Air‐sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?

To limit global warming below 2°C by 2100, we must drastically reduce greenhouse gas emissions and additionally remove ~100–900 Gt CO2 from the atmosphere (carbon dioxide removal, CDR) to compensate for unavoidable emissions. Seaweeds (marine macroalgae) naturally grow in coastal regions worldwide where they are crucial for primary production and carbon cycling. They are being considered as a biological method for CDR and for use in carbon trading schemes as offsets. To use seaweeds in carbon trading schemes requires verification that seaweed photosynthesis that fixes CO2 into organic carbon results in CDR, along with the safe and secure storage of the carbon removed from the atmosphere for more than 100 years (sequestration). There is much ongoing research into the magnitude of seaweed carbon storage pools (e.g., as living biomass and as particulate and dissolved organic carbon in sediments and the deep ocean), but these pools do not equate to CDR unless the amount of CO2 removed from the atmosphere as a result of seaweed primary production can be quantified and verified. The draw‐down of atmospheric CO2 into seawater is via air‐sea CO2 equilibrium, which operates on time scales of weeks to years depending upon the ecosystem considered. Here, we explain why quantifying air‐sea CO2 equilibrium and linking this process to seaweed carbon storage pools is the critical step needed to verify CDR by discrete seaweed beds and nearshore and open ocean aquaculture systems prior to their use in carbon trading.


INTRODUCTION
Since the industrial revolution, humans have released carbon, which had been stored on geological timescales as gas, oil and coal, into the atmosphere.The resulting increase in atmospheric CO 2 from pre-industrial levels of ~280 ppm to current values of 417 ppm is causing ongoing warming of the atmosphere and ocean, terrestrial and marine heatwaves, and altered weather patterns; the cumulative effects of these rapid environmental changes are altering natural systems and challenging human societies across the globe to adapt (Bindoff et al., 2019;Smale et al., 2019;Smith et al., 2022).
To keep warming below 2°C, CO 2 emissions must not only be drastically reduced, but also 100-1000 Gt CO 2 must be removed from the atmosphere over the present century (IPCC, 2018; 1 Gt CO 2 = a billion tonnes of carbon dioxide).Carbon dioxide removal (CDR) is defined as anthropogenic activities removing carbon dioxide (CO 2 ) from the atmosphere and durably storing it either in geological, terrestrial, or ocean reservoirs or in products (IPCC, 2022).Carbon sequestration can be defined as the storage of carbon-containing molecules, which have been removed from the atmosphere, for more than 100 years: sequestered carbon must be stored in a safe, reliable, and verifiable way (Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection, 2019).Ideally, carbon sequestration would permanently lock up CO 2 so that it does not re-enter the atmosphere in the future (Smith et al., 2023).Various strategies are being proposed for CDR.Ocean-based CDR includes ocean alkalinity enhancement (Renforth & Henderson, 2017) and increasing the amount of seaweed-derived carbon storage on a global scale via the expansion of aquaculture, including into the open oceans (ocean afforestation; Ross et al., 2023), by restoring seaweed ecosystems that have been lost due to climate change (Smale et al., 2019;Wernberg et al., 2011) and conserving existing seaweed ecosystems (Pessarrodona et al., 2023).
Seaweeds are highly productive primary producers of temperate rocky shores worldwide where they form substantial biomass in natural beds, provide energy to higher trophic levels, and create a habitat for invertebrates and fish (Hurd et al., 2014;Pessarrodona, Assis, et al., 2022;Pessarrodona, Filbee-Dexter, et al., 2022).They are being considered as a biological method of CDR and for eventual use in carbon trading as carbon removal offsets (Coleman et al., 2022;Cooley et al., 2022;Ross et al., 2023;Vanderklift et al., 2022;Figures S1-S7 in the Supporting Information): Indeed, some companies and NGOs are already selling seaweed carbon as offsets on the voluntary carbon market (Figures S2-S4).Ongoing projects include enhancing the scale of natural seaweed beds by restoring those that have been lost from climate change and local anthropogenic activities (Filbee-Dexter et al., 2022), protecting existing seaweed beds (Pessarrodona et al., 2023), and expanding the scale of seaweed aquaculture, including in the open ocean (N'Yeurt et al., 2012;Froehlich et al., 2019).Numerous start-up companies and NGOs are trialing growing seaweeds, particularly kelps (order Laminariales) but also pelagic Sargassum spp.(Order Fucales), in various aquaculture systems in the coastal and open oceans for CDR (Figures S1-S7).The size of artificial seaweed "farms" in some proposals is vast, for example, the creation of an ocean gyre-scale "Sargasso Sea" in the southern hemisphere (Figure S7).Some companies and NGOs are planning to sink seaweed biomass grown in the open ocean to the deep ocean for CDR (Figures S1,S4,S5,S7).An underlying assumption in all these trials (Figures S1-S7) is that the production of seaweed biomass leads to verifiable CDR that will cause a globally relevant reduction in (or result in avoided emissions of) atmospheric CO 2 .
Demonstrating long-term CDR and carbon sequestration by seaweeds is currently confounded by the lack of adoption of a clear definition of the term "carbon sequestration."The biogeochemical definitions of CDR and secure C-storage that we use (above) require the quantification and linkage of two processes, both of which are complex to measure: the removal of atmospheric carbon dioxide and the subsequent storage of the resulting organic carbon on long time scales (Bach et al., 2021(Bach et al., , 2023;;Hurd et al., 2022).However, in the literature, carbon sequestration has been variously considered as synonymous with net primary production (Chung et al., 2011(Chung et al., , 2017;;Eger et al., 2023), carbon assimilation (Chung et al., 2017), biomass production/standing stock (Filbee-Dexter & Wernberg, 2020;Sondak et al., 2017), or particulate organic carbon that is buried in sediments (Duarte et al., 2017).Although each of these processes is important for assessing carbon stocks, storage pools, fluxes, and budgets in seaweed systems and the carbon export to other systems, none are equivalent to CDR nor carbon sequestration, as they do not take into account the flux of atmospheric CO 2 from the atmosphere into seawater nor the fate of this CO 2 (Bach et al., 2021(Bach et al., , 2023;;Hurd et al., 2022).
Forensic Carbon Accounting (FCA) is a framework to assess the contribution of seaweed to both CDR and carbon sequestration and was developed to assist scientists, NGOs, and companies interested in assessing the potential of seaweeds for carbon offsets and credits (see the checklist in table 2, Hurd et al., 2022).Ensuring that carbon offset/credit schemes are legitimate is critically important for ensuring the integrity of carbon credits so that they can be used in carbon trading, that is, the removal of atmospheric CO 2 by seaweeds and its secure storage must be subject to a monitoring, reporting, and verification (MRV) process (Cadman & Hales, 2022).The FCA framework includes biological metrics, such as photosynthesis, standing stocks, and export of seaweed production to other systems, including nearshore sediments and the deep ocean (Hurd et al., 2022).It also incorporates an Earth system analysis because the carbon cycles of the atmosphere and oceans are linked on a global scale.There is much ongoing research to determine the contributions of seaweed carbon to various storage pools, particularly in the context of "Blue Carbon," including as living biomass, particulate and dissolved organic carbon (POC, DOC), and sedimentary seaweed carbon (e.g.Erlania et al., 2023;Gao et al., 2022;Li et al., 2022;Pedersen et al., 2020Pedersen et al., , 2021;;Perkins et al., 2022;Pessarrodona, Assis, et al., 2022;Pessarrodona, Filbee-Dexter, et al., 2022).In contrast, very few studies have considered air-sea CO 2 exchange (see Hurd et al., 2022).Here, we build on Hurd et al. (2022) by detailing why it is critical to link carbon storage pools with air-sea CO 2 exchange to quantify and verify CDR by seaweeds.

SE AWEED CARBON STOR AGE POOLS
Seaweeds contribute to a range of oceanic carbon storage pools including living biomass, as well as POC, DOC, and dissolved CO 2 that are released into the water column (Figure 1a).As seaweeds are short-lived with rapid turnover-times, they do not store carbon as living biomass on timescales relevant to sequestration (Hurd et al., 2022; Figure 1a).Seaweed beds do not create soil, which is the largest pool of sequestered carbon on land.A proportion of seaweed carbon is, however, exported (often laterallytermed out-welling) to other systems, where it may be stored on various times-scales, including in deep oceanic sediments (Figure 1a; Filbee-Dexter et al., in revision; Ould & Caldwell, 2022).As POC and DOC are transported via ocean currents beyond continental shelves (200 m isobath, Filbee-Dexter et al., in revision), a range of processes will regulate the fate of seaweed carbon including remineralization to CO 2 by heterotrophic bacteria and photochemical oxidation (Figure 1a; Filbee-Dexter et al., in revision).Depending on the oceanographic region, a substantial proportion of this remineralized CO 2 will be released back into the atmosphere, often within 50 years (Siegel et al., 2021).
POC ranges in size from tiny particles (100 μm and larger detritus) to large pieces of detached seaweed blade, holdfasts, stipes, or even entire detached seaweeds (Pedersen et al., 2020(Pedersen et al., , 2021)).It is an extremely important carbon-flux that supports higher trophic levels, being consumed within the seaweed bed by mobile and sessile herbivorous invertebrates (e.g., urchins, amphipods, isopods, snails, abalone; Figure 1a).A proportion of POC is exported to adjacent systems where it may also be consumed (Filbee-Dexter et al., 2020) or broken down into various biochemical components, some of which may reside in sediments on long timescales (Pedersen et al., 2021).The fate of kelp biomass will depend on various environmental factors that affect the rate at which it decomposes and the specific biochemicals the biomass is degraded to.Factors include temperature and whether or not the local environment is aerobic or anaerobic (Filbee-Dexter et al., 2022;Pedersen et al., 2021;Wright et al., 2022).In field studies of the kelp Laminaria hyperborea, breakdown times were between 150 and 300 days, with aerobic conditions leading to the complete degradation of the biomass into various substances including mannitol and polyphenols (both constituents of DOC), whereas under anaerobic conditions around 20%-30% of kelp material did not breakdown and was stored in the sediment (Pedersen et al., 2021).As larger pieces of POC transit offshore, they do so with a range of invertebrates and bacteria that will result in its decomposition (Boyd et al., 2022).
Living seaweeds release substantial amounts of DOC, which is defined as "any organic carbon particles that pass through a filter pore size 0.22-0.7 μm (GF/F filters)" (Paine et al., 2021(Paine et al., , p. 1376; Sondergaard & Middelboe, 1995; Figure 1).The release rate varies seasonally, with species and location (Paine et al., 2021(Paine et al., , 2023)).Dissolved organic carbon is a generic term for a wide range of molecules of various sizes and labilities.Labile DOC may be consumed almost immediately by bacteria in the water column.Recalcitrant and refractory DOC have much longer timescales of degradation and are the most likely form of DOC in long-term carbon storage pools: They are exported offshore to other systems, including shelf seas and the deep ocean, where DOC from various sources is considered an important carbon store on sequestration-relevant timescales (see Paine et al., 2021).Similar to POC, the composition of DOC will alter as it moves offshore and downwards.For example, the photochemical breakdown of DOC will likely increase in the optically brighter open oceans compared to turbid coastal waters (Wada et al., 2015).The contribution of seaweed carbon to the DOC pool is unknown, and we have little understanding of its transit time and fate offshore (Paine et al., 2021;Pessarrodona et al., 2023).
In addition to the natural fluxes of seaweed carbon beyond the continental shelf, there is considerable interest in enhancing these processes by growing seaweeds on artificial structures in the open ocean, termed "ocean (macroalgal) afforestation," and sinking resulting biomass to the deep ocean (Figure 1b S1, S4, S5, S7).As seaweeds sink, a range of processes (as above) will affect their degradation and the amount of biomass that reaches the deep ocean floor (Figure 1b).In some oceanic regions, once below the permanent pycnocline (~500 m), the seaweed carbon may be out of contact with the atmosphere for more than 100 years, but this residence time varies substantially with location (Siegel et al., 2021).In some respects, FCA for these aquaculture systems should be more straightforward, as they are monocultures (compared with highly diverse natural seaweed communities) with relatively few natural herbivores, so some biological processes needed for FCA may be easier to assess.Nevertheless, to quantify the amount of atmospheric CO 2 that sunken seaweed biomass has removed requires measurement of CO 2 equilibrium between the air and water in which the seaweed was growing prior to harvest or sinking.

AIR -SE A CO 2 EQUILIBR ATION
A key concept in assessing CDR by seaweeds is that there is constant flux of CO 2 into and out of seawater (Figure 2).Carbon dioxide enters (influx) and leaves (efflux) seawater as a gas, and it is the difference in the partial pressure of CO 2 in air and water that drives the movement of CO 2 .When considering CDR by seaweeds, it is important to recognize that CO 2 is constantly exchanged between the air and sea: Whether or not CO 2 enters or exits seawater, and the rate of this process, is essential in the assessment of CDR by seaweeds.However, this two-way movement of CO 2 to and from seawater has been overlooked in some conceptual figures of seaweed carbon sequestration potential (e.g., Coleman et al., 2022;Gao et al., 2022;Krause-Jensen & Duarte, 2016;United Nations Environment Programme, 2023).For example, figure 2 in Krause-Jensen and Duarte (2016) shows CO 2 entering the ocean as a result of seaweed biomass naturally sinking to the deep ocean, implying a direct link between sinking seaweeds, CDR, and carbon sequestration.This is misleading because these processes are not directly linked, as we illustrate later.
Processes governing the CO 2 -equilibration timescales between air and water are complex but need to be assessed to estimate CDR (Bach et al., 2023).If the partial pressure in air is greater than that in water, there will be a net influx of CO 2 and vice-versa if the partial pressure in water is higher than that in air.The chemistry of CO 2 in seawater is much more complex than that of other gases, such as O 2 and N 2 .When it enters the ocean, CO 2 does not simply dissolve, but it reacts with the water molecules, which starts a series of chemical reactions characterizing the seawater carbonate system (Figure 2).Bicarbonate (HCO − 3 ) is the predominant form of dissolved inorganic carbon (DIC) in the ocean with a concentration of ~1913 μmol·kg −1 compared with ~16 μmol·kg −1 for CO 2 (illustrative example, see legend of Figure 2).The series of reactions in the seawater carbonate system are reversible and act to buffer seawater and maintain seawater pH.However, air-sea equilibration time is about 20 times longer for CO 2 than for oxygen.The consequence is that the timescale for CO 2 to equilibrate with seawater is weeks to months in the coastal ocean, months to a year in the open ocean, and more than a year in the Arctic (Bach et al., 2023;Jones et al., 2014).These timescales of air-sea CO 2 -equilibration vary by geographic location (Jiao et al., 2018;Takahashi et al., 1997), and within each location they are modified by local biological conditions, particularly photosynthesis and respiration that can set the CO 2 gradient between air and water (Bach et al., 2023;Jiao et al., 2018); physical conditions, for example, bubble injection, low versus high wind sites that influences gas transfer efficiency, high versus low dynamics such as tides and currents (Takahashi et al., 1997); and chemical conditions, for example, slicks and surfactants (see figure 3 Hurd et al., 2022 for an overview).

QUANTIF YING CDR BY SE AWEEDS
The vast majority of seaweeds remove inorganic carbon (CO 2 or HCO − 3 ) from seawater and not the atmosphere (Figure 3).This photosynthetic uptake of CO 2 results in a deficit of CO 2 in the seawater within the seaweed bed, and in a second step, CO 2 enters seawater from the atmosphere via CO 2 equilibration (Figure 3a-c).The timescale of photosynthetic CO 2 uptake by seaweeds is seconds, whereas the timescale for equilibration is weeks to years (see above).Therefore, although seaweeds have removed CO 2 from seawater, it will take weeks to years for full equilibration depending on the system (Bach et al., 2021(Bach et al., , 2023)).The temporal mismatch between seaweed photosynthesis and CO 2 equilibrium means that knowledge of photosynthetic rates and the size of the various seaweed carbon storage pools does not equate to either CDR or carbon sequestration.To verify CDR, the critical step is to evaluate the air-sea CO 2 equilibration (Bach et al., 2023).
To determine CO 2 equilibration in the water body in which the seaweed was resident (natural or restored seaweed bed, coastal and open ocean aquaculture), the fate of the seawater that bears the CO 2 deficit needs to be followed.For example, if the seaweed bed/ aquaculture system is in a wave-exposed coastal site, which is typically beneficial for healthy seaweed growth (Hurd, 2000), then the seawater that bears the CO 2deficit will move rapidly to new locations with currents, along shore drift, and with tides and waves-such that the equilibration needed for CDR will not occur in the surface waters above the seaweed bed where the CO 2 was taken up by photosynthesis (Figure 3d,e).If seawater moves along the shoreline for several weeks and continues to stay in an autotrophic system, then it may result in CDR (Figure 3f1).If, however, the seawater moves over heterotrophic systems such as an oyster or mussel bed, then the water with the CO 2 -deficit will mix with high-CO 2 water, and there will be less net CDR; in some extreme cases (heterotrophic water column end-member), there may be a net release of CO 2 , although this net release should be smaller than if the seaweeds had not taken up CO 2 (Figure 3f2).In some marine systems, particularly the open ocean but also the coastal zone, the seawater bearing the CO 2deficit can subduct such that it is not in contact with the atmosphere for a time sufficiently long for equilibration and CDR (Figure 3f3; Bach et al., 2021;Orr & Sarmiento, 1992).
Therefore, the critical step in measuring and verifying CDR by seaweeds is to track the parcel of water from which seaweeds removed the CO 2 to quantify and verify that the CO 2 was removed by the seaweed bed or farm for which carbon removal offsets are being claimed.Only a few studies have recognized the need to measure air-sea CO 2 equilibrium by following the seawater that bears the CO 2 -deficit (Bach et al., 2021(Bach et al., , 2023;;Berger et al., 2023;Orr & Sarmiento, 1992;Watanabe et al., 2020).Methods to directly measure air-sea CO 2 flux are available (e.g., Dong et al., 2021;Takahashi et al., 1997;Yang et al., 2022), but their deployments are technically challenging ("an armada of sensors that cover much of the surface ocean") and unlikely to be economically feasible in the near future (Bach et al., 2023, p. 686).Although this is technically challenging, it could be achieved by including physical oceanographers and modelers into programs to assess CDR by seaweeds (see Berger et al., 2023; Van Dam F I G U R E 2 Air-sea CO 2 equilibrium.In its gaseous form, carbon dioxide (CO 2 ) is exchanged between seawater and the atmosphere.The rate and magnitude CO 2 flux into and out of the surface ocean is driven by the partial pressure of CO 2 and other processes (see text).Carbon dioxide dissolves slowly in seawater compared to other gases such as O 2 , and once dissolved it reacts with water (H 2 O) and is subject to a series of chemical reactions.The combined effects of the relative insolubility of CO 2 and the slow reactions times of the seawater carbonate system lead to a timeframe of equilibration between atmosphere and air of weeks to months (coastal ocean), to months to a year (open ocean), and to more than a year (Arctic).Illustrative values of the concentration of CO 2 and HCO − 3 were calculated using the following set of variables: pCO 2 (417 μatm), total alkalinity (2290 μmol·kg −1 ), salinity of 35, and a temperature of 15°C.[Color figure can be viewed at wileyonlinelibrary.com]CO 2 CO 2 (aq) 16 µM Air-sea CO 2 equilibrium and the fate of the water parcel that bears the CO 2 -deficit that resulted from seaweed net photosynthesis.Air-sea CO 2 exchange is indicated on the upper left side of each figure, but this process occurs across all water bodies illustrated.(a) Carbon dioxide (CO 2 ) in the atmosphere enters or leaves seawater (CO 2(g) ) as a gas.It dissolves into seawater to form CO 2(aq) , which is a component of the seawater carbonate system (see Figure 2).(b) Seaweeds remove CO 2 and bicarbonate (HCO − 3 ) from seawater on timescales of seconds, which (c) results in a CO 2 deficit in the seawater within the seaweed bed (gray shading).(d) The seawater bearing the CO 2 deficit moves away from the seaweed bed.For simplicity, it moves to the right of the picture (indicated by the arrow) and is replenished with new seawater (dashed line) from which seaweeds remove dissolved inorganic carbon.(e) The seawater bearing the CO 2 -deficit has moved completely away from the seaweed bed from where the CO 2 was photosynthetically fixed.We now consider three fates for the seawater bearing the CO 2 deficit.In (f1), the seawater has moved into another autotrophic (vegetated) system.In (f2), it has moved into a heterotrophic (e.g., an oyster bed) system.In both cases (f1 and f2), the seawater bearing the CO 2 deficit mixes with water in the new systems that will affect the direction and magnitude of air-sea equilibrium.In (f3), the seawater bearing the CO 2 deficit has moved offshore and subducted below another water parcel such that it is no longer in contact with the atmosphere.Subduction is most common in the open ocean where CO 2 equilibration times are months to a year and can result in incomplete CO 2 equilibration (see text).
[Color figure can be viewed at wileyonlinelibrary.com]  2023) propose a risk-assessment approach in which the global community works together to produce a "look up map" that makes available information on the amount and timescales of CO 2 equilibration in various oceanic regions over different seasons.This approach would be useful for open ocean afforestation but probably more difficult to apply in the more dynamic coastal waters where most seaweeds reside (see Berger et al., 2023;Van Dam et al., 2021).The importance of measuring airsea CO 2 equilibration is exemplified for seagrass systems by Van Dam et al. (2021), who found that four of six seagrass beds studied were net CO 2 sinks whereas two beds were net sources of CO 2 for the atmosphere.Despite the difficulties, such methods will need to be developed and applied to each seaweed system being considered for carbon credits or offsets.
F I G U R E 4 Schematic linking the two processes that are critical for MRV to assess whether discrete seaweed beds and onshore or offshore seaweed farms can be used in carbon trading schemes as offsets or credits.Air-sea CO 2 equilibrium is illustrated on the top left, but this process occurs in all water bodies illustrated.Black text and lines depict the various ways in which the seawater that bears the CO 2 deficit may move away from the seaweed bed in which the CO 2 was photosynthetically fixed.To assess CO 2 equilibrium, the water parcel must be followed for weeks to months in natural coastal systems (illustrated here), whereas in the open ocean (ocean afforestation) the time scale of tracking the water is months to years.In the coastal system illustrated, the seawater may move along the shore, staying within a large kelp bed (i.e., an autotrophic system, see Figure 3f1) for several weeks.It may also move along the shore but over a heterotrophic system (see Figure 3f2) or offshore where the seawater will be mixed with other water bodies with different partial pressures of CO 2(g) .The seawater may also subduct below a less-dense water mass, resulting in incomplete equilibration (see Figure 3f3).The white arrows and text illustrate the processes involved in assessing carbon storage pools for seaweed carbon that has moved across the continental shelf.Some proportions of DOC and POC from seaweeds will breakdown and re-enter the atmosphere as CO 2 .An estimated 16% of seaweed POC will transit the continental shelf (200 m isobath) and be stored in deep ocean sediments (Filbee-Dexter et al., in revision).A proportion of recalcitrant DOC that is resistant to degradation may be stored in the deep ocean, but this has not been quantified to date.If seaweeds are grown offshore at large spatial scales in ocean afforestation (Figure 1; Figures S1, S5-S7) and the resulting biomass sunk below the permanent pycnocline, then the biomass will reside on the ocean floor, out of contact with the atmosphere for more than 100 years: For MRV we need to quantify how much atmospheric CO 2 the sunken biomass has sequestered, which requires tracking of the water parcel in which the seaweeds were originally grown to assess the magnitude of air-sea CO 2 equilibration.[Color figure can be viewed at wileyonlinelibrary.com] Some of the complexities highlighted here for seaweeds are less problematic when measuring air-sea CO 2 equilibration caused by phytoplankton photosynthesis.Phytoplankton are ubiquitous throughout the euphotic zone of the global ocean and are mainly passive drifters that move with the water mass (Basterretxea et al., 2020).For these two reasons, equilibrium is more likely to occur following phytoplankton carbon fixation than in seaweed systems where the seaweeds, in discrete beds or farms, are attached to a substratum and the water moves away from where the CO 2 was photosynthetically fixed.Thus, there is a pronounced decoupling between seaweed C fixation and the water body that carries the "memory" of that fixation, which then must equilibrate with atmospheric CO 2 .The only naturally occurring free-drifting pelagic seaweeds are Sargassum spp. of the Sargasso Sea; the same species have recently formed (>decade ago) the Great Atlantic Sargassum Belt (GASB), which comprises a patchy floating seaweed system.Within the free-drifting GASB, the seawater from which the CO 2 was photosynthetically fixed has a residence time in the upper ocean (10-110 m) before the waters subduct to depth, which is 3-18-fold too fast relative to the equilibration time for atmospheric CO 2 .Hence, equilibration will be inefficient with only 6%-33% of potential CDR being achieved (Bach et al., 2021).

SUM MARY
Seaweeds are critically important to the health of coastal systems and provide substantial services to humanity.Important work is ongoing to restore seaweed beds already damaged by climate change (Eger et al., 2023).Seaweeds also have tremendous aquaculture potential for human food and animal feeds, novel nutraceuticals, and pharmaceuticals (e.g., Bleakley & Hayes, 2017;Wells et al., 2017: Yu et al., 2018).For example, seaweeds produce bioactive chemicals that are well known to improve soil qualities and agriculture that will have climate benefits in soil enhancing soil carbon (e.g., Cole et al., 2017;Roberts et al., 2015).The red seaweed genus Asparagopsis produces the chemical bromoform that when fed to sheep and cows, has shown promising signs of reducing methane emissions; there is evidence that chemical properties of brown seaweeds also play a similar role (Alvarez-Hess et al., 2023;Choi et al., 2021;De Bhowmick & Hayes, 2023).Seaweed biomass may therefore be useful to help offset climate change in ways that have a disproportionate influence on the amount of seaweed biomass used and for which offsets are more easily verifiable.Ongoing work to understand the role that seaweeds play in the coastal and open ocean carbon cycle, including the size of various storage pools, is extremely important for enabling predictions of how oceanic carbon cycles will change in the future, as temperatures rise and levels of dissolved CO 2 increase, causing ocean acidification (Gao et al., 2022;Pessarrodona et al., 2018).
The essential step that needs to be incorporated into all studies aimed at MRV for CDR and carbon sequestration by seaweeds is air-sea CO 2 equilibrium.Figure 4 highlights the fates of both the parcel of water bearing the CO 2 deficit and seaweed biomass.There is no doubt that on a global scale, seaweeds form long-term carbon storage pools in ocean sediments on timescales of 100 years or more, although their contribution is extremely small (<1%, 55 Tg C·y −1 ) compared to >99% from oceanic phytoplankton via the biological pump (5-15 Pg C·y −1 ; Boyd et al., 2019;Filbee-Dexter et al., in revision;Pessarrodona et al., 2023).For any seaweed system (natural seaweed bed, nearshore or open ocean aquaculture) under consideration for carbon credits/ offsets, two steps are required for MRV (Figure 4): (1) The parcel of seawater that bears the CO 2 deficit resulting from seaweed photosynthesis must be tracked over long timescales (weeks to months to over a year) to quantify the amount of atmospheric CO 2 that enters seawater, and (2) the amount of the organic carbon from the same seaweed system that enters long-term storage (>100 years; as POC, DOC) must be quantified.Our view is that the difficulty of linking these two steps for seaweed beds and farms currently precludes the use of seaweeds in carbon trading schemes.

SUPPOR T I NG I NFORMAT I ON
Additional supporting information can be found online in the Supporting Information section at the end of this article.

F
Seaweed carbon storage pools.(a) Most seaweeds store carbon as living biomass for <10 years.(1) Carbon from living and decaying seaweeds can be recycled within the seaweed bed via herbivory, autotrophic, and heterotrophic respiration and the release of particulate and dissolved organic carbon (DOC, POC).(2) Living and decaying seaweeds also contribute POC and DOC to adjacent systems via lateral transport.This transport may be within coastal systems or beyond the continental shelf to the deep ocean.As POC and DOC are transported offshore, they are broken down into carbon-containing molecules via various processes including photochemical oxidation (zig-zag arrow) and heterotrophic respiration.(b) Schematic of an offshore seaweed aquaculture system in which (1) seaweeds are grown in the photic zone, and (2) the resultant biomass is sunk below the permanent pycnocline (500 m) to the deep ocean.(3) Seaweeds that have been sunk below the permanent pycnocline are out of contact with the atmosphere, and seaweed carbon and breakdown products may not return to the atmosphere for more than 100 years.[Color figure can be viewed at wileyonlinelibrary.com] et al., 2021), although parameterization of different regions will require baseline data(Bach et al., 2023).Because the difficulties in measuring air-sea CO 2 equilibration are limiting the potential of marine CDR methods, Bach et al. ( C. L. Hurd: Conceptualization (lead); investigation (lead).J.-P.Gattuso: Conceptualization (equal); investigation (equal).P. W. Boyd: Conceptualization (equal); investigation (equal).ACK NOWLEDG M ENT S This work was supported by an Australian Research Council (ARC) Discovery Project DP200101467 to CLH and an ARC Laureate Fellowship FL160100131 to PWB.JPG was supported by the EU Horizon programme (FACE-IT project, grant agreement No 869154).We thank an anonymous reviewer whose insightful comments improved an earlier version of the manuscript.Open access publishing facilitated by University of Tasmania, as part of the Wiley -University of Tasmania agreement via the Council of Australian University Librarians.ORC I D C. L. Hurd https://orcid.org/0000-0001-9965-4917R E F E R E N C E S Alvarez-Hess, P. S., Jacobs, J. L., Kinley, R. D., Roque, B. M., Neachtain, A. S. O., Chandra, S., & Williams, S. R. O. (2023).Twice daily feeding of canola oil steeped with Asparagopsis armata reduced methane emissions of lactating dairy cows.
;Arzeno- Soltero et al., 2023; Ocean Visions and Monterey Bay  Aquarium Research Institute, 2022Ross et al., 2023;;  Figures S1.Running Tide.https:// www.runni ngtide.com/ carbo nremoval.Seeks to grow seaweeds on buoys that are subsequently released to sink the biomass to the deep ocean, for carbon sequestration and CDR and for use in carbon markets.Figure S2.Green Ocean Seaweed Farming.https:// www.green ocean farmi ng.com/ ocean -carbo n-bonds.php Selling Green Ocean Carbon Bonds, stating that £300 (Sterling) spent on Green Ocean Carbon Bonds will offset a household carbon footprint for one year.Figure S3.Carbon Kapture.https:// carbo nkapt ure.com/ .Public can sponsor a seaweed "rope" for a monthly donation of £2, £5, or £10. Figure S4.Seaweed Generation.https:// www.seawe edgen erati on.com/ .The aim is to collect and sink existing pelagic Sargassum to the deep ocean (>2,000 m) for carbon sequestration, and to grow Sargassum "sustainably and cheaply." Figure S5.Climate Foundation.https:// www.clima tefou ndati on.org/ .Growing seaweeds at scale using marine permaculture and artificial upwelling for carbon sequestration and a range of other seaweed-based products.Figure S6.Kelp Blue.https:// kelp.blue/ co2-remov al/ Seeks to "re-wild" the oceans by growing Macrocystis pyrifera at scale for carbon dioxide removal and carbon sequestration.Figure S7.Seafields.https:// www.seafi elds.eco/ Company aims to culture Sargassum sp. in an oceanic gyre of the south Atlantic ocean to create an artificial version of the massive seaweed bloom in the north Atlantic ("Great Atlantic Sargassum Belt").Hurd, C. L., Gattuso, J.-P., & Boyd, P. W. (2024).Air-sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?Journal of Phycology, 60, 4-14.https://doi.org/10.1111/jpy.13405