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Review
Potential role of seaweeds in climate change mitigation
Finnley W.R. Ross
a,
⁎, Philip W. Boyd
b
, Karen Filbee-Dexter
c,d
, Kenta Watanabe
e
, Alejandra Ortega
f
,
Dorte Krause-Jensen
g,h
, Catherine Lovelock
i
, Calvyn F.A. Sondak
j,k
, Lennart T. Bach
b
, Carlos M. Duarte
l,m
,
Oscar Serrano
n,o
,JohnBeardall
p,q
, Patrick Tarbuck
r
, Peter I. Macreadie
a
a
Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Burwood Campus, Burwood, VIC, Australia
b
Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia
c
Institute of Marine Research, 4817 His, Norway
d
UWA Oceans Institute, University of Western Australia, Crawley, WA 6009, Australia
e
Coastal and Estuarine Environment Research Group, Port and Airport Research Institute, 3-1-1 Nagase, Yokosuka 239-0826, Japan
f
King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
g
Department of Ecoscience, Aarhus University, Ole Rømers Allé, building 1131, Aarhus C 8000, Denmark
h
Arctic Research Centre, Aarhus University, Ole Worms Allé 1, Aarhus C 8000, Denmark
i
School of Biological Sciences, The University of Queensland, St Lucia, QLD 4072, Australia
j
Department of Oceanography, Pusan National University, Busan 46241, South Korea
k
Faculty of Fisheries and Marine Science, Sam Ratulangi University, Manado 95115, Indonesia
l
King Abdullah University of Science and Technology, Red Sea Research Center (RSRC), Saudi Arabia
m
Computational Bioscience Research Center (CBRC), Thuwal, Saudi Arabia
n
Centro de Estudios Avanzados de Blanes, Consejo Superior de Investigaciones Científicas (CEAB-CSIC), Blanes, Spain
o
School of Science &Centre for Marine EcosystemsResearch, Edith Cowan University, Joondalup, WA, Australia
p
School of Biological Sciences, Monash University, Clayton 3800, Australia
q
Faculty of Applied Sciences, UCSI University, Kuala Lumpur, Malaysia
r
Sea Green Pte. Ltd., 60 Paya Lebar Road #06-12, Paya Lebar Square, Singapore 409051, Singapore
HIGHLIGHTS GRAPHICAL ABSTRACT
•Seaweed carbon accounting is yet to be
fully constrained.
•Seaweed products have the potential to
lower industrial emissions.
•Seaweed farms sequester carbon at site but
at a limited scale to date.
•Quantifying carbon seqestration from wild
seaweed restoration remains ellusive.
•Sinking seaweed has scalability but carries
many risks and uncertainties.
Science of the Total Environment 885 (2023) 163699
⁎Corresponding author.
E-mail address: fwross@deakin.edu.au (F.W.R. Ross).
http://dx.doi.org/10.1016/j.scitotenv.2023.163699
Received 12 December 2022; Received in revised form 3 April 2023; Accepted 19 April 2023
Available online 4 May 2023
0048-9697/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
ABSTRACTARTICLE INFO
Editor: Kuishuang Feng
Keywords:
Blue carbon
Aquaculture
Kelp restoration
Conservation
Macroalgae
Carbon sequestration
Seaweed (macroalgae) has attracted attention globally given its potential for climate change mitigation. A topical and
contentious question is: Can seaweeds' contribution to climate change mitigation be enhanced at globally meaningful
scales? Here, we provide an overview of the pressing research needs surrounding the potential role of seaweed in cli-
mate change mitigation and current scientific consensus via eight key research challenges. There are four categories
where seaweed has been suggested to beused for climate change mitigation: 1) protecting and restoringwild seaweed
forests with potential climate change mitigation co-benefits; 2) expanding sustainable nearshore seaweed aquaculture
with potential climatechange mitigation co-benefits;3) offsetting industrial CO
2
emissions using seaweedproducts for
emission abatement; and 4) sinking seaweed into the deep sea to sequester CO
2
. Uncertainties remain about quantifi-
cation of the net impact of carbon export from seaweed restoration and seaweed farming sites on atmospheric CO
2
.
Evidence suggests that nearshore seaweed farming contributes to carbon storage in sediments below farm sites, but
how scalable is this process? Products from seaweed aquaculture, such as the livestock methane-reducing seaweed
Asparagopsis or low carbon food resources show promise for climate change mitigation, yet the carbon footprint and
emission abatement potential remains unquantified for most seaweed products. Similarly, purposely cultivating
then sinking seaweed biomass in the open oceanraises ecologicalconcerns and the climatechange mitigationpotential
of this conceptis poorly constrained.Improving the tracing of seaweed carbon export to ocean sinksis a critical step in
seaweed carbon accounting. Despite carbon accounting uncertainties, seaweed provides many other ecosystem ser-
vices that justify conservation and restoration and the uptake of seaweed aquaculture will contribute to the United
NationsSustainable Development Goals.However, we caution thatverified seaw eed carbon accou nting and associat ed
sustainability thresholds are needed before large-scale investment into climate change mitigation from seaweed
projects.
Contents
1. Introduction................................................................ 2
2. Challenge1-resolvingknowledgegapsintheseaweedcarboncycle ....................................... 3
3. Challenge 2 - developing technologies, computational models and tools to measure seaweed carbon fluxes....................... 6
4. Challenge3-understandingthepotentialcontributionofwildseaweedtoclimatechangemitigation......................... 7
5. Challenge4-contributionofseaweedaquaculturetoundeliberatecarbonsequestrationduringthefarmingprocess................... 8
6. Challenge5-understandingfutureopportunitiesfromseaweedaquacultureproductsforemissionabatement...................... 9
7. Challenge 6 - sinking seaweed into the deep seato sequester CO
2
......................................... 9
8. Challenge 7 - environmental benefitsandrisksofacceleratingseaweedaquaculturetoenhancecarbonsequestration ................. 10
9. Challenge 8 - understanding the key policy and governance considerations surrounding the potential role of seaweed for climate change mitigation .... 11
10. Nextstepsinthescienceofseaweedasameanstocounteractclimatechange ................................ 12
CRediTauthorshipcontributionstatement .................................................... 12
Dataavailability................................................................ 12
Declarationofcompetinginterest ........................................................ 12
Acknowledgements .............................................................. 12
References.................................................................. 12
1. Introduction
Atmospheric carbon dioxide has risen to 420 ppm from pre-industrial
levels of 280 ppm, triggering climate change impacts on biodiversity and
society (Bolin and Doos, 1989;Houghton, 1996;Bennett et al., 2016;Pecl
et al., 2017;IPCC, 2019;Pörtner et al., 2019;Smale et al., 2019;Straub
et al., 2019). Whereas reduced consumption, increasing energy efficiency
and transitioning towards renewable energy and more sustainable agricul-
tural systems are essential to achieve climate goals, enhancing removal of
excess atmospheric CO
2
is crucial to limit warming to 1.5 °C (IPCC, 2022).
Half of the global CO
2
fixation occurs in the oceans (Nellemann et al.,
2010). As such, management of blue carbon –the organic carbon that is
captured and stored by coastal and marine ecosystems, is of importance
as it provides opportunities to increase oceanic sequestration of atmo-
spheric CO
2
(Mcleod et al., 2011). Vegetated coastal ecosystems are in
the spotlight as their protection and restoration may offer pathways for
climate change mitigation, hereafter referred to as climate change mitiga-
tion, and adaptation in the coastal ocean (Duarte et al., 2013;Macreadie
et al., 2021).
Seaweeds are fast-growing marine macrophytes with modelling sug-
gesting a global area and net primary production comparable to the Amazo-
nian forest (Duarte et al., 2022). In addition to wild seaweed forests,
seaweed farming contributes about half of the production of marine
aquaculture (Duarte et al., 2021). Seaweed has been suggested to contrib-
ute to climate change mitigation through four potential pathways
(Krause-Jensen et al., 2018;Duarte et al., 2021;Troell et al., 2022):
1) protecting and restoring wild seaweed forests with potential climate
change mitigation co-benefits; 2) expanding sustainable nearshore seaweed
aquaculture with potential climate change mitigation co-benefits; 3) offset-
ting industrial CO
2
emissions using seaweed products for emission abate-
ment; and 4) sinking seaweed into the deep sea to sequester CO
2
.
Protection and restoration of blue carbon ecosystems has focussed on
mangroves, saltmarshes and seagrass –macrophytes that contribute carbon
in roots and other tissues to sediments and facilitate the accumulation of
organic-rich sediments below the habitat (Ouyang et al., 2017;Macreadie
et al., 2021). However, recent reviews suggest that seaweed-dominated
ecosystems are also important contributors to nearshore and offshore
carbon-sinks (Kennedy et al., 2010;Krause-Jensen and Duarte, 2016;
Macreadie et al., 2019;Ortega et al., 2019;Ortega et al., 2020b;
Williamson and Gattuso, 2022), leading to arguments that these ecosystems
should also be used for climate change mitigation (Krause-Jensen et al.,
2018;Duarte et al., 2022). The area of seaweed that grows on soft-
sediments, where organic carbon may accumulate, is estimated at 1.5 mil-
lion km
2
–relative to the 7.2 million km
2
of total global seaweed extent
(Duarte et al., 2022). Moreover, seaweed release and laterally export
dissolved and particulate organic matter beyond their habitat, thereby
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
2
providing a potential long-term carbon sink in the deep ocean (Hill et al.,
2015;Trevathan-Tackett et al., 2015;Krause-Jensen and Duarte, 2016;
Paine et al., 2021). Although the process of lateral carbon export and
potential sequestration beyond the habitat boundaries is inherent to all
coastal vegetated ecosystems (Duarte and Krause-Jensen, 2017), it consti-
tutes the main carbon storage pathway in seaweed ecosystems (Krause-
Jensen and Duarte, 2016).
Seaweed cultivation may also, during the growth period, contribute to
organic carbon sequestration in sediments beneath seaweed farms and
beyond, hence providing opportunities for climate change mitigation
(Duarte et al., 2017;Sondak e t al., 2017;Yong et al., 2022). Global seaweed
aquaculture has been expanding at 6.4 % annually from 2000 to 2019
(FAO, 2021), propelled by the development of new applications and
markets for their products (Duarte et al., 2021). Whilst there are clear
socio-economic benefits for coastal livelihoods, there are potential negative
consequences of upscaling seaweed aquaculture for coastal ecosystems, like
transfer of disease to wild seaweed populations (Campbell et al., 2019),
altered structure and genetics of wild populations (Graf et al., 2021), or
changes in biodiversity and thus it is important to examine environmental
and societal trade-offs as seaweed farming expands globally as a strategy
to mitigate climate change (Duarte et al., 2021).
While there is increasing interest in promoting blue carbon sequestra-
tion by seaweed through the four pathways described, currently there are
no standardised methodologies to verify and document seaweed carbon
sequestration like there is for other vegetated coastal ecosystems (Emmer
et al., 2015). It remains unclear which actions involving seaweed can be
viable for climate change mitigation. Challenges to defining whether sea-
weed projects are scalable for climate change mitigation include evidence
of permanence, additionality, verification, consequences and potential up-
take of the solution (Krause-Jensen et al., 2018;Macreadie et al., 2021;
Hurd et al., 2022). Moreover, there is a need to document that such solu-
tions are also sustainable and under which sustainability thresholds.
Here, we provide context, via eight key research and policy challenges, to
the viability of seaweed projects for climate change mitigation (Fig. 1).
The consensus of those challenges is summarised in Box 2 at the end of
this paper. The authorship represents a wide range of opinions on the
topic, hence not all statements in the paper were unanimously agreed
upon. However, collectively we convey some of the wide ranging issues
over which there is a diversity of viewpoints across the research commu-
nity. We explore the state of knowledge and key gaps related to these
eight key challenges and discuss the possibilities of seaweed projects
being viable for climate change mitigation. This review aims to provide
context for assessing the viability of four pathways to use seaweed for cli-
mate change mitigation, identifying when, where and what can be done
to unlock the potential of seaweed for climate change mitigation.
2. Challenge 1 - resolving knowledge gapsin the seaweed carbon cycle
First-order estimates of seaweed primary production on a global scale
have suggested that about 173 Tg OC yr
−1
(with a range of 68–268,173
TgC yr
−1
)(Krause-Jensen and Duarte, 2016) could be sequestered in the
oceans. Of this, about 97 % has the potential to be sequestered beyond sea-
weed habitats by outwelling (lateral) followed by downward export. How-
ever, there are few experiments and modelling studies that verify these
estimates. Quantifying carbon fluxes across all components of the seaweed
carbon cycle is difficult, owing to the large spatial and temporal scales
Fig. 1. Diagram detailing how the identified research challenges relate to either seaweed aquaculture and/or wild/naturally occurring seaweeds. The research challenges
have been abbreviated.
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
3
involved (Hurd et al., 2022). Understanding these carbon fluxes is crucial
for better constraining the carbon sequestration capacity of seaweeds.
The location (e.g. within different coastal geomorphologies, water
depths and ocean circulation patterns), and the life cycle of seaweeds
(e.g., growth, mortality, and reproduction) determines the magnitude of
net primary productivity and the potential for subsequent carbon flows
(Fig. 2a; Pessarrodona et al., 2018). Organic seaweed carbon (OC) that is
not grazed and/or decomposed or deposited in the habitats in which it is
producedcan be laterally exported as seaweed thalli or detritus (i.e. partic-
ulate organic carbon [POC]), and as dissolved organic carbon (DOC) from
coastal habitats (Gilson et al., 2021), to sites in the deep subtidal or deep
ocean (Filbee-Dexter et al., 2018;Ortega et al., 2019;Queirós et al.,
2019) and within other adjacent blue carbon ecosystems (Wernberg
et al., 2006;Ortega et al., 2020b)(Fig. 2c-g).
Although the offshore export of seaweed depends on oceanographic pro-
cesses (e.g., currents, waves, and tides) and ocean bathymetry (e.g., the
distanceto the deep sea), seaweed detritus can be transported thousands
of kilometres away from their sites of origin at varying depths (Nikula
et al., 2010;Kokubu et al., 2019;Ortega et al., 2019). The long-
distance transport of seaweed biomass and detritus depends on the
decomposition rate and physical characteristics of the seaweed mate-
rial, particularly the buoyancy and density (Trevathan-Tackett et al.,
2015;Filbee-Dexter and Wernberg, 2020;Smale et al., 2021). For ex-
ample, positively buoyant seaweeds, such as Sargassum,Macrocystis
and other Laminariales, can be transported as rafts offshore (Fig. 2c;
(Kokubu et al., 2019)). When floating seaweeds lose buoyancy and
sink, part of their biomass (OC) may be transported to carbon sinks in
shelf sediments or to the deep sea, depending on their size, currents,
sinking speed, and distance to the deep sea (Fig. 2h). Likewise non-
buoyantseaweedcanbeexportedinthebedload(Krause-Jensen and
Duarte, 2016). As OC is laterally exported, biotic and abiotic processes
continuously fragment the seaweed biomass, resulting in reduction in
the particle size and leading to continuous dispersal of POC, which in
turn influences the location of potential POC burial and the efficiency
of carbon sequestration (Fig. 2a, b, (Filbee-Dexter et al., 2020)). The
decomposition of buried seaweed OC in shelf, coastal and beach sedi-
ments also influences the rates of carbon sequestration (Fig. 2k). Anaer-
obic conditions reduce the decomposition rate of seaweed biomass,
which will potentially enhance the preservation of OC (Pedersen
et al., 2021), along with chemical compositions that render seaweed
organic material, such as their cell walls, recalcitrant to microbial deg-
radation (Costa et al., 2021).
The production and release rates of seaweed DOC has been measured
using in situ observations and lab experiments (Smale et al., 2021;Weigel
and Pfister, 2021). However, the subsequent transport and processes
Box 1
Seaweed carbon credits.
Blue carbon is becoming increasingly included in national carbon inventories (Herr
and Landis, 2016;Martin et al., 2016;Bertram et al., 2021). Carbon credits are
frequently referenced as a way to finance climate change mitigation. Voluntary
carbon market methodologies currently exist for other blue carbon ecosystems, for
example in the Verified Carbon Standard (Emmer et al., 2015), but not for seaweed.
The sale of seaweed carbon credits may make seaweed projects more profitable if
managers receive income for climate change mitigation (Duarte et al., 2021). Based
on our review, there appears to be five separate distinct methodologies for potential
seaweed climate change mitigation pathways, of which only two (A and C) are
immediately actionable; A) Carbon deposited in sediment below seaweed farms, B)
transport of POC and DOC to OC-sinks offshore from seaweed aquaculture, C)
emission abatement credits from seaweed products, D) restoration and protection of
wild seaweed habitats and their related carbon sinks, and E) sinking seaweed in the
deep sea.
Both D) seaweed restoration and E) sinking seaweed in the deep sea do not appear to
be viable for carbon credits in the short term due to concerns raised in Challenges 3, 6
and 7, as well as challenges of attribution of seaweed carbon sequestered beyond the
habitat boundaries to a particular action (Challenge 2). Likewise, due to concerns
around traceability and quantification of carbon in Challenge 2, (B) transport of POC
and DOC offshore from seaweed aquaculture is unlikely to be viable in the short term,
as issuing of carbon credits requires evidence that the carbon is in a stable form and
remains sequestered for a significant time period - usually over 100 years (Boyd et al.,
2019,Bach et al., 2021,Siegel et al., 2021). However, our review suggests there is
enough evidence to support a development towards a global standardised
methodology for the accumulation of carbon beneath sediments of seaweed farms (A)
and for seaweed product carbon abatement credits (C), providing that sustainability
standards for seaweed aquaculture are developed and adhered to. If seaweed farmers
can access this value via a global standardised methodology which enables payments
from sale of carbon credits as a co-benefit to their production, this may further
incentivize the expansion of seaweed aquaculture (Duarte et al., 2021). We note this
research is already underway via (https://www.oceans2050.com/seaweed and
https://verra.org/project/vcs-program/projects-and-jnr-programs/seascape-carbon-
initiative/). The development of methodology A should consider and subtract the
baseline carbon sequestration from natural ecosystems, e.g., phytoplankton in areas
where seaweed growth will compete for nutrients with other primary producers.
Similarly carbon credits for emission abatement via, for example, methane reduction
from seaweed as suggested by some industry groups (https://www.future-feed.com/-
faqs, www.weeklytimesnow.com.au%2Fagribusiness%2Fagjournal%2Fseaweed-may--
hold-key-to-red-meats-emissions-problem), will reduce costs and encourage uptake of
these projects in terrestrial farming systems. The scalability of projects, while rela-
tively unquantified, appears significant for methodologies B, C, D and E. For method-
ology A, at current seaweed aquaculture expansion rates of 6 % annually,
sequestration may only reach 5.5 TgCO
2
yr
−1
by 2050 (Duarte et al., 2021). Rapid
expansion of the seaweed aquaculture industry would be needed to meaningfully
contribute to climate change mitigation, for example 20 % annual growth could
sequester 239 Tg CO
2
yr
−1
by 2050 for methodology A (Duarte et al., 2021). The
current financial scope of methodology A is limited to $3,580,160 per year at $10 per
tonne of CO
2
(Sondak et al., 2017), with 358,000 t CO
2
sequestration per year (FAO,
2021, Challenge 4). This potential revenue from carbon credits could have a large
portion used up in administration, monitoring and research cost for verification.
Indeed, verification costs currently represent a significant hurdle for blue carbon
projects altogether (Macreadie et al., 2021), and a need for more cost-effective
methods is evident. We suggest that any development of a methodology to assess
carbon stored below seaweed farms uses the forensic carbon accounting approach
described in Hurd et al. (2022) and should be carefully considered against the poten-
tial scalability of this solution.
Table 1
Summary of key variables and pathways for the sequestration of carbon from wild
macroalgal beds. The structure follows an earlier review (Krause-Jensen and
Duarte, 2016) but values are updated based on more recent information.
Variable/pathway Mean Standard
deviation
Global brown algal area (million km
2
) 1.68
a
–
NPP (gC m
−2
yr
−1
) 536
b
706
Global NPP (TgC yr
−1
) 1320
a
–
Percentage of NPP buried within algal beds (%) 0.4
c
0.54
c
Percentage of NPP exported laterally from algal beds (%) 43.5
c
48
c
DOC exported laterally from algal beds (gC m
−2
yr
−1
)16–4400
b
–
Percentage of DOC exported below the permanent pycnocline (%) 30
d
–
POC exported laterally/offshore from algal beds (TgC yr
−1
) 323
e
907
e
Percentage of POC exported to the deep sea (%) 11
f
1.7
f
POC export retained in the shelf environment (TgC yr
−1
) 288
g
808
g
POC buried in shelf sediments (gC m
−2
yr
−1
) 4.65
h
2.47
h
Percentage of beach-cast macroalgae biomass converted into
semi liable or refractory DOC
18.3
i
–
a
(Duarte et al., 2022) with modelled individual estimates ranging 1.43–1.79
million km
2
.
b
(Pessarrodona et al., 2022).
c
From the mean and standard error reported in Duarte et al. (1996),forn=30,
where n is the number of observations.
d
This mean estimate is supported by the finding that the net oceanic primary
production (around 50PgC yr
−1
), of which about 13 % (6.5 PgC yr
−1
,Baines et al.,
1991) is released as DOC, supplies a downward DOC export of 2 PgC yr
−1
(approx-
imately 30 %) below the mixed layer(fig. 6.1 in Ciais et al. (2013)).We assume that
the same fraction of macroalgal DOC is exported below the mixed layer and poten-
tially reaches the deep sea.
e
Calculated as the total export−DOC export through the uncertainty analysis by
Krause-Jensen and Duarte (2016).
f
The mean and standard error of three independent studies reported by Krause-
Jensen and Duarte (2016).
g
Calculated as total POC export −POC exported to the deep sea through the
uncertainty analysis by Krause-Jensen and Duarte (2016).
h
Calculated from two experiments reported in Hardison et al. (2013).
i
From a case study in Perkins et al. (2022).
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
4
determining sequestration of DOC are poorly understood and indeed, diffi-
cultto determine.One of the mechanisms proposed to contribute to car-
bon sequestration is the persistence of refractory DOC in the water
column (Fig. 2c). Microbial decomposition experiments show that
about half (range, 28–85 %) of the DOC persists as potential recalcitrant
DOC (RDOC) over annual timescales (Wada et al., 2008;Watanabe
et al., 2020) and that the recalcitrance of seaweed DOC depends on
the species and environmental conditions (Paine et al., 2021), therefore
experiments on DOC cannot easily be extrapolated reliably. Future
studies could assess how variation in microbial assemblages and chem-
ical and physical conditions affect the long-term recalcitrance of DOC
during transport to offshore sink sites. Another mechanism contribut-
ing to carbon sequestration is the downward transport (downwelling)
of DOC into the deep sea (Fig. 2b). The fraction of seaweed DOC that
reaches the deep sea will largely depend on its recalcitrance and on hy-
drodynamic, oceanographic, and bathymetric factors.
If we consider the carbon cycle in seawater, CO
2
from the atmosphere
(CO
2(g)
) dissolves in water to form CO
2(aq)
, which can also be formed
from the breakdown of organic matter. CO
2(aq)
is in equilibrium with car-
bonic acid, bicarbonate and carbonate as shown below.
CO2gðÞ
↔CO2aqðÞ
þH2O↔H2CO3↔HCO3þHþ˙↔CO3
2þ2Hþ
Increasing CO
2(aq)
causes a decline in seawater pH (increase in H
+
)
thereby reducing carbonate ion concentration. Conversely, autotrophic as-
similation of CO
2
causes an increase in seawater pH (decrease in H
+
)and
an increase in CO
3
2−
. The other important component to consider is change
in alkalinity and there are a number of processes contributing to changes in
alkalinity in marine systems, see Table 1 of Sippo et al. (2016).Increasing
alkalinity shifts the equilibria towards HCO
3
−
and CO
3
2−
,thereby“making
new space”for CO
2
in seawater, which can be absorbed from the atmo-
sphere. Ocean alkalinity and the DIC system are considered in detail by
Middelburg et al. (2020) and the dynamics of DIC in coastal systems by
Ouyang et al. (2022). Seaweed OC reaching anaerobic sediments may
promote mineralization in ways that increase total alkalinity (Santos
et al., 2021 and references therein). If this alkalinity is transferred to the
water column through diffusion or outwelling, it would, as mentioned
above, shift the DIC equilibrium towards HCO
3
−
and CO
3
2−
and would
thus facilitate drawdown of CO
2
in oceanic waters.
Box 2
Summary of key issues and consensus statements for each challenge.
Challenge Key issues Consensus statement
1. Resolving knowledge gaps in the seaweed carbon
cycle
•DOC production and advective fluxes and fate.
•POC fluxes and fate.
•Controls on decomposition rate
•Benthic oxidation state.
•Alkalinity interactions.
The empirical quantification of seaweed carbon pools and cycles
remains elusive and needs technological developments in monitoring,
tracing and modelling seaweed organic carbon fluxes.
2. Developing technologies, computational models
and tools to measure seaweed carbon fluxes
•Tools to distinguish seaweed signatures
(e.g. eDNA).
•Remote sensing methods.
•Seaweed transport models.
Advances in tracing and measuring seaweed carbon fluxes are key for
inclusion of seaweed in the blue carbon framework.
3. Understanding the potential contribution of wild
seaweed to climate change mitigation
•Anthropogenic impact on seaweed forests
•Restoration techniques
•Refined estimates available for global seaweed primary
production.
•Contribution of kelp detritus to carbon storage.
•Effectsofprotectionandrestorationonclimatechange
mitigation and co-benefits.
Restoring and protecting wild seaweed forests is critical because of the
numerous co-benefits they provide, however, it is unlikely that the
climate change mitigation benefits of seaweed restoration will be
quantifiable/have a reliable accounting framework.
4. Contribution of seaweed aquaculture to
undeliberate carbon sequestration during the
farming process
•Potential contribution of shedded cultivated seaweed
to carbon sequestration under seaweed farms.
•Export of seaweed carbon from the aquaculture site to
sink sites beyond the farm.
Seaweed aquaculture accumulates carbon in the sediment below
seaweed farms, this has quantifiable climate change mitigation potential
at small scales.
Export of seaweed carbon during the growth phase has the same
quantification challenges as for wild seaweed, i.e. unlikely quantifiable
climate change mitigation benefit (see point 3).
5. Understanding future opportunities from seaweed
aquaculture products for emission abatement
•Current development of seaweed products.
•Asparagopsisand other species for reduction of
methane emission from ruminants.
•Future opportunities for avoided emissions from
seaweed products.
Many new seaweed aquaculture products are being developed and the
industry is growing rapidly. Seaweed aquaculture could have an
increasing climate change mitigation impact as seaweed aquaculture
products can potentially result in emission abatement by having a lower
carbon footprint than products they replace, as well as contributing to
reduce methane emission from ruminants
6. Sinking seaweed into the deep sea to sequester
CO
2
•Ethics of sinking seaweed.
•Risks of sinking seaweed.
•Effects on carbon storage.
Purposeful seaweed sinking remains controversial for climate change
mitigation although the scale of opportunity could be significant.
Major challenges around ethics, ecological risks, as well as carbon
accounting for offshore seaweed cultivation would need to be resolved
before a feasibility assessment would be relevant.
7. Environmental benefits and risks of accelerating
seaweed aquaculture to enhance carbon
sequestration
•Quantification of benefits and risks
•Seven possible impacts of seaweed farming were
identified.
Seaweed cultivation offers many co-benefits to society, but is relatively
limited in extent outside Asia.
While relatively few negative impacts have been documented to date
from seaweed aquaculture, caution should be given to the various
potential impacts of seaweed cultivation as it expands globally.
8. Understanding the key policy and governance
considerations surrounding the potential role of
seaweed for climate change mitigation
•Relevant international treaties.
•Global collaboration on seaweed.
•Assigning responsibility for seaweed management.
International guidelines and standards for seaweed protection/restoration
and sustainable seaweed aquaculture are needed to unlock the potentialof
seaweed for climate change mitigation and the numerous co-benefits from
seaweed activities while avoiding negative impact.
Box 1 - Seaweed Carbon credits •Five methodologies for potential seaweed-based
climate change mitigation are outlined.
•Only two of these are viable for development of
carbon credits.
•A seaweed carbon accounting methodology is lacking.
Activities linked to seaweed forest restoration, the export of carbon
offshore from seaweed farms during the farming process, or deliberately
sinking seaweed, do not appear to be ready to be implemented for
climate change mitigation while carbon sequestration in sediments
below farms and emission reduction via seaweed products may offer
potential carbon credits in the short term.
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
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The processes leading to increased total alkalinity under anaerobic con-
ditions include denitrification, reduction of metals such as manganese and
iron and, especially, sulphatereduction (Perkins et al., 2022, see also Fig. 2
in Santos et al., 2021). Alkalinity released through the anaerobic minerali-
zation of seaweed OC thus locks CO
2
into inorganic carbon ions in the
ocean, which would be one of the carbon pools to sequester carbon over
long time scales ((D) in Fig. 2)). Similarly the release of alkalinity due to
providing substrate for calcifiers could in theory drawdown CO
2
(Challenge
3). Another process that has been suggested to lead to increased alkalinity,
at least for mangrove ecosystems (Saderne et al., 2021), is the dissolution of
calcium carbonate (Ridgwell and Zeebe, 2005;Saderne et al., 2019)under
the lower pH values caused by enhanced pCO
2
from the breakdown of or-
ganic matter in sediments. Under current atmospheric CO
2
levels, forma-
tion of CaCO
3
results in ~0.6 mol of CO
2
being released per mole of
CaCO
3
produced (Frankignoulle et al., 1994;Macreadie et al., 2017), disso-
lution of CaCO
3
would result in increases in total alkalinity and similar
amounts of CO
2
being consumed. However, to what degree these processes
play a role in alkalinity release from sediments and CO
2
drawdown is cur-
rently unclear.
Few studies have shed light on the magnitude of these processes across
these carbon pools and fluxes, including OC in the deep sea, sedimentary
OC, RDOC and alkalinity release from seaweed processes. Importantly, de-
position of seaweed carbon does not necessarily directly equate to CO
2
re-
moval (CDR), as re-equilibration and capture of atmospheric CO
2
is an
essential step for CDR, which few studies have quantified (Hurd et al.,
2022), although subsaturated pCO
2
in waters over seaweed forests and
other seaweed communities imply capture of atmospheric CO
2
(Smith,
1981;Gazeau et al., 2005). We also note that seaweed research is strongly
biased towards some species of certain geographical locations that are more
accessible and therefore, little is known onseaweed carbon cycling in loca-
tions such as Antarctica, South America, Africa or many parts of Asia The
empirical quantification of seaweed carbon pools and cycles remains elu-
sive and needs technological developments in monitoring, tracing and
modelling seaweed organic carbon fluxes (Hurd et al., 2022). These limita-
tions are discussed in the next challenge.
3. Challenge 2 - developing technologies, computational models and
tools to measure seaweed carbon fluxes
Advances in tracing and measuring seaweed carbon fluxes are key for
inclusion of seaweed in the blue carbon framework (Krause-Jensen et al.,
2018). While there are available tools to quantify these flows, tracing sea-
weed carbon fluxes is complex and still unprecise. Detecting the presence
and quantifying stored seaweed carbon is essential to validate whether sea-
weed carbon reaches sink sites and how much of it arrives to depositional
habitats such as the deep ocean (Smith et al., 2015;Geraldi et al., 2019;
Ramirez-Llodra et al., 2021). Seaweed carbon has often been traced in sed-
iment cores using pigment signatures (e.g., pheophytin and fucoxanthin)
(Leiva-Dueñas et al., 2020;Ramirez-Llodra et al., 2021) and stable isotopes
(Duggins et al., 1989;Geraldi et al., 2019). However, these tools fail at
Fig. 2. Potential pathwaysfor sequestration of seaweed carbon. Net primaryproduction lowers CO2concentration in the surroundingwater (a), facilitating atmospheric CO2
uptake through fixation into macrophyte biomass(b). CO2* indicates the chemical equilibrium between dissolvedCO
2
, carbonicacid, carbonate ions, and bicarbonateions in
the ocean,which is controlledby total alkalinity (TAlk). Floating thalli (or POC whichis usually defined as particles >0.2 μm) of buoyantseaweed can be transported offshore
(c), where they sink to the deep sea when buoyancy is lost (h). Non-buoyant seaweeds sink near the sites of origin and are transported via bottom currents and accumulate
along bottom slopes (d). A portion of the biomass and detritus of floating and non-buoyant seaweeds is washed onto beaches (e). Particulate organic carbon (POC) and
dissolved organic carbon (DOC) are released and transported offshore (f, g). Seaweed biomass is continuously fragmented during these processes, promoting dispersal of
POC and DOC (i). For seaweed farms, most of the biomass is removed from nearshore waters when seaweed is harvested (j), but about half of their net primary
production is released to the environment as DOC and POC before harvest (Duarte et al., 2021). CO2 and CH4 gases and TAlk are released to the water column
depending on the seaweed decomposition pathways (k) (see text for details of DIC speciation and the role of alkalinity). Degassing of CO2 and CH4 gases (l) produced by
decomposition (see text for details of DIC speciation). The potential carbon pools which contribute to long-term carbon sequestration are: burial of OC in coastal, shelf
and deep sea sediments (A); OC transportedto the deep sea (B); and refractory DOC (RDOC) and DOC transported below 1000 m depth, and excess bicarbonate produced
from TAlk release (red dashed arrow) –which both remain in the water column (C, D). Key quantitative and semi quantitative values associated with Fig. 2.are
summarised in Table 1.
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
6
distinguishing seaweed signatures from certain types of phytoplankton
(e.g., diatoms). Recent studies document that environmental DNA (eDNA)
detect and provide estimations of seaweed abundance in marine sediments
and water samples (Ortega et al., 2019;Queirós et al., 2019;Frigstad et al.,
2021;Ørberg et al., 2021), evidencing seaweeds transport beyond
coastal habitats at even 5000 km offshore, and in all ocean basins (Ortega
et al., 2019). Long sediment cores can be sliced into layers and used to
detect the presence of DNA from dominant seaweed species, enabling a
semi-quantitative measure of seaweed-DNA concentrations over time
(Frigstad et al., 2021). Suitable primers for these eDNA techniques have
been developed for numerous seaweed genera which may give insight
into the presence, burial rate and abundance of seaweed carbon in marine
sediments (Ortega et al., 2020a, 2020b). Although eDNA can be used to es-
timate, for example, the proportion of various seaweed sequences in the
eDNA pool of sediments, those estimates remain semi-quantitative and an
accurate quantification of carbon deposition is currently challenging
(Ortega et al., 2020a;Ørberg et al., 2021).
Remote sensing using satellite imagery is an important emerging tool
for monitoring seaweed populations (Cavanaugh et al., 2021). For example,
it has been used to monitor giant kelp canopy biomass (Bell et al., 2020;
Marquez et al., 2022) and to track positively buoyant species such as float-
ing Sargassum, based on distinct wavelengths absorbed by brown seaweeds
(Ody et al., 2019). Improvements in the resolution of satellite images and
their post-processing technology means that satellites are increasingly
able to detect individual seaweeds of medium size floating on the surface
(Ody et al., 2019); this could greatly improve the ability to remotely track
surface floating seaweeds. Despite these advances, it is critical to under-
stand the source location of buoyant seaweeds, and where and when they
finally sink to the deeper ocean. Lossof buoyancy can be from remineraliza-
tion, vesicle rupture by waves, and downward mixing and fragmentation
(Johnson and Richardson, 1977;Vandendriessche et al., 2007). Experi-
ments and simulation models exploring such processes are required to im-
prove our understanding of the vertical flux of buoyant seaweeds.
Seaweeds are exported vertically in the water columnand along the sea
floor via passive processes. Early insights into these fluxes came from bot-
tom trawls that captured seaweeds in the deep sea, in submarine canyons
and along the continental shelf (Vetter and Dayton, 1999;Garden and
Smith, 2015;Krause-Jensen and Duarte, 2016). These methods have
allowed oceanographers to understand the role of large seaweeds in depos-
iting rocks entrained in their holdfasts to the sea floor (Emery and Tschudy,
1941;Garden and Smith, 2015). Although coarsely resolved, trawl records
can provide information on maximum transport distances, and even sea-
sonal changes in the flow of large seaweeds (Vetter and Dayton, 1998;
Filbee-Dexter et al., 2018). More recent technologies, such as ROVs and
other underwater camera systems can be used to generate data on relative
abundance, particle sizes, seasonal timing of detritus production, storm-
induced fluxes (Filbee-Dexter and Scheibling, 2012;Pries, 2020)and
areas of accumulation (Britton-Simmons et al., 2009). For example, re-
peated underwater surveys of detritus adjacent to coastal habitats can be
used to estimate when and how much seaweed is moving beyond shallow
habitats into deeper sinks (Filbee-Dexter et al., 2018;Smale et al., 2021).
Seaweed DOC export can be traced by sampling water at different depths
and using eDNA and other fingerprinting techniques (Geraldi et al.,
2019). However, a major limitation of observational studies is that they
do not directly link sources to sinks, but instead provide a snapshot evidenc-
ing transport of seaweed POC out of the coastal zone and its storage in the
deep oceans. Such source-to-sink links and associated quantification may
remain an unsolved challenge except in specific cases. For example, such
links can be potentially established below seaweed farms, quantifying sea-
weed sequestration rates by comparing locations without farming in nearby
sediments, or the sediment prior establishment of the farm.
There are a range of methods used to estimate the amount of seaweed
biomass transported from the coastal zone to the deep ocean. Morphologi-
cal characteristics (Vanderklift and Wernberg, 2008), physical tags
(Kirkman and Kendrick, 1997;Filbee-Dexter et al., 2018) and genetics
(Queirós et al., 2019) have been used to attribute seaweed detritus to its
source location. However, use of ocean circulation (i.e., currents) models
will likely provide the most comprehensive understanding of total seaweed
flux beyond their habitat (Hurd et al., 2022). Where ocean currents are
highly resolved, seaweed export can be quantified using particle tracking
models (Filbee-Dexter and Wernberg, 2020). In the past, ocean current
models were not suitable for modelling seaweed fluxes out of the coastal
zone because they did not have sufficient spatial resolution. However,
advanced regional models now have improved spatial and temporal resolu-
tion to better capture coastal zone processes (e.g., Regional Ocean Model,
(Haidvogel et al., 2008)). To parameterize models, estimates of settling
velocities of seaweed detritus (Filbee-Dexter and Wernberg, 2020)and
decomposition rates (Pedersen et al., 2021), as well as thresholds for
resuspension and movement across the seafloor are required (Oldham
et al., 2010).
Ultimately, linking management actions at the seaweed habitat remains
a key challenge for seaweed carbon accounting. The vast expanse of the
ocean and distance between source and sink sites will make accounting
for seaweed carbon transport difficult, even with improved monitoring
techniques. Accuracy for higher resolution on tracking and quantifying sea-
weed carbon is needed to enable and support research to meet carbon offset
assurance requirements (discussed in Challenge 8).
4. Challenge 3 - understanding the potential contribution of wild
seaweed to climate change mitigation
Seaweed forests (e.g., kelp, Sargassum) are globally significant ecosys-
tems with high biodiversity (Teagle et al., 2017). In some regions, wild sea-
weed is at risk from anthropogenic stressors (Wernberg et al., 2019),
including pollution (Munda, 1993) and overharvesting of wild populations
(Buschmann et al., 2017). Perhaps the most important stressors are ocean
warming and the increasing frequency and intensity of marine heat
waves, which have led to mortality of seaweed forests in some areas
(Filbee-Dexter and Wernberg, 2018). Simultaneously at the poles, recent
evidence points at a realised and projected poleward expansion of seaweed
with climate change (Deregibus et al., 2016;Krause-Jensen et al., 2019;
Goldsmit et al., 2021;Assis et al., 2022). Over the last 50 years, despite
high regional variability in seaweed forest expansion or contraction,
around 60 % of all long-term records of seaweed forests show a decline, at-
tributed tomultiple pressuresincluding harvesting, pollution, invasive spe-
cies, overgrazing and/or warming (Krumhansl et al., 2016;Wernberg et al.,
2019). There is a need to better identify trends in wild seaweed ecosystems
to target relevant sites for restoration and protection, which could include
the revegetation of lost areas or the facilitation of poleward expansion.
Local scale management can reduce the impacts of climate change on
seaweeds (Smale et al., 2019;Arafeh-Dalmau et al., 2020). For example,
coastal managers can support local seaweed forests by protecting these for-
ests from other threats like overharvesting, pollution and bottom trawling
(Norderhaug et al., 2020). Reducing the impact of these stressors can pro-
mote conservation of seaweed forests by increasing their resilience against
climate change impacts (Smale et al., 2013;Araújo et al., 2016;Rogers-
Bennett and Catton, 2019). Restoration of seaweed forests (Fig. 3)can
also be potentially facilitated through other means such as artificial reef
restoration (that provides substratum for new algal forests), by selective
harvesting of grazers like sea urchins, and by genetic modification to
increase resilience for stressors like heatwaves (Steneck et al., 2002;
Laffoley and Grimsditch, 2009;Hamilton and Caselle, 2015;Wood et al.,
2019;Layton et al., 2020;Eger et al., 2022). A new promising management
strategy for restoring kelp involves translocating small kelp planted on
gravel to be dropped from the surface, called ‘Green gravel’(Fig. 3)
(Fredriksen et al., 2020). While in some areas survival of green gravel
could be limited by being covered insand or sediment thisrestoration activ-
ity shows promise in several restoration projects (Fredriksen et al., 2020).
Similar towhat is currently being done for corals, seaweed conservation ef-
forts could also focus on species that are more resilient to future tempera-
ture stress to avoid local or regional extinctions (Smale et al., 2019;
Thomsen et al., 2019;Vranken et al., 2021). Marine protected areas may
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
7
also reduce compounding stressors on seaweed, for example by restoring
the populations of predators that control sea urchin abundance (Shears
and Babcock, 2002), and may subsequently protect blue carbon sequestra-
tion by seaweeds (Ling et al., 2009). The largest seaweed restoration project
in the world is led by the Korean government who has invested over US
$280 million from 2009 to 2019 to restore a total of 21,500 ha of seaweed
forests in Korea(Hwang et al., 2020). However, a recent review of 259 kelp
restoration projects shows that most have been relatively small in scale and
costly, and broad uptake of large scale restoration projects are yet to be
achieved (Eger et al., 2020,Eger et al., 2022).
Given the high estimates of seaweed forest primary productivity
(Duarte et al., 2022) and carbon sequestration potential (Krause-Jensen
and Duarte, 2016), their protection and restoration has been suggested to
have climate change mitigation co-benefits (Krause-Jensen et al., 2018).
Krause-Jensen and Duarte (2016), estimated that about 173 Tg OC
year
−1
(with a range of 61–268 Tg C year
−1
), corresponding to 11 % of
the estimated global net seaweed production, is sequestered by wild sea-
weed globally each year (Challenge 1). Wild seaweed forests are estimated
to be the most extensive vegetated coastal habitat globally with an extent of
7.2 × 10
6
km
2
(Duarte et al., 2022), and their associated net primary pro-
duction, which has recently been updated as 1.32 Tg C yr
−1
(Duarte et al.,
2022) from the previous estimate of 1.5 Tg C yr
−1
Krause-Jensen and
Duarte (2016), is also larger than that of other vegetated coastal habitats
by about 60 % (Krause-Jensen and Duarte, 2016;Duarte, 2017;Raven,
2018). Around 90 % of wild seaweed carbon sequestration is estimated to
be through export of carbon to the deep-sea and the remainder is buried
in coastal and deep sea sediments (Krause-Jensen and Duarte, 2016).
Another important consideration for carbon budgets of wild seaweed
forests, while poorly constrained as discussed in Challenge 1, is calcifica-
tion. This is because some seaweeds species are calcifiers and seaweed for-
ests can also provide habitat for calcifying organisms (Taylor, 1998;
Newcombe and Taylor, 2010;Thomsen et al., 2016). Calcification reduces
seawater alkalinity and subsequently lowers the CO
2
uptake capacity of
seawater (Frankignoulle et al., 1994;Macreadie et al., 2017). Seaweed
provides habitat for calcifiers and subsequently increases their biomass;
this may partially offset the seaweed CO
2
sequestration and any deposit
of organic carbon from calcifying species (Bach et al., 2021).
To quantify the impacts of any future change in carbon sequestration re-
lated to seaweed protection or restoration, estimates of historical and cur-
rent carbon sequestration of wild seaweed forests and the associated
change in these rates need to be improved through empirical evidence. Im-
proved baseline estimates should therefore consider changes due to sea ur-
chin overgrazing, ENSO cycles, extreme events, and rising CO
2
among
other environmental variables. Key next steps include quantification of
losses and quantification of risks of loss of seaweed habitats alongside asso-
ciated rates of carbon sequestration. This will inform the potential climate
change mitigation benefit of both preventing further loss and restoring al-
ready lost habitats. Research to support the management and restoration
of seaweed forests should help preserve and increase their role in blue car-
bon storage, although precise quantification will depend upon details de-
scribed in Challenge 2.
5. Challenge 4 - contribution of seaweed aquaculture to undeliberate
carbon sequestration during the farming process
Seaweed aquaculture is the fastest-growing component of global food
production (Duarte et al., 2017). Similar to ongoing carbon sequestration
by wild seaweed forests, carbon sequestration with seaweed aquaculture
could potentially be an additional CO
2
sink due to the incidental shedding
of biomass andDOC during growth, some of which would be sequesteredin
sediments below the farm. Moreover, farmed seaweed products may have
climate change mitigation benefits (discussed in Challenge 5). In 2019,
Fig. 3. Seaweed forest restoration. A - Laminaria ochroleuca on green gravelin Peniche, Portugal, B - Phyllospora comosa transplanted on a crayweed matt as part of operation
crayweed, C- Saccharina latissima attached to a wooden frame on artificial reefs in the Gulf of St Lawrence, Canada, D - Alaria esculenta and Saccharina latissima on artificial
reefs in Gulf of St Lawrence, Canada.
Photo credits: Nahlah A. Alsuwaiyan (A), John Turnball (B), Karen Filbee-Dexter (C, D).
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
8
seaweed aquaculture production had grown to 34.74 million tonnes fresh
weight (FAO, 2021). Therefore there would bea maximum theoretical car-
bon drawdown of 3.16 million tonnes CO
2
per year if all of the 34.74 mil-
lion tonne fresh weight produced contributed to carbon sequestration, or
a minimum carbon drawdown if 0 t if all seaweed biomass was used in
human consumption and respired. This is based on a dry weight content
of 10 % of fresh weight, and an average carbon content of 24.8 % of sea-
weed dry weight (Duarte et al., 2017). If all of this seaweed was consumed,
for example as food products and carrageenan, where the majority of car-
bon is released back into the atmosphere,there could be permanent carbon
sequestration from shedding of biomass and release of DOC during growth
prior to harvesting and subsequent sequestration of approximately 11 %
(Duarte et al., 2017). Assuming that the estimateof 11 % of carbon becom-
ing sequestered from wild seaweed forests is also appropriate for sea-
weed aquaculture (Duarte et al., 2021), 358,000 t CO
2
peryearmay
have been sequestered unintentionally in oceanic sink sites in 2019
during the farming process. Carbon sequestration can also only be esti-
mated during the seaweed farming season which varies by location.
This estimate does not consider potential carbon sequestration via sea-
weed products (see Challenge 5).
This estimate is a relatively small global contribution to CO
2
sequestra-
tion being only 0.04 % of the average 841 Tg (621–1064 Tg) potential CO
2
predicted to be sequestered annually by 2030 from all other blue carbon
habitats (Macreadie et al., 2021). It is also unlikely that the carbon seques-
tration rate of 11 % is accurate, owing to the many variables discussed in
Challenges 1 and 2, meaning this number could be either lower or higher
than for wild seaweed, depending on the setting. Additionally, the process
of seaweed aquaculture may reduce othernatural carbon sinks, forexample
by shading seagrass (Moreira-Saporiti et al., 2021), or nutrient robbing
from endemic phytoplankton ecosystems (Challenge 7; Bach et al., 2021),
and incur operational CO
2
emissions, from for example the use of boats.
The use of life cycle analyses (LCAs) of such projects is needed to identify
their net climate mitigation effect (Hasselström and Thomas, 2022).
While impacts on other species can be mitigated through management
and culture methods, project level estimates of abatementwould need to in-
clude emissions and loss of CO
2
sequestration from loss of these natural car-
bon sinks, where they may occur (Hurd et al., 2022).
The majority of seaweed aquaculture currently involves basic infra-
structure and it is located in sheltered coastal areas. If cost-effective andsus-
tainable offshore (beyond the shelf break) aquaculture can be developed,
the expansion potential for seaweed aquaculture may increase significantly
(Buck et al., 2017). Further identification of ecological effects and risks
(Challenge 7) of seaweed aquaculture, and assessment of new technologies,
both inshore and offshore, are critical next steps in determining the poten-
tial scale for seaweed aquaculture for climate change mitigation both lo-
cally and globally. Seaweed aquaculture is presently concentrated in Asia,
where it accounted for 97 % of global production in 2019. Of global sea-
weed production in 2019, 56.8 % occurs in China, 27.8 % in Indonesia
with other countries producing <5 % of global seaweed (FAO, 2021).
Therefore, detailed information on carbon fluxes from seaweed production
in Asia is crucial to assess their potential contribution to climate change. If
new species, aquaculture methods and geographies for seaweed aquacul-
ture become economically and ecologically viable within sustainability
limits (Duarte et al., 2021), more of the world's coastlines could become
suitable for seaweed aquaculture.
In Challenge 5 we move beyond the contributions of seaweed aquacul-
ture to blue carbon during the growth phase and instead look at specific
ways of using products from seaweed aquaculture for climate change miti-
gation by emission abatement.
6. Challenge 5 - understanding future opportunities from seaweed
aquaculture products for emission abatement
Seaweed aquaculture products can result in emissions abatement by
having a lower carbon footprint than the products they replace (Spillias
et al., 2023). Some seaweed products are substitutes for products from
various other industries. For example, seaweed-based bioplastics could be
used as a substitute to petroleum-based plastics (Rajendran et al., 2012).
However, the scale of the potential substitution is unknown. Importantly,
the lifecycle of most seaweed aquaculture products currently includes de-
composition into CO
2
and methane emissions which are released back to
the atmosphere. At present, 90 % of cultivated seaweeds are consumed as
food or as additives (Duarte et al., 2021). The degree to which seaweed
aquaculture products are substitutes for products from land-based sources
differs between product types. In some instances, there may be an opportu-
nity for increased emission abatement from expanding seaweed aquacul-
ture industries so they increasingly compete with markets that produce
similar products at a higher environmental and carbon footprint, such as
using biofuel to partially replace fossil fuels (Milledge et al., 2014;Chen
et al., 2015;Yong et al., 2022). Generally, bio-oils and bioethanol are
more likely to be competitive than biodiesel (Chen et al., 2015). While
biofuels can be derived from seaweed, lowering costs to compete with
other fuels, particularly where petroleum based fuel receives subsidies, re-
mains a significant barrier (Soleymani and Rosentrater, 2017). Soleymani
and Rosentrater (2017) suggests an area of 129,500 ha of seaweed needs
to be cultivated with a yield of 680,000 dry tonnes annually to reach a
relatively competitive price of US$ 0.93/L of ethanol. For comparison,
the current extent of seaweed aquaculture is approximately 198,300 ha
(Duarte et al., 2021). Key impasses to unlocking emission abatement from
seaweed as a biofuel include the development of cost-efficient aquaculture
using kelp species, and lowering the cost and energy requirements, thereby
lowering the carbon footprint of aquaculture and bioprocessing.
Recently, the red seaweed Asparagopsis has been recognised asa poten-
tial methane reducer in livestock (Roque et al., 2021). Asparagopsis can re-
duce enteric methane emissions when added as a supplement to the diet of
ruminants, and can lead to increases in meat production (Roque et al.,
2021). However, bromoform, the anti-methanogenic compound within
Asparagopsis may have some toxicological effects and can be excreted in
milk (Muizelaar et al., 2021), and at scale could be a small contributor to
ozone depletion (discussed in Challenge 7). Additionally, Asparagopsis
must be fed daily, which may not be feasible to distribute to animals in
low density pastoral grazing systems until it is available in new forms like
salt licks. Nevertheless, enteric methane emissions from livestock comprise
14.5 % of global emissions (Gerber et al., 2013) and, therefore, there is
significant potential, should Asparagopsis supplements to ruminant feed
be confirmed to have no negative effects, to contribute to emissions abate-
ment. We also note that many seaweed species, including Asparagopsis,
contain toxic components such as bromoform (Min et al., 2021) and there-
fore, other species than Asparagopsis should be investigated in the future for
methane reduction in livestock.
These seaweed aquaculture markets are becoming increasingly attrac-
tive as demand for, and price of, land rises globally (Cotula, 2012). There-
fore identifying emission abatement opportunities from new sustainable
seaweed aquaculture sources remains a key research gap in establishing
the potential scope for seaweed aquaculture. Determining the carbon foot-
print and life cycle analysis of seaweed aquaculture-derived products will
allow for more accurate estimations of the carbon abatement potential for
seaweed aquaculture derived products compared to land-derived ones.
This will enable calculations of potential carbon sequestration and emis-
sions avoidance per unit of seaweed biomass from different types of sea-
weed aquaculture and the fate of seaweed biomass, helping to decide
which seaweed aquaculture markets may be most profitable for both indus-
trial and climate change mitigation purposes. Overall, the demand and sub-
sequent emission abatement opportunities for emerging seaweed products
will depend upon the price, fungibility, type and fate of these products.
7. Challenge 6 - sinking seaweed into the deep sea to sequester CO
2
‘Ocean afforestation’is the concept of facilitating the transport and
sinking of coastal seaweed species into the deep sea to sequester CO
2
(Ritschard, 1992;Antoine de Ramon et al., 2012;Boyd et al., 2022;Ross
et al., 2022). This concept was investigated in the 1980's, and was
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
9
considered unfeasible then (Ritschard, 1992). The idea is currently seeing
a revival due to escalating needs for atmospheric CO
2
removal at
the gigatonne-scale (IPCC, 2019), and has been proposed by several compa-
nies (https://www.runningtide.com,https://southernoceancarbon.com,
https://pulltorefresh.team). However, sinking seaweed is controversial
(Ricart et al., 2022) as both numerical modelling and empirical investiga-
tions found high levels of uncertainty regarding its ability to sequester
carbon (Orr and Sarmiento, 1992;Bach et al., 2021;Wu et al., 2022;
Berger et al., 2023;DeAngelo et al., 2023), along with concerns on impacts
on deep-sea biology and oxygen budgets. There are also ethical concerns on
purposefully sinking biomass that could contribute to alleviate hunger and
sustainably produce animal feed (Duarte et al., 2021;Ricart et al., 2022).
This would be the case if the implementation of this concept meant reduced
yield from, or growth of, the seaweed aquaculture industry due to resource
competition between industries.
Nutrient reallocation (discussed in Challenge 7), is a key consideration
for the sinking of seaweed offshore, whereby seaweed species compete
with phytoplankton for nutrients and reduce their baseline carbon seques-
tration (Orr and Sarmiento, 1992;Bach et al., 2021). Therefore, for the con-
cept of sinking seaweed for climate change mitigation, selecting species
with a favourable C:N ratio, not just high growth rates would be important
to consider. Additionally, adverse ecological impacts seaweed sinking
(discussed in Challenge 7) may be significant. A key constraint on feasibil-
ity of seaweed sinking for climate change mitigation will be the return (CO
2
sequestered) on investment. While seaweed sinking remains controversial
for climate change mitigation (Orr and Sarmiento, 1992,Bach et al.,
2021,Wu et al., 2022,Berger et al., 2023,DeAngelo et al., 2023), the
scale of opportunity could be significant given the expanse of the ocean.
“Froehlich et al. (2019) calculated a hypothetical upper limit to the area
possible for seaweed cultivation to be 48 million km
2
, however this was
only constrained by nutrients and temperature and did not consider sustain-
ability thresholds, potential negative effects (Campbell et al., 2019)or
other limiting factors such as irradiance, grazing, competition and currents.
Spillias et al. (2023) #911@@author-year} constrained their even larger
estimate of hypothetical global seaweed farming area to a total area of
6.5 million km
2
by considering water depth (<200 m), distance from
ports, sea ice extent, wave energy, native algal distribution and avoiding
marine protected areas and shipping traffic. Large challenges around car-
bon accounting for offshore seaweed cultivation would need to be resolved
before an accurate feasibility assessment can be conducted for this concept.
8. Challenge 7 - environmental benefits and risks of accelerating
seaweed aquaculture to enhance carbon sequestration
Seaweed aquaculture can provide positive environmental effects
(Zheng et al., 2019), including reduced local ocean acidification and deox-
ygenation (Xiao et al., 2021), alleviation of eutrophication, and the provi-
sion of marine habitat restoration (Fernand et al., 2017). Increasing the
scale of seaweed aquaculture also leads to job creation and social develop-
ment, contributing to multiple United Nations Sustainable Development
Goals (Valderrama, 2012;Boettcher et al., 2019;Larson et al., 2021). In
coastal communities of developing nations, livelihoods through sea-
weed aquaculture may provide alternatives to sources of income from,
for example, unsustainable commercial fishing (Valderrama, 2012).
Seaweed may also have a higher albedo (reflectance) than oceanic
water, and this therefore could lower the potential for warming (Bach
et al., 2021). However, in the long term this effect is potentially less im-
portant than the reduction of radiative forcing by removing CO
2
due to
the long life-time of CO
2
in the atmosphere and also because this albedo
effect could be strongly modified through cloud-forming substances
seaweeds may release (Bach et al., 202 1). Furthermore, the change in al-
bedo from seaweed is unknown and will likely vary significantly with
their proximity to the ocean surface (i.e., floating or fixed to slightly
subsurface rafts and, more importantly, with the extent of the seaweed
and amount of substances they release that can affect cloud albedo
(Brooks and Thornton, 2018).
While the benefits from increasing the scale of seaweed aquaculture
and wild seaweed forest restoration are for the most part widely known,
the risks remain relatively unquantified. Currently, there is little evidence
of negative consequences from seaweed cultivation (Boettcher et al.,
2019). Yet the enormous extent of seaweed aquaculture in Asian nations
may have some yet uncharacterised environmental consequences and
knowledge of the natural ecosystem state pre-seaweed-aquaculture may be
unknown, therefore impacts are difficult to quantify. However, seaweed
aquaculture is relatively new outside Asia (Nayar and Bott, 2014;Ferdouse
et al., 2018) and has not yet been successfully implemented offshore. If off-
shore and scalable seaweed aquaculture is to be developed, consideration
should be given to possible negative environmental and ecological impacts
(Campbell et al., 2020;Bach et al., 2021;Boyd et al., 2022).
If novel large scale seaweed aquaculture is to take place, there are
possible impacts on wild seaweed forests and other marine ecosystems in-
cluding seagrasses. For example, a high abundance of spores of particular
species or genotypes could in theory reach endemic seaweed forest commu-
nities and lead to changes in community composition, therefore cultured
seaweed species could be considered a threat as an invader by impacting
biodiversity (Wikström and Kautsky, 2004;Williams and Smith, 2007;
Russell et al., 2012). This is because there have been instances of seaweed
invasions having a negative impact on coastal communities (Johnson,
2008), for example the spread of Undaria pinnatifida in Europe (Epstein
and Smale, 2017) and spreading to coastal ecosystems from aquaculture
sites in New Zealand (James and Shears, 2016). Some wild seaweed forest
ecosystems are undergoing significant decline (Smale and Wernberg, 2013;
Thomsen et al., 2019;Layton et al., 2020). Where this is the case, increased
spore release from nearby seaweed aquaculture of similar kelp species
could, in theory, aid seaweed forest recovery in a similar way to the spore
bag method (Choi et al., 2000), whereby seaweed spores are released
close to areas targeted for restoration. This will depend on the proximity
of cultivation to coastal seaweed ecosystems and the life histories of culti-
vated and naturally occurring seaweed species. For this reason, it would
be preferential to cultivate native or endemic macroalga species to reduce
threats of invasion from non-native species (Boettcher et al., 2019;Shan
et al., 2019), although the presence of cryptic species and variation in cul-
tured and natural genotypes may add complexity (Zanolla et al., 2018;
Altamirano-Jeschke, 2021). If seaweed species were genetically modified
or engineered to be more resistant to marine heatwaves or for any other
reason, they may face major regulatory challenges in the aquaculture indus-
try, given a potential ecological risk of invasion in coastal ecosystems
(Robinson et al., 2013;Kim et al., 2017;Cheney et al., 2019). However de-
velopment of this technology may offer opportunitiesfor restoring seaweed
forests under global change threats.
We have identified seven key potential negative environmental and eco-
logical impacts of offshore seaweed farming on natural ecosystems.
1) Large-scale production of anti-methanogenicseaweeds could potentially
release volatile halocarbons that deplete the ozone layer. While this link re-
mains somewhat unclear, it is extremely unlikely to reach any meaningful
scale (Keng et al., 2020;Duarte et al., 2021). 2) Seaweeds release dissolved
organic carbon (Paine et al., 2021), so their cultivation could theoretically
lead to an increase of DOC in offshore and onshore ecosystems, which could
have varying ecological impacts (Boyd et al., 2022), although this has
not been quantified to date. Similarly if coastal seaweeds are cultivated
offshore they may have different chemical ecology and could bring ‘passen-
gers’, such as their microbiome, whichcould both have negative impacts on
offshore ecology (Boyd et al., 2022). 3) Microorganisms convert CO
2
to
methane in the deep sea, so there may be an unknown risk of methane re-
lease due to GHG additions in the deep sea (Whiticar, 1999;Kotelnikova,
2002;Sivan et al., 2007). However, this risk may be partially offset as
the likelihood of methanogenesis may be low where high sulphate concen-
trations occur in the deep ocean (Sivan et al., 2007). Complex deep sea bio-
geochemical pathways for seaweed decomposition need to be better
understood. 4) An increased abundance of dead organic matter on the sea-
floor will also consume more oxygen via decomposition, resulting in lower
oxygen concentrations or hypoxia (Wyrtki, 1962). Therefore dumping of
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
10
seaweed could also lead to localised hypoxia (Lapointe et al., 2018;Wu
et al., 2022). 5) Impacts of seaweed and associated debris on deep sea ben-
thic communities are difficult to monitor (Danovaro et al., 2020), and may
be significant (Wolff, 1962;Filbee-Dexter and Scheibling, 2014;Baker
et al., 2018). If deposition of seaweeds can be directed to relatively hypoxic
areas with low biodiversity, these impacts may be smaller (Levin, 2002;
Helly and Levin, 2004). 6) Interactions with marine life, seabirds and mam-
mals are possible risks, as entanglement has been associated with other
aquaculture systems (Würsig and Gailey, 2002;Kemper et al., 2003;
Young, 2015) but has been deemed low risk for offshore renewable energy
mooring systems (Harnois et al., 2015). Entanglement remains a risk along-
side polluting coastlines with debrislike growing lines from seaweed farms.
7) Seaweeds will likely compete for nutrients with phytoplankton which
could have diverse and relatively unstudied ecological impacts (Bach
et al., 2021). While seaweeds generally consume carbon more efficiently
than phytoplankton relative to their N and P consumption (Baird and
Middleton, 2004), the nutrients that would otherwise have been used by
phytoplankton and their associated carbon sequestration should be
subtracted from any seaweed carbon sequestration (Bach et al., 2021).
This is because if seaweeds compete with phytoplankton for nutrients in
ecosystems where there are no excess nutrients, they may reduce the
potential baseline carbon sequestration of phytoplankton. The capacity of
seaweed to absorb nutrients, increase water clarity and reduce phytoplank-
ton biomass have been observed by satellite remote sensing of ‘green tides’
(Ulva blooms) (Xing et al., 2015). However, this increase in water clarity
will also increase the euphotic depth and therefore potentially enhance
phytoplankton biomass in deeper layers. These seven impacts are depicted
in Fig. 4.
9. Challenge 8 - understanding the key policy and governance
considerations surrounding the potential role of seaweed for climate
change mitigation
The feasibility of engineering techniques, ecological carrying capacity,
consent and permits to operate seaweed aquaculture in the coastal zone
and open ocean are also all key considerationsfor the expansion of seaweed
aquaculture (Barbier et al., 2019;Kim et al., 2019;van den Burg et al.,
2020) and, specifically, the deliberate sinking of seaweed biomass in deep
sea waters. International ocean-related treaties will influence implementa-
tion of seaweed aquaculture in international waters, including components
of The Law of the Sea, TheInternational Seabed Authority, The London Pro-
tocol, International Maritime Organization and the UN Convention on Bio-
logical Diversity. For example, under the Law of the Sea, there are laws
guiding marine research and mineral exploration in international waters.
The London Convention is an international treaty designed to protect the
marine environment from pollution caused by dumping materials into the
ocean. The Article III b ii) of the London Convention states: “Dumping”
does not include: Placement of matter for a purpose other than the mere dis-
posal thereof, provided that such placement is not contrary to the aims of
this Convention”. In addition, if nutrient additions are considered for
fertilisation, the Ocean Fertilisation Assessment Framework should be
considered (Boettcher et al., 2019).
Global collaboration to restore and recover natural seaweed ecosystems
should include national and international policies to preserve and monitor
seaweed ecosystems that can be stimulated regardless of their potential car-
bon sequestration. Quantitative information on the provision of ecosystem
services and associated co-benefits for social development with seaweed
aquaculture or restoration can provide compelling justification for restora-
tion andprotection of both habitat and sink sites, as in other blue carbon
ecosystems (Vanderklift et al., 2019). Where seaweeds cross transna-
tional boundaries, collaboration among nations will be important, as
with the Great Atlantic Sargassum Belt (Bach et al., 2021). Also, if natu-
ral deposition of seaweed at carbon sink sites are at risk from trawling in
international waters, some of these carbon stocks could potentially be
released during trawling, or conversely, be protected by trawling bans
(Legge et al., 2020), and international agreements on seabed manage-
ment would be required.
As seaweed ecosystems are often spatially disconnected from carbon
sinks, it will make policy difficult to develop as sinking sites are likely to
be outside the Economic Exclusive Zone (EEZ) or jurisdiction of the zone
Fig. 4. Potential risks from increasing extent of seaweed aquaculture for carbon sequestration. 1. Possible release of volatile halocarbons; 2. Release of DOC interacting with
phytoplankton and bacteria communities; 3. Possible Methane release from seaweed deposition; 4. Deposition of seaweed causing hypoxia; 5. Ecological interaction of
seaweed with benthic communities; 6. Risk of entanglement for marine life; and 7. Nutrient competition with phytoplankton communities.
F.W.R. Ross et al. Science of the Total Environment 885 (2023) 163699
11
where the carbon project originated. Hence, key next steps are to determine
if seaweed carbon is stored within the countries' EEZs, and how such stor-
age impacts the contributions to national GHG inventories. This requires
a policy that enables accounting for blue carbon outside of blue carbon pro-
ject boundaries (remote sequestration), as well as mechanisms in place to
prevent double counting of sequestration. An international approach to
managing sink sites would need to fall outside the jurisdiction of a country
or within Exclusive Economic Zones, thereby including the open ocean
which is usually not managed or owned by any country. This will also
apply to the OC exported from other blue carbon ecosystems (e.g. man-
groves, tidal marshes and seagrasses), which is also not considered by
current global blue carbon methodologies (Emmer et al., 2015).
Some countries may have a greater interest in global seaweed carbon
policy because they have extensive seaweed forests. Similarly, some coun-
tries may have a greater interest given their large seaweed aquaculture in-
dustry, or plans to develop such industry - which todate is at anembryonic
stage in theglobal north. Thismay determine whichcountries are more mo-
tivated to advance the importance of seaweed for climate change mitiga-
tion at the United Nations Framework Convention on Climate Change.
While detection and attribution of restoration of carbon sequestration to
management action is currently challenging, there are many reasons to re-
store seaweed forests and promote seaweed aquaculture other than just
blue carbon, including biodiversity and fisheries benefits (Claisse et al.,
2013;Layton et al., 2020;Hynes et al., 2021), along with contributions to
many United Nations Sustainable Development Goals (Duarte et al.,
2021). Carbon sequestration should, therefore, be treated as a co-benefit
from seaweed restoration rather than the primary driver of the expansion
of this industry. The carbon crediting market in place, however, can incen-
tivize large scale uptake of seaweed for climate change mitigation.
10. Next steps in the science of seaweed as a means to counteract
climate change
Industries and governments are increasingly being compelled to
find strategies for climate change mitigation, and seaweed will continue
to be considered as one of the potential climate change mitigation solu-
tions. However, seaweed carbon accounting remains poorly developed
(Hurd et al., 2022) and is more complex than accounting of terrestrial car-
bon sinks and other vegetated blue carbon sinks (seagrass, mangroves and
tidal marshes). Such established blue carbon systems have standardised
methodologies to document carbon sequestration (Emmer et al., 2015),
whereas similar methodologies are not in place for seaweed projects.
This lack of methodologies is due to fundamental knowledge gaps of
fluxes and sinks, limited proj ects that have demonstrated carbon seques-
tration and CO
2
influx in response to a particular activity (restoration,
seaweed aquaculture), and the lack of effective monitoring and evalua-
tion techniques.
Based on the science and policy knowledge gaps reported in this
paper and summarised in Box 2, activities linked to seaweed forest
restoration, the export of carbon offshore from seaweed farms during
the farming process, or deliberately sinking seaweed, do not appear to
be ready to be implemented for climate change mitigation. However,
there are many social and environmental reasons why both restoration
of seaweed forests and expansion of seaweed aquaculture should be en-
couraged, regardless of carbon sequestration (Chung et al., 2017;
Duarte et al., 2021;Filbee-Dexter et al., 2022). Emission abatement
from seaweed products remains a promising way for seaweed aquacul-
ture to reduce industrial emissions, as proven by the methane-
reducing seaweed Asparagopsis.Defining the environmental and social
impact of seaweed products compared to terrestrial alternatives is an
important next step in unlocking seaweed aquaculture for climate
change mitigation. Finally, investment is now needed to accurately
quantify carbon sequestration that occurs below seaweed farms using
forensic carbon accounting (Hurd et al., 2022), and subsequently accu-
rately determine the scalability of this solution. Overall, seaweed shows
promise for climate change mitigation, however caution should be
given to ensure the interest from industry groups and governments in
seaweed for climate change mitigation does not outpace the science.
CRediT authorship contribution statement
The idea for the manuscript was conceived by FR, PM and PT. FR
organised and directed the writing of the manuscript. All other authors
contributed to the writing and editing of the manuscript.
Data availability
No data was used for the research described in the article.
Declaration of competing interest
The authors declare the following financial interests/personal relation-
ships which may be considered as potential competing interests: Peter
Macreadie reports financial support was provided by Australian Research
Council Discovery Project. Dorte Krause-Jensen reports financial support
was provided by Independent Research Fund Denmark. Dorte Krause-
Jensen reports financial support was provided by European Union H2020.
Karen File-Dexter reports financial support was provided by Norwegian
Blue Forest Network. Karen Filbee-dexter reports financial support was pro-
vided by Australian Research Council. Catherine Lovelock reports financial
support was provided by Australian Research Council. Lennart Bach reports
financial support was provided by Australian Research Council. Philip Boyd
reports financial support was provided by Australian Research Council.
Acknowledgements
This project was supported by Deakin University and Sea Green Pte Ltd.
PIM thanksthe support of an Australian Research Council Discovery Project
(DP200100575). DKJ acknowledges support from the Independent
Research Fund Denmark through the project ‘CARMA’(reference: 8021-
00222B) and from the European Union H2020 (FutureMARES, contract
#869300). OS was supported by I + D + I projects RYC2019-027073-I
and PIE HOLOCENO 20213AT014 funded by MCIN/AEI/10.13039/
501100011033 and FEDER. KFD was supported by the Australian Research
Council (LP1931001500, DE1901006192, DP220100650) and the
Norwegian Blue Forest Network. CEL acknowledges support of the
Australian Research Council (FL200100133). The authors in this study
were supported by Australian Research Council by Future Fellowship
FT200100846 (to L.T.B.) and Laureate Fellowship FL160100131 (to
P.W.B.).
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