Carbon Sequestration
by Seaweed:
Background paper for the Bezos Earth FundEDF workshop on seaweed carbon sequestration
Environmental Defense Fund
Natural Climate Solutions
1

CARBON SEQUESTRATION BY SEAWEEDCarbon Sequestration by
Seaweed:Background paper for the Bezos Earth Fund — EDF workshop
on seaweed carbon sequestration
Authors
Rod Fujita
Jamie Collins
Kristin Kleisner
Douglas Rader
Annie Mejaes
Simona Augyte
Poppy Brittingham
Environmental Defense FundContact
Rod Fujita - rfujita@edf.org
Acknowledgments
The authors acknowledge contributions that helped build a foundation for this work, including the work of
Laura Chavez Anderson and Aukai Manson. We also appreciate discussions with Kristin Kleisner, Camrin Braun,
Sarah Cooley, Doria Gordon and the many scientists and experts who participated in the EDF/Bezos Earth Fundconsensus workshop on natural climate solutions in the open ocean.

How to cite this report
Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon
sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon
sequestration. Environmental Defense Fund, New York, NY. URL
Version 2, 3/28/22

2

CARBON SEQUESTRATION BY SEAWEEDTable of Contents
About this report

4

Executive Summary

5

Definitions

5

Introduction

6

Carbon sequestration by natural seaweed stands

7

Carbon sequestration via seaweed farming

11

Co-benefits associated with carbon sequestration by seaweed

14

Interventions to increase carbon sequestration by seaweed, andtheir potential social and economic effects

15

Readiness for implementation

3

18

Conclusions

21

Appendix

23

References

26

CARBON SEQUESTRATION BY SEAWEEDAbout this report
This is one of three reports produced by theEnvironmental Defense Fund (EDF) ocean scienceteam as part of a two-year EDF project on natural
climate solutions (NCS). With financial support fromthe Bezos Earth Fund, EDF seeks to build consensus
around the scientific readiness, market suitability,
socioeconomic dimensions and pathways to largescale uptake of NCS within four major parts of theearth system – tropical forests, temperate forests,
working (agricultural) lands and the oceans. The
ultimate objective of EDF’s work is to identify scalable
interventions that could preserve or magnify NCS
pathways and that are ready to implement – i.e.,interventions that are likely to result in durable carbon
sequestration via a NCS pathway, are likely to generateco-benefits and that present low risk of adverse social,economic or ecological adverse impacts. We also
identify where further scientific and policy research isneeded to result in NCS that meet these criteria.
Within the ocean system specifically, EDF is examining
three sets of potential NCS interventions:• Interventions in the open ocean, including carbon
sequestration via the rebuilding of biomass in large
marine mammals and epipelagic fishes, and thepotential for avoided emissions by restricting or
limiting new fishing in the mesopelagic ocean orbenthic trawling,• various interventions to conserve, restore andincrease the productivity of macroalgal (seaweed)
systems (natural beds and farms) to avoid GHG
emissions and sequester more carbon (C) and• interventions to conserve, restore and manage
vegetated, coastal blue carbon ecosystems such as
mangroves, marshes and seagrasses to avoid GHG
emissions and increase C sequestration.

4

CARBON SEQUESTRATION BY SEAWEED

The present report attempts to describe thestate of the science, including key uncertainties,
surrounding the second set of pathways – those
based on macroalgal ecosystems and aquaculture.
EDF has prepared companion reports on the stateof the science surrounding the open ocean andcoastal blue carbon pathways. Together, these oceansystem reports served as inputs for a series ofcomplex systems mapping workshops in which EDF
engaged more than 60 outside experts to criticallyevaluate our initial findings; to identify co-benefits,risks, tradeoffs and equity concerns associated withthe various pathways; and identify any promising
additional pathways for carbon sequestration or
avoided emissions. As such, the present report is justa starting point for discussion and exploration of thescientific and socioeconomic dimensions surrounding
macroalgal NCS pathways, and does not necessarilyreflect the consensus of EDF’s macroalgal workshop
participants. EDF is separately investigating themarket readiness of pathways associated with forest
and agricultural systems.

Executive Summary
The purpose of this paper is to provide a review ofthe literature on what is both known and unknown
about seaweed as a Natural Climate Solution (NCS),
with respect to the need to characterize current andpotential future carbon sequestration by seaweeds.
We first summarize the existing literature on carbon
sequestration by seaweeds. We then propose three“interventions” aimed at increasing seaweed-basedcarbon sequestration, and evaluate the potential of

UNSPLASH | BEN WICKS

each intervention for enhancing carbon sequestration
by seaweed and for generating co-benefits (suchas habitat provisioning and the alleviation of ocean
acidification), social impacts and ecological impacts.
Finally, we describe the degree to which eachintervention may be ready for implementation byevaluating them against criteria defining a high quality
carbon credit (i.e., how credible carbon removal wouldbe) (EDF et al., 2020 and 2021).

Definitions:
Carbon market: A market in which units —allowances or credits — are traded between
entities. When units are used for voluntary
purposes or where carbon credits are certified
solely by voluntary programs or standards, themarket is often referred to as a “voluntary” carbon
market. Where units are used to satisfy legal
compliance obligations, this is often referred to asa “compliance” market.
Nature-based solutions (NBS): The fullrange of values humans derive from natural
systems, defined by IUCN (2020) as “actions
to protect, sustainably manage, and restore
natural or modified ecosystems, that address
societal challenges effectively and adaptively,
simultaneously providing human well-being andbiodiversity benefits”.
Natural climate solutions (NCS): A subset of
NBS that directly addresses the GHG reduction
benefits (i.e., increase carbon storage and/or avoid
greenhouse gas emissions) that humans derivefrom natural systems via conservation, restoration,
and/or improved management actions.

5

CARBON SEQUESTRATION BY SEAWEED

Carbon Dioxide Removal: process in which carbon
dioxide gas (CO2) is removed from the atmosphereand sequestered for long periods of time.
Monitoring, Reporting and Verification: A systemor protocol for tracking specific methods andoutcomes, transparently communicating specificinformation, and validating that the information isaccurate and complete. Often abbreviated as MRV.
Carbon capture and storage: the process oftrapping carbon dioxide produced by burning fossil
fuels or any other chemical or biological process
and storing it in such a way that it is unable toaffect the atmosphere.
Blue Carbon: carbon sequestered mangrove
forests, seagrass beds and tidal marshes (Mcleod
et al., 2011). More recently, some have broadened
the definition to include all carbon “captured by
the world’s ocean and coastal ecosystems” (NOAA
NOS, 2021).

Introduction
In this paper, we review existing literature on carbon
sequestration by natural seaweed stands andseaweed farms, the co-benefits that are associatedwith seaweed, and risks that could be associatedwith seaweed as a NCS. We also evaluate three
proposed interventions aimed at expanding carbon
sequestration by seaweed against criteria for ahigh quality carbon credit (as defined by EDF et al.,2021) in order to describe the degree to which eachintervention may be ready for implementation andwhat activities may be required to develop high quality
carbon credits based on seaweed.
In a recent workshop, NCSs based on marine
ecosystems – namely mangroves, marshes,
seagrasses, epipelagic fishes, mesopelagic
communities, large marine mammals, and seaweed –were compared with respect to carbon sequestration
potential, co-benefits, risks, and the degree to whichinterventions comport with the attributes of a highquality carbon credit. We anticipate that the outputsof this exercise will inform the development of policies,
blue carbon markets/offsets, research programs, andother activities aimed at implementing ocean-based
NCSs, including those based on seaweed. We also
hope to identify activities that may be necessary todevelop high quality carbon reduction projects basedon seaweed.

6

CARBON SEQUESTRATION BY SEAWEEDUNSPLASH | BENJAMIN JONES

Additionally, we convened a separate working groupand models for workshop participants who wishedto work together to improve estimates of carbon
sequestration by seaweeds in different types ofseaweed farms. To facilitate this, we have developed
seaweed farm archetypes (see Appendix – Seaweed
Farm Archetypes for Data and Models Working
Group) to account for context-dependent factorsthat strongly influence carbon sequestration suchas nutrient availability, strength of advection to
sediments and deep water, harvest cycle, products
and uses of the seaweed, etc. The outputs of thisworking group would ideally serve as an input to thedevelopment of a robust protocol for monitoring andverifying carbon sequestration by seaweeds.

Carbon sequestration by natural seaweed stands
Seaweeds take up dissolved carbon (C) from seawaterand convert it to organic compounds and biomass(Paine et al., 2021) via photosynthesis at rates thatare among the highest on the planet (Packer 2009).
The dissolved C removed by seaweed is slowly
replaced by atmospheric CO2 (NASEM 2021 report andreferences therein), resulting in a flux of CO2 from theatmosphere into the ocean.
While seaweeds fix C rapidly, and store it for a time inbiomass, only a portion of this C is sequestered on atimescale of centuries or millennia. Some of the fixed
C is grazed, then remineralized to CO2. In some cases,
grazing can remove a large fraction of the seaweedbiomass. For example, Krumhansl and Scheibling(2012) estimate that 82% of kelp productivity is

Remineralized 0.57

Grazing 0.51

Macrophyte NPP
1.5(1.0 - 2.0)

Exported DOC
0.36 (0.19 - 0.49)

Remineralized in shelf0.24

removed by grazing. Some of the carbon fixed byseaweeds is exuded as dissolved organic C (DOC),
a portion of which is also remineralized to CO2 insurface waters. The total quantity of C sequestered byseaweeds is thus affected by many factors, including
the gradient in CO2 concentration between the ocean
and the atmosphere, the amount of loss to grazing
and DOC remineralized in surface waters, the amountof seaweed C that is transported to sediments or deep
waters as fragments and C adsorbed onto particles
or as DOC (Krause-Jensen and Duarte 2016). KrauseJensen and Duarte (2016) present a useful diagram
depicting flows of C through a generalized seaweed
stand:

Exported below
mixed layer
0.12 (0.03 - 0.19)

Burial in algal bed
0.006 (0 - 0.001)

Exported POC
0.32 (0 - 0.79)

Buried in shelf Remineralized in shelf
0.01 (0 - 0.3)
0.27
Exported to deep sea
0.04 (0 - 0.8)

Total C Sequestration
0.17(0.06 - 0.27)
FIGURE 1.

Synthesized pathways of carbon export and sequestration by natural macrophytes if these seaweed popilations
could be grown to their maximal extent (values in Pg C/yr). The blue text represents exported carbon thatis thought to be sequestered for long times; black text is rapidly remineralized and exchanged with theatomosphere. Values in parentheses represent 25 percent and 75 percent quartile uncertainty levels.
Source: Redrawn from Krause-Jensen and Duarte (2016).
From NASEM 2021.
7

CARBON SEQUESTRATION BY SEAWEEDThe rate at which DOC is exuded from seaweeds
and the degree to which it is remineralized strongly
influence how fast and how much C is sequestered.
It is estimated that 30% – 50% of the net primary
production of seaweeds is released as DOC. Someof the DOC exuded by seaweed is considered labileand has a short lifespan of days to weeks as it isconsumed by microbes and remineralized as CO2.
Some of the DOC can be refractory – i.e., resistant torapid microbial degradation – and can be exported
to the deep sea via advection (flow) (Carlson et al.,
1994, Hansell 2013). In this paper, we take “refractory”
to mean resistant to degradation and release of C
on a timescale of hundreds of years, given our focus
on carbon sequestration. The proportion of labile
to refractory DOC exuded by seaweeds remains
uncertain and probably depends on species andenvironmental conditions. Watanabe et al. (2020)
estimated that 56% - 78% of the DOC released bytemperate Sargassum stands is refractory after 150
days. However, the relevant timescale for carbon
sequestration is >100 years, so more studies will benecessary to ascertain the fraction of refractory DOC
on this timescale.
The rate of C sequestration by seaweeds remains
uncertain but it is estimated that if all the suitable
habitats for seaweed were actually inhabited byseaweed (e.g., if large-scale seaweed afforestation
were to occur), these ecosystems would sequester
about 0.173 Pg C yr-1 (equivalent to 0.635 Pg CO2 yr-1)– about 11% of their net productivity (Krause-Jensen
and Duarte 2016). This estimate of potential carbon
sequestration via seaweed afforestation was reportedwith extraordinarily high uncertainty; the authors
estimated the sequestration rate could be as high as
0.268 Pg C yr-1 (0.98 Pg CO2 yr-1), or as low as 0.061 Pg
C yr-1 (0.22 Pg CO2 yr-1). Nevertheless, seaweeds may
be sequestering more carbon than some other oceanbased biological pathways; the seaweed sequestration
rate and associated uncertainty are of the same
magnitude as the 0.077 – 0.204 Pg C yr-1 (0.28 – 0.75
Pg CO2 yr-1) estimated to be sequestered by all coastal
blue C ecosystems (mangroves, tidal marshes andseagrass stands) combined (Howard et al., 2017).
Various methods have placed the extent of naturalseaweed stands, including kelp forests, at between
1.4 × 106 ha (minimum estimated extent) (Duarte etal., 2013) and 5.7 × 106 ha (maximum possible extent,
based on modeled habitat) (Gattuso et al., 2006). The
3,500,000 km2 reported by Krause-Jensen and Duarte(2016) is based on a Monte Carlo bootstrap simulationthat combined all available literature values. The
amount of “additional” C sequestered via restorationof seaweed stands will depend on the amount of initialstanding stock, the area reforested, the degree to
8

CARBON SEQUESTRATION BY SEAWEED

which standing biomass can be maintained and otherfactors.
Different types of seaweed process C differently,
with implications for the overall rate of sequestration
by different macroalgal ecosystems. For example,
calcifying algae assimilate C via both photosynthesis
and calcification; the latter process releases one
molecule of CO2 for every molecule of calcium
carbonate that is produced, offsetting at least
some of the C sequestration benefit attributable tophotosynthesis alone. The carbonate minerals fixedin this way are an important component of carbonate
sands, which eventually become consolidated into
carbonate rocks. Calcifying algae can also grow onseaweed, complicating the estimation of carbon
sequestration by a seaweed bed. Depending on theratio of calcification to photosynthetic C fixation,
this could even result in a net flux of CO2 into thewater under certain conditions for species suchas Sargassum (Bach et al., 2021). Gallagher etal. (2022) noted that the net C budget of a given
naturally occuring seaweed ecosystem – i.e.,,
whether the system ultimately serves as a net sinkor source of C to the atmosphere – is determinedby the balance between a number of different,
simultaneous biogeochemical processes that include
photosynthesis, calcification, microalgal uptake of
DOC exuded by seaweeds and export of both dissolved
and particulate C. Importantly, Gallagher et al. (2022)
also found that the net contribution of a seaweed
bed to the global C cycle depends heavily on the way
ecosystem boundaries are defined; some systems
that exchange C heavily with other parts of the ocean,or in which rates of heterotrophic respiration are
substantial, may serve as net sources of C rather thansinks.
The role of pelagic seaweed in C sequestration is lesswell understood. Accumulations of pelagic seaweeds
can become quite large due to ocean gyres and eddies
that concentrate floating species like Sargassum.
These seaweeds can form very large floating masses(Davis et al., 2021). It has been estimated that about
10% of the production of Sargassum in the Atlantic
sinks to the seafloor in deep water as fragments, andthere are also massive episodic injections of pelagic
seaweed biomass into deep water during storms(Krause-Jensen and Duarte 2016). Both the chronicand episodic advection of this biomass presumably
result in the sequestration of at least some of the C
in that biomass. There is considerable uncertainty
over the amount of C stored in pelagic seaweed
aggregations, with Hu et al. (2021) suggesting that
Sargassum in the North Atlantic could potentially store
up to 3.61 x 10-3 Pg C. Pelagic seaweed aggregations

can sometimes have adverse impacts as the biomass
starts to decompose after reaching shallow waters
and beaches (Davis et al., 2021), resulting in damage
to coastal ecosystems, undesirable odors and othernuisance factors, as well as some release of C to
the atmosphere. To sequester the carbon stored
in this seaweed biomass, it would be necessary toharvest the seaweed and direct the biomass into
a sequestration pathway. However, this may pose
ecological risks as pelagic seaweeds are important
habitats for many species, both at sea and after thebiomass strands on beaches (Piñeiro-Corbiera et al.,2021; Bustamante et al., 2017). These seaweeds arealso an important source of fixed carbon and nutrients
to coastal ecosystems, including beaches (SAFMC
2002).
The context-specific factors that influence C fluxes
through seaweed stands must be better understoodto accurately quantify C sequestration. Rates that
determine C sequestration by seaweeds – including
CO2 flux from the atmosphere to the ocean, C fixation
into biomass, labile and refractory DOC exudation
rates, fragmentation rates and rates of advection– are all likely to vary with dominant species, bed
density, harvest intensity, nutrient availability, grazing
intensity, season and other factors. They will also likely
vary with environmental conditions, such as whether
the seaweed is subjected to high current and wave
energy, steepness of the gradation of the bottom todepth, etc. (NASEM 2021).
While standing stocks of natural seaweed stands havebeen widely studied, the amount of C present withinseaweed stands is highly uncertain. We could not findestimates of total seaweed biomass that take into
account pelagic seaweed biomass. Calcifying algae
and the sediments derived from them may represent
a significant pool of biologically sequestered C –some portion of which could be stored for millennia
as carbonate minerals – but the magnitude of C
stored in this pool is unknown (NASEM 2021). We
have developed a preliminary estimate of the total
amount of C present in natural seaweed stands (1.3
– 9.5 Pg C) by combining data reported in several
sources on the potential coastal area seaweeds may
occupy based on their light requirements and lightavailability on the seafloor (Duarte et al., 2017; Gattuso
et al., 2006; Graham et al., 2007; Muraoka 2004),
and by assuming that elemental composition andareal density can be estimated by a representative
species, the kelp Laminaria spp. Seaweed species vary
markedly in C content and areal density; however,additional data and models accounting for thesedifferences are needed for a more accurate estimate
of C stocks in natural seaweed stands.
9

CARBON SEQUESTRATION BY SEAWEEDNatural seaweed stands probably sequester relatively
small amounts of C relative to other NCS pathways
for several reasons. While seaweeds take up C rapidly
and may represent a fairly large (but highly variable)
stock of C, turnover rates are high relative to other C
stocks such as forests, or even large fish and whales(compare 1 – 2 years for some kelp forests with 50– 100 years for the woody biomass in some tropicalforests).
Natural seaweed populations have declined very
steeply in certain regions due to climate change,
fishing pressure, excessive sedimentation and otherfactors (e.g., a 93% decline in kelp cover was observed
off California from 2008–2014), with global kelp
forest declines occurring two and four times fasterthan coral reefs and tropical forests, respectively(Feehan et al., 2021). These declines may be offset byincreases in other areas, resulting in a relatively small
global average decline rate of 1% or 2% (Krumhansl etal., 2016). If the average decline rate is indeed small,this might equate to low C sequestration potential
via seaweed stand conservation and restoration since
restoration potential, on average, would also be low.
On the other hand, the potential for additional C
sequestration via seaweed stand restoration would behigh for specific stands, depending on the availabilityof suitable habitat and the extent of seaweed biomass
loss. Restoration of seaweed stands that have been
lost offers an opportunity to store a one-time pulseof “new” or “additional” C into seaweed biomass foras long as the biomass can be maintained. However,
because many factors affect biomass levels inseaweed stands they can be highly variable, increasing
the vulnerability of the C stored in the biomass
to remineralization. Ultimately C sequestration byseaweed on timescales that influence atmospheric
CO2 concentrations depends on the fate of thebiomass and of the dissolved carbon that seaweeds
exude.
Carbon stored in standing biomass via restorationof natural seaweed stands may prove to be easier
to monitor and verify than C stored via exudationof refractory DOC or fragments to deep waters or
sediments, as the seaweed biomass would occur inrelatively shallow nearshore waters, making remotesensing or even field surveys feasible.
Conservation, restoration and afforestation efforts
aimed at increasing C storage and sequestration by
natural seaweed stands could be challenging becauseof context-specific differences in the factors that
limit seaweed productivity and longevity of storage,
including climate change, depletion of predatorsof grazing species, sediment input, disease, etc. A
recent meta-analysis of seaweed restoration studies

suggests a relatively high rate of success (Eger etal., 2022); however, many of these have been small inscale, and the results perhaps do not reflect failuresthat are unreported in the literature. Eger et al. (2022)
report on some large-scale restoration successes aswell. The Nature Conservancy offers useful guidanceon how to develop kelp forest restoration plans(Gleason et al., 2021). Based on these considerations,
we conclude that conservation of seaweed stands
will be essential to prevent further C loss frommarine systems, and that there is potential tostore “additional” C via seaweed bed restoration.
Seaweed conservation and restoration would also
generate many co-benefits and relatively few social
or ecological risks. Emerging restoration models may
be capable of delivering social and economic benefits
to small-scale fishers (e.g., the Urchinomics model,
in which fishers are paid to harvest “empty” urchinsfrom urchin barrens, which are then fed in cultivation
systems to be sold into premium markets) (UN The
Ocean Decade 2022; Verbeek et al., 2021).

10

CARBON SEQUESTRATION BY SEAWEED

The following questions remain regarding carbon
sequestration by natural macroalgae stands:• Which of the carbon fluxes relevant to C
sequestration by seaweeds (atmosphere to ocean,
ocean to seaweed, seaweed to microbial andother food webs, seaweed to deep water) are wellquantified and which are not?
• What is the best way to measure carbon
sequestration rates by natural seaweed stands?
• How might the potential C sequestration by naturalseaweed stands compare with other ocean-based C
sequestration pathways?
• Is it feasible to sequester C via restoration ofnatural seaweed stands for long enough to help
stabilize the climate?

Carbon sequestration via seaweed farming
Carbon sequestration by natural seaweed stands
is constrained by productivity, areal extent, highturnover rate of seaweed biomass, losses of C to
DOC exudation and grazing, and the rate of natural
advection of refractory DOC and fragments tosediments and deep waters. Seaweed farming can,in concept at least, reduce each of these constraints,
thereby dramatically increasing carbon sequestration.
Seaweeds are already highly productive, but
productivity can be increased by farming practices
that optimize spacing, choice of species or cultivars,
timing of seeding and harvesting, and otheroperational aspects. Some seaweeds, such as thekelps Macrocystis and Laminaria, have very high rates
of productivity on the order of ≥3000 g C m-2
year-1 or 30x106 g ha-1 year-1. A number of otherspecies have productivity rates per unit area of 1,000
g C m-2 year-1 (Chung et al., 2011). Selective breeding
and farming practices can make them even moreproductive (Umanzor et al., 2020; Wade et al., 2020).
Several breeding programs are ongoing with thekelp Saccharina and Macrocystis, with the goal ofenhancing productivity by 20% - 30% (MARINER),
while ranges of 30% - 131% have been reported forspecies cultivated in Asia (Patwary and van de Meer
1992).
Farming seaweeds can also be used to overcome
limitations on production imposed by the need forsuitable habitat by providing artificial habitat for spore
settlement and grow-out. In theory, seaweed can begrown anywhere in the ocean where sufficient lightand nutrients are available. Froehlich et al. (2019)
estimate that about 48 million km2 may be suitable forseaweed farming, based on suitable temperature andnutrient availability. However, the extent of potential
farmable ocean area remains poorly understood (e.g.,there are many potential user conflicts including
vessel traffic, fishing, military operations, etc.).
Moreover, we still do not have a full understanding of
relevant biogeochemical processes or the possible
ecological consequences of seaweed farming
conducted at a scale that could remove a significant
amount of atmospheric CO2. However, despite highuncertainty, it seems clear that because less than
0.01% of the suitable ocean area estimated by
Froehlich et al. (2019) is currently being used forfarming seaweed, all in nearshore waters (Duarte
et al., 2017; Froehlich et al., 2019), carbon fixation
by seaweeds could be greatly expanded. The
quantity of C sequestered by new seaweed farms
is likely to vary dramatically depending on siting,operations and the ways in which the yield is used
11

CARBON SEQUESTRATION BY SEAWEED(see Appendix: Seaweed Farm Archetypes for Data
and Models Working Group). Hence, the range of C
sequestration that may be possible as a result of
expanded seaweed farming will likely be very large,
even once sufficient data are available on rates of Cabsorption, remineralization and storage to estimate
C sequestration from seaweed farms. At very large
scales, nutrient availability and conflicts with otherclaims on marine space could constrain offshore
seaweed farming. Nutrient availability constraints
could be overcome via artificial upwelling; however,this could entail added expense and energy inputs(NASEM 2021).
Limitations to carbon sequestration resulting fromrapid biomass turnover and natural advection rates
in natural seaweed stands can also be overcome inseaweed farms. Seaweed farms can be monitored
and regularly harvested, which could result in theconversion of seaweed biomass into refractory
products with long lifetimes, such as bioplastics(Sudhakar et al., 2020; Lim et al., 2021), productsthat result in avoided greenhouse gas emissions,such as biofuels, and to products that may resultin some C sequestration in soils, such as soil
amendments (Ramya et al., 2015; Wang et al., 2018).
A comprehensive database of seaweed products(Phyconomy 2022) has been assembled, andresearchers are evaluating the climate and ecologicalimpacts of sinking seaweed, converting it to biofuel, or
making food and animal feed products (Davis 2022).
Seaweed biomass can also potentially be sunk to theocean bottom or injected into deep water or stablegeological formations to dramatically increase C
storage duration (Wu et al., 2021; Sandalow et al., 2021;
Zhang et al., 2020; Chopin 2021).
The amount of C stored and the duration of C storage
in products currently made from seaweed is probably
fairly small, as 40% of seaweed was used as food for
direct consumption in 2012 (Bjerregaard et al., 2016).
Consumption of seaweed results in rapid return of the
C sequestered in seaweed biomass to the atmosphere.
Another 40% of seaweed production in 2012 was used
to make agar, carrageenan and other hydrocolloid
products that are used in processed foods and otherproducts (Bjerregaard et al., 2016). These products
also have relatively short lifespans, resulting inlimited C sequestration and effects on atmospheric C
concentrations. During the extraction of hydrocolloids,
the majority of the seaweed biomass goes unused,
and thus only a negligible amount of C is expected
to be sequestered. However, markets for direct
consumption are growing as demand for substitutes

for high-impact foods (e.g., meat substitutes) grows;
this creates some scope for reducing future GHG
emissions.
The remaining 20% of global seaweed yield was usedin other nonfood related seaweed products, such as
pharmaceuticals, cosmetics and animal feed. Like thefood products, these products are also not expected
to provide much direct C sequestration due to theirrelatively short life cycles. Hence, we omit the C
stored in seaweed products from our estimate of total
C storage in seaweed (Bjerregaard et al., 2016).
Because sinking seaweed grown in farms may resultin a C sequestration pathway that is easier to measurethan other ocean-based pathways, there has been
considerable interest in this pathway by scientists,
entrepreneurs and investors (Duarte et al., 2017; Bever
2021; Froehlich et al., 2019). The 2021 study board
on ocean carbon dioxide removal (CDR) convened by
the U.S. National Academies of Science, Engineeringand Medicine (NASEM) estimated that farming a 100m wide continuous belt of ocean along 63% of global
coastline (about 72,900 km2) could sequester 0.1 Gt
CO2/yr if all the biomass were sunk directly into thedeep ocean. This would represent a 45-fold increase
from current areal extent of seaweed farming to about
0.2% of the total suitable farmable area estimated by
Froehlich et al. (2019). This estimate rests on several
simplifying assumptions: (1) 8% of DOC exuded by theseaweed is refractory (undefined in the NASEM 2021
report) and becomes sequestered (low contributionof refractory DOC to sequestration results fromharvesting and sinking the biomass); (2) 20% of
the biomass is lost to breakage, grazing and otherfactors; (3) yield will approximate that of a naturalkelp forest (1 kg dry weight m-2); and (4) 1.5 harvests
will be completed each year. Nutrient availability
may constrain the expansion of large-scale seaweedfarming. To illustrate the potential scale of nutrient
uptake by large-scale seaweed farming: a recent study
estimates that by 2026, seaweed farms will use all of
the anthropogenic nutrient runoff in China (Xiao et al.,2017). There is concern that shading and the highly
efficient uptake of nutrients by large-scale seaweedfarming could reduce phytoplankton productivity insome areas, with adverse impacts on marine foodwebs (Campbell et al., 2019; Wu et al., 2022).
Seaweed farming currently involves some fossil fueluse, resulting in GHG emissions. Operating processes(e.g., harvesting) and value-add processes (e.g.,
drying) could be optimized to reduce or prevent
emissions; for example, via conversion to fuel efficientor zero emission boats, changes in flushing rates, theuse of low energy intensive drying methods, etc. (Kim

12

CARBON SEQUESTRATION BY SEAWEEDet al., 2017, Stévant and Rebous 2021). GHG emissions
could also be reduced by altering the mix of productsthat are made from seaweed, since different productsupply chains have very different emissions profiles
and store the C absorbed by the seaweed for different
lengths of time. Many of these changes could probably
be applied to existing seaweed aquaculture operations
or incorporated at baseline into new farming activities.
Carbon sequestration is not the only way seaweedfarms could help avert catastrophic climate change. In
fact, uses of seaweed that result in avoided emissionsof CO2 and other more powerful greenhouse gasses
may present options for slowing climate changethat are easier to document and verify than carbon
sequestration. Application of seaweed-based soil
conditioners or biochar may result in some carbon
sequestration in soils and perhaps spare the releaseof GHGs associated with chemical fertilizer production(Smith 2002). Use of seaweed-based soil conditioners
may indirectly influence N20 production by changing
soil chemistry, especially in slightly acid soils, althoughmore research is needed (Thomson et al., 2012).
There is some evidence that the addition of seaweed
to soil can also improve crop yields (Roberts et al.,2015; Zacharia et al., 2015; Cole et al., 2015) and crop
quality (El-Salhy et al., 2017), which would reduce GHG
emissions associated with fertilizer production (Smith
2002) to the extent seaweed-based amendments canreplace chemical fertilizers. This application, however,may be limited by the relatively small concentrationsof macronutrients present in seaweed biomass(Roleda and Hurd 2019).
Use of seaweed as a biofuel could reduce GHG
emissions to the extent it is substituted for fossil
fuel or as a feedstock in a bioenergy with C captureand storage (BECCS) system (Hughes et al., 2012).
Consumption of seaweed as food could also reduce
GHG emissions, to the extent that seaweed replaces
other foods that have higher GHG footprints. Several
studies suggest that inclusion of small amounts ofseaweed in ruminant feeds can result in substantial
reductions in methane emissions (Kinley et al., 2016;
Maia et al., 2016); the safety and efficacy of thispathway are under active investigation.
There are trade-offs involved with the generationof many of the benefits of seaweed farming. For
example, managing a seaweed farm to maximize
carbon sequestration by sinking seaweed biomass
into deep ocean waters would be expected toreduce the generation of other benefits such as
food production, and may pose ecological risks to
deep sea ecosystems (NASEM 2021). Conversely,
producing food, hydrocolloids and other valuableproducts from seaweed would likely result in lesscarbon sequestration, due to the short life cycle of
the C in these products. Farms that grow and harvest
seaweeds seasonally will likely reduce in situ benefitsof seaweed farming, such as amelioration of ocean
acidity (Campbell et al., 2019), habitat provisioning(Langton et al., 2019; Campbell et al., 2019) andfishery enhancement (Bertocci et al., 2015).

The following questions remain regarding the effects
of seaweed farms on carbon sequestration and GHG
emissions avoidance:• Are there estimates of GHG emissions and C
sequestration associated with seaweed farms?
• Do seaweed farms differ significantly in their ability
to sequester carbon?
• If so, what are the factors that influence carbon
sequestration in seaweed farms?
• Which of these factors are well documented andwhich remain uncertain?
• Which carbon flows relevant to quantifying
C sequestration by seaweed farms are well
documented and which remain uncertain?

13

CARBON SEQUESTRATION BY SEAWEEDCo-benefits associated with carbon sequestration by seaweed
Seaweed can generate a number of benefits formarine ecosystems, which are regarded in thispaper as co-benefits to the main benefit of carbon
sequestration. However, it is important to note thatsome of these benefits are so significant (while thecarbon sequestration benefits are still so uncertain)that they could rightly constitute the main benefits ofseaweed farming, with carbon sequestration as a cobenefit. We summarize some of the co-benefits that
could be generated by natural seaweed stands anddifferent types of seaweed farms that we believe are
either quite common or may become common in thenear future.
Seaweed farms and natural seaweed stands have thecapacity to ameliorate ocean acidification somewhat(at least locally) as a result of rapid uptake of CO2,
which increases seawater pH (Mongin 2016). They
can also contribute to biodiversity (Theuerkauf
et al., 2021), habitat provisioning (Langton et al.,2019), heavy metal and nutrient pollution removal(Zheng et al, 2019; Jiang et al., 2020), and fisheryenhancement (Rimmer et al., 2021; Theuerkauf etal., 2021). Seaweeds may also improve water clarity
by facilitating the settlement of fine sediments
originating from soil erosion (Jiang et al., 2020).
In addition to these ecological benefits, seaweed cangenerate several social and economic co-benefits.
These include job creation, which can be especially
important in coastal communities that are highly
dependent on fisheries and need to reduce harvests
temporarily in order to allow fish stocks to recoverto sustainable levels. Seaweeds may also enhance
fisheries production under certain conditions (e.g.,where habitat is limiting fish productivity or habitat
connectivity) by improving habitat conditions and by
attracting aggregations of target species (Theuerkauf
et al., 2021). Seaweeds can be the basis for circularmarine bioeconomies, which recycle waste products
and generate multiple benefits (Zhang and Thomsen2019).

14

CARBON SEQUESTRATION BY SEAWEED

Clearly, seaweeds are capable of generating manyco-benefits, but the degree to which they generatethem depends on the harvesting intensity, as well assiting and other factors. Natural seaweed stands thatare not harvested can provide consistent biodiversity
and habitat provisioning co-benefits year-roundbut of course generate fewer social and economic
benefits than harvested stands. Annual harvests ortrimming to increase social and economic benefits
would be expected to reduce the ecosystem benefits
somewhat, while frequent harvests could dramaticallyreduce them. Natural seaweed stands can also remove
nutrient and heavy metal pollution and consistently
ameliorate ocean acidification.
Tropical seaweed farming typically involves a longgrowing season, with multiple harvests each year.
A tropical seaweed farm that is trimmed, rather
than completely harvested, during multiple annual
harvests may provide more stable, continuous habitat
provisioning. Water clarity and nutrient extraction
benefits could also be consistent and especially
important in tropical waters, where seagrass meadowsand coral reefs depend on high water quality.
Temperate seaweed farming typically has shorter
growing seasons, with a single annual harvest;
however, diversifying cultivated species may allow forextended farming periods. Typically, the single annual
harvest of temperate seaweed farms would remove
any habitat available for fish species, so any in situ cobenefits would be temporary or seasonal.
See the Appendix (Seaweed Farm Archetypes for Data
and Models Working Group) for more details on how
growing season, harvest cycle, nutrient availability
and other factors could impact carbon sequestrationand the generation of co-benefits in different kinds ofseaweed farms.

UNSPLASH | BRADEN COLLUMInterventions to increase carbon sequestration by seaweed, andtheir potential social and economic effects
To compare NCS and develop guidance for investors,
entrepreneurs and policymakers, it is useful todescribe the interventions that would be required toprotect or enhance each pathway so GHG emissions
are avoided, or that more atmospheric carbon issequestered. In this section we describe threeinterventions for implementing seaweed-basedcarbon sequestration or GHG avoidance. We attempt
to characterize the potential to contribute to climate
stabilization, co-benefits, social and economic effects
and ecological risks that could be associated witheach intervention.

Intervention 1: Conserve existing seaweed
stands and restore stands that havedeclined.
This would likely involve research to ascertain contextspecific factors causing declines or limiting restoration
potential, followed by context-specific threat reduction
activities (e.g., harvesting grazing organisms whose
populations have exploded due to lack of predation;
regulations to reduce fishing pressure on predators ofgrazing species; reduction of pollution, etc.).
These activities would require policy and regulatory
reform in some cases, increased funding or pressurein other cases and new policies and regulations inother cases. There is also an intriguing business model
developed by the for-profit company Urchinomics
for restoring kelp stands that have declined due toovergrazing by urchins, a common driver of kelp
decline (UN The Ocean Decade 2022). The urchins,
which are low quality due to lack of sufficient food,

15

CARBON SEQUESTRATION BY SEAWEED

are harvested and then fed to produce very highquality urchin roe, which is then sold into high-end
markets for premium prices (Verbeek et al., 2021).
The next phase of this business model will dependon the development of markets for C sequestrationand nutrient removal by the restored kelp stands tofinance the removal of more urchins, resulting in more
kelp restoration (UN The Ocean Decade 2022; Verbeek
et al., 2021).
Implementing entities and specific restoration actions
would vary depending on the nature of the threats. For
example, if overfishing of predators of herbivores isa major factor limiting seaweed restoration at a site,
fishery management agencies could reduce fishingpressure on the predatory species which are often
valued fishery targets. On the other hand if excessive
turbidity related to deforestation or agricultural
practices were determined to be major factors limiting
seaweed restoration, entities with jurisdiction overthese practices would be engaged, affecting different
sets of stakeholders. Programs to build capacity
among small-scale seaweed harvesters and connect
harvest operations with markets could result in a more
equitable distribution of the benefits of conservation
and restoration of seaweed stands. Specificinterventions would occur in nearshore tropical andtemperate waters with sufficient light, nutrients, andsubstrate availability to support seaweed growth.
Carbon reduction via seaweed bed restoration within
exclusive economic zones (EEZs) that are sufficiently
well documented could be credited in Nationally
Determined Contribution accounts.

Potential for C sequestration:
Conservation of existing seaweed stands would
not result in additional C sequestration, unlike
afforestation of seaweed stands. This remains
highly uncertain, but it has been estimated that if
large-scale seaweed afforestation were to occur,these ecosystems might sequester about 0.173
Pg C yr-1 (equivalent to 0.635 Pg CO2 yr-1) (KrauseJensen and Duarte 2016). Currently, most of theyield from seaweed harvesting is used for food andcolloid production, limiting the amount of C that issequestered by natural seaweed stands to the C thatis advected to sediments or deep water. Use of larger
fractions of harvest from natural seaweed stands
to make refractory products or products such as
bioplastics, biofuels or fertilizers that result in avoided
GHG emissions would enhance the role of naturalseaweed stands in climate stabilization, but thepotential would likely be low relative to other NCSs.
Co-benefits:
High levels of habitat and biodiversity provisioning,
local amelioration of ocean acidification, water quality
improvement and fishery enhancement.
Social and economic effects:
This intervention could benefit fisheries andecotourism, as seaweed stands can be importantnurseries for sport and commercial target species andmany are attractive dive tourism locations. However,it could also have negative impacts on short-termfishery revenues in contexts which require reductions
in fishing pressure to restore predator populations
capable of regulating grazing pressure, if that is whatis needed to restore a seaweed stand.
Ecological risks:
We do not anticipate large adverse ecological impacts
from the conservation and restoration of naturalseaweed stands. There may be a risk of genetic
bottlenecks if restoration is based on a highly inbred
strain, or on only a few such strains.

Intervention 2: Increase productivity andcarbon sequestration performance ofexisting seaweed farms
by identifying and addressing factors that constrain
productivity, such as disease, strain selection,
pollution, capacity and financing, and address
constraints to the development of farms that optimize
for carbon sequestration, such as the quantificationof carbon sequestration, research to address otheraspects of a high quality carbon credit (EDF et al.,2020) and the enhancement of markets for refractory
seaweed products and uses. It could be undertaken
largely by seaweed farmers, processors, blue carbon
16

CARBON SEQUESTRATION BY SEAWEED

accrediting entities and buyers, supported by NGOs
and government agencies which could provide
extension services and oversight to ensure that efforts
to increase productivity do not harm public trust
resources or the interests of other stakeholders. In thenear term, this intervention would be largely limited
to farms located in nearshore tropical and temperatewaters, as that is where the vast majority of seaweedfarms are currently sited.
Potential for C sequestration:
Potential productivity increases in current seaweedfarms would vary depending on local conditions,
including nutrient availability, water flow, temperatureand other factors. They would also likely depend on
local technical capacity. Duarte et al. (2021) assume
an average yield of 16t DW ha-1 when estimating
potential C sequestration by seaweed, and note that
this is nearly 10-fold lower than maximum productivity
reached under intensified farming conditions. Froelich
et al. (2019) use an estimate of 2000t DW km-2 (20t
DW ha-1) and also note that this is uncertain due tovariability in productivity at the farm level. These
considerations suggest that potential productivity
increases could be large. This in turn could resultin proportionally larger amounts of C storage infarmed seaweed biomass; however, the rates ofseveral processes that influence C sequestration
could change with increasing productivity, including
DOC exudation rate, the fraction of the DOC that isrefractory and fragmentation rate. We could find no
empirical data on how C sequestration rate scaleswith increased seaweed productivity.
Co-benefits:
Relatively low levels of additional seaweed habitatand biodiversity, with some enhanced ameliorationof ocean acidification and habitat/biodiversity
provisioning depending on siting and operations.
Social and economic effects:
Increased productivity of seaweed farms could benefit
smallholders who currently dominate seaweed farming
operations, as well as operators of offshore farms. This
intervention, if highly successful, could also reduce
seaweed prices by increasing supply, impacting mostly
smallholders who currently dominate production.
Ecological risks:Higher productivity seaweed farms located innutrient-poor water could reduce phytoplankton
production. Sequestering C by sinking seaweed into
deep waters would increase the advection of organic
C into deep water which could alter food webs,
species composition and oxygen levels via microbial
decomposition and respiration.

Intervention 3: Expand seaweed farming.
Most of the expansion of seaweed farming is expected
to occur in offshore waters both within and beyond
national jurisdictions. Some nations are developing
governance systems for offshore aquaculture (e.g.,the U.S.), but governance is largely absent as very few
commercial ventures currently exist. Research into thesocial and ecological risks that could be associatedwith offshore seaweed farms would be required to set
performance standards, accountability measures andother elements of an effective offshore governancesystem. Any governance system would also have to
reduce barriers to expansion without compromisingsocial and economic goals. For example, thecomplexity and cost of aquaculture permitting in
U.S. waters is widely recognized as a significant
barrier to industry expansion. Solutions may include
the establishment of aquaculture enterprise zones
where farmers could take advantage of governmentsubsidized research to prepare permit applications,or operate under an umbrella permit supported
by several farms, performance standards andaccountability measures to mitigate risks of adverse
social, economic and ecological impacts. National
plans of action and the incorporation of seaweed C
sequestration into Nationally Determined Contribution
accounts could also be elements of this intervention
that incentivize the expansion of types of seaweedfarming that sequester more C than other types.
This intervention would also include an effort toremove barriers to the development of seaweed farms
that sequester more C than others, e.g., by quantifying
carbon sequestration by different types of seaweedfarms, doing research to address other aspects ofa high quality carbon credit (EDF et al. 2020) andenhancing markets for products and uses of seaweedthat sequester carbon and result in avoided GHG
emissions.
Sites and therefore jurisdictions are uncertain andwould depend on many factors, including light andnutrient availability, proximity to shoreside support
facilities and markets, areas where risk of interactions
with marine wildlife is high and others. Siting will also
depend on existing claims on marine space and theneed to minimize adverse ecological impacts, e.g.,
reductions in phytoplankton production resulting fromnutrient uptake by farms. While these considerations
will limit the potential farmable area of the ocean, thisis still likely to be quite large. For example, in the U.S.
Caribbean and Florida, 80% of the total area within
10–100 m depth is potentially available for seaweedfarming or has no conflicts with navigation, natural
resources, oceanographic conditions, etc. (NOAA
OceanReports 2022). For sugar kelp, Saccharina
17

CARBON SEQUESTRATION BY SEAWEED

latissima, in Alaska and New England, that number
drops to around 20% of the total area available. But
for Alaska in particular, a considerable amount offarmable area remains (over 3.5 million ha) (NOAA
OceanReports 2022). The large and fragmentary
nature of these areas (or intentional siting for thispurpose) could mitigate adverse impacts caused bynutrient uptake by the farms.
Potential for C sequestration:
NASEM (2021) estimate that farming about 72,900
km2 of seaweed and injecting the biomass into thedeep ocean could sequester 0.1 Pg CO2/yr. This could
scale dramatically with increasing use of the suitable
farming area of the ocean, barring other limiting
factors, such as negative impact on phytoplankton
productivity, congestion at sea, etc.
Co-benefits:
Most of this expansion would probably occur offshore
due to existing claims on nearshore marine space,pollution and other factors. Because floating debris
and seaweed are known to attract marine life, it seems
likely that this intervention could result in substantial
amounts of habitat and biodiversity provisioning.
Large-scale seaweed farming could also result inlarger-scale amelioration of ocean acidification.
Social and economic effects:
This intervention may primarily benefit entrepreneurs
with access to capital and technical expertise, perhaps
to the detriment of smallholders, as a result ofcompetition in some markets. To the extent different
products are produced offshore, this competition
would be reduced. Larger-scale seaweed farming
could also pose hazards to navigation, fishing andother uses of offshore waters. This intervention, if
highly successful, could also reduce seaweed prices
by increasing supply, impacting mostly smallholders
who currently dominate production.
Ecological risks:More seaweed farms could translate into higher risk ofwildlife entanglement. Sequestering 0.1 Pg CO2/yr via
sinking seaweed into deep waters would increase theinjection of organic C into deep water by about 25%,a very significant increase (NASEM 2021) that could
alter food webs, species composition and oxygen
levels via microbial decomposition and respiration.

Readiness for implementation
We propose using criteria defining high-integrity
carbon credits (EDF et al., 2020) to evaluate which,if any, of the proposed interventions could generate
high-integrity carbon credits, and to identify
appropriate actions for getting interventions ready for
implementation.

rate of advection of fragments and refractory DOC
to deep water, and factors that affect losses of Cabsorbed by seaweed such as grazing and exudationof labile DOC, and reduced phytoplankton production(with reduced C sequestration) resulting fromcompetition for macronutrients.

The criteria are:

Intervention 2: Increase productivity of seaweedfarms. High additionality if operations leave seaweed
in the water or products and uses are refractory.
Increased productivity would increase yield but would
only increase C sequestration if biomass is left in thewater long enough for fragmentation and advection
to occur. If the increased yields are used to produce
food, hydrocolloids or other products with short life
cycles, additional carbon sequestration via increased
productivity would be minimal. On the other hand, if
increased yields are sunk, buried or incorporated into
soils (i.e., products and uses are refractory) resulting in
long-term C storage, additionality would be high.

• Additionality (carbon reduction would not have
happened without the project)• Vulnerability of the carbon sequestration pathway• Robust quantification of emission reductions andremovals• Significance of risks to permanence of carbon
sequestration• Robustness of approaches to achieve permanent
carbon sequestration• Adoption of low, zero or negative emissions
technology• Contribution to adaptation and resilience including
support of poorest and most vulnerable people
affected by climate change• Social and economic impacts• Avoidance of double issuance, use, claims against
domestic and international targets, double
counting in carbon markets• Demonstration of host country commitment toglobal carbon reduction goals• Program governance• Third-party auditing• Transparency and stakeholder consultation
The following evaluation is based on the criteria fromEDF et al. (2020) that seem amenable to scientificanalysis (bolded in the list above). The remaining
criteria relate to governance and accountability, which
we did not explore as part of this report.

Additionality
Intervention 1: Conserve and restore natural seaweed
stands. Additionality for restoration, but not forconservation. While conserving existing seaweeds
would not result in additional carbon sequestration,
unlike restoring seaweed stands that have declined
relative to a recent historical baseline. The amountof additional carbon sequestered via seaweed bed
restoration would depend on a number of contextspecific factors, including the ability to maintainseaweed biomass over time, proximity to deep water,

18

CARBON SEQUESTRATION BY SEAWEEDIntervention 3: Expand seaweed farming. Highadditionality if operations leave seaweed in thewater or products and uses are refractory. Increased
seaweed production associated with the expansion ofseaweed farming would increase yield but would only
increase C sequestration if biomass was left in thewater long enough for fragmentation and advection to
occur, or if products and uses were refractory.

Vulnerability of the carbon sequestration
pathwayIntervention 1: Conserve and restore natural seaweed
stands. High vulnerability. Because many factors caninfluence natural seaweed productivity, C loss and C
sequestration, this pathway seems quite vulnerable.
Climate change can result in dramatic declines inseaweed productivity (Bennett et al., 2015) and hence
in C sequestration (Krause-Jensen and Duarte 2016).
Increased grazing pressure resulting from overfishingof predators or other factors can also result innatural seaweed bed decline, which could also reduce
C sequestration via DOC and seaweed fragment
advection. Some of the C fixed by the seaweed will
be stored in grazers and microbes, but on shorttimescales.
Intervention 2: Increase productivity of seaweedfarms. Moderate vulnerability. Because many factors(including climate change) can influence C loss
and C sequestration by farms, this pathway could
be vulnerable. Vulnerability could be reduced tosome extent via strain selection, changes in farming
practices,and changes in product disposition.

Harvesting frequently and using the yield to makerefractory products, sinking the yield, burying theyield or using the yield to make soil conditioners toincorporate C into soil would reduce vulnerability.
Intervention 3: Expand seaweed farming. Moderate
vulnerability. Because many factors (including climatechange) can influence C loss and C sequestration byfarms, this pathway could be vulnerable. Vulnerability
could be reduced to some extent via strain selection,changes in farming practices and changes in productdisposition. Harvesting frequently and using the yield
to make refractory products, sinking the yield, burying
the yield or using the yield to make soil conditioners toincorporate C into soil would reduce vulnerability.

Robust quantification of emission
reductions and removals
Intervention 1: Conserve and restore natural seaweed
stands. No robust quantifications yet. Estimates ofcarbon removal by natural seaweed stands are highly
uncertain currently. Context-specific research on C
flow from the atmosphere through seaweed stands
that differ in conditions hypothesized to affect C
sequestration (e.g., with differing nutrient availability,
delta pCO2, advection to deep water, proximity to
sediments, grazing pressure, fragmentation frequency,etc.) will be required to quantify carbon sequestration
by seaweed bed restoration projects.
Intervention 2: Increase seaweed farm productivity.
No robust quantification yet.
Intervention 3: Expand seaweed farms. No robust
quantification yet. However, Oceans 2050 andother groups of researchers are conducting studies
designed to quantify C sequestration by seaweedfarms (Froehlich et al., 2019; Duarte et al., 2017; GSP2022).

Significance of risks to permanence ofcarbon sequestration
Intervention 1: Conserve and restore naturalseaweed stands. High risk. Declines in biomass after
restoration, reduced fragmentation rate, reduced
fraction of refractory DOC exudation and reduced
advection rates to sediments and deep water can all
reduce the duration of carbon sequestered by naturalseaweed stands.
Intervention 2: Increased seaweed farm productivity.
Moderate risk. The duration of carbon sequestration
resulting from increased farm productivity would bevulnerable to the extent that sequestration depends
on natural processes (production of refractory DOC
and advection of seaweed fragments to sediments or
deep waters). Currently, reliance on these processes
19

CARBON SEQUESTRATION BY SEAWEED

is high. However, reliance and therefore risk to
the duration of C sequestration could be greatly
reduced by shifting to the production of refractory
products and uses. Purposeful sinking of all or mostof the seaweed yield from a farm could reduce risks
to the permanence of C sequestration, but could
pose risks to deep ocean ecosystems. Storage ofseaweed biomass into geological formations orapplying seaweed to soils would also reduce risks topermanence.
Intervention 3: Expand seaweed farming. Moderaterisk. The duration of carbon sequestration resulting
from increased farm productivity would be vulnerable
to the extent that sequestration depends on naturalprocesses (production of refractory DOC andadvection of seaweed fragments to sediments or deep
waters). Currently, reliance on these processes is high.
However, reliance and therefore risk to the duration of
C sequestration could be greatly reduced by shifting
to the production of refractory products and uses.
Purposeful sinking of all or most of the seaweed yieldfrom a farm could reduce risks to the permanenceof C sequestration, but could pose risks to deep
ocean ecosystems. Storage of seaweed biomass into
geological formations or applying seaweed to soils
would also reduce risks to permanence.

Robustness of approaches to achieve
permanent carbon sequestration
Intervention 1: Conserve and restore natural seaweed
stands. Moderately robust. Natural seaweed stands,
both existing and restored, sequester carbon byexporting refractory DOC and biomass fragments
to deep water. In addition, restored seaweed stands
would store the C absorbed during growth following
restoration as long as this biomass is maintained.
Bundles of kelp degrade fairly rapidly even at 1700 m
depth, but this results in very little change in sediment
C concentration, suggesting that most of the seaweed
carbon is converted to DIC and DOC (Bernardino
et al., 2010 – cited in a NASEM 2021 report). Ocean
circulation plays an important role in determining
how long seaweed carbon would remain in deep
water. Most of the C is expected to stay in the deep
ocean for more than 100 years, except perhaps in thewestern north Atlantic. The Pacific and Indian ocean
basins are likely to sequester this carbon for longer
periods of time than the Atlantic and Southern ocean
basins (NAS 2021).
Intervention 2: Increase productivity of seaweedfarms. Highly robust. This could result in robust C
sequestration if products and uses are refractory(i.e., have long lifetimes or do not result in theremineralization of C back into CO2).

Intervention 3: Expand seaweed farming. Highlyrobust. This could result in robust C sequestration
if products and uses are refractory (i.e., have long
lifetimes or do not result in the remineralization of C
back into CO2).

Intervention 2: Increase productivity of seaweedfarms. High contribution. Habitat provisioning withresulting biodiversity enhancement and increased
resilience could result from this intervention to theextent that biomass is left in the water (e.g., via lessfrequent harvesting or year-round production).

Adoption of low, zero or negative emissions Intervention 3: Expand seaweed farming. Hightechnology
contribution. Habitat provisioning with resulting
Intervention 1: Conserve and restore natural seaweed
stands. Low emissions. To the extent that vessels
would be necessary to restore seaweed stands, lowor zero emissions engines could be used to minimize
GHG emissions.
Intervention 2: Increase productivity of seaweedfarms. Moderate emissions. Compared withconservation and restoration of natural seaweed
stands, farming – even with increased productivity –would be expected to release more GHG emissions.
Vessels and other machinery associated with seaweedfarms could use low or zero emission engines tominimize GHG emissions. Drying seaweed prior toshipping or processing is the most energy intensive
process in most seaweed farm operations. Drying
technology can also be converted to use low or zero
emission fuels. Transport of the seaweed to markets
or sequestration locations may also result in GHG
emissions.
Intervention 3: Expand seaweed farming. Moderate
emissions. It seems likely that expansion of seaweedfarming into offshore waters would entail higher
GHG emissions relative to nearshore farming orconservation/restoration of natural seaweed stands.
However, vessels and other machinery associated with
seaweed farms could use low or zero emission engines
to minimize GHG emissions. Drying seaweed prior toshipping or processing is the most energy-intensive
process in most seaweed farm operations. Drying
technology can also be converted to use low or zero
emission fuels. Transport of the seaweed to markets
or sequestration locations may also result in GHG
emissions.

Contribution to adaptation and resilience
Intervention 1: Conserve and restore natural seaweed
stands. High contribution. Restoration of naturalseaweed stands will likely enhance biodiversity,
functional redundancy and other attributes of resilient
ecological systems. To the extent that restoration
also results in larger fish populations and ecotourism
opportunities, seaweed bed restoration could also
increase social and economic resilience by diversifying
livelihood opportunities and revenue streams.

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CARBON SEQUESTRATION BY SEAWEED

biodiversity enhancement and increased resilience
could result from this intervention to the extent thatbiomass is left in the water (e.g., via less frequent
harvesting or year-round production).

Support of poorest and most vulnerable
people affected by climate changeIntervention 1: Conserve and restore natural seaweed
stands. High support. Some of the poorest and mostvulnerable people affected by climate change livein tropical coastal communities. Many are highly
dependent on fishing, which in turn depends on themaintenance of high quality spawning, rearing andgrow-out habitats, including seaweed stands. Naturalseaweed stands also allow for seaweed gathering,which is an important source of income in somecoastal communities.
Intervention 2: Increase productivity of seaweedfarms. High support. Almost all seaweed farms are
currently located in nearshore waters and operatedby smallholders, including some fishers, many ofwhom are poor and quite vulnerable to climate change
due to the lack of economic opportunities in theircommunities and declines in fish stock abundance due
to climate change, particularly in the tropics.
As a result, increased seaweed farm productivity
would therefore likely benefit these groups, unlessthese improvements result in the displacement ofsmallholders by individuals or firms with greater
access to capital and technical expertise.
Intervention 3: Expand seaweed farming. Low support.
While some expansion could occur in nearshore
waters, potentially benefiting smallholders andfishing-dependent communities, most expansion
is likely to occur in offshore waters. This will likely
benefit those with access to sufficient capital andtechnical expertise to carry out offshore operations,rather than smallholders. In addition, a major increase
in seaweed supply could disrupt existing markets
dominated by smallholders to the extent the products
compete in the same markets.

Conclusions
Seaweeds are, in concept, capable of sequesteringsignificant amounts of atmospheric carbon dioxide.
However, the amount of carbon sequestered depends
on a number of variables, many of which are contextdependent.
The literature seems to indicate that the major
processes that influence carbon sequestration byseaweeds are fairly well understood conceptually, but
significant data gaps exist. Estimates of the amountof carbon being sequestered by seaweeds currently
depend on extrapolations of the rates of C uptake,
C pool size in seaweed biomass, DOC exudation,
microbial mineralization, losses to grazing andsenescence and advection of C fixed by seaweeds
to sediments or deep waters from limited studies for
only a few species in a few locations; moreover, dataon these fluxes are not often collected simultaneously.
However, these rates and pools vary widely, depending
on context and species. More empirical, parallel
studies of these rates and pools with more species
under different environmental conditions would beneeded to improve global estimates of seaweed
carbon sequestration. It seems likely that the rate of
CO2 drawdown resulting from C uptake by seaweeds
will also vary due to variation in CO2 concentrationdifferentials between the ocean and the atmosphere indifferent parts of the ocean and at different times ofthe year.
Quantifying carbon sequestration by specific seaweedfarms with respect to the creation of high quality
credits (EDF et al. 2020, 2021) requires these data
gaps be filled, plus some additional data streams will
be required, many of which are also context-specific.
These include data on harvest frequency, fractionof biomass harvested, seaweed density, productdisposition, GHG emissions associated with seaweed
production and processing, and impacts of the farm
21

CARBON SEQUESTRATION BY SEAWEEDWIKIMEDIA COMMONS

on other sources and sinks of GHGs. Such data arelargely lacking; however, several research efforts areunderway to fill these data gaps.
Seaweed stands and seaweed farms already produce
many social, economic and ecological benefits,
including food and hydrocolloids, livelihoods forsmallholders who dominate the seaweed farming
industry, alternative livelihoods and supplemental
incomes for people and communities with limited
economic opportunities, biodiversity provisioning, local
amelioration of ocean acidification and some carbon
sequestration. However, there are tradeoffs inherent
in the generation of these benefits. For example,
farms that use seaweed to make food products orhydrocolloids would be expected to sequester lesscarbon than farms that sequester seaweed biomassin the sea or on land. On the other hand, farms thatmaximize carbon sequestration would probably
generate less food and other valuable products, andco-benefits. A portfolio of farms producing different
types of products and uses with different harvest
cycles would probably be required to realize all thebenefits that seaweeds are capable of providing.
There are several ways in which carbon sequestration
by seaweeds could be enhanced: Conservation andrestoration of existing seaweed stands, increasing
productivity of seaweed farms and expansion ofseaweed farming to offshore waters with the creationof enabling conditions that incentivize more C
sequestration.
Conservation of existing seaweed stands will be
important to avoid C loss from the ocean to theatmosphere and to conserve the many benefits that
seaweed generates, but would not result in additional
carbon sequestration. Restoration of depleted stands
would result in additional C absorption and storageas long as the seaweed biomass is maintained, whichmay prove challenging given that many factors
influence seaweed standing stock. Some of the C fixedby current and restored seaweed stands would besequestered via advection of fragments and refractory
DOC to sediments and deep waters.
This intervention would not likely result in a large
increase in additional carbon sequestration because
the areal extent of such efforts is constrained byavailable nearshore habitat. However, it would likely
result in a large number of substantial co-benefits,
including food production, fishery enhancement,
biodiversity provisioning and increased socioecological resilience with very low risk of adverse
social, economic or ecological impact. This
intervention is already underway. Acceleration of C
sequestration by seaweeds via this intervention would
require research to identify context-specific threats
to natural seaweed stands and specific areas where
threats are amenable to threat reduction actions
such as the reduction of fishing pressure or pollution(rather than threats that require international action)
and appropriate substrate (Fredriksen et al., 2020),
coupled with enhanced efforts to counter thosethreats. This can also be expanded to areas that havenot traditionally had seaweed farms or have not had
natural stands for >20 years (e.g., TBF 2022).
There is also scope for increasing carbon
sequestration by increasing the productivity ofseaweed farms through improved strain selection
and other practices, although it is not known how
much additional C sequestration would result. This
intervention would result in additionality for existing
farms, but carbon sequestration would be constrainedby the relatively small area that is currently farmed.
Additional productivity in the near term would likely
drive the production of carbon labile products such as
food and hydrocolloids, given current market structure.
This would also constrain the amount of additional
carbon that could be sequestered. This intervention
is also currently underway. Enhancement of carbon
sequestration via this intervention would require more
research on high-yield strains for many more species,
as well as research on how to improve cultivation
methods and remove constraints on productivity suchas disease. Capacity building would also be required tomainstream practices that improve farm productivity,
particularly in tropical regions where temperatures
are already high. New policies and a robust C offset
market would probably be necessary to encourage
the development of refractory products and uses (e.g.,
sinking, storage in geological formations, or use as soil

22

CARBON SEQUESTRATION BY SEAWEED

amendments) that could leverage increased seaweed
productivity to result in increased C sequestration or
avoid GHG emissions (e.g., biofuels, bioplastics anduse as feed additives to reduce ruminant methane
emissions if further research indicates that this is asafe and effective pathway).
Expansion of seaweed farming into offshore waters
is perhaps the most promising way to increase C
sequestration by seaweeds at a scale that could
significantly contribute to climate stabilization.
Offshore seaweed farming is becoming feasible given
rapid advances in infrastructure, farm operations andmonitoring. Farmable area is constrained by roughseas, light and nutrient availability, as well as byproximity to shore-side support infrastructure, claims
on marine space, and the need to avoid negative socialand ecological impacts. While estimates of farmablearea are uncertain, recent exercises that take someof these factors into account suggest that farmablearea would be very large relative to the area currently
farmed. Further advances in how to safely sequester
seaweed carbon, in offshore farming infrastructure
and in understanding the fate of carbon absorbed
by seaweed grown in different types of farms will
likely be necessary to make this NCS viable (NASEM
2021). Significant data gaps must also be filled, andwhile it may be possible to improve estimates of
the C sequestration potential of different types offarms (see Appendix – Seaweed Farm Archetypes
for Data and Models Working Group), many of these
uncertainties can only be reduced by building highly
monitored farms in diverse ocean settings that are
optimized for carbon sequestration (NASEM 2021).
As is the case for nearshore seaweed farming, there
will be tradeoffs between benefits that offshore
seaweed farms could generate. In the near-term,
seaweed yield from offshore farms would likely flow
into food production, hydrocolloid production andperhaps into some newer high-value products suchas nutraceuticals given current markets. This would
constrain carbon sequestration in the absence ofinterventions to incentivize sequestration, suchas policy directives, government funding and highintegrity markets for carbon offsets based on seaweed
carbon sequestration. Collaboration between entities
such as Oceans 2050, Ostrom Climate Solutions,
Verra, EDF and others working on protocols forquantifying carbon sequestration via seaweed farming
would probably be helpful in this respect by reducing
uncertainty about what constitutes credible carbon
accounting for seaweed farms.

Appendix
Seaweed Farm Archetypes for Data and Models Working GroupAt our workshop, we proposed the formation of aseaweed data and models working group for scientists
who would like to work together to refine estimates ofcurrent levels of C sequestration by seaweed farms
and develop monitoring and verification protocols
for seaweed-based C sequestration. This group willattempt to agree on which carbon fluxes must be
measured in order to quantify carbon sequestration
by seaweed stands and seaweed farms, and on how
to monitor and verify a seaweed-based carbon offset
project based on the criteria for a high quality carbon
credit. The group will also strive to create an inventoryof existing data streams, identify necessary additional
data streams, create an inventory of existing andplanned models, and identify ways that we cancontinue to work together to add value to the global
dialogue around carbon sequestration by seaweeds.
Many context-specific factors influence C
sequestration by seaweed farms. These include
growing season, nutrient availability, harvest cycleand end uses of the yield. While ultimately it will benecessary to monitor every farm that is being used tooffset carbon emissions, it may be helpful to define
seaweed farm archetypes that capture variation in themost important factors that influence C sequestration
to serve as frameworks that could guide thedevelopment of common monitoring and verification
protocols for different kinds of farms.

Selected seaweed farm archetypes
Consideration of just a few of the factors that
influence carbon sequestration by seaweed farms –proximity to carbon sequestration pathways such as
deep water, nutrient availability, duration of growing
season, harvest patterns and product disposition –yields dozens of possible seaweed farm archetypes.
Since this is far too many to evaluate within thescope of our project, we propose a focus on thearchetypes that are now common or seem likely to
be implemented and which capture a wide range ofvariation in these factors. We refer to products anduses in which C turns over on a timescale of months
to years (e.g., food and hydrocolloids) as “labile,”
and to products and uses in which C turns over ona timescale of hundreds of years (e.g., sinking or the
production of very durable products) as “refractory”
for brevity.
1. Tropical (long growing season, multiple annual
harvests), nearshore, low nutrient, carbon-labile
products and uses. This type of seaweed farming
is very common in countries that produce large
amounts of seaweed (e.g., Indonesia and thePhilippines) (Rimmer et al., 2021; Trono and Largo
2019). We anticipate that seaweed productivity
would be quite high despite low nutrient availability
given observed tropical seaweed growth rates
under these conditions, with some loss to grazing.
Advection of refractory DOC and fragments
could be relatively low due to intensive grazing,
remineralization and distance to deep water. We
anticipate that most nearshore tropical seaweed
23

CARBON SEQUESTRATION BY SEAWEED

farms will continue to produce carbon-labile
products because they are valuable, and because
carbon markets and policies do not yet incentivize
practices and products that would result in large
amounts of carbon sequestration.
2. Tropical (long growing season, multiple annual
harvests), close to sequestration pathways, lownutrient availability, refractory products and uses. If
carbon markets develop, carbon sequestration andother aspects of a high quality carbon credit canbe verified, and policies that encourage seaweedbased carbon sequestration are emplaced, wewould anticipate these types of farms to increase in
number and scale.
3. Temperate (short growing season, single annual
harvest), close to sequestration pathways, highnutrient availability, labile products and uses.
Longer harvest cycles typical of temperatewater seaweed farms could result in more C
sequestration via fragmentation and advection, as
well as co-benefits such as habitat provisioning,fishery enhancement and amelioration of ocean
acidification. However, the production of labile
products in response to existing price signals andmarkets would limit C sequestration benefit.
4. Temperate (short growing season, single annual
harvest), close to sequestration pathways, highnutrient availability, refractory products and uses.
Longer harvest cycles could result in more C
sequestration via fragmentation and advection, aswell as co-benefits such as habitat provisioning,fishery enhancement and amelioration of ocean
acidification. If carbon markets develop, carbon
sequestration and other aspects of a high quality
carbon credit can be verified, and policies thatencourage seaweed-based carbon sequestration
are emplaced, we would anticipate these types offarms to increase in number and scale.
We chose these archetypes to facilitate thedevelopment of carbon flow maps for use indeveloping quantification and monitoring programsfor different types of seaweed farms. We eliminatedarchetypes that seem uncommon, such as tropical,
high nutrient and close to deep water. We also
eliminated archetypes based on types of products anduses that are not directly related to C sequestration.

For example, seaweed farms that produce biomass
for conversion to biofuel can spare CO2 emissions
to the extent that the biofuel is substituted for fossil
fuel, but this will not result in net CO2 sequestrationand so are not considered in this analysis. Seaweedfarms that produce biomass to be used as a ruminantfeed supplement could reduce emissions of methane(Min et al., 2021), a potent greenhouse gas, but thispathway would require a separate system mapping
exercise. Similarly, it would be necessary to separately
map carbon flow through systems that include
seaweed farms that produce biomass to be used assoil amendments. We will discuss with the data andmodels group whether there is interest in addingarchetypes that capture these pathways for avoiding
GHG emissions.

TABLE 1:
Seaweed farm archetypes based on location, climate regime and nutrient availability.

MeasureLocation

24

Type

Definition / Description

Nearshore• Proximate to other nearshore ecosystems• Most versatile regarding farm structure, including bottom, off-bottom,
rafting and long-line culture• Close to land
• Shallower waters• User conflicts• Proximity to deep water and upwelling• Current movement• Can utilize agricultural runoff as nutrient source• Provide ecosystem services to coastal marine life (enhanced
fisheries, biodiversity, ocean acidification remediation)

Offshore• Typically stronger currents and rougher conditions• Not able to utilize bottom or off-bottom culture• Can be installed in combination with other structures, such as windfarms and oil rigs
• Out-of-state waters – EEZ at three miles from coast• Depth profile is much deeper• Less user conflicts

Land-based• This includes the cultivation of seaweed in ponds or tanks, indoors
and outdoors, where water is pumped to the algae.
• Usually, land-based systems also include a higher control ofenvironmental parameters (including light, water velocity, water
residence time, and temperature and nutrient types and amount)when compared to the other types• Control over carbohydrate / protein production• Zero pollution of effluent• May have high costs• Constrained by land, has to be closer to the coast

CARBON SEQUESTRATION BY SEAWEED

MeasureType

Definition / Description

Tropical• Eucheuma, Gracilaria, Hydropuntia, Hypnea, Kappaphycus,
Cladosiphon and Caulerpa are primarily cultivated in the tropics andsubtropics• Tropical seaweed farms are characterized by year-round growth andmultiple harvesting cycles yearly• Oligotrophic waters, low nutrients and low advection to deep water

Temperate• Agardhiella, Gelidium, Gigartina, Porphyra, Saccharina, Laminaria,
Undaria, Monostroma and Ulva are primarily cultivated in temperatezones• Temperate seaweed farms are characterized by a limited harvestingcycle• Eutrophic waters and seasonal pulsing of nutrientsHigh• Proximity of the seaweed farm to high-nutrient sources that can benatural (i.e., upwelling) or anthropogenic sourced (i.e., from fish farms,
agricultural runoff, sewage, eutrophication and IMTA / co-culture)• Artificial upwelling

Low• The absence of nutrient sources proximate to the seaweed farm,
resulting in lower nutrient uptake by the seaweed and thus slower
growthClimateNutrient
input

It will be important to consider scale for all these farm archetypes, as this will also affect processing capacityand export vs. keeping this in the local areas. Planting density and structure are also important considerations.
Scaling up requires strain selection, genetic diversity, as well as diversifying species for cultivation vs. monocropping.

Seaweed Typologies Schematic

Nearshore

TropicalTemperate

Nutrient input

25

Offshore

TropicalTemperate

Nutrient input

CARBON SEQUESTRATION BY SEAWEED

Land-based

TropicalTemperate

Nutrient input

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