries, it does not offer a significant leap in mechanistic understanding, whilst inadvertently supporting current misconceptions on the role of the air sea flux and ignoring the impact of biological calcareous production.

1. Lack of Novelty:

The authors essentially "re-discover" the limitations already voiced by Prof. David Ho and Philip Boyd regarding iron limitation (74% drop) and low air-sea efficiency (27%). By modeling "global" seaweed farming, they are merely showing that the open ocean is an even less efficient place for mCDR than the coastal zones, which is an expected result rather than a novel insight.

1. Physical and Chemical Omissions:

The manuscript ignores two major developments in recent literature and a category error:

• The Carbonate Counter-Pump within oceanic seaweed ecosystems: As established by Lennard Bach, you cannot model macroalgae sequestration without an alkalinity budget from calcifying epibionts. Furthermore this is compounded in regards to C mitigation relative to baseline in the replacement (partial to total) of areas with coccolithophore production that likely inflates the reported C mitigation efficacy.
• Non-Equilibrium Dynamics: The paper relies on the "standard model" of down welling as the cause of low efficiency. However, the true bottleneck, is the permanent kinetic lag of CO2 invasion as supported by my own work on seaweed and blue carbon systems and citation therein (Nishihara et al. 2025), the driver of this through seasonal surface CO2 deficit variation that requires a input resolution of less than 1 the  month, not used in their application of the NEMO-MEDUSA model. All this leading to a misunderstanding that down welling is the primary driver of sequestrtaion inefficiency is primarily down welling. Thus leading to wrong conclusions on where of seaweed C sequestration effciency would be the worst or best.
• The use of NPP as the biological that ignore community respiration outside algal metabolism, when  it needs to be NEP (Gallagher et al. 2022).

Recommendation:

In its current state, the paper risks being a "replication study" that repeats the structural errors of previous models (treating the ocean as a leaky bucket rather than a kinetically limited system). I have recommended a Major Revision. To merit publication, the authors must move beyond "gross" sequestration figures and provide a Net System Analysis that accounts for:

• Surface water community respiration for NEP as the biologically driven CO2 deficit ((Ito & Reinhard 2025)
• The displacement of natural phytoplankton (Borum & Sand-Jensen 1996) "zero-sum" NPP.
• The carbonate chemistry penalty (Bach et al. 2021).
• The kinetic re-equilibration lag on seasonal scales (Ito & Reinhard 2025; Jiang et al. 2019) for which I add my own daily simulation in a supplementary document to clearly illustrate surface water dynamics. This is a figure from our unpublished article currently in submission, using a similar but higher resolution and likely NEP for seaweed variation. 

If the authors cannot integrate these non-equilibrium and competitive dynamics, the paper’s "global potential" maps are more misleading than helpful for the mCDR.

References

Bach LT, Tamsitt V, Gower J, Hurd CL, Raven JA, Boyd PW (2021) Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt. Nature Communications 12:

Borum J, Sand-Jensen K (1996) Is total primary production in shallow coastal marine waters stimulated by nitrogen loading? Oikos 76(2): 406-410

Gallagher JB, Shelamoff V, Layton C (2022) Seaweed ecosystems may not mitigate CO2 emissions. ICES Journal of Marine Science 79(3): 585-592

Ito T, Reinhard CT (2025) A New Framework for the Attribution of Air‐Sea CO2 Exchange. Global Biogeochemical Cycles 39(2):

Jiang ZP, Cai WJ, Lehrter J, Chen B, Ouyang Z, Le C, Roberts BJ, Hussain N, Scaboo MK, Zhang J, Xu Y (2019) Spring net community production and its coupling with the CO2 dynamics in the surface water of the northern Gulf of Mexico. Biogeosciences 16(18): 3507-3525

Nishihara GN, Sato Y, Eger AM, Gallagher JB, Hurd C, Kawai H, Kuwae T, Pessarrodona A (2025) Expert opinions regarding the concept of blue carbon in seaweed systems. Phycological Research: