| 01 Dec 2025
The Impact of Large-Scale Macroalgae Cultivation and Harvesting Strategies on the Marine Carbon Dioxide Removal Efficacy and Marine Biogeochemistry
Abstract. The large-scale cultivation of macroalgae has been proposed as a marine carbon dioxide removal (mCDR) strategy, yet its efficiency and consequences for ocean biogeochemistry remain uncertain. Using a new macroalgae aquaculture module within an ocean biogeochemistry model, NEMO-MEDUSA, we investigate carbon removal potential and biogeochemical feedbacks under hypothetical global-scale macroalgae cultivation with varying harvest strategies, loss rates, and iron availability. Overall cultivation enhances air–sea CO2 uptake by 11.0 Pg C yr-1, but only ∼ 27 % of macroalgal production results in additional CO2 uptake. Furthermore, natural phytoplankton and zooplankton biomass is suppressed by almost 50 % and is geographically displaced by significant surface nutrient changes. Sinking of the harvested biomass to the deep ocean drives widespread oxygen depletion (-20 % globally), creating new suboxic zones in deposition regions. When macroalgal growth is not supplemented with iron micronutrient, its production declines sharply (-74 %), revealing a significant limitation for large-scale feasibility. Collectively, our results reveal that large-scale macroalgal cultivation offers low mCDR potential, that it is both spatially extensive and locally intensive, and its unintended biogeochemical consequences can be substantial. Our findings highlight the urgent need to assess nutrient constraints and ecological trade-offs before considering this method as a viable large-scale mCDR strategy.
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?