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Suggested Citation: United Nations Environment Programme (2023). Seaweed Farming: Assessment on the Potential of
Sustainable Upscaling for Climate, Communities and the Planet. Nairobi

Author: Colette J. Feehan, PhD, Department of Biology,
Montclair State University, New Jersey, USA
Editors: Linda Jonsson (United Nations Environment
Programme), Hally Voi Blanchard (United Nations Environment
Programme)
Layout and Cover: Hally Voi Blanchard (United Nations
Environment Programme)

Reviewers: Alejandro Buschmann (Universidad de Los Lagos), Junning Cai (Food and Agriculture Organization of theUnited Nations), Alan Critchley (The Verschuren Centre),
Halley Froehlich (University of California, Santa Barbara),
Dorte Krause-Jensen (Aarhus University), Flower Msuya(University of Dar es Salaam), and Jihong Zhang (Chinese
Academy of Fishery Sciences)

ABBREVIATIONS
AMPEP
BECCS
CO2
DIC
DOC
EEZ
EIA
emLab
FAO
GESAMP
GHG
HEA
IMTA
IPBES
IPCC
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NTC
POC
PPP
RCP
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TEEB
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UVWHOAcadian Marine Plant Extract Powder
Bioenergy with Carbon Capture and Storage
Carbon Dioxide
Dissolved Inorganic Carbon
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Exclusive Economic Zone
Environmental Impact Assessment
Environmental Market Solutions Lab
Food and Agriculture Organization of the United Nations
Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection
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Habitat Equivalency Analysis
Integrated Multi-Trophic Aquaculture
Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services
Intergovernmental Panel on Climate Change
Marine Protected Area
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The National Academies of Sciences, Engineering, and Medicine
Not in My Backyard
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Particulate Organic Carbon
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Strengths, Weaknesses, Opportunities, and Threats
The Economics of Ecosystems and Biodiversity
United Nations
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United Nations Framework Convention on Climate Change
United States Dollar
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World Health Organizationii

TABLE OF CONTENTS
Foreword
Executive Summary
Highlightsv
1
1

PART 1: INTRODUCTION

4PART 2: REVIEW OF EVIDENCE FOR THE POTENTIAL CLIMATE BENEFITS OF SEAWEEDFARMING

7

2.1. Potential for Natural Carbon Sequestration
2.1.1. Absorption of Carbon Dioxide
2.1.2. Carbon Built into Biomass
2.1.3. Estimates of Natural Carbon Sequestration
2.1.4. Summary of Potential for Natural Carbon Sequestration

8
8
8
9
9

2.2. Potential Commercial Use Pathways for Climate Mitigation
2.2.1. Displacing Carbon-Intensive Energy
2.2.2. Reducing Livestock Methane Emissions
2.2.3. Displacing Carbon-Intensive Food Production
2.2.4. Intentional Deep Ocean Sinking
2.2.5. Land-Based Carbon Sequestration
2.2.6. Summary of Potential Commercial Use Pathways for Climate Mitigation

10
10
12
14
15
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17

PART 3: REVIEW OF EVIDENCE FOR POTENTIAL ENVIRONMENTAL AND SOCIOECONOMIC COBENEFITS AND RISKS
19
3.1. Potential for Environmental Co-Benefits
3.1.1. Marine Biodiversity
3.1.2. Water Quality
3.1.3. Coastal Protection
3.1.4. Summary of Potential for Environmental Co-Benefits

20
20
22
24
24

3.2. Potential for Environmental Risks
3.2.1. Habitat Competition
3.2.2. Spillover of Pathogens and Invasive Species
3.2.3. Genetic Pollution
3.2.4. Organic Matter Over-Deposition
3.2.5. Marine Megafauna Entanglement
3.2.6. Marine Pollution
3.2.7. Short-Lived Halocarbon Emissions
3.2.8. Summary of Potential for Environmental Risks

25
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29

3.3. Potential for Socioeconomic Co-Benefits
3.3.1. Income Generation and Diversification of Livelihoods
3.3.2. Gender Equity
3.3.3. Nutrition and Global Food Security
3.3.4. Cultural Services
3.3.5. Summary of Potential for Socioeconomic Co-Benefits

31
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34

3.4. Potential for Socioeconomic Risks
3.4.1. Adverse Health Effects
3.4.2. Low Income
3.4.3. Spatial Use Conflicts

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37

iii

3.4.4. Summary of Potential for Socioeconomic Risks

PART 4: FEASIBILITY OF UPSCALING GLOBAL SEAWEED FARMING PRODUCTION

38

39

4.1. Biophysical Factors that Limit Seaweed Farming Production
4.1.1. Conditions Required by Seaweed Crops
4.1.2. Crop Diseases and Pests
4.1.3. Crop Genetic Erosion
4.1.4. Climate Change
4.1.5. Summary of Biophysical Factors that Limit Seaweed Farming Production

40
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42

4.2. Potential Approaches for Upscaled Production
4.2.1. Marine Spatial Planning
4.2.2. Polyculture, Crop Rotation and Selective Breeding
4.2.3. Advanced Technology and Infrastructure
4.2.4. Global Biosecurity
4.2.5. Market and Product Development
4.2.6. Economic Incentives and Regulatory Support
4.2.7. Summary of Potential Approaches for Upscaled Production

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PART 5: SITUATIONAL ANALYSIS FOR SUSTAINABLE EXPANSION

50

5.1. Strengths
5.2. Weaknesses
5.3. Opportunities
5.4. Threats

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53
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57

PART 6: CONCLUSIONS

59

References
Appendix A: Tables
Units and Symbols

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76

iv

FOREWORD
The United Nations Environment Programme (UNEP) recognizes the existence of a triple planetary crisis – climate
breakdown, biodiversity loss and rampant pollution – which puts humanity at risk of irreversibly damaging our relationshipwith the natural world. Tackling these challenges to achieve climate stability, living in harmony with nature and moving
towards a pollution-free planet will require a recalibration of our economies and societies towards more sustainable,
circular and equitable models.
The UN Decade on Ecosystem Restoration, the Global Biodiversity Framework and the Sustainable Development Goals
point to the need to take a whole-of-society and whole-of-government approach that values the vast potential of the ocean
to help tackle this crisis. This means that we must rapidly accelerate solutions such as seaweed aquaculture. Marine andcoastal ecosystems are a bedrock of resilience, delivering powerful nature-based solutions to climate change, supporting
lives and livelihoods, and providing shelter and food for communities and a diverse range of life forms.
Modern scientific enquiry must scrutinize the exploitation of marine and coastal ecosystems and their biodiversity toidentify pathways that are underpinned by a sustainable blue economy; an economy in which activities that rely on theocean are nature and climate positive, as well as beneficial to the communities whose livelihoods depend on its health.
Given the rapid growth and expansion of the seaweed farming industry, we must proceed with cautious optimism. We
appreciate that the potential benefits of seaweed farming must be balanced against the risks of human interference in themarine environment and ultimately to the communities dependent on healthy blue ecosystems.
I am excited by the outcome of this report. The evidence showcases a range of possible opportunities to utilize seaweed
and, importantly, clearly articulates the potential to open a pathway for sustainably upscaling seaweed farming if we takea well-informed precautionary approach and take swift action to fill key environmental and socioeconomic research andpolicy gaps.
I hope this publication can help set in motion closer global collaboration on the topic of seaweed farming. By bringing
together local, national, regional and international stakeholders from both the public and private sectors in a meaningful
and inclusive way, we can more safely explore the possibilities while embracing the safeguards and best practices that
ensure we deliver sustainable upscaling for communities, people and the planet.

Leticia Carvalho
Head of Marine and Freshwater Branch
United Nations Environment Programmv

EXECUTIVE SUMMARYThe United Nations Environment Programme (UNEP)
recognizes the growing global interest in seaweed farming
as a potentially scalable ocean-based climate changesolution that may provide environmental and social cobenefits as part of the advancement of resilient and climate
smart aquaculture. To critically examine this potential, areport based on an in-depth literature review and situational
analysis was commissioned to scientifically assess thepotential for the sustainable expansion of seaweedfarming to deliver climate benefits with minimal
environmental and social risks.
This report collates and scrutinizes existing research on
the quantifiable climate benefits, as well as the associatedenvironmental and social risks and benefits, of global
seaweed farming. The scope of the report includes aninvestigation into the full value chain of seaweed farmingwith an emphasis on the potential for climate benefits
realized through various natural and commercial use
pathways, and the feasibility of upscaling global farmed
seaweed production. The findings herein are synthesized ina situational analysis with a SWOT design (Strengths,
Weaknesses, Opportunities, and Threats) for sustainable
expansion of global seaweed farming.

HIGHLIGHTS
Potential for Climate Benefits:
Seaweeds absorb carbon dioxide, converting it into
seaweed biomass. The fate of farmed seaweed biomass
will in large part determine the potential for climate
benefits. Research on the climate benefits of seaweedfarming is ongoing and the focus of the literature has beenon 2 primary routes by which seaweed farming could
contribute to climate mitigation:
Natural Carbon Sequestration: A component of the carbon
built into seaweed biomass is exported into the marine
environment during cultivation as particulate and dissolved
organic carbon. Some of this “unseen production” will be
consumed or remineralized in ocean food webs andreleased as carbon dioxide. Another component may reachlong-term oceanic carbon sinks, providing natural carbon
sequestration. Research indicates that substantial
upscaling of seaweed farming would be required formeaningful climate mitigation to occur via this route. Suchupscaling could have unforeseen effects on the Earth
System and ocean ecology, and there is therefore a needfor dedicated research on the environmental effects oflarge-scale seaweed farming.

Commercial Use Pathways for Climate Mitigation: The useof seaweed biomass post-harvest will also determine thepotential for climate benefits, and various seaweedcommercial use pathways have been proposed for carbon
sequestration and greenhouse gas emissions reduction.
These pathways have had varying levels of scientificassessment and development towards implementation.
Progress towards each pathway is summarized below:
Seaweed Biofuels and Other Low Emissions Products:
Seaweed biofuels have the potential to be a carbon
negative energy replacement for land-based biofuels andfossil fuels, particularly if they are co-produced with
seaweed-based replacements for other emissionsintensive commodities, such as land-based proteins orsynthetic fertilizers. Cost and energy efficiency have been
identified as barriers to the commercial viability ofseaweed biofuels, and implementation has occurred atonly pilot and experimental scales. Areas for future
progress include economies of scale for seaweed
cultivation, technological innovations for increased energy
conversion efficiency of seaweeds to biofuels, andincreased processing capacity for multiple seaweed
products in a biorefinery concept.
Seaweed-Enhanced Livestock Feed: Several studies indicate
the potential for select seaweeds (primarily Asparagopsis
species), added in small quantities to ruminant livestock
feed, to substantially decrease enteric emissions ofmethane, a powerful greenhouse gas and contributor toclimate change. The experimental trials conducted to date,although limited in scope, indicate little to no negative
effects on agricultural production and the co-benefit ofhigher production in some cases. The seaweed-enhanced
livestock feed pathway is limited to ruminant livestock thatare nourished by feeds (i.e., excluding free-range livestock),and studies examining safety, efficacy, and practicality atcommercial scales are needed.
Intentional Deep Ocean Sinking: The intentional sinking of
farmed seaweeds into the deep ocean has the potential tosequester carbon. Cost, technical feasibility, and social
license are considered some of the limiting factors to thispathway, and research and development is needed to
improve cost efficiency, establish new technology andinfrastructure, confirm the veracity of the carbon
sequestration, and assess the environmental risks. The
intentional sinking of farmed seaweeds for climate
mitigation also comes with ethical considerations given thepotential for seaweeds to be used as food, feed, or fuel.

1

Environmental and Socioeconomic Co-Benefits and Risks:
Seaweed farming has several potential environmental andsocioeconomic effects that should be considered whenassessing whether seaweed farming is a sustainable
practice and scalable approach for climate mitigation:
Marine Biodiversity: Research indicates that seaweedfarms are linked to altered/enhanced biodiversity when
they supersede a less complex habitat (e.g., sandy bottom),
but reduce biodiversity when they supersede a naturally
biodiverse, complex habitat (e.g., seagrass bed or coralreef). In tropical waters, herbivorous fishes are positively
associated with seaweed farms, likely due to theprovisioning of food (i.e., farmed seaweeds). Studies on thebiodiversity effects of seaweed farming have been largely
small-scale and short-term, and large gaps exist inscientific understanding of the effects of seaweed farming
on regional-scale patterns of biodiversity, including theeffects of farms on nearby ecosystems, and how
biodiversity is affected by the scale of seaweed farming
and the timing of cultivation and harvesting.
Water Quality: Seaweeds absorb pollutants (e.g., nutrients
including nitrogen and phosphorous, and heavy metals) incoastal waters. Hence, large-scale seaweed farming in
China likely absorbs most of the phosphorus inputs fromland and approximately 6 per cent of the nitrogen inputs. At
a global scale, substantial upscaling of seaweed farming
would be required to significantly mitigate nutrient
pollution; an upper estimate for global nitrogen removal bycurrent seaweed farming represents only 0.06 per cent of
the total global agricultural nitrogen fertilizer usage
estimated for the year 2022. Yet, large spatiotemporalvariability in ocean nutrients may still constrain seaweedfarming expansion. Given that seaweeds absorb heavy
metals in polluted waters, strict safety standards for theuse of seaweeds (e.g., for consumption) are needed.
Seaweed farms also produce oxygen and remove dissolved
carbon dioxide, and may thereby locally mitigate ocean
deoxygenation and acidification, respectively.
Other Environmental Effects: Seaweed farming has various
environmental risks, including competition with wild
habitats for nutrients and light, spillover of diseases andinvasive species and genetic pollution from farms to theenvironment, and entanglement of marine megafauna fromseaweed farming infrastructure such as ropes. Mitigation
measures can help to protect against some of theseenvironmental risks (e.g., carefully siting seaweed farms);
however, disease spillover and genetic pollution are
considered difficult to mitigate given the high dispersal of
biological material in the ocean. Although there is limited
evidence available for negative environmental effects ofseaweed farming at the levels of marine populations,communities, and ecosystems, sustainability standards
should require the use of preventative measures to avoid
risks, and investigation of environmental risks should be an
ongoing process.
Socioeconomic Effects: Research on the socioeconomic
effects of seaweed farming has focused primarily on smallscale farming in poor rural coastal communities ofdeveloping countries (e.g., in Indonesia, the Philippines,
and the United Republic of Tanzania). The studies indicate
generally positive outcomes of seaweed farming forpeople, including increased income and standards of living,
diversification of livelihoods, food security, and gender
equity. Health problems (e.g., respiratory disease due tostoring seaweeds in households) and income below thepoverty and extreme poverty line (e.g., due to the low value
placed on raw seaweed) have, however, been reported byseaweed farmers, and additional research is needed to
examine and address these adverse effects. Seaweedfarming can provide a source of high-quality food, with thepotential to contribute to global food security; however,variation in the nutritional value of seaweeds and the riskof heavy metal exposure must be carefully controlled.
Cultural services may be provided by seaweed farming,such as enhanced cultural heritage and social cohesion;
however, to realize these benefits, local stakeholders mustbe fully engaged in the industry and spatial use conflicts
must be avoided.
Patterns of Expansion:
Of the greater than 12,000 described marine seaweed
species, a small number (approximately 8 genera)
dominate the farmed production. Nearly all (99.5 per cent)of farmed seaweed production occurs across 9 countries
in East and Southeast Asia. Research indicates that inglobal regions where the seaweed farming industry has
grown rapidly, but access to scientific advancement islimited, growth of the industry has slowed (e.g., due to crop
diseases and pests). In regions where seaweed farming iswell established and science is advancing, the seaweedfarming industry has continued to grow; however, thisgrowth may be threatened by ecosystem carrying
capacities and climate change. Where seaweed farming isa nascent industry, barriers to growth include spatial use
conflicts, lack of social license, lack of legislative policies
and regulatory environments to support seaweed farming
and seaweed consumption, and lack of strong domestic
markets for high value seaweed products.
SWOT Analysis:
A SWOT analysis (Strengths, Weaknesses, Opportunities,
and Threats) for sustainable expansion of global seaweedfarming is presented, summarizing the expert view of the2author across the themes of climate benefits,environmental co-benefits/risks, socioeconomic cobenefits/risks, and the feasibility of upscaling production.
A total of 13 strengths, 18 weaknesses, 19 opportunities,
and 9 threats are presented. The SWOT analysis provides
context for managing the seaweed farming industry to

maximize the potential for climate benefits and other cobenefits, while minimizing risks. The results highlight anurgent need for additional research and sustainability
standards, including analysis of carrying capacities forseaweed farming at local and global scales.

Fig. ES1. Graphical abstract. Credit: UNEP

3

PART 1: INTRODUCTION
The global increase in atmospheric carbon dioxide (CO2)and other greenhouse gases (GHGs), arising fromanthropogenic activities, is driving global climate change
and associated degradation of ocean- and land-based
ecosystems upon which humans depend (IPCC 2019a;
IPCC 2019b; IPCC 2021). To avoid the worst outcomes ofclimate change and limit warming of the planet to 2°C, aslaid out in the Paris Agreement on Climate Change(UNFCCC 2015), negative emissions technologies areneeded to recapture the substantial GHGs already released
into the atmosphere (i.e., 100–900 gigatons), in addition to
curbing further GHG emissions (Bach et al. 2021; IPCC
2021). This knowledge has prompted increasing interest to
identify nature- and ocean-based solutions to capture andsecurely store (sequester on >100-year timescales;
GESAMP 2019) atmospheric CO2 and reduce carbon
emissions, while also providing co-benefits such as
biodiversity enhancement (Hoegh-Guldberg et al. 2019).
Protection and restoration of “blue carbon” ecosystems(i.e., vegetated coastal and marine ecosystems that
support carbon sequestration) has received growing
attention as a nature-based solution to mitigate climatechange while provisioning biodiversity (Duarte et al. 2013;
Macreadie et al. 2022). Traditional blue carbon
ecosystems include mangrove forests, salt marshes, andseagrass beds, which sequester carbon in the soft
sediments below them (Duarte et al. 2013). More recently,
seaweed forests, despite growing primarily on rocky
substrates, have been suggested to contribute to carbon
sequestration in sinks beyond the locality in which they aresituated (Krause-Jensen and Duarte 2016; Ortega et al.
2019). Seaweeds are primary producers in shallow
nearshore waters (less than 50 meters depth) that support
marine biodiversity by provisioning food and habitat(Dayton 1985). Protection and restoration of seaweed
forests therefore also represents a promising naturebased solution to climate change, although it ischallenging to quantify the contribution (Krause-Jensen etal. 2018; Hurd et al. 2022).
Along with the potential to provide nature-based solutions
to climate change, there is increasing evidence that theoceans can supply food to support the growing humanpopulation, helping to safeguard future food security(Duarte et al. 2017; Costello et al. 2020; Free et al. 2022).
Ocean-based foods are particularly relevant in the face ofincreasingly limited resources for land-based foodproduction, including arable land, freshwater, and fertilizer,
and the carbon-intensive nature of land-based foodproduction (e.g., via land use changes) (Vermeulen et al.

2012). Given that many of the world’s capture fisheries are
considered overexploited, the advancement of resilientand climate smart aquaculture has come to the forefront
as a solution for future food security (Duarte et al. 2022a;
Free et al. 2022).
Seaweed farming, or the aquacultural production of marine
multicellular photosynthetic algae, is a rapidly growing
sector of the aquaculture industry, currently accounting forover half of marine aquaculture production (FAO 2020;
Chopin and Tacon 2021). Seaweeds are diverse organisms
encompassing
over
12,000identified
species,taxonomically assigned to 3 phyla: reds (Rhodophyta),
browns (Ochrophyta including kelps), and greens(Chlorophyta) (Hurd et al. 2022). The enormous biologicaldiversity represented across these taxonomic groups isbest exemplified by the fact that they do not share acommon multicellular ancestor; rather, multicellularity
arose separately across the groups (akin to the separate
origins of multicellularity in land plants and animals) (Cocket al. 2010). Seaweeds are also highly diverse in theirenvironmental tolerances, life cycles, growth forms,
habitats, physiology, and chemical composition, including
their nutritional value to humans (Roleda and Hurd 2019;
FAO and WHO 2022).
Farmed seaweeds are an important component of humandiets in several Asian countries (FAO 2020). 97 per cent of
the approximately 30 million tons of seaweed biomass
used by humans in 2018 was farmed, and the estimatedfarm gate value of the global seaweed farming industry is
USD 13.3 billion (FAO 2020). This represents 5 per cent of
the estimated USD 250 billion value of aquatic animal
aquaculture, including finfish (FAO 2020). Yet, seaweedfarming occurs in a relatively small number of countries,
which in order of annual production include China,
Indonesia, the Republic of Korea, the Philippines, theDemocratic People's Republic of Korea, Japan, Malaysia,
the United Republic of Tanzania, Chile, Viet Nam, Solomon
Islands, Madagascar, India, and the Russian Federation(FAO 2020). The top 9 producing countries, all of which arein East or Southeast Asia, account for 99.5 per cent of
production (FAO 2020; Chopin and Tacon 2021).
Most of the global farmed seaweed production isprocessed for direct human consumption or hydrocolloids(i.e., carrageenan, alginate, and agar) used as stabilizing,
thickening, suspending, and gelling agents in food,cosmetics, and pharmaceuticals (Valderrama et al. 2013;
Marquez et al. 2015; Charrier et al. 2017). A limited number4

of seaweeds make up 96.8 per cent of global production1(Chopin and Tacon 2021). Three groups dominate fordirect human consumption:
Saccharina/Laminaria(kombu), Undaria (wakame), and Porphyra/Pyropia (nori)(Chopin and Tacon 2021). A species of kelp2 and a groupof tropical red seaweeds3 are the most farmed in terms ofwet weight production (FAO 2020). The kelp species isgrown primarily in China for food and the extraction ofalginate, while the tropical red seaweeds are grownprimarily in Indonesia for carrageenan that is processedlargely in China from imported raw seaweed (Porse andRudolph 2017; FAO 2020; Chopin and Tacon 2021). Europe
and North America (primarily the United States of America)
are the largest international markets for carrageenan(Valderrama et al. 2013).
Seaweed farming does not require the use of arable land,
freshwater, or fertilizer (Roleda and Hurd 2019). Seaweeds
extract inorganic nutrients (e.g., nitrogen and phosphorus)
directly from the marine environment to produce biomass(Roleda and Hurd 2019). Hence, seaweed farming is
considered an extractive form of aquaculture (as opposedto fed aquaculture such as finfish), requiring no addition of
feed (FAO 2020). The primary methods of seaweed
cultivation are the fixed-off-bottom method and thefloating longline, net, or raft method (Robledo et al. 2013;
Buschmann et al. 2017) (Fig. 1). Clonal seaweeds4 arecultivated through vegetative propagation, a form of
asexual reproduction, while unitary seaweeds, such as
kelps5, require sexual reproduction in a hatchery or nurseryfor cultivation (Buschmann et al. 2017).

While global seaweed production tripled from 2000 to
2018, the growth rate of the industry has slowed since2015, largely due to a reduction in tropical seaweed
production in Southeast Asia, and primarily Indonesia (FAO
2020). Seaweed farming production also has declined in
Chile since 2000 (FAO 2020). While seaweed farming isreported to occur at pilot and pre-commercial scales inseveral countries in Europe and North America(Buschmann et al. 2017), data are limited for these regions
due to data confidentiality (FAO 2020). Seaweed farming
is an emerging industry in several African countries,
including Kenya, Mozambique, and Namibia, with aperceived potential for expansion in these regions (Msuya
et al. 2022). Available data for Europe indicate that 68 percent of seaweed producers/companies currently use wild
stocks rather than farmed seaweeds, indicating a nascent
European seaweed farming industry (Araújo et al. 2021).
However, Vincent et al. (2020) predict substantial growthin the European seaweed farming industry within the next
decade. Similarly, while 95 per cent of edible seaweed
products in the United States of America are currently
sourced from overseas, an economic analysis for the stateof Maine indicated that the regional seaweed farming
industry could grow by 12–15 per cent annually over the
next approximately 15 years (Piconi et al. 2020). Such
growth assumes a growing market for seaweed products
and the capacity for marine ecosystems to sustain
seaweed farms (Piconi et al. 2020; Vincent et al. 2020;
Msuya et al. 2022).

Fig. 1. Two primary methods for seaweed cultivation: fixed-off-bottom method and floating method. Dimensions, depths, materials,
and anchorage weights will vary. Reproduced from Robledo et al. (2013) with permission.

Saccharina/Laminaria, Eucheuma/Kappaphycus, Gracilaria,
Porphyra/Pyropia, Undaria, and Sargassum
2 Saccharina japonica (previously Laminaria japonica)
1

Eucheuma/Kappaphycus
For example, Kappaphycus
5 For example, Saccharina japonica
3
4

5

The seaweed farming full value chain runs from seaweed
harvesters to the users of seaweed products, and typically
includes 4 stages: cultivation, post-harvest treatment,
trading, and processing (Cai et al. 2013). Cultivation
involves production and harvest of fresh seaweed biomassin the ocean (Cai et al. 2013). Post-harvest treatment
involves removing impurities (e.g., sand, shells, and litter)and drying fresh seaweeds (Cai et al. 2013; Wei et al. 2013).
Removal of impurities is necessary to avoid equipment
problems during the processing stage (Marquez et al.
2015). Trading involves consolidating seaweed frommultiple sources and delivering it to processors dried or
powdered (Cai et al. 2013). Processing involves turning theraw seaweed into commercial products, which in addition
to human food and hydrocolloids, may include animal
feeds, fertilizers and biostimulants, bioactives, pigments
and colorants, bioplastics, and biofuels (Holdt and Kraan
2011; Cai et al. 2013; Rouphael et al. 2018; Torres et al.
2019; FAO 2020; Thomsen and Zhang 2020; Chopin andTacon 2021). There is a lack of information on thetransportationoffarmedseaweeds;
however,transportation of wild harvested seaweed, which can have
80–85 per cent moisture content, has been cited as a
generally costly stage in that value chain (Bruton et al.
2009).

seaweed commercial use pathways. This is followed by areview of the environmental and socioeconomic cobenefits and risks of seaweed farming. Information is thenreviewed on the feasibility of upscaling seaweed farming,taking into consideration current environmental, social,and economic limitations to production. The report ends
with a situational analysis for sustainable expansion of
global seaweed farming following a SWOT design(Strengths, Weaknesses, Opportunities, and Threats). The
inclusion herein of any environmental or socioeconomic
benefit/risk or commercial use pathway does not imply
endorsement by UNEP.

It has been posited that seaweed farming represents the
expansion (afforestation) of seaweed habitats that can
provide climate benefits and other ecosystem services ifsustainability standards are considered (Duarte et al. 2017;
Duarte et al. 2022a). Various global entities such as the
Lloyd’s Register Foundation, United Nations Global
Compact, and World Resources Institute have therefore
identified seaweed farming as a promising option to
mitigate and adapt to climate change, feed the world, andprovide additional co-benefits, such as provisioning
biodiversity and ameliorating coastal pollution (e.g.,
Hoegh-Guldberg et al. 2019; Doumeizel et al. 2020). In
recognition of this growing global movement, UNEP has
solicited a literature review and situational analysis to
scrutinize existing evidence for the potential of a wellmanaged seaweed farming industry to address multiple
United Nations (UN) Sustainable Development Goals(SDGs) (including SDG2: zero hunger; SDG13: climate
action; and SDG 14: life below water), and to contextualize
the risks and benefits of seaweed farming so that, if
necessary, interventions/guidance can be delivered tomaximize the benefits and avoid undue harm toecosystems and people.
To this end, this report begins with an in-depth review ofthe potential for climate benefits of seaweed farming,
including examination of the full value chain with anemphasis on the carbon sequestration and GHG emissions
reduction potential during cultivation and through various

6

PART 2: REVIEW OF EVIDENCE FOR THE
POTENTIAL CLIMATE BENEFITS OF
SEAWEED FARMING

7© Unsplash / Caleb Kastein2.1. Potential for Natural Carbon Sequestration
Several studies indicate the potential for wild seaweed
forests to contribute to ocean carbon sequestration (e.g.,
Krause-Jensen and Duarte 2016; Ortega et al. 2019; Fenget al. 2022; Filbee-Dexter et al. 2022). Under an
atmospheric CO2 removal and sequestration scenario forseaweed forests, seaweeds absorb dissolved inorganiccarbon (DIC) in surface waters, reducing the partialpressure of aqueous (i.e., dissolved) CO2, and eventuallydriving the flux of CO2 from the atmosphere into the ocean(Delille et al. 2000). The inorganic carbon converted byseaweeds to organic carbon (i.e., built into seaweedbiomass) is sequestered when a component of thebiomass that is not consumed or remineralized in ocean
food webs is exported as dissolved and particulate organiccarbon (DOC and POC) to long-term carbon sinks in ocean
sediments and the deep ocean (greater than 200 meters
depth) (Krause-Jensen and Duarte 2016). Yet, tracking
carbon across the air-sea interface, into seaweed forests,
and to oceanic carbon sinks is highly complex, andtherefore questions remain regarding the extent of carbon
sequestration by seaweed forests (Macreadie et al. 2019;
Gallagher et al. 2022; Hurd et al. 2022). Accordingly,
research is underway to understand the extent to which
seaweed farms can sequester carbon in the ocean throughatmospheric CO2 removal and the export of seaweed DOC
and POC from farms to oceanic carbon sinks (Chung et al.
2011; Duarte et al. 2017).
Since farmed seaweed biomass is removed from theocean during harvest, the post-harvest usage of seaweeds
will also determine the potential for climate benefits. For
example, a component of seaweed carbon could be storedin soils following seaweed biochar or biofertilizer
application (Chung et al. 2011; Duarte et al. 2017; Sondak
et al. 2017), or climate benefits could occur if farmed
seaweeds are used to replace more carbon-intensive
commercial products, such as seaweed biofuels replacing
fossil fuels (Thomsen and Zhang 2020). In this section,
current understanding of the potential for carbon
sequestration to occur in the ocean during seaweed
cultivation is reviewed, referred to here as “natural carbon
sequestration”. Other potential pathways for climate
mitigation that may occur post-harvest are discussed
under section 2.2 Potential Commercial Use Pathways for
Climate Mitigation.
2.1.1 Absorption of CO2
The potential for absorption of CO2 by seaweed farms wasassessed by Jiang et al. (2013) in Lidao town, China. They
found that seaweed farms6 had significantly lower ocean
surface dissolved CO2 and greater air-sea CO2 flux (i.e.,
Saccharina japonica and Gracilaria lemaneiformis
Gracilaria tikvahiae and Saccharina latissima
8 Ecklonia cava and Ecklonia stolonifera

increased CO2 flux into the ocean) as compared toreference areas outside seaweed farms, indicating an
atmospheric CO2 sink (Appendix A, Table A1). The extent
of the atmospheric CO2 sink varied throughout the year in
response to the seasonal cultivation of seaweeds, with thegreatest sink occurring during the period of fastest
seaweed growth and a reduction in the sink during
summer, likely due to seaweed biomass decay (Jiang et al.
2013). Given the localized nature of this study and thecomplexity of air-sea gas exchange globally, additional
research is needed to fully assess the extent to which
seaweed farms can accomplish atmospheric CO2 removal(Hurd et al. 2022).
2.1.2 Carbon Built into Biomass
Various studies have assessed the capacity of seaweedfarms to build carbon into seaweed biomass (Chung et al.
2013; Kim et al. 2014; Kim et al. 2015; Zheng et al. 2019).
Kim et al. (2014; 2015) found that seaweed farms7 in Long
Island Sound and Bronx River Estuary, United States ofAmerica, held up to 300–1,800 kilograms of carbon perhectare during cultivation (Appendix A, Table A1). In the
Republic of Korea, Chung et al. (2013) estimated that apilot-scale seaweed farm8, purposed with removing CO2
from the atmosphere, held approximately 10 tons of CO2
per hectare per year during cultivation (Appendix A, Table
A1). Across China, Zheng et al. (2019) estimated that
seaweed farms, of various species, hold 421.78 tons ofcarbon per square kilometer per year (Appendix A, Table
A1). Kelp9 is responsible for most of the carbon held in
seaweed farms annually in China due to the magnitude of
total production (Zheng et al. 2019; Gao et al. 2022).
However, a red seaweed10 has the greatest carbon holding
capacity per unit area farmed based on its relative carbon
content (Gao et al. 2022).
Given that farmed seaweed biomass is harvested, thecarbon storage period is limited to the period of cultivationand does not represent natural carbon sequestration (e.g.,
as defined by GESAMP 2019). However, some seaweed
production that is released during cultivation into theenvironment as DOC and POC and reaches long-term
oceanic carbon sinks can be sequestered. It has been
posited that the magnitude of natural carbon
sequestration from DOC export during seaweed cultivation
is similar (per unit area) to that of wild seaweed forests;
however, this assumption is still being tested (Hughes etal. 2012; Gao et al. 2022) (Appendix A, Table A1). Studies
have examined the release of POC from seaweed farms
due to breakage, erosion, and loss of seaweed fronds;
however, the fate of this material was not assessed (Zhang
et al. 2012; Fieler et al. 2021). At a seaweed farm11 in Sungo
Bay, China, 58 per cent of gross wet weight and 61 per cent
Saccharina japonica
Gracilariopsis lemaneiformis
11 Saccharina japonica

6

9

7

10

8

of carbon gross production were exported as POC during
cultivation (Zhang et al. 2012) (Appendix A, Table A1). At
seaweed farms12 in Norway, 8–49 per cent of gross dry
weight and 63–88 grams of carbon per square meter per
year were released as POC (Fieler et al. 2021) (Appendix A,
Table A1).
2.1.3 Estimates of Natural Carbon Sequestration
Regional estimates of the total carbon built into farmed
seaweed biomass annually during cultivation (see section
2.1.2 Carbon Built into Biomass) have been extrapolated byvarious authors to estimate the upper-bound global carbon
sequestration potential of seaweed farming. These upperbound global estimates assume that all the carbon built
into farmed seaweed biomass is sequestered (anunrealistic scenario but useful for considering the current
maximal potential for sequestration). They range from0.67 to 0.78 teragrams of carbon per year (Turan and Neori
2010; Duarte et al. 2017; Kim et al. 2017; Sondak et al.
2017) (Appendix A, Table A1) and equate to only
approximately 0.4 per cent of the estimated natural carbon
sequestration of global wild seaweed forests (Duarte et al.
2017). These results indicate that under even the mostidealistic scenario, substantial upscaling of farmed
seaweed production would be necessary to accomplish a
globally relevant carbon sink.
In a more realistic scenario, Gao et al. (2022) used the ratioof natural carbon sequestration to overall productivity ofwild seaweed forests from various literature sources toestimate that 58.6 per cent of farmed seaweed carbon
across China is sequestered in oceanic carbon sinks(Appendix A, Table A1). This estimate considers that a
portion of seaweed DOC and POC released into the ocean
is consumed and remineralized, which does not contributeto sequestration. Yet, the estimate of Gao et al. (2022) isstill considered coarse, and other calculations of naturalcarbon sequestration from seaweed farms range greatly
from 0–205 kilograms of CO2 per megaton of seaweed dryweight produced per year (emLab 2019) (Appendix A,
Table A1). As a result, a life cycle analysis for seaweedfarming, that considered both this potential range ofnatural carbon sequestration and the pre-processing
seaweed farming carbon footprint13, yielded net carbon
emissions from seaweed farming ranging from a carbon
sink to a carbon source (emLab 2019) (Appendix A, Table
A1). This indicates the great need for additional researchthat refines estimates of natural carbon sequestration atseaweed farms to fully assess the potential for carbon
negative farmed seaweed production.
The development of new methods to analyze seaweed
carbon dynamics in the ocean, including fingerprinting
seaweed carbon beyond the habitat (or farm), is
12

Saccharina latissima

considered a high research priority in the medium-term (by2025) to accomplish quantifiable ocean-based climate
mitigation (Hoegh-Guldberg et al. 2019). Such technology
will be needed to verify the natural carbon sequestration
function of seaweed farms and parameterize life cycle
analyses for the carbon footprint of seaweed farming. It
will also be critical to develop a clearer understanding of
how seaweed farming effects air-sea CO2 flux (i.e.,
determining when and where air-sea exchange occurs attimescales relevant for atmospheric CO2 removal) (Bach etal. 2021; Hurd et al. 2022). Moreover, the trade-offs
between farmed seaweed production and the primary
productivity of wild ecosystems that currently drawdown
CO2 (i.e., competition with phytoplankton), the effects offarms on ocean albedo and associated radiative forcing,
and the effects of farm-associated fauna on overall carbon
budgets (given their respiration and calcification), also will
be necessary to fully assess the climate mitigation
potential of seaweed farming (Bach et al. 2021; Hurd et al.
2022).
2.1.4 Summary of the Potential for Natural Carbon
Sequestration
There is evidence that wild seaweed forests sequester
carbon in the ocean through the export of DOC and POC tooceanic carbon sinks (Krause-Jensen and Duarte 2016;
Feng et al. 2022). While the extent of this sequestration ischallenging to estimate, protection and restoration of wild
seaweed habitats represents a promising nature-based
solution to climate change (Duarte et al. 2013; Hurd et al.
2022; Macreadie et al. 2022). Accordingly, it has been
posited that seaweed farms also can sequester carbon in
the ocean through the export of DOC and POC during
cultivation (Gao et al. 2022). Yet, estimates of naturalcarbon sequestration during seaweed cultivation remain
coarse and are based largely on an unconfirmed
assumption that seaweed farms function similarly to wild
seaweed forests. Hence, it is currently difficult to discern
when and where seaweed farming acts as a carbon sink
(emLab 2019). Notwithstanding, even upper-boundestimates for the global natural carbon sequestration
potential of current seaweed farming represent a small
climate benefit when compared to the estimated naturalcarbon sequestration of wild seaweed forests, indicating
limited climate mitigation potential through natural carbon
sequestration at current farming scales. Scientificadvancements that allow for accurate measurements ofnatural carbon sequestration by seaweed farms, and the
direct and indirect effects of seaweed farming on
atmospheric CO2 concentrations, will be crucial for
implementing upscaled seaweed farming as a climate
mitigation tool.

143–287 kilograms of CO2 per megaton of seaweed dry weight
produced
13

9

The evidence reviewed here on the natural carbon
sequestration potential of seaweed farming indicatesvarious inherent strengths and weaknesses of seaweedfarming as well as opportunities for the future that can betaken advantage of to maximize the benefits of seaweedfarming expansion.

2.2 Potential Commercial Use Pathways for
Climate Mitigation
In addition to the potential for seaweed farming to yield
climate benefits through natural carbon sequestration in
the ocean (see section 2.1. Potential for Natural Carbon
Sequestration), various post-harvest commercial uses ofseaweeds have been discussed in the literature for carbon
sequestration and GHG emissions reduction. In thissection, information is reviewed on these commercial use
pathways, and progress and limitations towards
implementation of each pathway are discussed.
2.2.1 Displacing Carbon-Intensive Energy
Fossil fuels are a finite resource, and their continued use isconsidered unsustainable due to the production of GHGs
that are major contributors to climate change (CO 2, sulfur
dioxide, and nitrogen oxides) (Wei et al. 2013; IPCC 2021).
Climate benefits could be realized from seaweed farming
through the replacement of fossil fuels with seaweed
biofuels (Duarte et al. 2017). While various hurdles exist to
implementing commercial scale seaweed biofuels, there isgreat interest in seaweed biofuels as a renewable energy
source that could bring about energy security and“bioenergy with carbon capture and storage” (BECCS), a
category of negative emissions technology (Hughes et al.

2012; Wei et al. 2013; Kraan 2013; Marquez et al. 2015;
Pechsiri et al. 2016; Melara et al. 2020). Duarte et al. (2017)
estimate that the broad adoption of seaweed biofuels has
the potential to avoid fossil fuel emissions of 1,500 tons of
CO2 per square kilometer of farmed area per year
(Appendix A, Table A2). Yet, production of seaweed
biofuels remains at experimental and pilot scales (FAO
2020; Mouritsen et al. 2021).
Unlike first-generation biofuels that are derived from landbased food crops (e.g., corn grain, sugar cane, soybean oil,or oil palm), second- and third-generation biofuels do notcompete with human food production by utilizing either thewastes of food crops or novel non-agricultural/non-staple
crops, respectively (Turan and Neori 2010; Wei et al. 2013;
Mouritsen et al. 2021). While the use of seaweeds forbiofuels must be traded off against other potential uses,
such as human food (Duarte et al. 2022a), seaweeds are
considered a viable option as a third-generation biofuel
feedstock given their fast growth rates and lack of need forarable land, freshwater, and fertilizer for growth (Hughes etal. 2012; Kraan 2013; Wei et al. 2013; Marquez et al. 2015).
Moreover, seaweed biofuels are considered less toxic andproduce fewer pollutants than fossil fuels (Wei et al. 2013).
Technologies considered feasible for seaweed biofuel
production include anaerobic digestion for biogas, ethanol
fermentation, and hydrothermal liquefaction for bio-oil, as
these methods each allow for the use of wet seaweed10

biomass, avoiding the energy intensive seaweed drying
process (Marquez et al. 2015).
Modelling studies indicate that seaweed biofuels have thepotential to be carbon negative energy sources; however,estimates are highly variable (Hughes et al. 2012; AlvaradoMorales et al. 2013; emLab 2019; Thomsen and Zhang
2020). For example, a life cycle analysis for seaweedbioethanol production indicates a carbon footprint ranging
from a carbon sink to a carbon source14 when used toreplace corn grain ethanol, and a carbon sink15 when used
to replace gasoline (emLab 2019). Replacing global
gasoline production with seaweed bioethanol would
require an annual 6.5 gigatons of seaweed dry weight
farmed across 0.6 per cent of the ocean surface, which
could reduce global annual carbon emissions by 2.9 percent (emLab 2019) (Appendix A, Table A2). However, acost analysis indicates that for seaweed bioethanol to becompetitive against corn grain ethanol and gasoline, thecost of seaweed farming would have to decrease by 77 and69 per cent, respectively (e.g., a liter of seaweed ethanol
would sell for approximately USD 3.03) (emLab 2019).
Farm infrastructure (e.g., long-lines and support lines)
contributed most of the carbon emissions (72–98 percent) during the cultivation stage (from propagule
production to ocean cultivation and harvesting), indicating
the potential for materials innovation to improve thecarbon footprint of seaweed production (emLab 2019).
The development of emissions efficient ships for
harvesting and farm maintenance could further reduce theseaweed farming carbon footprint (NASEM 2021).
Thomsen and Zhang (2020) also present a life cycle
analysis for seaweed bioethanol production when used toreplace gasoline, wherein seaweed proteins are used toreplace soy protein and excess carbon is stored in soils
through seaweed biofertilizer. This scenario resulted in a

net climate change reduction of 10 kilograms of CO2
equivalent per hectare for a 1 hectare farmed area
(Thomsen and Zhang 2020) (Appendix A, Table A2). A
seaweed biogas production scenario had an even greater
net climate change reduction of 1,870 kilograms of CO2
equivalent per hectare for the same 1 hectare area,
indicating the sensitivity of life cycle analyses to the typeof biofuel produced, among other factors (Thomsen andZhang 2020) (Appendix A, Table A2).
Modelling of methane biogas production by anaerobic
digestion indicates that if the digestate (waste) resulting
from seaweed biofuel production is used as soil improver,
the refractory component of organic carbon (resistant tobiological degradation) in the digestate is sequestered in
the soil (Hughes et al. 2012). Based on an estimated 7 percent refractory component in kelp, the sequestration
potential from seaweed biofuel was estimated as 0.78
tons of carbon per hectare of seaweed farmed16 (Hughes
et al. 2012) (Appendix A, Table A2). This value is in thesame order of magnitude as the estimated sequestration
potential of low-input high-diversity grassland biofuel17(Hughes et al. 2012).
A life cycle analysis of seaweed anaerobic digestion forbiogas also indicates a carbon negative energy source(Alvarado-Morales et al. 2013). The seaweed grow-out
phase of cultivation was the most energy intensive phasein the analysis (50–57 per cent of total energy demand)due to fossil fuel use for farm maintenance. However, thiswas offset by the climate benefit of using digestate as
biofertilizer to replace synthetic fertilizers with a higher
carbon footprint (Alvarado-Morales et al. 2013) (Fig. 2).

Fig. 2. Biogas production by anaerobic digestion of seaweed can be a carbon negative energy source when thedigestate is used as biofertilizer. Two stages in the seaweed farming value chain are shown (cultivation andprocessing). Based on Alvarado-Morales et al. (2013).
14
15-226 to 130 kilograms of net CO2 per megaton of seaweed dry weight-239 to -41 kilograms of net CO2 per megaton of seaweed dry weight

0.42 tons of carbon per hectare of farm from digestate in soil plus
0.36 tons of carbon per hectare of farm from the release of refractory
DOC in the ocean, estimated at 15 per cent of total DOC released
17 1.2 tons of carbon per hectare
16

11

For seaweed biofuels to be a commercially viable methodof BECCS, they must both have a net negative carbon
footprint and yield net energy production (Pechsiri et al.
2016; Melara et al. 2020). A modelling study of methane
biogas production by anaerobic digestion with biofertilizer
production found that a small-scale farm in Sweden (0.5hectares) consumed more energy than it produced basedon a conservative estimate for biogas yield of 40 per cent,indicating a negative energy return on investment (Pechsiri
et al. 2016). Upscaling to 10 hectares of farmed area
resulted in only a small improvement, with slightly moreenergy produced than consumed (Pechsiri et al. 2016).
Similarly, a modelling study in the United States ofAmerica, focusing on 3 locations (West Coast, East Coast,and Gulf of Mexico), found a low probability (5 per centchance) of achieving a combination of both net carbon
sequestration and net energy production from seaweed
biofuel (Melara et al. 2020). There was high carbon output
and low energy return on investment from anaerobic
digestion, indicating the need for innovation to increasebiomass conversion efficiency (Melara et al. 2020). Indeed,a more optimistic biogas yield estimate of 90 per cent at
the 10 hectare scale in the Swedish study resulted in net
energy production that nearly met the energy return on
investment required for commercial viability and provided
GHG savings of 60 per cent as required by the European
Union Renewable Energy Directive (Pechsiri et al. 2016).
Improvements to anaerobic digestion that increase biogas
yield, and economies of scale, particularly during seaweed
cultivation, have the potential to make seaweed biofuel amore viable BECCS method (Pechsiri et al. 2016; Melara etal. 2020).
Calculations indicate the theoretical potential to offset
transportation fuel use with seaweed biofuels (Fernand etal. 2017). For example, replacing 60 per cent of thetransportation fuel needs of France would require seaweedfarming over a small fraction (0.5 per cent or 54,795 square
kilometers) of the total 11 million square kilometer French
exclusive economic zone (EEZ) (Fernand et al. 2017).
Similarly, in Israel, the Netherlands, and Norway, farming
across 3,250, 17,368, and 1,034 square kilometers of theirrespective total 27,000, 154,011, and 787,000 square
kilometer EEZs would be required for 60 per centtransportation fuel replacement (Fernand et al. 2017). By
contrast, in Germany, farming across 69,683 square
kilometers would be required to replace 60 per cent oftransportation fuel use, which is greater than twice thetotal 33,100 square kilometer German EEZ (Fernand et al.
2017). Biofuel obligation targets for transport exist, forexample, in Ireland (10 per cent biofuel use by 2020; Bruton
et al. 2009), indicating the value of such theoretical
calculations. Yet, these calculations do not consider the

18

Approximately 10 years

cost and feasibility of large-scale seaweed farming forbiofuels (Fernand et al. 2017).
As with any commercial use pathway herein, advanced
technology and infrastructure will be required for largescale seaweed biomass production and processing (seesection 4.2.3 Advanced Technology and Infrastructure).
Specific to biofuels, there is a need for identification oflocations and seaweed strains with maximal capacity forbiofuel production, and technological innovations for highyield conversion of seaweed biomass to biofuels (Turanand Neori 2010; Kraan 2013; Wei et al. 2013; Kerrison et al.
2015; Marquez et al. 2015). Identification of novel
microbes, particularly from the ocean, for use infermentation of the diverse and unusual carbohydrates
found in seaweeds (e.g., galactose, rhamnose andarabinose, and sugar acids), could increase biomass
conversion efficiency for biofuel production (Fernand et al.
2017).
These advancements will likely require political, private,
and public support that may include government
regulations, public-private partnerships (PPPs), private
investment into research and innovation, and thedevelopment of educational infrastructure (Turan andNeori 2010). At the large farming scales required toeconomically produce seaweeds for commercial uses(including for biofuels), the potential negative effects ofseaweed farming on ocean ecosystems must be
considered (see section 3.2 Potential for Environmental
Risks). Finally, accurate accounting of atmospheric CO2
removal and natural carbon sequestration during seaweed
cultivation will be necessary to account for the overall
climate benefits of seaweed biofuel production (as withany commercial use pathway; see section 2.1 Potential for
Natural Carbon Sequestration).
2.2.2

Reducing Livestock Methane Emissions

The livestock sector contributes 14.5 per cent of total
global anthropogenic GHG emissions, and methane
generation by ruminant livestock (e.g., cows and sheep)
accounts for 39 per cent of these emissions (Gerber et al.
2013). Methane is a short-lived18 but powerful19 GHG andis a by-product of the enteric fermentation of plant material
by microbes in ruminant livestock (Gerber et al. 2013; Vijn
et al. 2020). Enhancement of livestock feeds with small
quantities of some seaweed species can reduce theenteric methane emissions from ruminant livestock(Machado et al. 2016; Li et al. 2016; Roque et al. 2019;
Kinley et al. 2020), providing a potential pathway forclimate mitigation (Duarte et al. 2017; emLab 2019). Yet,the use of seaweed feeds to reduce enteric methane
emissions has occurred at only experimental scales, andadditional research and development are needed for this19

28 times higher global warming potential than CO2

12

pathway to reach commercial scale (Vijn et al. 2020;
Wasson et al. 2022).
Including as little as 2 per cent organic matter of theseaweed Asparagopsis taxiformis in livestock feed reduced
methane production in steers (Bos indicus) in vitro by 99
per cent (Machado et al. 2016) (Appendix A, Table A2). The
2 per cent dose had minimal effect on the ruminantfermentation process, suggesting a suitable feed additive
with the potential to reduce methane emissions (Machado
et al. 2016). These results were supported in vivo for sheep,and dairy and beef cattle (Li et al. 2016; Roque et al. 2019;
Kinley et al. 2020). Asparagopsis taxiformis added to thefeed of sheep fed a high-fiber diet reduced methane
production in a dosage-dependent manner, with an in vivo
80 per cent reduction in methane production in the highest
addition treatment (3 per cent seaweed in feed), and withno change in live weight gain (Li et al. 2016) (Appendix A,
Table A2). A congeneric seaweed species, Asparagopsis
armata, added to the feed of lactating cows at a rate of 0.5
and 1 per cent reduced methane emissions in vivo by 26
and 67 per cent, respectively, with a modest decrease inmilk production in the latter treatment (Roque et al. 2019)(Appendix A, Table A2). The study did not document anincrease in bromoform (a chemical potentially hazardous
to humans) in milk and there was no decrease in the
nutrient quality of the feed. However, the authors
cautioned that mineral concentrations (e.g., iodine) in milk
from seaweed-fed cows should be monitored to ensurethat they remain within recommended daily allowances(Roque et al. 2019).
In beef cattle fed a high-grain diet, addition of Asparagopsis
taxiformis at a rate of 0.1 and 0.2 per cent reduced
methane production in vivo by 40 and 98 per cent,respectively, and increased weight gain over 90 days by 53
and 42 per cent, respectively (Kinley et al. 2020) (Appendix
A, Table A2). Kinley et al. (2020) found no negative effects
on meat quality, and bromoform was not detected in themeat. These authors concluded that seaweed addition tobeef cattle feed had potential to both help mitigate climatechange and benefit the livestock sector through increased
meat production (Kinley et al. 2020). Other seaweed
species, including Bonnemaisonia hamifera, Euptilota
formisissima, Plocamium cirrhosum, Vidalia colensoi, andEcklonia radiata also have been shown to reduce entericmethane emissions in vitro at inclusion rates of up to 10
per cent of feed organic matter, indicating the need forfollow up studies in vivo (Mihaila et al. 2022).

pathway is constrained by the number of fed livestock on
the planet (i.e., excluding free-ranging livestock), if selectseaweed species were added to the feed of all cows
around the world, global annual carbon-equivalentemissions could be reduced by 2.3 per cent (emLab 2019)(Appendix A, Table A2). This would require farming 33
million megatons of seaweed dry weight across 0.003 percent of the ocean surface (emLab 2019). Removal of 1
megaton of CO2 equivalent through addition of seaweed tolivestock feed was estimated to be less expensive thanother climate mitigation pathways of solar energy, windenergy, land reforestation, and land afforestation (emLab2019). However, the cost of seaweed farming would haveto decrease by 77 per cent to be competitive as areplacement for current feed additives (i.e., corn–the likely
feed ingredient to be replaced) (emLab 2019). Thus,
innovation to improve the cost efficiency of seaweedfarming would be needed to realize the seaweed feed
pathway for climate benefits (emLab 2019).
Research into the seaweed feed pathway indicatespotential for climate mitigation (Vijn et al. 2020; Wasson etal. 2022). However, this pathway has been tested forlimited seaweed species and under limited livestock
production systems and scales. Additional research istherefore needed to identify a variety of seaweed species
for cultivation and conduct animal trials in a variety ofcontexts to further assess the safety and efficacy of thepathway and build the regulatory, industry, and consumer
confidence needed for large-scale implementation (Vijn etal. 2020; Wasson et al. 2022) (Fig. 3). Inclusion of entericmethane emissions in national emissions credit schemes
could provide a financial route for the seaweed feed
pathway (Vincent et al. 2020). Moreover, the cost
efficiency of this pathway could be improved when wastefrom other seaweed products is used as feed (Wan et al.
2019) (e.g., in a seaweed biorefinery system wherein
additional high value products are produced) (see section
4.2.5 Market and Product Development).

A life cycle analysis of enteric methane reduction via theseaweed feed pathway found a carbon footprint ranging
from a major carbon sink to a carbon source20, indicating
a potentially potent climate mitigation strategy (emLab2019). Though the maximum climate benefit of this-36,774 to 3,160 kilograms of net CO2 equivalent per megaton
seaweed dry weight20

13

Fig. 3. A commercial scale seaweed feed pathway will require expanded research to identify additional
seaweed species for cultivation and examine the enteric methane mitigation, animal performance, andanimal safety, etc. in animal trials. Collaboration will be required across the seaweed and livestockindustries. Adapted from Vijn et al. (2020), distributed under a CC BY license.

2.2.3 Displacing Carbon-Intensive Food Production
Expansion of land-based agricultural food production to
support the growing human population is constrained byarable land and freshwater (Duarte et al. 2009; Falkenmark
et al. 2009). Moreover, land-based agriculture currently
accounts for 70 per cent of global freshwater usage and30 per cent of global GHG emissions (Vermeulen et al.
2012; Steffen et al. 2015). In combination, this information
provides a strong impetus to find alternative pathways forhuman food production that require limited or no land,
freshwater, or fertilizer, and that have a lower carbon
footprint than land-based agriculture (Duarte et al. 2022a).

production in terms of dry weight (Radulovich et al. 2015a).
Similarly, a green seaweed23 in the Arctic was found tohave a similar nutritional profile to select fruits, vegetables,
nuts, and grains, contained sufficient dietary minerals, andcontained iodine and heavy metals within safe limits(Roleda et al. 2021). In China, the largest seaweed
producer in the world, cultivation of approximately 2 million
tons of seaweed in 2015 is estimated to have saved theequivalent of 62,492 hectares of land resources (Zheng etal. 2019) (Appendix A, Table A2). Yet, the risk of heavy
metal exposure in seaweeds and their variation in dietary
value, must not be overlooked (for further discussion see
section 3.4.1 Adverse Health Effects).

Seaweeds have, in some cases, been deemed a suitablereplacement for land-based agricultural crops (Radulovich
et al. 2015a). For example, in Costa Rica, crude proteinfrom native seaweed farming21 was similar to that of grain
crops, and it was estimated that tropical seaweedfarming22 could replace up to 15 per cent of national foodSeaweeds can also be suitable as a primary feed inaquaculture24 (Bolton et al. 2009) or feed additive inaquaculture (Hua et al. 2019) and agriculture (FAO 2018),
when included at modest rates25 (Hua et al. 2019). Use of
seaweeds in animal feed can indirectly support humanfood production, while offsetting the pressure on arable
land, freshwater, fertilizer, and wild fish stocks (Wan et al.

21

0.91 tons per hectare per year
Codium, Gracilaria, Sargassum, and Ulva
23 Ulva lactuca

24

22

25

For example, abalone
For example, less than 10 per cent in commercial finfish feed142019; Cottrell et al. 2021). Moreover, seaweed addition toanimal feed can enhance animal growth rates and provide
immune system stimulant effects that reduce the need for
antibiotics (e.g., in finfish) (Hoseinifar et al. 2018; Mohan
et al. 2019; Wan et al. 2019). Optimizing these benefits will
likely require farming a broader diversity of seaweed
species, processing seaweeds to increase protein content
and reduce indigestible polysaccharides, and conducting
additional research to test for bioactives in seaweeds thatenhance animal health and production (Øverland et al.
2019; Hua et al. 2019; Wan et al. 2019). It is worth notingthat for some seaweed species, growth in nitrogenenriched (i.e., eutrophic) waters can yield increased protein
content (Gordillo et al. 2006).
Producing 1 kilogram of grain or beef for food requires
between 1000–2000 and 15,000–200,000 liters offreshwater, respectively (Forster and Radulovich 2015). Ata rate of 1 liter of freshwater per kilocalorie of food, humanfood consumption currently requires an average of 2000
liters of freshwater per person per day (Forster andRadulovich 2015). For every ton of food produced at sea, 4million liters of freshwater can be saved (Radulovich 2011)(Appendix A, Table A2). Additionally, producing grain forbiofuels competes with human food and requires 1000s ofliters of freshwater, which could be avoided by substituting
grain with seaweed as a biofuel feedstock (Forster andRadulovich 2015). Bioplastics made from corn, vegetables,or starch also require arable land, freshwater, and fertilizer,
making seaweed bioplastics a viable substitute to reserve
these resources for global food production (Mouritsen etal. 2021).
Proteins extracted from seaweeds could replace carbonintensive land-based meat, egg, soy, and milk proteins(Sadhukhan et al. 2019). For example, a life cycle analysis
indicates that production of seaweed protein yields a
climate change savings of 12 kilograms of CO2 equivalent
per kilogram of seaweed protein produced (Sadhukhan etal. 2019) (Appendix A, Table A2). Yet, when produced as asingle-stream product, seaweed protein is not able tocompete with the cost of soy protein26 (Emblemsvåg et al.
2020). Replacement of lower-cost land-based proteins
with seaweed proteins could be made economicallyfeasible through a seaweed biorefinery system with limited
or zero waste (Sadhukhan et al. 2019) (see section 4.2.5
Market and Product Development).
Fertilizers used in agriculture are a major contributor to
global emissions of nitrous oxide, a powerful GHG, due tosoil microbial processes (Gerber et al. 2013). Replacementof land-based crops with farmed seaweeds, which do notnecessarily require fertilization, could therefore reduce
nitrous oxide emissions (Duarte et al. 2017). For example,
Zheng et al. (2019) found that in Chinese coastal waters,
26

the cultivation of approximately 2 million tons of seaweed
in 2015 had the potential to save 16,554 tons of nitrogen,5,503 tons of phosphorous pentoxide, and 7,255 tons ofpotassium oxide fertilizers, as well as 1,873 tons ofpesticides (Appendix A, Table A2). Moreover, given that
inorganic fertilizers are sourced primarily from fossil
resources, replacing these fertilizers with seaweed-derived
nutrients (e.g., through a biorefinery system) could reducethe carbon footprint of land-based agriculture (Sadhukhan
et al. 2019). A life cycle analysis indicates that biorefining
seaweed into inorganics (including for fertilizer) has a
climate change savings of 1 kilogram of CO2 equivalent per
kilogram of inorganics produced (Sadhukhan et al. 2019)(Appendix A, Table A2). Use of seaweed biofertilizer also
has the potential co-benefits of biostimulant effects (e.g.,
seed germination), controlling plant pathogens, andremediating pollution in contaminated soils (Nabti et al.
2017; Ali et al. 2021).
While there is potential for seaweeds to displace carbonintensive land crop production (Cai et al. 2013; Mahadevan
2015; Mouritsen et al. 2021), as with other seaweedcommercial use pathways, additional research anddevelopment are needed, particularly towards increased
cost efficiency (see sections 4.2.3 Advanced Technology
and Infrastructure and 4.2.5 Market and ProductDevelopment). Hoegh-Guldberg et al. (2019) suggest thatwithin the current decade, policy decisions could
incentivize a shift to seaweed foods, and this may include
exploring the impact of a carbon tax on more carbonintensive foods.
Establishing inclusive policies which incorporate
participation of women and men farmers would further
guide the sustainable growth and development of thesector (Cottier-Cook et al. 2021). To help protect vulnerable
groups, seaweed farming expansion strategies can be
developed in line with an inclusive policy environment(Cottier-Cook et al. 2021). Some policy recommendations
emphasize the need to develop assessment tools thatbalance environmental and economic risks with thepotential benefits of seaweed production (Cottier-Cook et
al. 2021). This should be done while channeling support for
long-term investments to promote the beneficial aspectsof the industry such as alternative livelihoods for women incoastal communities (see section 3.3.2 Gender Equity).
2.2.4 Intentional Deep Ocean Sinking
The intentional sinking of seaweed biomass into the deep
ocean has been discussed as a potentially scalable carbon
sequestration strategy (emLab 2019; Froehlich et al. 2019;
NASEM 2021; Wu et al. 2022a). In concept, seaweedbiomass farmed in surface waters would absorb CO2 and,
when intentionally subducted into the deep ocean,

For example, a current primary finfish feed15

sequester carbon over climate relevant timescales(greater than 100 years) (emLab 2019; NASEM 2021; Troellet al. 2022; Wu et al. 2022a). Yet, the feasibility of thispathway as a climate mitigation strategy remains underinvestigation (Bach et al. 2021; Ricart et al. 2022).
A target depth for deposition of greater than 2000 meters
has been suggested to accomplish maximal sequestration
potential (NASEM 2021). Accordingly, a modelling study
showed that when delivered to greater than 3000 meters,
more than half of the seaweed carbon remained
sequestered at year 3000 (Wu et al. 2022a). In concept, therates of seaweed degradation and sedimentation on theseafloor would determine the proportion of the depositedbiomass that is sequestered as respired CO2 versus POC
in sediments (NASEM 2021). Timescales for release of
respired CO2 from the deep ocean may depend on ocean
circulation and mixing, which vary regionally and acrossocean basins (NASEM 2021).
It has been suggested that deep ocean sinking of seaweed
has limited economic viability, as there would be greater
economic gain from harvesting and processing seaweeds
into commercial products27 (emLab 2019; Chopin andTacon 2021; Ricart et al. 2022; Troell et al. 2022). Moreover,authors have raised ethical questions regarding the sinkingof seaweed biomass that could be used for food, feed, or
fuel, with the potential to address multiple UN SDGs28(Ricart et al. 2022) (Fig. 4). Troell et al. (2022) and Ricarte
et al. (2022) each conclude that rapid investment in deep
ocean sinking as a carbon sequestration strategy should
not deflect resources away from research to assess the
economic and environmental sustainability of thispathway.
A life cycle analysis of intentional deep ocean sinking
indicates the potential to sequester 787–945 kilograms of
CO2 per megaton of seaweed dry weight (emLab 2019).
Sequestering 2017 global emissions would require 57
gigatons of seaweed dry weight farmed acrossapproximately 5 per cent of the ocean surface (emLab2019). Given the size of the global carbon market, deep
ocean sinking currently could accomplish removal of at
most approximately 0.43 per cent of total global emissions(Appendix A, Table A2), and authors have concluded thatthe cost of this pathway29 would have to be significantly
reduced30 to be feasibly funded through carbon offset
markets (emLab 2019).

Fig. 4. Carbon sequestration through intentional deep ocean sinking (light blue) and alternatives that are considered optimal toaccomplish multiple UN SDGs (dark blue). Release of CO2 will occur during the seaweed cultivation and harvesting/transport phases.
Adapted from Ricart et al. (2022), distributed under a CC BY 3.0 license.

27
28

For example, food and pharmaceuticals
SDG2: zero hunger and SDG7: affordable and clean energy

2930

USD 709 per megaton of CO2
88–99 per cent16

The efficient transfer with minimal loss of seaweedbiomass to deep ocean sinks would likely require
specialized equipment (NASEM 2021). New technology
would be required to confirm the veracity of the carbon
storage (NASEM 2021; Ricart et al. 2022). As with any
seaweed commercial use pathway discussed herein, there
may be environmental risks of upscaled seaweed farming
for this pathway (see section 3.2 Potential for
Environmental Risks). Risks specific to deep ocean sinking
include acidification, deoxygenation, and eutrophication of
the deep ocean due to enhanced respiration of depositedbiomass (emLab 2019; NASEM 2021; Wu et al. 2022a).
Finally, as with any atmospheric CO 2 removal andsequestration strategy, intentional deep ocean sinking
must consider the residence time of surface waters
relative to the air-sea CO2 equilibrium timescale to ensure
an effective carbon sink (Bach et al. 2021; Hurd et al.
2022).

pathways would require robust and cost-efficient seaweed
supply and production chains and the social license tooperate. Regarding supply and production, economies ofscale and a seaweed biorefinery concept wherein multiple
high and low value seaweed-based products are coproduced, with limited or no waste, have potential toincrease cost efficiency (Marquez et al. 2015). Regarding
social license, additional research is needed to further
assess the environmental and socioeconomic benefits
and risks of each pathway.
The evidence reviewed here on the potential seaweedcommercial use pathways for climate mitigation indicatesseveral strengths and weaknesses of seaweed farming andopportunities and threats that could be planned for under
future expansion.

2.2.5 Land-Based Carbon Sequestration
While research into seaweed carbon sequestration on land
is limited, it is expected that carbon in seaweed biochar
and biofertilizer in soils will persist for months to years(Clark et al. 2021). Yet, estimates vary, and the refractorycomponent of seaweed carbon could persist in soils fortimescales of decades to centuries (Hughes et al. 2012;
Troell et al. 2022). This is in contrast to the seaweeds
consumed as food or feed that should result in the releaseof carbon in a matter of days to weeks (Clark et al. 2021).
The carbon stored in seaweed bioplastics is expected topersist for years to decades (Clark et al. 2021). Seaweedbioplastics can be used to produce materials such as films
for food packaging, edible containers, and biodegradable
drinking straws (Mouritsen et al. 2021). Seaweedbioplastics are produced from the polysaccharides inseaweeds, including carrageenan, alginate, and agar, andtend to be highly durable and heat resistant (Mouritsen etal. 2021). However, single-use bioplastics that are highlybiodegradable may have shorter lifespans. The use of
seaweeds to produce bioplastics currently occurs at
experimental and pilot scales (FAO 2020).
2.2.6 Summary of Potential Commercial Use Pathways
for Climate Mitigation
Seaweed commercial use pathways have been proposedfor carbon sequestration and GHG emissions reduction,with varying levels of progress towards implementation.
These pathways include displacing carbon-intensive
energy with seaweed biofuels, reducing livestock methane
emissions with seaweed feed, displacing land-based crops
with seaweed foods, providing ocean carbon
sequestration through intentional sinking of seaweeds,
and providing land carbon sequestration in seaweed
products (Marquez et al. 2015; Sadhukhan et al. 2019; Vijn
et al. 2020; Clark et al. 2021; NASEM 2021). Each of these

17

18

PART 3: REVIEW OF EVIDENCE FORPOTENTIAL ENVIRONMENTAL AND
SOCIOECONOMIC CO-BENEFITS AND RISKS

19© Unsplash / Oleksandr Sushko

3.1 Potential for Environmental Co-Benefits
Various environmental co-benefits may be associated with
seaweed farming. In this section, information relating tothese potential co-benefits is reviewed and contextualized,
and knowledge gaps are identified.
3.1.1

Marine Biodiversity

Climate change is driving the rapid loss of biodiversity onland and in the sea (IPCC 2019a; IPCC 2019b), and there isgreat interest to determine whether seaweed farming can
support biodiversity (Duarte et al. 2017). Yet, studies
examining the biodiversity effects of seaweed farming arerare, and results thus far are mixed (Forbes et al. 2022).
While a global meta-analysis found an overall positiveeffect of seaweed farms on tropical fishes and mobile
macroinvertebrates (Theuerkauf et al. 2022), the effects ofseaweed farms on biodiversity can be highly context
dependent (Kelly et al. 2020) and some authors haveconcluded that seaweed farms do not provide the same
biodiversity benefits as wild seaweed forests (Forbes et al.
2022). The identity of the habitat superseded by a seaweedfarm may be highly relevant. This section reviews recent
progress on understanding the biodiversity effects ofseaweed farms.
Farming of kelp in temperate waters tends to attract
biodiversity (Kerrison et al. 2015; Walls et al. 2016; Walls et
al. 2017; Wood et al. 2017; Visch et al. 2020; Hancke et al.
2021). At an experimental kelp farm over sandy bottom inthe west of Ireland, holdfasts of farmed kelp31 provided abiogenic habitat for native macroinvertebrates32, fostering
higher species diversity and a “novel” species assemblage
as compared to wild holdfasts (Walls et al. 2016)(Appendix A, Table A3). The authors suggested that kelp
harvesting approaches could be employed that retain theholdfast and associated biodiversity (Walls et al. 2016).
Also, in the west of Ireland, an 18-hectare kelp farm33
grown above a seagrass bed34 had no significant negative
effect on seagrass biomass over a two-year period ascompared to reference sites (Walls et al. 2017). However,there was lower sediment total organic matter and smaller
grain size beneath the farm likely due to consumption of
detritus by fouling organisms and physical baffling ofwater currents by the farm (Walls et al. 2017). In Northern
Ireland, benthic marine organisms35 colonized theanchoring structures of an experimental kelp farm on
sandy bottom (Wood et al. 2017). On the west coast of
Sweden, kelp farming36 had no effect on benthic mobile
Laminaria digitata
42 species total
33 Alaria esculenta and Saccharina latissima
34 Zostera marina
35 Seaweeds, tunicates, razor clams, and crabs
36 Saccharina latissima

fauna but increased the abundance and diversity of benthic
infauna as compared to sandy bottom reference sites(Visch et al. 2020). Kerrison et al. (2015) reported grazing
by snails on farmed kelp37 in the United Kingdom. In
Norway, Hancke et al. (2021) determined that while small(0.4–4 hectare) kelp farms act as artificial habitats formarine species, they support lower species abundanceand diversity than wild kelp forests. This is likely because
farmed kelp is harvested seasonally and does not reach amature kelp forest state, and when placed over sandy
bottom, lacks the extensive rocky habitat of a wild kelp
forest (Wood et al. 2017). The kelp detritus released byfarms38 may stimulate biodiversity through subsidies tobenthic food webs (Zhang et al. 2012; Wu et al. 2016; Fieler
et al. 2021), but may conversely negatively affect benthic
fauna due to biogeochemical changes to the seafloor(Zhou 2012; Hancke et al. 2021) (see section 3.2.4 Organic
Matter Over-Deposition).
In Zanzibar, United Republic of Tanzania, seaweed farming
occurs primarily in shallow water that is accessible fromshore at low tide, and 92 per cent of seaweed farms are atleast partly located on seagrass habitat (Hedberg et al.
2018). Small-scale seaweed farming is considered to havea limited impact on seagrass ecosystems, as extensive
seagrass beds remain in between farms (Eklöf et al. 2012).
However, negative effects of seaweed farms on seagrassbeds have been documented, likely due to competition forlight and nutrients, and upscaling of seaweed farming
could have greater impacts (Eklöf et al. 2005; Eklöf et al.
2006a; Eklöf et al. 2012; Lyimo et al. 2006; see section 3.2.1
Habitat Competition). For example, Eklöf et al. (2005) foundthat seagrass beds beneath seaweed farms39 had lessseagrass and wild seaweeds and finer sediment with lessorganic matter than reference sites. Similarly, amanipulative study in Zanzibar found that experimental
seaweed farm plots had lower abundance of seagrasses40
as compared to control plots (Eklöf et al. 2006a). The
effects of seaweed farming on seagrasses in Zanzibar canbe species-specific, with a study documenting a negative
effect on the abundance of Enhalus acoroides but not
Thalassia hemprichii (Eklöf et al. 2006a) and another study
documenting the reverse pattern (Lyimo et al. 2006). Given
recorded reductions in sediment organic matter atseaweed farms placed over seagrass beds (e.g., Eklöf etal. 2005), it is worth considering how seaweed farms atlarge scales could affect regional seagrass carbon
sequestration.

Saccharina latissima
Up to 49–58 per cent of productivity in some studies (Appendix A,
Table A3)
39 Eucheuma and Kappaphycus
40 40 per cent lower biomass

31

37

32

38

20

The effects of seaweed farming on benthic fauna intropical seagrass beds have generally been negative, likely
due to the loss of seagrass habitat (Eklöf et al. 2005; Eklöf
et al. 2006b; Ólafsson et al. 1995; Eklöf et al. 2012). For
example, in Zanzibar, seagrass beds beneath seaweedfarms41 had less benthic macrofauna42 than reference
seagrass sites (Eklöf et al. 2005). Experimental farm plots
in Zanzibar also had lower abundance of associatedbenthic epifauna43 as compared to control plots (Eklöf etal. 2006b). Furthermore, all major benthic meiofauna taxa
were less abundant within seaweed farms44 in Zanzibar ascompared to reference sites (Ólafsson et al. 1995). It has
also been noted that tropical seaweed farming requires
mangrove poles as part of the farm infrastructure, and thismay contribute to unsustainable mangrove harvesting(Sievanen et al. 2005; Msuya et al. 2007). However, this risk
has not been quantified and was not discussed in a recent
review on the environmental effects of tropical seaweedfarming (Kelly et al. 2020).
While tropical seaweed farms are associated withenhanced local abundance of herbivorous fishes (likely
due to provisioning of food), the abundance of carnivorous
fishes45 tends to decrease when seaweed farms
supersede seagrass beds. This is likely due to losses ofbenthic invertebrates that are dependent on seagrass
habitat (Eklöf et al. 2012). For example, in Kenya, therelative abundance of herbivorous and carnivorous fishes
increased and decreased, respectively, following
development of seaweed farms on patchy seagrass
habitat (Mirera et al. 2020). Similarly, in Zanzibar,
herbivorous fishes were more abundant at seaweed farms,
while carnivorous fishes were more abundant at seagrass
habitats (Eklöf et al. 2006b). At a national scale, Hehre andMeeuwig (2016) found a positive correlation between
landings of herbivorous reef fishes46 and seaweed
production in Indonesia, Malaysia, and the Philippines.
However, this pattern did not hold for the United Republicof Tanzania and Fiji, possibly due to their sporadic/lower
seaweed production (Hehre and Meeuwig 2016). On thesouth coast of Kenya, non-native farmed seaweed wasfound in the guts of fish at a reference site nearby theseaweed farm, indicating a pathway for farmed seaweed
into local food webs (Anyango et al. 2017).
In general, seaweed farming is associated with increasedfish abundance when farms supersede a lower complexity
habitat (e.g., sandy bottom), but lower fish abundance
when farms supersede a higher complexity habitat (e.g.,
seagrass bed or coral reef) (Hehre and Meeuwig 2015;
Kelly et al. 2020). For example, in Zanzibar, seaweed farms
Eucheuma denticulatum and Kappaphycus alvarezii
Primarily suspension-feeding bivalves
43 Sea urchins and sponges
44 Eucheuma spiniosum
45 For example, those that eat invertebrates

had similar fish abundance as nearby seagrass beds, but
fish abundance was 3–7 times higher at the farm than at a
nearby sandy bottom site (Eklöf et al. 2006b). In Costa
Rica, greater fish counts47 and species diversity48 wereobserved within a farm plot of mixed seaweed species ascompared to two sandy bottom reference areas, andinvertebrates including gastropods and crabs were seen
grazing on farmed seaweeds (Radulovich et al. 2015a)(Appendix A, Table A3). In the Philippines, a negative
association was found between fish biomass andseaweed farming at healthy coral reefs in a marine
protected area (MPA), but there was no effect of seaweedfarming on fish biomass at degraded coral reefs impacted
by blast fishing outside the MPA (Hehre and Meeuwig2015). At a seaweed farm49 in Belize, there was nodifference in fish diversity or abundance as compared toreference areas dominated by a mixed seaweed and coral
habitat (de Carvalho et al. 2015).
It is unknown to what extent enhanced abundance anddiversity of species at seaweed farms is due to additional
production supported by the farm or attraction of species
away from nearby wild habitats (Eklöf et al. 2012; Gentry et
al. 2020; van den Burg et al. 2020). Unlike mature seaweed
forests, seaweed farms provide a temporary habitat, giventhat seaweed biomass is harvested (Skjermo et al. 2014).
Seaweed farming may therefore be particularly beneficial
when cultivation coincides with the spawning or early lifehistory stages of marine species, given that availability ofseaweed habitat is particularly critical during these stages(Skjermo et al. 2014). Yet, harvesting of farmed seaweeds
during critical stages could have negative effects on
populations, and these ecological dynamics require further
examination (Forbes et al. 2022). Seaweed farms also may
harbor pests or invasive species that pose a threat tonearby ecosystems (see section 3.2.2 Spillover ofPathogens and Invasive Species). Additional research isneeded to quantify the biodiversity effects of seaweedfarming over large spatial (e.g., entire habitats andecosystems) and temporal (e.g., before, during, and after
cultivation and harvesting) scales. Given that seaweedfarming represents seaweed afforestation rather thanprotection or restoration of an existing ecosystem,
biodiversity changes should be carefully scrutinized fortheir positive versus negative effects on marine
ecosystems.

Siganids
Approximately 100 versus 5
48 14 versus 3
49 Kappaphycus alvarezii

41

46

42

47

21

3.1.2

Water Quality

Ocean acidification, deoxygenation, and coastal pollution50
are major coastal water quality issues that can havenegative knock-on effects for marine biodiversity (Beman
et al. 2005; Howarth et al. 2011; IPCC 2019b). Through theuptake of CO2 and nutrients and release of oxygen during
photosynthesis, wild seaweed forests increase ocean pH,
reduce nutrient levels, and oxygenate the water column,
respectively (Krause-Jensen et al. 2015).

Moreover, seaweeds passively absorb heavy metal ions(e.g., arsenic, cadmium, and lead), providing heavy metal
bioextraction (Zeraatkar et al. 2016). Like wild seaweed
forests, seaweed farms may have the potential to improvewater quality through the mitigation of ocean acidification,
deoxygenation, and coastal pollution (Duarte et al. 2017;
Langton et al. 2019) (Fig. 5).

Fig. 5. Potential for mitigation of ocean acidification, deoxygenation, and coastal eutrophication by seaweed farms
due to the absorption of CO2, production of oxygen, and removal of nitrogen and phosphorus, respectively, by
seaweeds. Nutrient depletion is a related environmental risk. Adapted from Langton et al. (2019) with permission.

Ocean acidification is caused by increased anthropogenic
CO2 concentrations in the ocean and leads to low aragonite
saturation states that can impact the growth, development,
and survival of calcifying organisms (IPCC 2019b). While
direct measurements of aragonite saturation states atseaweed farms are limited (Gentry et al. 2020), Xiao et al.
(2021) found generally higher aragonite saturation51,
higher pH52, and lower dissolved CO253 at seaweed farms54
as compared to reference areas in China (Appendix A,
Table A3). Jiang et al. (2020) also observed higher pH in

the vicinity of a seaweed farm55 as compared to areference area in China. Mongin et al. (2016) used anumerical model to assess the potential for seaweedfarms to buffer the impacts of ocean acidification on coral
reefs at the Great Barrier Reef through removal of
dissolved CO2. They found that an optimally located,
approximately 2 square kilometer seaweed farm56
increased aragonite saturation by 0.1 units over a 24
square kilometer area (Mongin et al. 2016) (Appendix A,
Table A3). The theoretical farm had the potential to buffer

For example, eutrophication
0–0.29-unit increase
52 0.03–0.1-unit increase
53 27.3–113.9 micro atmosphere decrease
54 Saccharina japonica, Gracilariopsis lemaneiformis, and Porphyra
haitanensis

55

50
51

Saccharina japonica, Gracilariopsis lemaneiformis, and Porphyra
haitanensis
56 With seaweed harvested at a rate of 42 grams of nitrogen per square
meter per week (equivalent of 6.6 tons of carbon per hectare per week)22

the coral reef from ocean acidification for approximately
7–21 years of projected future conditions (IPCC RCP 8.5–2.6) (Mongin et al. 2016).
The combination of land-based nutrient inputs and climatechange-induced ocean surface warming are causing
ocean deoxygenation due to the associated increase inoxygen demand by marine life and decrease in ocean
mixing, respectively, and this can compress suitablemarine habitats and cause mortality of marine organisms(IPCC 2019b). Enhanced dissolved oxygen has beenmeasured in the vicinity of seaweed farms in China as
compared to reference areas (Zheng et al. 2019; Jiang etal. 2020; Xiao et al. 2021; Gao et al. 2022). Zheng et al.
(2019) determined that seaweed farming in Chinesewaters produces a total of 1,440,612 tons of oxygen peryear in a region with high chemical oxygen demand(Appendix A, Table A3). Gao et al. (2022) found a higher
estimate of 2,533,221 tons of oxygen per year generated
by seaweed farming across China and determined that thisis sufficient to counteract deoxygenation in surface waters(Appendix A, Table A3). Xiao et al. (2021) also observed
enhanced dissolved oxygen57 at seaweed farms ascompared to reference areas in China (Appendix A, Table
A3).
Some farmed seaweeds are very efficient at removing
nitrogen and phosphorus (Kim et al. 2014; Kim et al. 2015;
Kim et al. 2017; Wu et al. 2017; Xiao et al. 2017; Zheng etal. 2019; Gao et al. 2022) and heavy metals (Zeraatkar etal. 2016) from polluted waters. Regarding the latter,
seaweeds can be used in environmental heavy metal
remediation and water treatment (Zeraatkar et al. 2016).
However, where farmed seaweeds are harvested forhuman food or food production58, heavy metals, such as
inorganic arsenic and cadmium, pose a risk to humanhealth (Besada et al. 2009) (see section 3.4.1 Adverse
Health Effects). As with all food products, monitoring for
harmful elements in farmed seaweeds is required, andcontaminated seaweeds should be considered for use innon-food related products (Duarte et al. 2022a).
Uptake by seaweeds of nitrogen and phosphorus does not
impact palatability to humans and can mitigate coastal
eutrophication by removing excess nutrients that mayotherwise lead to harmful algal blooms (Yang et al. 2015;
Duarte et al. 2017). In Long Island Sound and Bronx River
Estuary, seaweed farming59 resulted in nitrogen removal of
up to 28–180 kilograms of nitrogen per hectare (Kim et al.
2014; Kim et al. 2015). In Jiangsu Province, China, seaweed
0.02–0.35 milligram per liter increase
For example, feed or fertilizer
59 Gracilaria tikvahiae and Saccharina latissima
60 Pyropia yezoensis (formerly Porphyra yezoensis) also containing Ulva
61 Up to 59.07 tons of nitrogen per square kilometer per year
62 Up to 7.50 tons of phosphorus per square kilometer per year

farms60 removed 3,765 tons of nitrogen and 103 tons ofphosphorus in a harvest season (Wu et al. 2017).
Farming of various seaweed species across Chinaremoves an estimated annual 70,000–75,000 tons of
nitrogen61 and 8,500–9,500 tons of phosphorus62 (Xiao et
al. 2017; Zheng et al. 2019; Gao et al. 2022) (Appendix A,
Table A3). Based on industry growth trajectories, seaweedfarming has the potential to remove 100 per cent ofphosphorus inputs to Chinese waters by 2026 (Xiao et al.
2017). By contrast, complete removal of land-based
nitrogen inputs from Chinese waters would require an
estimated 17 times greater area of seaweed farming,
necessitating substantial expansion (Xiao et al. 2017). It is,however, unclear where the excess phosphorus would
come from to support the seaweed productivity required toabsorb total nitrogen inputs. Gao et al. (2022) calculated
that the nitrogen and phosphorus pollution produced byfish farms in China could be mitigated by a seaweedfarming area 2 and 3 times larger, respectively, than thecurrent scale. Farmed kelp63 currently removes the mostnitrogen and phosphorus from Chinese waters due to a
combination of total harvest yield and tissue nutrient
content (Zheng et al. 2019; Gao et al. 2022). Under an
upscaling scenario, a red seaweed64 had the greatest
nitrogen removal capacity (Gao et al. 2022).
A life cycle analysis of seaweed farming for biofertilizer
production in Denmark indicated that biofertilizer fromseaweeds grown offshore provided a sustainable circularnutrient management approach with a net decrease incoastal eutrophication (Seghetta et al. 2016). The use of
farmed seaweeds as biofertilizer could also assist in
addressing the growing global shortage of phosphorus for
fertilization of land crops, given that seaweeds are very
efficient at absorbing, and therefore recycling, excess
phosphorus released into the ocean (Bjerregaard et al.
2016).
The estimated annual nitrogen removal by seaweedfarming in coastal China65 is similar to the total global
nitrogen removal66 estimated by Kim et al. (2017) for 5
primary farmed seaweed taxa67 (Appendix A, Table 3A).
Given that China accounts for approximately 2/3 of global
seaweed farming production (FAO 2020), this global value
is likely still correct within an order of magnitude, meaningthat current upper estimates of nitrogen removal represent
a small fraction of the total global fertilizer use estimatedSaccharina japonica
Gracilariopsis lemaneiformis
65 70,000–75,000 tons of nitrogen per year
66 65,000 tons of nitrogen per year
67 Saccharina/Undaria, Pyropia/Porphyra, Gracilaria,
Kappaphycus/Eucheuma, and Sargassum

57

63

58

64

23

for the year 202268 (FAO 2019), up to 30 per cent of which
may end up in the ocean (Kim et al. 2017; González-Rivas
et al. 2020). Hence, while there may be strong local
benefits of seaweed farms for nutrient mitigation,
substantial upscaling would be required to accomplish
globally relevant remediation.
Current estimates of nutrient removal by seaweed farms
assume that all nutrients stored in seaweed biomass are
removed from the ocean during harvesting and thenutrients are not released into the ocean later in the value
chain. Yet, some of the nutrients fixed by farmed seaweeds
will be exported into the marine environment during
cultivation as seaweed detritus (Zhang et al. 2012). For
example, in Sungo Bay, China, Zhang et al. (2012) foundthat 54 per cent of nitrogen gross production fromseaweed farming69 was released through detrital export(Appendix A, Table A3) and the fate of the nitrogen remains
unexplored (Zhang et al. 2012). Data on nutrient mitigation
by tropical seaweed farms is also lacking (Kelly et al.
2020). Future research should examine the fate of farmed
seaweed nutrients in the environment throughout the value
chain and assess the effects of tropical seaweed farming
on water quality, particularly near seagrass beds and coral
reefs that are highly vulnerable to coastal eutrophication(Kelly et al. 2020). The potential for depletion of nutrients
upon which native ecosystems depend is also an
important consideration for seaweed farming (see section
3.2.1 Habitat Competition).
3.1.3 Coastal Protection
Climate change-induced sea level rise and increased storm
activity threaten vulnerable coastlines with wave
inundation and erosion (IPCC 2019b), and there is evidence
that wild seaweed forests provide coastal protection by
mitigating surface waves (Mork 1996; Gaylord et al. 2007).
It is therefore theorized that seaweed farms may similarly
dampen coastal waves, providing a climate change
adaptation benefit (Duarte et al. 2017).

combination with inshore submerged aquatic vegetation(Zhu et al. 2020).
The potential for seaweed farms to provide coastal
protection may depend on several variables, including
farming method, species farmed, timing of harvest, andlocation and orientation of the farm (Clark et al. 2021).
Thus, additional site-specific research is needed to assessthe universality of this potential co-benefit (Clark et al.
2021). Moreover, seaweed farms have the potential to alter
ocean currents, and therefore nutrient flows, presenting a
potential risk to natural ecosystem functioning (seesection 3.2.1 Habitat Competition). The risk–benefit profile
of alterations to waves and currents by seaweed farms
requires further research.
3.1.4 Summary of Potential for Environmental Co-Benefits
Seaweed farms have the potential to provide
environmental co-benefits, including altering/enhancing
biodiversity, improving water quality, and protecting
coastlines (Kim et al. 2014; Kim et al. 2015; Walls et al.
2016; Walls et al. 2017; Zhu et al. 2020). However, thesebenefits can be highly context dependent, with evidence
for some neutral or negative effects of seaweed farming
on biodiversity, and limited research on coastal protection
benefits. While the local environmental benefits ofseaweed farming can be strong (as is the case for coastal
nutrient mitigation), the global scale benefits likely remain
small at current farming scales. In areas where seaweedfarming is expected to expand, there is great need for
additional research on the environmental effects,
particularly to biodiversity and nutrient cycling.
The evidence reviewed here on the potential forenvironmental co-benefits of seaweed farming indicatesvarious inherent strengths and weaknesses of seaweedfarming as well as opportunities for the future.

While studies on coastal protection by seaweed farms arelimited (Gentry et al. 2020), a modelling study for thenortheastern United States of America showed thepotential for suspended shellfish and seaweed farms to
reduce wave energy during a storm event (Zhu et al. 2020).
The study indicated that the effectiveness of a suspended
farm in mitigating waves was less impacted by sea level
rise than that of submerged aquatic vegetation, given that
there was less bottom baffling by vegetation at higher sea
levels (Zhu et al. 2020). There was a strong benefit forwave mitigation of offshore suspended farms in

68
69

111,591,000 tons of nitrogen
Saccharina japonica

24

3.2 Potential for Environmental Risks
Various environmental risks may be associated with
seaweed farming, and these risks should be weighed when
considering the sustainability of expanded seaweedfarming. In this section, information on these potential
risks is reviewed and contextualized, and the possible
mitigation measures are discussed.
3.2.1

Habitat Competition

Seaweed farms may compete with wild habitats forresources such as sunlight and nutrients (Campbell et al.
2019). While data are generally limited on the effects ofseaweed farms on light and nutrients, depletion of theseresources is considered a medium- to high-risk impact
driver of environment change as seaweed farming
expands (Campbell et al. 2019).

Field observations of light and nutrient competition
between seaweed farms and wild ecosystems indicate
mixed results (Johnstone and Ólafsson 1995; Jiang et al.
2020; Visch et al. 2020; Hancke et al. 2021). On the west
coast of Sweden, a maximum of 40 per cent lightattenuation occurred at 5 m depth under a 2-hectare
seaweed farm70, but there were no negative effects on
dissolved nutrients or biodiversity (Visch et al. 2020)(Appendix A, Table A4). A study of open water seaweedfarming in Norway found no effects of small farms (0.4–4
hectares) on the growth of phytoplankton or the local
availability of nutrients (Hancke et al. 2021). In Zanzibar,
there were no significant reductions in bacterial
production, bacterial abundance, or primary productivity inthe water column beneath seaweed farms in a region withgreater than 100 hectares of farms (Johnstone andÓlafsson 1995). In a study in China, seaweed farming71
yielded increased phytoplankton diversity due to increased
water clarity in the vicinity of the farm (Jiang et al. 2020).
Modelling studies of ocean nutrients indicate the potential
for nutrient competition between seaweed farms and wild

70

Saccharina latissima

71

Saccharina japonica

25

ecosystems (Shi et al. 2011; Aldridge et al. 2012; Stephenset al. 2014). For example, a modelling study in China
indicated competition between farmed kelp andphytoplankton for dissolved inorganic nitrogen at a typicalfarm site (Shi et al. 2011). The seaweed farm also
decreased water flow from the open ocean72, which could
further affect nutrient supplies (Shi et al. 2011) (Appendix
A, Table A4). A modelling study of an approximately 20
square kilometer seaweed farm in Scotland showed asignificant reduction in phytoplankton productivity,
indicating the potential for nutrient competition between
farmed seaweed and phytoplankton in nutrient-limited
waters (Aldridge et al. 2012). Reduced phytoplankton
productivity could alter natural carbon flux to deep waters
and energy flow to higher trophic levels73 (Bach et al. 2021;
NASEM 2021). Modelling also indicated the potential for
kelp farms in Scotland to compete with wild kelp forests
for nutrients at a site level (Stephens et al. 2014).
Benthic organisms such as seagrasses may be more
affected than phytoplankton by light and nutrient
competition at seaweed farms, as phytoplankton drifts
through the farm and is exposed to farm conditions only
temporarily while seagrasses are continuously exposed tofarm conditions (Campbell et al. 2019). Decreased
seagrass biomass under seaweed farms in Zanzibar has
been linked to shading and nutrient competition in multiple
studies (Eklöf et al. 2005; Eklöf et al. 2006a). Yet, one study
found no difference in seagrass growth or photosynthetic
performance under farms versus at reference sites, and it
is possible that mechanical abrasion or deliberate removalof seagrasses by farmers were the cause of reduced
seagrass abundance in this case (Lyimo et al. 2006).
Removal of seagrasses to initiate seaweed farming has,for instance, been documented in Zanzibar (Trono 1992;
Eklöf et al. 2012; Kelly et al. 2020). Additional researchshould act to clarify the primary driver(s) of negative
habitat effects of seaweed farming on seagrassecosystems.
Site-specific field studies and additional modelling of
nutrient budgets and hydrodynamical-biological coupling
are needed to clarify the nutrient relationship between
seaweed farms and wild ecosystems (Campbell et al.
2019). Such information could aid in siting seaweed farms
to avoid the depletion of nutrients and to mitigate nutrientsfrom aquaculture, agriculture, or urban wastewater(Campbell et al. 2019). The effects of habitat competition
also should be quantified at levels of entire marine
populations, communities, and ecosystems. In themeantime, researchers recommend that seaweed farming
employ a precautionary approach of not placing farms overSurface currents decreased by 40 per centFor example, fisheries
74 For example, seagrasses
75 Gracilaria salicornia

habitats dominated by native primary producers74(Campbell et al. 2019; Kelly et al. 2020; Eggertsen andHalling 2021) and establishing focused ecosystemmonitoring programs, including for phytoplankton(Campbell et al. 2019).
3.2.2 Spillover of Pathogens and Invasive Species
Seaweed farming often involves the transfer of non-native
seaweed species or strains to new farming areas. With thistransfer, there is the potential for pathogens and invasive
species to spread and impact wild ecosystems, particularly
given the high rates of dispersal of biological material in
the ocean (Valero et al. 2017; Wood et al. 2017; Campbell
et al. 2019; Clark et al. 2021).
Although there are no known reports of pathogen spreadfrom seaweed farms to the environment, facilitation ofseaweed pathogens is nonetheless considered a high-risk
impact driver of environmental change as seaweedfarming expands (Campbell et al. 2019) and diseases area primary cause of crop loss at seaweed farms (seesection 4.1.2 Crop Diseases and Pests). Quarantine ofseaweed seed prior to cultivation could prevent pathogen
introduction (Kelly et al. 2020); however, the risk of disease
spread is generally considered difficult to mitigate(Loureiro et al. 2015; Campbell et al. 2019; Msuya et al.
2022) (see section 4.2.4 Global Biosecurity).
The spread of invasive species from seaweed farms to theenvironment has been documented and represents a highrisk impact driver of environmental change as seaweedfarming expands (Campbell et al. 2019). Farmed seaweed
species/strains are often selected for their tolerance tolocal conditions and fast growth, and these are traits that
increase their invasive risk (Valero et al. 2017). For
example, in Hawaii, United States of America, a non-native
seaweed75 farmed for agar production in the 1970s
escaped cultivation and now competes with local flora andfauna on coral reefs (Smith et al. 2004). In Panama, a nonnative seaweed76 was introduced for farming and now has
high coverage77 in seagrass, mangrove, and coral patch
habitats where it smothers the native flora and fauna(Sellers et al. 2015) (Appendix A, Table A4). A non-native
seaweed78 has similarly overgrown and smothered reefbuilding corals in India (Chandrasekaran et al. 2008). As aresult, farming of non-native carrageenophytes is bannedin various countries, including Cuba, Columbia, and the
Marshall Islands (Kelly et al. 2020). Non-native species
also cannot be farmed in Europe or New Zealand, unlessthe invasive species is already present in the environment(Campbell et al. 2019; Cunningham et al. 2020).
Kappaphycus alvarezii
Greater than 30 per cent78 Kappaphycus alvarezii

72

76

73

77

26

Abandonment of seaweed biomass at decommissioned
farms can increase the risk of species invasions (Campbell
et al. 2019; Kelly et al. 2020). Yet, even if non-native
seaweeds do not escape cultivation, they can still alter
local ecosystems through indirect effects on species
interactions (Stimson et al. 2001). For example, in Hawaii,
farmed non-native seaweed79 attracts herbivorous fishes
away from coral reefs, reducing the grazing pressure onother invasive seaweeds that compete with corals(Stimson et al. 2001). Moreover, native seaweed species
that escape from cultivation and colonize local wild
habitats, such as coral reefs, can also pose a problem
where nutrient loading facilitates fast seaweed growth ornative herbivores that control seaweed growth are patchily
distributed or have declined80 (Kelly et al. 2020).
Recurrent large-scale81 green tides have occurred in the
Yellow Sea since 2007, partially attributed to intensive
seaweed farming82 and the release of epiphytes83 into theocean as a byproduct during harvesting (Liu et al. 2009; Huet al. 2010; Xing et al. 2019, but see Pang et al. 2010; Duanet al. 2012). Ocean disposal of epiphytes from seaweedfarms also occurs in the tropics, but the environmental
risks there have not been assessed (Mariño et al. 2019).
The cost to remediate a 2008 green tide in China84 wasalmost twice the annual value of the seaweed farming85
industry (USD 53 million) (Hu et al. 2010). Research
indicates that green tides can be minimized throughimproved seaweed farming management practices,
including harvesting rather than discarding epiphytes (Xinget al. 2019).
In China, a non-native kelp86 escaped from cultivation andis now considered to play an important ecological role onshallow rocky reefs as a habitat former (Shan et al. 2019).
By contrast, a kelp87 native to Asia and intensively farmedin China, Japan, and Korea, is considered a major nuisanceinvasive species globally (Epstein and Smale 2017). The
primary modes of introduction are believed to have beenon ship hulls and by shellfish aquaculture, aside from asingle known introduction event through seaweed farmingin Brittany, France (Epstein and Smale 2017). Following its
colonization of habitats, the non-native kelp88 has been
considered as a possible option for seaweed farming (e.g.,
in New Zealand) (Cunningham et al. 2020). Yet, farming of
established invasive species could still be problematicwhen it undermines attempts to curb regional spread(Campbell et al. 2019).
Kappaphycus
For example, due to overfishing
81 Greater than 1000 square kilometers
82 Porphyra yezoensis
83 Ulva
84 USD 100 million
85 Porphyra yezoensis

Additional research is needed to determine the frequencyof spillover of pathogens and invasive species fromseaweed farms to the environment (Campbell et al. 2019;
Kelly et al. 2020). The risk of these events could in themeantime be reduced by limiting farming to native
seaweed species/strains, and this will require
identification of new seaweeds for commercial production(Campbell et al. 2019). Strategic siting of seaweed farms
and preventative ecosystem management practices also
could act to mitigate these risks (Kelly et al. 2020).
3.2.3 Genetic Pollution
The high rate of dispersal of material in the ocean could
facilitate gene flow from farmed to wild seaweed
populations, leading to maladaptation (Valero et al. 2017;
Wood et al. 2017; Campbell et al. 2019; Clark et al. 2021).
Moreover, farmed seaweeds that have undergone artificial
selection could escape from farms and colonize local
habitats, outcompeting wild seaweeds (Valero et al. 2017).
As a result, the release of reproductive materials fromseaweed farms is considered a high-risk impact driver ofenvironmental change as seaweed farming expands(Campbell et al. 2019).
Genetic pollution from farms to the wild is poorly studied
and limited data exist (Loureiro et al. 2015; Valero et al.
2017; Campbell et al. 2019). In China, a study of an
extensively farmed kelp89 found no marked effect in termsof gene flow of the farmed population on a historicallyintroduced wild population (Zhang et al. 2017). This was
similarly true for another kelp90 farmed in China, where awild population had nearly no genetic membership fromfarmed populations (Li et al. 2020). By contrast, kelp
farming91 in China has been negatively affected by geneflow from wild to farmed populations, as wild phenotypes
are considered less desirable for farming (Shan et al.
2018).
The potential to mitigate genetic pollution is consideredlimited (Campbell et al. 2019). Thus, various researchers
advocate for using locally sourced ecoregion-specificseaweed seed for farming (Campbell et al. 2019; Evankow
et al. 2019; Clark et al. 2021; Hancke et al. 2021). The useof sterile seaweed strains should also be technologicallyfeasible and could prevent genetic pollution if developed
for widespread use (Campbell et al. 2019). The risk ofgenetic pollution also could be reduced by differences in
Saccharina japonica
Undaria pinnatifida
88 Undaria pinnatifida
89 Saccharina japonica
90 Undaria pinnatifida
91 Undaria strains

79

86

80

87

27

the reproductive timing of wild and farmed populations
through selective breeding of farmed strains (Shan et al.
2019) or harvesting of farmed seaweeds before they reach
reproductive maturity (Clark et al. 2021). Yet, risks likely
still exist under these circumstances (Campbell et al.
2019).
3.2.4 Organic Matter Over-Deposition
Over-deposition of seaweed organic matter on the seafloor
has the potential to cause oxygen deficiency, toxic sulfide
production, and the loss of seafloor fauna (Campbell et al.
2019; Clark et al. 2021; Hancke et al. 2021). Althoughstudies of these effects are limited, over-deposition oforganic matter is considered a low-risk impact driver ofenvironmental change as seaweed farming expands given
the incentive to minimize crop biomass loss at farms(Campbell et al. 2019).
A case study in Norway indicated that greater than 90 per
cent of released kelp detritus92 ended up on the seafloor
within 4 kilometers of the seaweed farm but did not affectseafloor biodiversity (Hancke et al. 2021). However, a field
experiment in the same region showed that the release oflarge quantities of kelp detritus onto the seafloor,
simulating a predicted upper limit of export from kelp
farms93, led to a decrease in animal biodiversity likely due
to oxygen depletion and toxic sulfide production (Hancke
et al. 2021). Similarly, a study in Sandu Bay, China showed
higher sedimentary acid volatile sulfide content94 beneatha kelp farm95 versus a control area, but the study foundlittle effect of kelp farming on the benthic community, withstronger effects observed at fish farming locations (Zhou2012) (Appendix A, Table A4). Beneath intensive kelp
farms96 in Sungo Bay, China, the effect on the benthic
community was low and sediment oxygen conditions
appeared unaffected (Zhang et al. 2009). A modellingstudy in China showed a strong stimulating influence ofkelp farming on some benthic organisms (e.g., sea
cucumbers), likely explained by energy subsidies tobenthic communities in the form of kelp detritus (Wu et al.
2016). On the west coast of Sweden, no change in benthic
oxygen flux was observed beneath a 2-hectare kelp farm97(Visch et al. 2020). Buschmann et al. (2014) also found no
significant trend in organic matter over 3 years under a 21hectare kelp farm98 in Chile, and few kelp fronds wereobserved on the ocean bottom.
Dispersal of seaweed fragments away from farms bycurrents may act to dissipate the effects of organic matterover-deposition (Clark et al. 2021). Thus, differences in
Alaria esculenta and Saccharina latissima
More than 8 kilograms of fresh kelp per square meter
94 1.22 versus 0.14 milligram per gram dry weight
95 Saccharina japonica
96 Saccharina japonica

ocean currents across farming sites could lead todifferences in the effects of seaweed farms on thebenthos. There is a need for additional research to assess
the environmental effects of organic deposition as afunction of farm location and size (Wood et al. 2017;
Campbell et al. 2019). It is expected that these effects will
be mitigated in part by industry practices that limit the lossof harvestable biomass, such as strategic farm siting andharvest timing (Campbell et al. 2019). The co-culture ofseaweeds with shellfish and sea cucumbers, which
consume seaweed fragments, could also act to reduce
seaweed deposition on sediments (Cottier-Cook et al.
2016).
3.2.5 Marine Megafauna Entanglement
Seaweed farming infrastructure may risk entangling
marine megafauna99, which can lead to drownings, and thisis considered a high-risk impact driver of environmental
change as seaweed farming expands (Campbell et al.
2019). The attraction of megafauna to seaweed farms also
may increase negative human-wildlife interactions, forexample with green turtles100 that are considered a
nuisance grazer in tropical carageenophyte farming (Kelly
et al. 2020). Reports of entanglement events at seaweedfarms are rare101 but this could be due to a lack of reporting(Campbell et al. 2019). Risk of entanglement could be
mitigated through proper farm planning and management,
including siting farms away from foraging, reproduction,and migration areas and using infrastructure thatminimizes entanglement risk (Campbell et al. 2019).
Regulations should require that infrastructure is wellmaintained and fit for purpose to avoid accidental damage
or loss that increases risks to marine megafauna(Campbell et al. 2019).
3.2.6 Marine Pollution
Synthetic polymer ropes, plastic ties, and other materials
used in seaweed farming are designed to withstanddegradation in the environment, and therefore have thepotential to contribute to marine plastic pollution when
discarded or lost (Campbell et al. 2019; Hurtado et al.
2019). Anecdotal reports indicate that some tropicalseaweed farmers use empty plastic bottles as floats onseaweed lines, where lost/discarded materials may enter
marine food webs (Campbell et al. 2019; Duarte et al.
2022a). However, plastic pollution is considered a low-risk
impact driver of environmental change as seaweedfarming expands that could be mitigated throughmandatory reporting of infrastructure losses and the
Saccharina latissima
Macrocystis pyrifera
99 Such as marine mammals and turtles
100 Chelonia mydas
101 See Poonian and Lopez (2016) for an incident with a dugong

92

97

93

98

28

development of innovative material substitutes such as
seaweed polymers (Campbell et al. 2019; Duarte et al.
2022a).
Noise pollution from machinery and increased vessel
traffic is also considered a low-risk impact driver ofenvironmental change as seaweed farming expands andcan similarly be avoided by placing seaweed farms awayfrom sensitive locations, such as marine mammal colonies(Campbell et al. 2019).
3.2.7 Short-Lived Halocarbon Emissions
Short-lived halocarbons deplete ozone in the troposphereand stratosphere, increasing harmful ultraviolet (UV)
irradiance on the earth’s surface. Seaweeds commonly
farmed in the tropics emit high levels of bromocarbons, atype of halocarbon (Leedham et al. 2013). The risk level(high/medium/low) of halocarbon emissions as seaweedfarming expands was not formally assessed in a recent
review (Campbell et al. 2019).
At current farming scales in Malaysia and Southeast Asia,
short-lived halocarbon emissions are considered small(Leedham et al. 2013). For bromoform, seaweed farmingat current scales contributes only approximately 2 per centof the estimated regional wild seaweed emissions(Appendix A, Table A4). Yet, upscaled seaweed farming
could contribute a substantial proportion of regional wild
seaweed emissions (up to 20 per cent), particularly if the
primary farmed species are rhodophytes (Leedham et al.
2013) (Appendix A, Table A4). Placed in a global context, it
is estimated that wild seaweeds in the tropics contributeonly 2–9 per cent of the global short-lived halocarbon
budget, suggesting a more substantial contribution by theopen ocean, including phytoplankton (Leedham et al.
2013). Consistent with this, it has been argued that if
seaweed farming were increased to 50 times the current
area, overall short-lived halocarbon emissions would
increase by only 1 per cent and are therefore of low
concern (Duarte et al. 2022a) (Appendix A, Table A4). Thismay be a conservative estimate, as it assumes that farmed
seaweeds emit similar levels of halocarbons as wild
seaweeds, which is true only if a representative group of
taxa are farmed (Duarte et al. 2022a).

the greatest risks are likely nutrient depletion, spillover ofpathogens and invasive species, genetic pollution, andentanglement of marine megafauna (Campbell et al. 2019;
Clark et al. 2021) (Fig. 6). Aside from the spillover ofpathogens and genetic pollution, the risks of seaweedfarming could be lessened with currently available
mitigation measures (Campbell et al. 2019). Following areview of literature focused on the European Union, Woodet al. (2017) recommended against the term environmental“impact” when referring to seaweed farming given thepaucity of data for negative changes to marine populationsin that region. They instead suggest the term
environmental “effect”, meaning a change in theenvironment, until further data on population level effects
are collected (Wood et al. 2017). In accordance, HoeghGuldberg et al. (2019) state with “high confidence” that
small-scale seaweed farming has low levels ofenvironmental risks. However, environmental risks should
be carefully assessed as seaweed farming scales expand.
The extent of negative environmental effects are likely
both site and scale dependent, and additional research isneeded, particularly in data poor regions such as thetropics.
The evidence reviewed here on the potential forenvironmental risks of seaweed farming indicates various
weaknesses of seaweed farming and threats for the futurethat should be protected against as seaweed farming
expands.

Emissions of volatile gases such as iodine should also be
considered as seaweed farming expands, specifically kelp
farming. Large quantities of these may lead to theformation of cloud nuclei that affect climate and radiative
forcing (Wood et al. 2017).
3.2.8 Summary of Potential for Environmental Risks
Various environmental risks are associated with seaweedfarming, and although variation exists among farm sites,29

Fig. 6. Summary of the potential for environmental risks of seaweed farming, including nutrient and lightdepletion, spillover of pathogens and invasive species, genetic pollution, over-deposition of the seabed,marine megafauna entanglement, and marine pollution. Adapted from Clark et al. (2021) withpermission.

30

3.3 Potential for Socioeconomic Co-Benefits
Various socioeconomic co-benefits may be associatedwith seaweed farming. In this section, information on thesepotential co-benefits is reviewed and contextualized, andknowledge gaps are identified.
3.3.1 Income Generation and Diversification of Livelihoods
Seaweed farming has provided a source of income thathas diversified the livelihoods of coastal people indeveloping countries, including India, Indonesia, Kenya,
Malaysia, the Philippines, the Solomon Islands, and theUnited Republic of Tanzania (Msuya et al. 2007; Krishnan
and Kumar 2010; Msuya 2011a; Zamroni et al. 2011;
Valderrama 2012; Cai et al. 2013; Nor et al. 2017;
Steenbergen et al. 2017; Mirera et al. 2020). In Indonesia,
the seaweed farming industry supports at least 267,000
households (Langford et al. 2021), in the Philippines atleast 100,000–150,000 people (Hurtado 2013), in the
United Republic of Tanzania at least 30,000 people (Msuya
et al. 2022), and in India the industry is expected to support
up to 200,000 households in the near-future (Krishnan andKumar 2010) (Appendix A, Table A5). Overall, the global
seaweed farming industry is estimated to support 6 million
small-scale farmers and processors (Cottier-Cook et al.
2021) and creates employment opportunities across thevalue chain, including for local consolidators, small
traders, and administrators102 (Cai et al. 2013).
In coastal communities with high rates of poverty, theestablishment of seaweed farming has been shown toincrease standards of living and food security (Valderrama
2012; Cai et al. 2013). In particular, seaweed farming canprovide supplemental income or an alternative and morestable livelihood to fishing where capture fisheries areoverexploited (Smith and Pestano-Smith 1980; Padilla andLampe 1989; Crawford 2002; Msuya 2011a; Zamroni et al.
2011; Valderrama 2012; Eklöf et al. 2012; Cai et al. 2013).
Although labor-intensive, seaweed farming in developing
countries has short production cycles with a relatively fast
return on investment103 and a low barrier to entry based on
low capital costs and material requirements (Cai et al.
2013; Piconi et al. 2020; Neish 2021). This simple method
practice is known as “adaptive phyconomy” (Neish 2021).
Establishment of seaweed farming may also reducepressure on local fish stocks in areas of overexploitedfisheries (Smith and Pestano-Smith 1980). However, due
to continued fishing efforts made possible by differencesin the timing of seaweed farming versus fishing and therecruitment of women into the seaweed farming
workforce, several examples indicate that seaweed102
103

At laboratories and government offices
Within months

farming does not always lead to decreased fishingpressure (Crawford 2002; Sievanen et al. 2005; Hill et al.
2012). Seaweed farming has benefited women in coastal
developing countries by providing an employment
opportunity where income opportunities for women areotherwise limited (Sievanen et al. 2005; Msuya 2011a;
Valderrama 2012) (see section 3.3.2 Gender Equity).
Nonetheless, seaweed farming is considered a low-income
livelihood, and farmers in coastal developing regions canbe at risk of poverty or extreme poverty when the industry
is not appropriately managed (Krishnan and Kumar 2010;
Fröcklin et al. 2012; La Ode et al. 2018; Mariño et al. 2019).
Measures to mitigate such risks and protect farmers
against low income exist, and when in place, can enhance
the socioeconomic sustainability of seaweed farming (seesection 3.4.2 Low Income).
3.3.2 Gender Equity
Seaweed farming contributes to gender equity by providing
women with an accessible employment opportunity(Msuya 2011a). Employment as seaweed farmers hasreportedly given women recognition and power withinsociety and their families in various coastal developing
countries, including India, Indonesia, Kenya, and the United
Republic of Tanzania (Msuya 2011a; Zamroni et al. 2011;
Periyasamy et al. 2014; Mirera et al. 2020; Larson et al.
2021).
Establishment of seaweed farming industries dominatedby women in Zanzibar and Kenya (75.2–90 per cent
women) has generated training opportunities for women todevelop entrepreneurial and business skills and create
value-added products104 that can substantially increase
income (Msuya 2011a; Eklöf et al. 2012; Mirera et al. 2020)(Appendix A, Table A5). Shallow water seaweed farming inIndia is also dominated by women and provides anopportunity for income in what is considered a generally
safe environment (Cai et al. 2013). Membership of womenin India in so-called “self-help groups” has led to theimplementation of best practices that substantially
increase seaweed farming income (Periyasamy et al.
2014). However, as women farmers are often limited toshallow water due to a lack of training on boats orswimming skills, their crops may be at particularly high risk
from diseases and pests that are linked to warming sea
temperatures (UNEP 2022) (see sections 4.1.2 Crop
Diseases and Pests and 4.1.4 Climate Change).
A study in Indonesia indicates that the equity effects
brought about by seaweed farming have improved the life
satisfaction of both seaweed farmer and non-farmer
104

Such as seaweed soaps and lotions

31

women (Larson et al. 2021). Children also tend to haveroles on small-scale seaweed farms alongside theirmothers or family members (Cai et al. 2013). While thisdoes not appear to lead to exploitation or abuse, time spentworking on the seaweed farm may detract from thechildren’s education (Cai et al. 2013). Alternatively, children
from families that enter into seaweed farming may be
more likely to continue with their secondary and tertiary
education due to an increase in household income thatenables families to pay school fees (Steenbergen et al.
2017; Mirera et al. 2020; Larson et al. 2021).
Given the generally positive effects of small-scale
seaweed farming on women in coastal developing
countries, it is important that upscaling of seaweedfarming does not displace existing farmers (Larson et al.
2021). For example, an unmanaged offshore expansion ofseaweed farming in coastal developing regions could
exclude many women who are untrained to handle boats
or swim (UNEP 2022).
Although research is limited, there is evidence that
seaweed farming also can enhance gender equity indeveloped countries, such as the United States of America(McClenachan and Moulton 2022). In the state of Maine,
there was at least 4 times greater participation by womenin seaweed farming than in wild-caught fisheries due tofactors including flexible working hours (McClenachan andMoulton 2022). Gender equity may be enhanced as
seaweed farming replaces the declining wild-caught
fisheries that are male dominated (McClenachan andMoulton 2022).
To ensure development of an equitable seaweed farming
industry, it is important to recognize the relevance ofgender and social inclusion and address the different
needs of women and men seaweed farmers by using
gender disaggregated data in decisions (Asri et al. 2022).
This will help develop understanding of how different
farmers respond to different economic, environmental, andhealth risks (Asri et al. 2022).
3.3.3 Nutrition and Global Food Security
Seaweed farming can deliver indirect and direct benefits
for nutrition (Cai et al. 2013). In impoverished regions, theestablishment of a small-scale seaweed farming industry
can improve nutrition indirectly by providing income that
increases food security (see section 3.3.1 Income
Generation and Diversification of Livelihoods). In regions
where seaweeds are broadly consumed by humans105,
seaweeds provide high-quality food that is rich in nutrientsand contains the only non-fish source of natural omega-3
long-chain fatty acids (Radulovich et al. 2015b; Wells et al.
105

2017; FAO 2020; Mouritsen et al. 2021). Yet, while
seaweeds are rich in calcium, iodine, iron, magnesium,
phosphorus, potassium, selenium, and zinc (Forster andRadulovich 2015), they can also contain dangerous levelsof heavy metals that must be carefully monitored andregulated (see section 3.4.1 Adverse Health Effects).
Importantly, seaweeds concentrate iodine, which is an
essential micronutrient for healthy pregnancy, childhood
development, and thyroid function (WHO 2004). In 54
countries around the world, the population is considerediodine deficient (WHO 2004). Zheng et al. (2019) found that
seaweed farming in China produced a total of 4,954
kilograms of iodine per square kilometer farmed per year
(Appendix A, Table A5). Based on the dietary
recommendations for iodine, this production is sufficient
to support approximately 100 billion people for a year,
meaning that only a small amount of seaweed productionneeds to be allocated to iodine to support human health(Zheng et al. 2019). A modelling study in the Netherlands
and Portugal recorded a near doubling of iodine intake
when seaweeds replaced 10 per cent of the human diet,and this increase was considered beneficial to the
populations (Vellinga et al. 2022) (Appendix A, Table A5).
However, excessive intake of iodine from a seaweed richdiet has the potential for adverse health effects and should
therefore be monitored alongside heavy metals, andguidance and recommendations for intake are needed(Banach et al. 2020).
Future global expansion of seaweed farming has thepotential to assist in meeting the nutritional requirementsof the growing human population for complex
carbohydrates, proteins, fats, and other organic nutrients(Forster and Radulovich 2015). Seaweed farming could
become particularly relevant for global food security as
resources for land-based agriculture become increasingly
limited and global requirements for food continue toexpand (Forster and Radulovich 2015). Currently,
aquaculture is focused on growing marine animals forhuman consumption and these animals are in at least thesecond trophic level of the food chain, while seaweed is in
the first trophic level (Forster and Radulovich 2015). Given
that approximately 90 per cent of energy from primary
producers is lost with each step up the food chain (Fig. 7),
shifting from animal to seaweed-based aquaculture foodproduction has the potential to provide significant
increases in food energy (Forster and Radulovich 2015).

For example, China and Japan32

Fig. 7. Trophic levels. Illustration of the loss in energy from producers to primary consumers where 90
per cent is lost in 1 step. Credit: UNEP

The potential health benefits of consuming seaweeds
range from regulation of blood sugar and cholesterol levels
to promotion of intestinal health and gut microbiota(Jaspars and Folmer 2013; Cherry et al. 2019). Yet, basedon the mechanisms by which some of these benefits
occur106, the benefits may be disproportionately felt by wellor overfed populations rather than those that areundernourished. As a result, the bioavailability of nutrientsin seaweeds should be closely examined when evaluating
their nutritional value to various populations, and when
considering seaweeds as a food to address global foodsecurity (Forster and Radulovich 2015; Wells et al. 2017;
FAO and WHO 2022). The nutritional composition ofseaweeds varies with species, region, and season, as wellas the method of harvesting, holding, and processing, andtherefore, research is needed to assess the dietary value ofseaweeds across a range of factors (Wells et al. 2017; FAO
and WHO 2022). Moreover, consumer willingness to eat
seaweeds must be considered, particularly in western
societies that do not currently have high intake rates (seesection 4.2.5 Market and Product Development). Thus, work
remains underway to fully establish the potential forseaweeds as a global food source (FAO and WHO 2022).
3.3.4 Cultural Services
The cultural services provided by coasts can include
recreation, aesthetic value, science and education, cultural
heritage, inspiration, and natural heritage (TEEB 2010).
Seaweed farming may contribute to cultural services when
it reinvigorates coastal villages that were in decline due toa shift in fisheries from small to large scale (Hasselström
et al. 2018). An example is in Sweden, where seaweedfarming has inspired research, business ideas, andsustainability innovations (Hasselström et al. 2018).

106

Seaweed farming may also provide non-use cultural
services when the perception of farming is that it
contributes positively to the local environment andeconomy (Hasselström et al. 2018).
In coastal developing countries, small-scale seaweedfarming is reported to increase social cohesion throughcooperation among seaweed farming families (Cai et al.
2013; Hurtado 2013; Kronen 2013). Seaweed farming canalso lead to collaboration at a global scale (Hwang et al.
2020). For example, a global demand for agar from farmed
seaweeds prompted collaboration among 14 countries on
research towards the seaweed Gelidium, a primary agarproducing taxon (Hwang et al. 2020). The project involved
Belgium, Canada, Chile, China, Germany, India, Japan, theNetherlands, the Democratic People's Republic of Korea,
the Russian Federation, the Republic of Korea, Spain, theUnited Kingdom of Great Britain and Northern Ireland, andthe United States of America (Hwang et al. 2020). The
seaweed farming industry can, however, be considered a
nuisance to other coastal industries and activities (e.g.,
due to aesthetic changes to the seascape), and therefore
resistance from local stakeholders can present a major
obstacle to seaweed farm development that requires
careful consideration (see section 3.4.3 Spatial Use
Conflicts).
To ensure the broadest socioeconomic benefits ofseaweed farming, the industry should be developed in amanner that is gender-sensitive and inclusive, supporting
farmers’ diverse needs (Ramirez et al. 2020). This will
ensure that both women and men seaweed farmers haveequitable access to seaweed farming resources andmarkets (Ramirez et al. 2020). The differing benefits, risks,and vulnerabilities of women and men in seaweed farming

For example, low protein digestibility and high soluble dietary fiber33

is crucial (Asri et al. 2022). A better understanding of thebenefits and risks seaweed farmers face could help in
determining their constraints and identifying coping
strategies to increase benefits and reduce risks (Asri et al.
2022). This could further inform policies and development
programs to achieve the UN SDGs and Agenda 2030 (Asriet al. 2022).
3.3.5 Summary of the Potential for Socioeconomic CoBenefits
Most studies of the socioeconomic benefits of seaweedfarming have occurred in coastal developing countries andfocus on small-scale community-based seaweed farming.
These studies indicate generally positive outcomes forincome and the diversification of livelihoods, which lead toincreased standards of living, food security, and gender
equity (Msuya et al. 2007; Krishnan and Kumar 2010;
Msuya 2011a; Zamroni et al. 2011; Valderrama 2012; Caiet al. 2013; Periyasamy et al. 2014; Nor et al. 2017;
Steenbergen et al. 2017; Mirera et al. 2020; Larson et al.

2021). Yet, low income can remain a risk for seaweed
farmers (Eklöf et al. 2012). While seaweeds provide asource of food with the potential to contribute to global
food security, limitations to the nutritional quality of someseaweeds and the presence of contaminants must be
considered (Cai et al. 2013; FAO and WHO 2022). Seaweedfarming may also provide cultural services to humans,
including enhanced cultural heritage and social cohesion
but risks such as spatial use conflicts must be avoided (Caiet al. 2013; Hurtado 2013; Kronen 2013; Hasselström et al.
2018; Hwang et al. 2020; Clark et al. 2021).
The evidence reviewed here on the potential forsocioeconomic benefits of seaweed farming indicatesseveral strengths of seaweed farming and opportunities
that can be taken advantage of in the future. Additionally,a threat has been identified that if protected against in the
future could maximize the socioeconomic benefits ofseaweed farming expansion.

34

3.4 Potential for Socioeconomic Risks
Various socioeconomic risks may be associated with
seaweed farming, and these risks should be weighed when
considering the sustainability of expanded seaweedfarming. In this section, information on these potential
risks is reviewed and contextualized, and the possible
mitigation measures are discussed.
3.4.1

Adverse Health Effects

Human health risks have been reported in relation toseaweed farming and seaweed foods (Fröcklin et al. 2012;
Cai et al. 2013; Bruhn et al. 2016; Banach et al. 2020;
Vellinga et al. 2022). For example, in Zanzibar, various
health conditions occurred at a higher incidence in women
seaweed farmers than non-farmers, including fatigue,
respiratory problems, general eye problems due toexposure to sun and glint, parasites, and injuries fromanimal hazards in the water such as sea urchins (Fröcklin
et al. 2012). Children were also reported to show adverse
health responses when seaweeds were stored in the home,potentially due to toxic vapors from hydrogen peroxide andhalogenated compounds produced while seaweeds dry(Fröcklin et al. 2012). In general, there are limited studies
on the health effects of the seaweed farming livelihood,indicating an area of future research need (Fröcklin et al.
2012; Cai et al. 2013).
The accumulation in seaweeds of heavy metals, such as
cadmium and inorganic arsenic, presents a health risk
when seaweeds are grown in contaminated waters andshould be closely monitored and regulated for humanconsumption (Forster and Radulovich 2015; Duarte et al.
2017; Banach et al. 2020). While studies of heavy metal
concentrations in seaweeds farmed for humanconsumption are limited, a study in Spain found that
cadmium concentrations in most of the 9 seaweed species
regionally commercialized for consumption107 exceeded
the safety standards stipulated in legislation in France–
one of the few European countries with legislation specificto seaweed consumption (Besada et al. 2009) (Appendix
A, Table A6). Inorganic arsenic levels were also above thelimit for one species108 (Besada et al. 2009) (Appendix A,
Table A6). A study of seaweed food products109 imported
to the United Kingdom of Great Britain and Northern Ireland
showed high levels of inorganic arsenic in samples of hijiki
seaweed110, particularly when the preparation instructions
on the packaging were not followed, such as soaking theseaweed prior to consumption (Rose et al. 2007)(Appendix A, Table A6). A modelling study in the
Netherlands and Portugal examined the effects on foodsafety of a 10 per cent replacement of the human diet with
107
108

Likely sourced from both farming and wild harvest
Hizikia fusiforme

seaweed and found no effects on sodium intake or
cadmium, lead, and mercury exposure, but recorded anincrease in arsenic exposure that exceeded the benchmark
dose lower confidence limit (Vellinga et al. 2022)(Appendix A, Table A6). Farm location is likely an importantconsideration for heavy metal exposure, and locationappropriate safety standards should be implemented tomitigate the risk of human exposure to toxic substances(Bruhn et al. 2016; Banach et al. 2020).
Exposure to excess iodine from consumption of seaweeds
is also a concern, particularly for high-risk groups such as
pregnant and breastfeeding women (FAO and WHO 2022).
Moreover, seaweeds consumed in uncooked raw form, forexample in salads, may pose a health risk due tomicrobiological hazards (FAO and WHO 2022).
3.4.2 Low Income
As a natural resource and commodity traded on the global
market, seaweed production and prices can be volatile,affecting the economic well-being of seaweed farmers(Valderrama 2012; Valderrama et al. 2013). Accordingly,
income below the poverty and extreme poverty line has
been reported by seaweed farmers in Indonesia (La Ode etal. 2018; Mariño et al. 2019) and the United Republic of
Tanzania (Fröcklin et al. 2012).
Low income has been linked to the low value placed onseaweeds caused in part by monopolistic buyers who
reduce the power of farmers to negotiate prices (Cai et al.
2013). Low income is further exacerbated by low farm
productivity caused by crop diseases and pests and a lackof informational and material resources for farmers (Eklöf
et al. 2012; Cai et al. 2013; La Ode et al. 2018; Mariño et al.
2019; Msuya et al. 2022; UNEP 2022) (see section 4.1.2
Crop Diseases and Pests). Farmers may lack information
on appropriate site selection, cultivation techniques, andharvest/post-harvest practices (UNEP 2022). Farmers canbe heavily dependent on the farming materials provided bybuyers, which compromises the capacity of farmers tonegotiate seaweed prices (Msuya et al. 2007; Zamroni etal. 2011). Profit maximization by global companies canalso decrease farmer income indirectly by lowering pay toexporters who then pay less to farmers and provide fewer
free materials which is a tactic initially used as an incentive
for coastal communities to start farming seaweeds (Eklöf
et al. 2012).
Under poor management, small-scale seaweed farming
can have transitory socioeconomic benefits, displaying aboom-and-bust pattern (Smith and Pestano-Smith 1980;
Steenbergen et al. 2017). This occurs when rapid
109
110

Likely sourced from both farming and wild harvest
Scientific name not given

35

expansion of farming leads to seaweed overproduction
and a concomitant price drop (Smith and Pestano-Smith
1980). It can also occur when farming overwhelms the
carrying capacity of the local environment such that
production declines (Steenbergen et al. 2017). For
example, in Hingotanan, Philippines, the establishment of
a seaweed farming industry increased the number of
middle-class households in the community within a year
but the benefits to seaweed farmers collapsed when highseaweed production drove down prices (Smith andPestano-Smith 1980). Given the cultural changes that
occur following the establishment of a seaweed farming
industry, such as greater reliance on a cash economy for
food, it can be difficult for households to shift back to lessreliable, alternative livelihoods111 (Steenbergen et al.
2017).

When seaweed farming income is low, feedback
mechanisms may lock seaweed farming into a “lowincome state” (Eklöf et al. 2012) (Fig. 8). For example, lowfarmer income can drive farmers to increase seaweed
stocking densities, increasing the incidence of diseases
that decimate the crop, further driving down income (Eklöf
et al. 2012). Additionally, low farmer income candisincentivize farming as a livelihood, leading farmers to
produce less seaweed and increasing the costs to export
companies, which respond by paying farmers less for theirproduct and further disincentivizing farming (Eklöf et al.
2012) (Fig. 8).

Fig. 8. Feedback mechanisms resulting from interactions between environmental andeconomic factors that may lock seaweed farming into a “low-income state”. Reproduced
from Eklöf et al. (2012) with permission.

111

Such as capture fisheries

36Low income may be mitigated in part through productdiversification that adds value to seaweeds, or small
government loans to seaweed farmers, provided that theloan payment schedule is consistent with the production
and sale cycle of seaweeds (Msuya et al. 2007). Notably,
production of seaweeds for direct consumptioncommands greater prices than processing forcarrageenan, but currently the latter dominates theindustry in poor rural areas of developing countries(Buschmann et al. 2017). Over 80 per cent of carrageenan
produced globally is used in only 3 types of products:
processed meats, dairy, and desserts and jellies, andapproximately 50 per cent of seaweed dry matter fromsingle-stream carrageenan production is wasted (Hurtado
et al. 2019). Diversification with high value products via abiorefinery concept has the potential to increase theincome of thousands of small-scale farmers in poor rural
areas (Fernand et al. 2017) (Appendix A, Table A5). For
example, in the United Republic of Tanzania, only
approximately 1 per cent of seaweed production is used invalue-added products, including cosmetics and food, butwhere present, this activity has substantially increased theeconomic viability of seaweed farming (Msuya et al. 2022)(see section 4.2.5 Market and Product Development).
Community or government management may increase theeconomic viability of the seaweed farming industry (Smithand Pestano-Smith 1980; Nor et al. 2017; Msuya et al.
2022). For example, in Zanzibar, establishment of a
farming cooperative enhanced the sales and profits fromseaweeds and seaweed-based products for some farmers(Msuya 2011a). By contrast, in Malaysia, an attempt by thegovernment to establish a seaweed cooperative wasviewed as generally unsuccessful due to the exclusion ofindigenous seaweed farmers, a lack of acceptance of newtechnology by farmers, and a lack of participation by
farmers in decision making (Nor et al. 2017). However, theprogram had the benefits of increasing the social status offarmers and reducing operating costs (Nor et al. 2017). Fullintegration of local indigenous seaweed farmers into theconception and implementation of community andgovernment programs will be crucial for such programs tobe successful (Nor et al. 2017).
Training programs intended to transfer technicalknowledge to seaweed farmers should provide flexible
training schedules that accommodate the domestic duties
of women given their large role in the industry (UNEP2022). Programs for job skills training should specificallytarget local women’s groups to prepare women seaweed
farmers to respond to environmental change112 and build
capacity113 (Brugere et al. 2020). Priority should be placedon effectively training women in novel seaweed farming
For example, training in basic sea survival for offshore seaweedfarming
112

techniques to overcome crop losses in order to empower
women seaweed farmers and secure their livelihoods(Brugere et al. 2020).
3.4.3 Spatial Use Conflicts
Spatial use conflicts are an important socioeconomic risk
factor when considering the placement of seaweed farms(Cabral et al. 2016). Indeed, there is evidence that thepositive effects of seaweed farms on ecosystem services
tend to occur at regional scales, while negative effects
tend to occur at local scales (Hasselström et al. 2018).
This has the potential to negatively affect the well-being ofcoastal communities and drive local resistance toseaweed farming (Hasselström et al. 2018). Accordingly, a300-hectare seaweed farming project in the NormandBreton Gulf region of France was put on hold, despite
administrative approval, due to resistance from the local
community (Cabral et al. 2016). Similarly, in the UnitedStates of America, resistance to seaweed farming has
occurred due to the so-called “NIMBY” (Not in My
Backyard) phenomenon, wherein farms lack the social
license to operate (Kim et al. 2019). A study in Scotland
showed that local stakeholders are more amenable to thedevelopment of seaweed farming when it focuses on asmall-scale approach that provides local benefits, such as
job creation, rather than a large-scale multi-nationallyowned seaweed farming industry (Bjørkan and Billing2022).
Spatial use conflicts can arise due to negative effects ofseaweed farming on cultural services, such as recreation,
tourism, and the aesthetic value of the coastline, due tophysical changes or noise pollution from farms (Wood etal. 2017; Hasselström et al. 2018). For example, beach castseaweed may impact tourism due to the appearance andsmell (Wood et al. 2017), and buoys may cause
undesirable visual changes to the seascape (Hasselström
et al. 2018). To avoid conflict with local stakeholders,evidence-based decision making should be supplemented
with informed discussions with local stakeholders at an
early stage and throughout the farm development process(Cabral et al. 2016). Some negative effects can be
mitigated. For example, white or gray, rather than brightly
colored buoys, can diminish the visual footprint of a farm
(Hasselström et al. 2018). A marine spatial planning (MSP)
approach also can be used to identify the development
sites least likely to conflict with other human uses (Cabral
et al. 2016) (see section 4.2.1 Marine Spatial Planning).

113

For example, on seaweed health and farming techniques373.4.4 Summary of Potential for Socioeconomic Risks
The socioeconomic risks of seaweed farming include
adverse health effects, low income, and human use
conflicts. Some of these risks could be mitigated throughcommunity and government interventions, such as
cooperatives, training programs, and regulations that fully
integrate local stakeholders, including women seaweed
farmers (Smith and Pestano-Smith 1980; Msuya 2011a;
Forster and Radulovich 2015; Cabral et al. 2016; Nor et al.
2017; Banach et al. 2020; UNEP 2022). The health impactsof the seaweed farming livelihood remain poorly
understood and further research is needed to determine
how best to mitigate risks to workers (Fröcklin et al. 2012).

The evidence reviewed here on the potential forsocioeconomic risks of seaweed farming indicates several
weaknesses of seaweed farming and threats for the futurethat should be protected against. Additionally, several
opportunities have been identified that if taken advantage
of could increase the socioeconomic sustainability of
expanded seaweed farming.

38

PART 4: FEASIBILITY OF UPSCALINGGLOBAL SEAWEED FARMING PRODUCTION

39© Unsplash / Paul Levesley4.1 Biophysical Factors that Limit Seaweed
Farming Production
In addition to the environmental and socioeconomic risksof seaweed farming that may present barriers tosustainable farming (see sections 3.2 Potential for
Environmental Risks and 3.4 Potential for Socioeconomic
Risks), various biophysical factors can be major
determinants of the production capacity of seaweedfarming. In this section, information on these factors isreviewed to provide context for the potential limits to future
upscaling.
4.1.1

Conditions Required by Seaweed Crops

The environmental conditions required by seaweed
species for growth vary greatly (Campbell et al. 2019). As
a result, the global ocean contains many locations from thetropics to the poles that are suitable for seaweed growth(Duarte et al. 2022b) and potentially, seaweed farming(Froehlich et al. 2019).
Based on the availability of adequate nutrients andtemperature as the only constraints, it is estimated that 48
million square kilometers of the ocean could theoreticallysupport seaweed farming (Froehlich et al. 2019). This area
estimate does not consider ocean currents and waves that
can affect seaweed growth, or environmental andsocioeconomic risk factors such as the displacement ofmarine habitats and spatial use conflicts (Froehlich et al.
2019). Moreover, it is likely considered undesirable to altersuch large swaths of ocean area, as has occurred inagriculture on land (Froehlich et al. 2019). At the current
industry growth rate114, it would take approximately 2
centuries to reach a 48 million square kilometer farmed
area, an area 10-fold greater than the estimateddistribution of wild seaweed habitats (Duarte et al. 2022a).
This is also considerably larger than the estimated 220,000
square kilometers required to produce 440 megatons ofseaweeds115 and contribute significantly to global foodsecurity (Forster and Radulovich 2015).
Variability in environmental conditions can have a large
effect on farmed seaweed productivity (Bruhn et al. 2016;
Forbord et al. 2020; Largo et al. 2020). For example, a study
in Denmark showed a 10-fold difference in the biomassyield of farmed kelp116 across sites that varied in light andnutrients (Bruhn et al. 2016). A study in Norway also foundsubstantial differences in growth rates of kelp117 across
6.2 per cent per year
Adding 5 per cent to overall global food production
116 Saccharina latissima
117 Saccharina latissima
118 Ice-ice disease
119 Polysiphonia-Neosiphonia complex
120 Oomycetes
114
115

sites of varying salinity (Forbord et al. 2020). These studies
indicate the importance of careful site selection toincrease farmed seaweed production (Bruhn et al. 2016;
Forbord et al. 2020). Environmental conditions can also
moderate the occurrence of seaweed crop diseases andpests (Largo et al. 2020). For example, in Zanzibar, abacterial disease118 and epiphytes119 cause outbreaks
during the hot-dry season but almost disappear during thewet season, likely due to variations in local sea
temperatures (Largo et al. 2020) (see sections 4.1.2 Crop
Diseases and Pests and 4.1.4 Climate Change).
While more refined estimates of the ocean area available
for seaweed farming are needed, it is nonetheless a uniquetrait of seaweed farming that expansion of the industry is
not limited by arable land or freshwater, as is the case forland-based agriculture (Froehlich et al. 2019; Duarte et al.
2022a). Yet, the capacity of marine environments tosustain seaweed farms is not limitless and understanding
ecosystem carrying capacities is an area of great researchneed.
4.1.2 Crop Diseases and Pests
Diseases and pests have caused substantial losses toseaweed crops, with particularly detrimental economic
effects on seaweed farmers in poor rural areas of coastal
developing countries, including Indonesia, the Philippines,
and the United Republic of Tanzania (Msuya 2011b;
Valderrama 2012; Radulovich et al. 2015b). Pathogens ofconcern include bacteria and water molds120 (Hurtado etal. 2019; Ward et al. 2020), and problematic pests include
epiphytes, biofouling organisms, and herbivores(Radulovich et al. 2015a; Bannister et al. 2019; Hurtado etal. 2019). It is difficult to control seaweed crop diseases
and pests and illegal use of algicides and pesticides may
present a growing concern (Cottier-Cook et al. 2016) (seesection 4.2.4 Global Biosecurity).
Seaweed farm production losses of 15–30 per cent havebeen recorded in China, the Republic of Korea, and thePhilippines due to disease outbreaks (Ward et al. 2020). In
China alone, over 30 diseases have been identified across
8 farmed seaweed species121 (Ward et al. 2020). In thePhilippines, a bacterial disease122 and epiphytes123 havesignificantly impacted farming of carrageenophytes124(Largo et al. 1995a; Largo et al. 1995b; Hurtado et al. 2019).
For example, the combined effects of disease andepiphytism led to a decrease in the number of seaweed
Particularly problematic are the oomycete pathogens Pythium
porphyrae and Olpidiopsis porphyrae that cause red rot disease in theseaweed Pyropia yezoensis
122 Ice-ice disease (bacterial complex Vibrio-Aeromonas and CytophagaFlavobacterium)
123 Polysiphonia-Neosiphonia complex
124 Kappaphycus and Eucheuma
121

40

farmers at Calaguas Island, Camarines Norte, Philippinesfrom approximately 300 to less than 15 over 4 years(Hurtado et al. 2006). Across the Philippines, there was an
overall 15 per cent loss of seaweed production125 due todisease and epiphytism between 2011 and 2013 (CottierCook et al. 2016).

example, in Costa Rica, Radulovich et al. (2015a) foundthat herbivory was extreme on 3 of at least 6 seaweed
species130 in newly established seaweed farms, limiting
the usefulness of some species for farming.

Disease126 and epiphytism also impact seaweed farms inIndonesia, Malaysia, and Zanzibar (Vairappan et al. 2009;
Largo et al. 2020), with spread from the Philippines likely
caused by the transfer of infected seedlings between
farms (Hurtado et al. 2019). Epiphyte infection across all
regions alters the physical structure of the seaweedcausing secondary infections with opportunistic bacteria(Vairappan et al. 2009). To reduce the impacts on seaweedfarming and limit the spread of crop diseases and pests,
farmers may need to farm in deeper waters and adopt
strict biosecurity measures (Msuya et al. 2007; Hurtado etal. 2019) (see section 4.2.4 Global Biosecurity). Treatment
of seaweed with a biostimulant/bioeffector or nitrogen canelicit a natural defense response to epiphytes (Loureiro etal. 2017; Hurtado et al. 2019). Treatment of tropicalseaweed seed127 with Acadian Marine Plant Extract
Powder (AMPEP), produced from a temperate seaweed128,
has been shown to experimentally reduce epiphyte
infestation (Loureiro et al. 2017; Hurtado et al. 2019).

Genetic erosion, or the loss of genetic diversity, in farmed
seaweed populations can increase the risk of crop failure
due to outbreaks of diseases and pests or environmental
change (Hurtado et al. 2019). Genetic erosion has been
documented in various farmed seaweeds131 (Huh et al.
2004; Voisin et al. 2005; Niwa and Aruga 2006; Zhang et al.
2017; Hurtado et al. 2019). For example, Zhang et al. (2017)
found that the genetic diversity of non-native farmed
kelp132 populations in China was lower than wild
populations in Japan, likely due to selective breeding andgenetic drift. The authors suggest that genetically distinct
or geographically isolated populations can be used toenhance genetic diversity and increase productivity (Zhang
et al. 2017). In tropical carrageenophytes133, repeatedvegetative propagation from a limited genetic pool has
resulted in populations with desirable traits regarding sizeand coloration but very low genetic diversity (Hurtado et al.
2019; Ward et al. 2020). This has led to devastating
outbreaks of diseases and pests (Hurtado et al. 2019; Ward
et al. 2020). Limited genetic background of other farmed
seaweeds134 has also increased their susceptibility todiseases and pests (Valero et al. 2017; Hurtado et al. 2019;
Largo et al. 2020) (see section 4.1.2 Crop Diseases andPests).

Biofouling organisms reduce seaweed farm production
through light, space, and nutrient competition, or throughphysical damage to seaweeds and farm infrastructure(Bannister et al. 2019). For example, suspension feeders,
including bryozoans, ascidians, and juvenile mussels cancoat the surface of farmed kelps, increasing theirsusceptibility to breakage by ocean waves (Skjermo et al.
2014; Bruhn et al. 2016). To mitigate these effects, farmed
kelps can be harvested in the spring prior to summer
outbreaks of biofouling organisms (Skjermo et al. 2014;
Bruhn et al. 2016). Given that kelps store carbohydrates
over the summer and autumn months, early harvesting canaffect the biochemical content of the harvested kelp
(Skjermo et al. 2014). Biofouling by nuisance seaweeds
occur on seaweed farms in various locations129 and has
caused farmers to abandon seaweed farming sites or
resulted in high costs for remediation (Hu et al. 2010;
Msuya 2011b). These seaweed blooms may be linked towarming ocean temperatures and coastal pollution (Hu etal. 2010; Msuya 2011b).
Herbivory during seaweed cultivation can decimate selectseaweed crop species (Radulovich et al. 2015a). For
268,000 ton reduction of Kappaphycus
Ice-ice disease
127 Kappaphycus
128 Ascophyllum nodosum
129 For example, Cladophora in the United Republic of Tanzania and Ulva
in China

4.1.3 Crop Genetic Erosion

4.1.4 Climate Change
Climate change is driving major shifts in the distribution of
the world’s wild seaweed forests (e.g., Filbee-Dexter andWernberg 2018; Assis et al. 2022). An increase in thefrequency or severity of thermally mediated crop diseases
and pests could pose a threat to seaweed farming,
particularly in shallow warm waters (Largo et al. 2017;
Largo et al. 2020; Ward et al. 2020) (see also section 4.1.2
Crop Diseases and Pests). Women seaweed farmers may
be at the greatest risk from climate change, particularly inregions where women farmers are limited to warmer
shallow waters due to a lack of access to offshore cooler
deep waters (Asri et al. 2022).
Increases in the frequency or severity of climate driven
natural disasters, such as tropical cyclones, may also
negatively affect seaweed farming (Largo et al. 2017). For
Eucheuma isiforme, Gracilaria cervicornis, and Caulerpa racemose
Undaria, Ulva, Porphyra, Saccharina, Kappaphycus, and Eucheuma
132 Saccharina japonica
133 Kappaphycus and Eucheuma
134 For example, Gracilaria and Pyropia

125

130

126

131

41

example, in the Philippines, a strong tropical cyclone
destroyed an entire seaweed stock135 5 years after farmestablishment, requiring complete replanting (Hurtado etal. 2006). In China, ocean warming and acidification underclimate change are considered major risks to seaweedfarming, and it has been recommended that efforts are
made to preserve existing wild and domestic seaweed
genetic diversity, establish stress-resistant seaweedstrains, develop novel cultivation methods, and identify
suitable sites for seaweed farming under changing
environmental conditions (Hu et al. 2021). It is possiblethat ocean warming could increase the number ofsubtropical regions suitable for tropical seaweed farming(Largo et al. 2017), yet the area suitable for tropicalseaweed farming may correspondingly decrease. Finally,
farmed seaweed productivity in some regions could
theoretically increase under increased CO2 concentrations,which have yielded increased seaweed growth rates andnutrient assimilation in laboratory experiments136 (Zou2005).
4.1.5 Summary of Biophysical Factors that Limit Seaweed
Farming Production
The primary biophysical factors limiting the production
capacity of seaweed farming include ocean conditions
required by seaweed crops, outbreaks of diseases andpests, genetic erosion of farmed seaweeds, and climate

135

Kappaphycus alvarezii

change. While large swaths of the ocean could in theory
support seaweed farming (Froehlich et al. 2019), carefulsite selection is important to minimize environmental risks
and maximize seaweed productivity (Bruhn et al. 2016;
Froehlich et al. 2019; Forbord et al. 2020). Outbreaks ofcrop diseases and pests have substantially impacted
seaweed farm production, particularly in coastal
developing countries (Msuya 2011b; Valderrama 2012;
Radulovich et al. 2015b; Bannister et al. 2019). Genetic
erosion has occurred in various farmed seaweeds, yielding
decreased genetic diversity, increased susceptibility tooutbreaks of diseases and pests, and decreased
productivity (Hurtado et al. 2019; Ward et al. 2020). Finally,
climate change poses a risk to seaweed farming due toreductions in native seaweed populations as genetic
sources, alteration of environmental conditions at farming
sites, and intensifying outbreaks of diseases and pests andnatural disasters, such as tropical cyclones (Largo et al.
2017; Hu et al. 2021).
The biophysical factors limiting seaweed farming
production reviewed here indicate a strength and several
weaknesses of seaweed farming and threats that should
be protected against for future sustainable expansion.

136

For example, in Hizikia fusiforme

42

4.2

Potential Approaches for Upscaled
Production

Various approaches are discussed in the literature as
having the potential to overcome the broad range ofenvironmental, social, and economic factors that limit
global seaweed farming production. In this section,
information on these approaches is reviewed andcontextualized, and knowledge gaps are identified.
4.2.1

Marine Spatial Planning

An MSP approach could be useful in determining the best
locations for seaweed farms (Cabral et al. 2016). These
locations may maximize production while minimizing
human use conflicts and negative effects on ecosystems(Cabral et al. 2016). Available quantitative biophysical (e.g.,salinity, nutrients, and temperature), ecological (e.g.,
eutrophication and species richness), and socioeconomic(e.g., cultural activities and other human uses) indicators
can be combined to identify appropriate locations (Fig. 9).
In addition, methods such as the Habitat Equivalency Anal-ysis (HEA) can be used to assess the cost required tocompensate for the ecosystem services impacted byseaweed farm development (Cabral et al. 2016). Thismethod uses expert knowledge to assess the presence of
ecosystem services within the farm area and calculate thecost of damage137 (Cabral et al. 2016). Direct engagementwith local stakeholders will remain important, as thevarious ecological and socioeconomic indicators may
have different value to local people and should beweighted accordingly (Cabral et al. 2016).
Stakeholder-based MSP could also yield synergies acrossmarine sectors through ocean multi-use (Msuya et al.
2007; Cottier-Cook et al. 2016; Gimpel et al. 2016;
Eggertsen and Halling 2021; NASEM 2021). Ocean multiuse could include the expansion of seaweed farming intowind farms (Gimpel et al. 2015; Buck et al. 2018), MPAs (LeGouvello et al. 2017), and Integrated Multi-Trophic
Aquaculture (IMTA) systems (Troell et al. 2009; Buck et al.
2018). Developing and piloting offshore multi-use areas forseaweed farming is considered by Hoegh-Guldberg et al.
(2019) as a short-term high priority by 2025 for oceanbased climate mitigation.

Fig. 9. MSP approach to identifying appropriate seaweed farm locations based on thebiophysical limitations to farming (such as salinity, nutrients, and temperature), thepotential for a farm to provide ecosystem services to humans (such as nutrient
remediation and biodiversity), and socioeconomic factors that may determine tolerance
to farm development (such as existing human uses). Based on Cabral et al. (2016).

Given the strong competition for inshore space, offshore
expansion may be necessary for upscaled seaweedfarming to meet a significant portion of global resource
needs (Forster and Radulovich 2015; Stévant et al. 2017;
Buck et al. 2018). Potential challenges to offshore seawe-

ed farming include the need for robust infrastructure in
deep water with strong waves and currents, and the highcost of operating offshore (Troell et al. 2009; Eklöf et al.
2012; Forster and Radulovich 2015). It may therefore be
strategic to leverage wind farm infrastructure andresources such as labor and equipment (Buck et al. 2018).

For example, USD 24,700–123,548 per hectare for maerl beds inNormand-Breton Gulf, France
137

43

Biophysical criteria should be evaluated offshore, including
temperature, salinity, nutrient, and flow requirements forseaweeds (Gimpel et al. 2015; Azevedo et al. 2019). For
example, a modelling study along the Norwegian coast
indicates strong potential for kelp farming in offshore
areas with high and consistent nutrient supplies (Hancke
et al. 2021). High offshore currents could enhanceseaweed productivity through increased nutrient cycling
and by limiting outbreaks of diseases and pests (Eklöf etal. 2012; Peteiro and Freire 2013; Kerrison et al. 2015;
Bannister et al. 2019).
Where governments wish to facilitate seaweed farming
expansion offshore, full integration of stakeholders,
including indigenous farmers, will be necessary (Nor et al.
2017). For example, an attempt in Malaysia to transition
seaweed farming to deeper water was met with resistancefrom women farmers who lacked seafaring skills (Nor etal. 2017). Research will be needed to identify and mitigate
spatial use conflicts and environmental effects and to
identify cultivation methods and seaweed species best
suited for offshore farming (Roesijadi et al. 2008).
Co-location of seaweed farms within MPAs has beensuggested as an approach to improve the use of coastal
space while enhancing biodiversity (Le Gouvello et al.
2017). However, few studies exist on the effects ofseaweed farms on biodiversity within MPAs, and negative
effects of seaweed farms have been documented for coral
reef fishes in MPAs in the Philippines (Hehre and Meeuwig2015) (see section 3.2.1 Habitat Competition). In countries
such as Germany, expansion of aquaculture is limited byregulations related in part to MPAs, which may makeoffshore seaweed farming a more feasible option (Fernand
et al. 2017).
Nutrient limitation on seaweed growth may be addressed
through cultivation of seaweeds as part of an IMTA system(Neori et al. 2004). IMTA is an aquaculture system wherein
extractive species, such as seaweeds, are grown alongside
fed species, such as finfish (Troell et al. 1997; Neori et al.
2004; Kim et al. 2017; Buschmann et al. 2017). IMTA may
decrease pollution from aquaculture, as most of the feed
provided to fed species becomes waste and some of thiswaste could be converted by seaweeds into biomass(Troell et al. 1997; Neori et al. 2004; Sanderson et al. 2012;Buschmann et al. 2017). For example, farming of
seaweed138 in open water in the state of Maine initially
failed due to insufficient nutrients for seaweed growth, butlater succeeded following co-location near an Atlantic
salmon farm139 (Kim et al. 2017). Nutrient limitation onseaweed productivity has also been observed in Denmark,Pyropia yezoensis
Salmo salar
140 Palmaria palmata and Saccharina latissima
141 Gracilaria chilensis

even in eutrophic waters (Bruhn et al. 2016). In Scotland,
farming of seaweeds140 near a salmon farm increased
seaweed growth by 48–61 per cent and seaweed biomassyields were 27–63 per cent higher (Sanderson et al. 2012).
Similarly, in Chile, seaweed141 farmed alongside salmon
had a 40 per cent greater growth rate (Troell et al. 1997).
IMTA may present an environmentally sustainable method
to manage pollution from fed aquaculture and avoid theuse of fertilizers on seaweed farms. Fertilizers could
contribute to eutrophication and are currently not
permitted by law for seaweed farming in some regions142(Campbell et al. 2019).
IMTA is already practiced on a commercial scale in China
and the Republic of Korea, resulting in the efficient
production of large quantities of food, with the co-benefit
of improving coastal water quality (Cottier-Cook et al.
2016). Although there are experimental-scale offshore
IMTA systems under evaluation, only a single commercialscale system exists143 (Buck et al. 2018). While IMTA
provides a potentially promising approach for seaweedfarming expansion, potential differences in the timing ofpeak production of wastes from fed aquaculture and peakgrowth of seaweeds must be considered, particularly whenbackground nutrient levels are limiting for seaweed growth(Park et al. 2018).
4.2.2 Polyculture, Crop Rotation, and Selective Breeding
Innovations in seaweed cultivation could enhance farm
production and may include seaweed polyculture, crop
rotation, and selective breeding (Jiang et al. 2013; Park andHwang 2014; Roleda and Hurd 2019).
Polyculture is the synchronous cultivation of multiple crop
species to maximize resource use and farm production(Roleda and Hurd 2019). Roleda and Hurd (2019) reviewed
the nutrient physiology of seaweeds and argued thatpolyculture of seaweeds that have differing nitrogen
physiologies144 could increase seaweed farming
production through niche partitioning that optimizes
nitrogen use and limits competition. Further studies areneeded to identify candidate species for seaweed
polyculture, and seaweeds145 with high nitrogen demand
may be more appropriately farmed as monocultures(Roleda and Hurd 2019).
Seaweed farm production could be facilitated by seasonal
crop rotation (Jiang et al. 2013). Some seaweed species
undergo sexual reproduction that is seasonal and havestrict seasonal environmental conditions required for
growth, both of which constrain the period over which
For example, Europe
In the Yellow Sea
144 Preference for ammonium versus nitrate
145 For example, Porphyra and Ulva

138

142

139

143

44

seaweed farming can occur (Charrier et al. 2017). To
optimize farm yield, farmers can rotate seaweed species
with different life histories and environmental tolerances
throughout the year. For example, in Lidao town, China, tooptimize yield, kelp146 is farmed from November to May,
while a red seaweed147 is farmed from June to October(Jiang et al. 2013). Biotechnological innovations thattarget the reproductive cycles of seaweeds could also
remove constraints on seasonal cultivation to increase
annual production (Charrier et al. 2017).
A primary goal of the seaweed farming industry is toestablish strains with combinations of fast growth, highyield, consistency and stability of chemical composition,and disease and pest resistance, which may be
accomplished through selective breeding (FAO 2018;
Wang et al. 2018). For example, selective breeding has led
to improvements in disease tolerance in various farmed
seaweed species (Park and Hwang 2014; Cottier-Cook et
al. 2016; Hwang et al. 2019). Two strains of a redseaweed148 have been developed in the Asia-Pacific region
that are partially resistant to red rot disease149 (Park andHwang 2014). Disease resistant strains of kelp150 havealso been developed in China, prompted by the diseaseinduced reduction in yield (Cottier-Cook et al. 2016). Chenet al. (2015a) developed a protocol for transferring foreign
genes into seaweeds151 as a step towards transgenic
disease resistance.
Selective breeding for seaweed productivity, and more
recently quality, is commonly practiced in the Republic ofKorea, Japan, and China (Hwang et al. 2019) but isgenerally absent in tropical developing countries, which
may in part explain a recent decline in seaweed productionin those countries (Hurtado et al. 2019; FAO 2020). In
regions of low surface nutrients, seaweed strains could intheory be developed that grow well under low nutrient
conditions (NASEM 2021). Selective breeding is, however,limited in countries such as Norway where regulations
prevent the use of hybridized or bred strains (Skjermo et al.
2014).
In general, to accomplish polyculture, crop rotation, andselective breeding, additional fundamental research will beneeded on the biology of seaweeds, including nutrient
physiologies, life cycles, environmental tolerances, growthstrategies, and disease resistance (Charrier et al. 2017;
Fernand et al. 2017; Roleda and Hurd 2019). These
innovations should occur with careful consideration for the
potential environmental risks, including genetic pollution
from farms to wild ecosystems (Campbell et al. 2019) (seesection 3.2.3 Genetic Pollution).
Saccharina japonica
Gracilaria lemaneiformis
148 Pyropia yezoensis
149 Pythium porphyrae

4.2.3 Advanced Technology and Infrastructure
Seaweed farming is centuries behind land-based
agriculture, and extensive investment in technology andinfrastructure is likely needed for upscaling (Forster andRadulovich 2015; NASEM 2021). To achieve economies ofscale and support large production volumes of seaweed,
efficient farm engineering and operation152 and robust
farm infrastructure are likely required (Campbell et al.
2019; NASEM 2021). Where surface nutrients are limited,farms could be engineered to move seaweeds into deeper
waters at night for nutrient absorption and shallower
waters during the day for light absorption (NASEM 2021).
Alternatively, technology could be developed to artificially
upwell deep nutrient rich seawater to the surface (NASEM
2021). This in principle could substantially increase
seaweed production (Wu et al. 2022a) but could also bringCO2 enriched seawater in contact with the atmosphere,
compromising the climate (NASEM 2021). Sometechnology and infrastructure needs could be met throughocean multi-use (see section 4.2.1 Marine Spatial
Planning).
Given the low cost of labor in countries where the seaweedfarming industry has matured, for instance in China andother parts of Asia, emphasis on technology andinfrastructure to mechanize farming has been low(Campbell et al. 2019). For example, transporting wet
seaweed biomass to drying sites in countries such as theUnited Republic of Tanzania occurs by human power andhas been identified as a major challenge for seaweed
farmers (Msuya 2013). Air drying of seaweeds has been
noted as particularly problematic during the wet season in
the tropics and can decrease the quality and market valueof seaweeds (Ali et al. 2017; Largo et al. 2020). Given that
seaweed farming is currently a seasonal activity, there is aneed for methods to preserve biomass following
harvesting to provide a continous annual supply forcommerical applications such as food, feed, and fuel(Skjermo et al. 2014). Vacuum packaged blanching prior tofreezing has been found to maintain the quality of kelp for
consumption for up to at least 6 months (AkomeaFrempong et al. 2022).
There is a geographical disconnect between rapid
seaweed farming scientific advancement in developed
countries and rapid seaweed farming industry expansion
in developing countries (Mazarrasa et al. 2013; Mazarrasa
et al. 2014). The countries with fast expansion but low
rates of scientific advancement are experiencing
concomitant declines in production (Mazarrasa et al. 2014;

Saccharina japonica
Pyropia
152 Including mechanization of seeding and harvesting

146

150

147

151

45

FAO 2020). This indicates the need for global scientificcollaboration to grow the industry.
Furthermore, women need more access to new knowledge
in seaweed production, including the use of digitaltechnologies (Ramirez et al. 2020). Enhanced gender
equity and social inclusion that includes the fullparticipation of women in seaweed farming development
could act to strengthen traceability for seaweed quality
and improve production sustainability (Ramirez et al.
2020).
4.2.4 Global Biosecurity
Biosecurity planning can be used to prepare for outbreaksof crop diseases and pests and environmental changes
that can be devastating to seaweed farms, and to protect
wild ecosystems from pathogens and invasive species(Cottier-Cook et al. 2016; Campbell et al. 2019; CottierCook et al. 2021).
Examples of biosecurity measures to protect seaweedfarms include training in quarantine procedures and farmmanagement practices to prevent the introduction andspread of diseases and pests, development andimplementation of diagnostic tools for early detection, andbreeding to maintain genetic diversity and resistance todiseases and pests and environmental change (CottierCook et al. 2016; Campbell et al. 2019; Ward et al. 2020).
Biosecurity planning can also include capacity building forthe control of outbreaks of disease and pests when theyoccur (Cottier-Cook et al. 2016). This may include accessto government-funded regional facilities for quarantine(Kambey et al. 2020).
National and international seed banks can be used topreserve and maintain the genetic diversity of farmed andwild seaweeds (Buschmann et al. 2017; Campbell et al.
2019; Barbier et al. 2020; Wade et al. 2020). Moreover,
government insurance schemes can provide farmers withthe opportunity to rebuild or treat farms following
outbreaks of disease and pests or natural disasters, rather
than abandoning the farm which can lead to environmental
problems (Cottier-Cook et al. 2016). Insurance schemes
can also encourage seaweed farming as a livelihood byreducing the investment risk for farmers (Cottier-Cook et
al. 2016).
Currently, European legislation requires biosecurity
measures for seaweed farming that prevent the spread of
pathogens and invasive species (Campbell et al. 2019). In
Madagascar, farmers receive biosecurity training from theprivate sector (Msuya et al. 2022). However, in somecountries, biosecurity measures are generally lacking(Kambey et al. 2020; Rusekwa et al. 2020) and this has
been linked to limits in legislative recognition, up-to-datescientific evidence, and recognition of the biosecurity
hazards related to seaweed farming (Kambey et al. 2020;
Rusekwa et al. 2020).
Biosecurity measures specific to farming establishedinvasive species include farming only in regions of heavy
infestation, careful containment of stock during landtransportation, decontamination of land-based water
intakes and outflows, standardized monitoring of
associated non-native diseases and pests for early
detection, and continuous monitoring for negative effectsof farming on the environment (Cunningham et al. 2020).
While global safeguards exist for terrestrial agricultural
crops and animal aquacultural species, global
coordination for the protection of seaweed crops is yet tooccur (Cottier-Cook et al. 2021). For global seaweedfarming biosecurity implementation, future research needs
include continued development of diagnostic tests fordiseases and pests of concern, capacity building tomanage outbreaks when they occur, identification of local
seaweed species/strains for cultivation, and developing
breeding and cultivation technologies for those species(Cottier-Cook et al. 2016). While some biosecurity
protocols have been developed for regional seaweedfarming, broader implementation of those protocols
through training, reporting systems, and incentives isneeded (Cottier-Cook et al. 2021).
4.2.5 Market and Product Development
The development of new markets for seaweeds anddiversification of seaweed products could drive theexpansion of the seaweed farming industry (Forster andRadulovich 2015). Market and product development may
include the expansion of seaweeds in diets around theworld and the development of novel high value products
and biorefinery processes for value addition (Palmieri andForleo 2020; Duarte et al. 2022a; Msuya et al. 2022).
Seaweeds have been consumed for centuries in China,
Japan, and the Republic of Korea, and popularity is growing
in western diets following an increased interest in plantbased diets and environmentally sustainable food (Forster
and Radulovich 2015; FAO 2018; FAO 2020; Palmieri andForleo 2020; Vincent et al. 2020). Yet, many countries withcoastlines potentially suitable for seaweed farming lack astrong market for seaweed products (Barbier et al. 2020;
Lähteenmäki-Uutela et al. 2021). In these areas, a complete
list of seaweed species that are authorized as food could
be useful to grow public confidence in seaweed
consumption (Barbier et al. 2020). Including seaweeds aspart of recommended daily dietary guidelines could also
increase consumer acceptance of seaweeds as a foodsource (Palmieri and Forleo 2020). Broad regulatory
guidelines for allowable levels of heavy metals in46

seaweeds are needed but currently exist for only a small
subset of countries153 (Hayes 2015). Product safety andconsumer protection must be a priority, and legislation on
limits of contaminants in seaweeds is needed for publicconfidence (Barbier et al. 2020; Lähteenmäki-Uutela et al.
2021; FAO and WHO 2022). In terms of the various
purported health benefits of seaweeds (see section 3.3.3
Nutrition and Global Food Security), research is needed toestablish the intake amounts and frequencies required torealize those benefits for various products (Barbier et al.
2020; Lähteenmäki-Uutela et al. 2021).
Though research on consumer behavior regarding
seaweeds is generally sparse, a case study in Australiaindicated that the odds of eating seaweeds wereassociated with familiarity with seaweed products, having
a university degree, and health-conscious eating behaviors(Birch et al. 2019). Neophobia, or the fear of new things,
can contribute to a lack of interest in consuming seaweeds(Losada-Lopez et al. 2021). Other considerations among
consumers are the way in which seaweed food products
are packaged and promoted, including their environmental
sustainability (Young et al. 2022).
Forster and Radulovich (2015) cite 3 potential pathways
for market development of seaweed as food: 1) wealthy
consumers in developed countries, recognizing the healthand environmental benefits of seaweeds, pay a premiumfor seaweed-based foods, supporting the seaweed foodindustry despite its early stage of development; 2)seaweeds are farmed and used directly for food indeveloping countries where seaweed consumption iscurrently limited or absent; and 3) global seaweed farming
production is upscaled to the extent that costs are reduced
and seaweeds are globally processed into low costingredients. According to the authors, the first scenario
may hold the most promise to rapidly accelerate global
seaweed production and processing in the short-term
given that capital and infrastructure are already in place indeveloped countries (Forster and Radulovich 2015).
Establishment of domestic processing infrastructure,
distribution networks, and consumer demand would beneeded for this pathway (Piconi et al. 2020). Yet, thepotential to increase food security in developing countries
through local seaweed farming and consumption (second
scenario) should not be overlooked (Forster andRadulovich 2015). Finally, substantial research anddevelopment would be required for economies of scale tobe realized in the third scenario (Forster and Radulovich2015).
Diversification of seaweed products also could act to growthe seaweed farming industry (Piconi et al. 2020). This
would likely require a societal shift from a simple linear
153

one-species one-process product model to a severalspecies several-processes biorefinery model within acircular bioeconomy, where wastes and by-products are
utilized as commercial products (Chopin and Tacon 2021;
Duarte et al. 2022a). For example, the value chain for
carrageenan production is typically single stream, andcarrageenan competes on an existing global hydrocolloid
market rather than opening new market space withdiversified products (Neish and Suryanarayan 2017). The
waste products of seaweed hydrocolloid production arehigh in protein, fat, and minerals that could be extracted forfood or feed (Forster and Radulovich 2015). The chemical
diversity of seaweeds indicates opportunities for the
production of multiple high value products withcollaboration across broad economic sectors (Hafting etal. 2015; Chopin and Tacon 2021). Bioactives could be
used in functional foods, cosmeceuticals, nutraceuticals,
and pharmaceuticals but this would require greater
standardization of seaweed quality than is currently
required by the industries driving seaweed farming(Hafting et al. 2015). A biorefinery system could include theproduction of biofuels through multiple processes (Wei etal. 2013; Chen et al. 2015b; Marquez et al. 2015). Residual
seaweed biochar can be used as soil conditioner for a zerowaste production process (Sadhukhan et al. 2019). A study
in the state of Maine indicated a net revenue of USD 0.72
per cubic meter wastewater treated when seaweedfarming (for human consumption) replaced a water
resource recovery facility upgrade (Wu et al. 2022b).
To accomplish ocean-based climate mitigation, theestablishment of biorefining techniques to sequentiallyextract seaweed products is considered a high priority by
2023 (Hoegh-Guldberg et al. 2019). Seaweed biorefinery
systems may be more cost effective than terrestrial
lignocellulosic biorefinery systems (Sadhukhan et al. 2019)
but research and development towards seaweedbiorefineries are in infancy (Kostas et al. 2021).
Government support for the development of domestic
processing plants in regions with high seaweed productionand export could enhance opportunities for seaweed
products (FAO 2018; Msuya et al. 2022).
4.2.6 Economic Incentives and Regulatory Support
Expansion of the seaweed farming industry may befacilitated by financial incentives or tax deductions forfarmers that consider the environmental benefits ofseaweed farming, such as nutrient and carbon removal(Duarte et al. 2022a). If the ecosystem services contributedby seaweed farming were monetized, this could increase
the value of the seaweed aquaculture sector (Chopin andTacon 2021). Assuming a nitrogen, phosphorus, andcarbon content of seaweeds of 0.35 per cent, 0.04 per cent,For example, Australia and France47

and 3 per cent, respectively, the value from bioremediationof wastewater is USD 10–30 per kilogram of nitrogen and
USD 4 per kilogram of phosphorus, and the carbon tax
value is USD 30 per ton of carbon (Chopin and Tacon2021). This yields an additional global value from seaweedfarming of between USD 1.214 and 3.482 billion, which
represents 26 per cent of the current global seaweedfarming industry154 (Chopin and Tacon 2021). The
establishment of nutrient trading credits (NTCs) could leadto a fairer price for seaweeds that incentivizes production(Chopin and Tacon 2021). The biodiversity benefits ofseaweed farming, if appropriately quantified (see section
3.1.1 Marine Biodiversity), could also be incorporated into
markets for biodiversity and nature credits, which are
currently emerging for blue carbon ecosystems(Macreadie et al. 2022).
Voluntary carbon markets could provide economic
incentives for seaweed farming in lieu of domestic
regulated seaweed carbon trading markets155 (Clark et al.
2021). A benefit of seaweed farming for carbon creditingis that there should be little debate over who owns the
carbon since the carbon can be harvested directly from afarm lease. This is unlike wild blue carbon ecosystems,
where the right to transact carbon credits can be contested
when the habitat traverses private and public coastal lands
or extends beyond the EEZ, or if the carbon sink occurs at
distance from the source of production, as in the case ofexported coastal detritus (Macreadie et al. 2022). A
necessary step for successful development of seaweedfarming carbon credits will be to accurately account for
natural carbon sequestration by farms, which remains anarea of ongoing research (see section 2.1 Potential for
Natural Carbon Sequestration).
As markets for seaweed products grow, seaweed farming
may expand as a direct response to overexploitation ofwild seaweed populations and regulatory restrictions on
their harvest (Duarte et al. 2007; Buschmann et al. 2014).
This has been observed for carrageenophyte farming in thetropics, kelp farming in Norway, and red seaweed farmingin Chile (Valderrama et al. 2013; Buschmann et al. 2017).
For example, carrageenan was sourced from wild seaweed
harvest in temperate and tropical waters until demand
outpaced supply from wild stocks, at which point tropicalfarming met the demand (Valderrama et al. 2013).
In developed countries, regulatory pathways to establish aseaweed farming industry are often lacking, limiting the
expansion of the industry (Wood et al. 2017; Barbier et al.
2020). For example, throughout Europe, there is no
regulatory process specific to seaweed farming, as marine
leasing and licensing are directed towards shellfish and154

USD 13.3 billion

finfish farming (Wood et al. 2017; Barbier et al. 2020).
There is a need to update current marine activities
guidance to consider seaweed farming, taking into
consideration risks for farmers and regulators (Wood et al.
2017; Barbier et al. 2020). Of particular interest is todetermine the circumstances under which anenvironmental impact assessment (EIA) is required, as thiscan be a costly process (Wood et al. 2017). Exemption ofseaweed farms from the marine licensing process in the
United Kingdom of Great Britain and Northern Ireland, as is
the case for shellfish farming, has been proposed (Woodet al. 2017). Given the limited evidence available fornegative environmental effects of seaweed farming in the
United Kingdom of Great Britain and Northern Ireland,
Wood et al. (2017) suggest a “survey, deploy and monitor”
approach to assess environmental effects while avoiding
an expensive regulatory process, as is used for offshore
wind development.
4.2.7 Summary of Potential Approaches for Upscaled
Production
Various approaches have been proposed to overcome
limitations on seaweed farming production. An MSP
approach has the potential to avoid spatial use conflicts
and lead to synergies across marine sectors (Msuya et al.
2007; Cottier-Cook et al. 2016; Gimpel et al. 2016).
Innovations in polyculture, crop rotation, and selective
breeding have the potential to increase seaweed farming
production (Jiang et al. 2013; Park and Hwang 2014;
Roleda and Hurd 2019). Technological and infrastructural
breakthroughs could increase the efficiency of seaweedfarming (NASEM 2021). Global biosecurity could preventand mitigate hazards to seaweed farming and theenvironment (Cottier-Cook et al. 2016; Campbell et al.
2019; Ward et al. 2020; Cottier-Cook et al. 2021).
Development of new markets for seaweeds and diversified
products could increase seaweed demand and value(Forster and Radulovich 2015; Palmieri and Forleo 2020;
Msuya et al. 2022). Finally, economic incentives andregulations, such as nutrient trading schemes andstreamlined marine licensing processes, have the potential
to support the seaweed farming industry (Chopin andTacon 2021; Duarte et al. 2022a).
The approaches to upscale seaweed farming production
reviewed here indicate several opportunities that can betaken advantage of in the future under seaweed farming
expansion.

Selling seaweed farming carbon credits is not currently permitted insome countries, such as New Zealand
155

48

49

PART 5: SITUATIONAL ANALYSIS FORSUSTAINABLE EXPANSION50© Unsplash / Kier in Sight

A situational analysis was performed to assess thepotential for expansion of seaweed farming to deliver
climate benefits and other co-benefits while avoiding
environmental and socioeconomic risks. The situational
analysis considers how factors both internal and external
to seaweed farming affect the potential for sustainableindustry expansion. The format used is that of a SWOT
analysis of global seaweed farming expansion. Based on
the evidence reviewed in this report, the analysis focuses
on climate benefits, environmental co-benefits and risks,socioeconomic co-benefits and risks, and production
upscaling aspects of seaweed farming. Strengths andopportunities are factors considered helpful to thesustainable expansion of seaweed farming, while
weaknesses and risks are considered harmful. Strengths
and weaknesses are internal factors inherent to seaweedfarming, while opportunities and threats are external
factors that can either be taken advantage of(opportunities) or protected against (threats) in the future.

5.1 Strengths
Several strengths of seaweed farming have been identified(Fig. 10). From a climate perspective, seaweed farms haveseveral strengths, including their capacity to absorb CO2
and build it into seaweed biomass, the potential for selectcultivated seaweeds to be processed into carbon negative
biofuels that do not compete with land-based food crops,
and the potential for select seaweeds to reduce entericemissions of methane as ruminant livestock feed. Selectcultivated seaweeds can also contribute to human foodintake in place of more carbon-intensive land-based crops.
Moreover, seaweed products such as biofertilizers andbiochar can sequester some carbon on land and candisplace more carbon-intensive products. From anenvironment perspective, seaweed farms over sandy
bottoms tend to attract marine biodiversity, and seaweedfarms release oxygen and absorb excess nutrients andheavy metals. From a socioeconomic perspective, there
can be a low barrier to entry into seaweed farming indeveloping countries due to low capital and material costs
and the simplicity of methods, and income generated byseaweed farming can lead to improvements in standardsof living and food security. Moreover, gender equity isenhanced in regions where women dominate the seaweedfarming industry. Seaweed farming can also produce highquality food, rich in nutrients, minerals, and vitamins, whenseaweed quality and safety are carefully considered.
Moreover, seaweed farming can provide cultural services
to coastal communities. Finally, in terms of upscaling
production, seaweed farming has the strength of not being
limited by the availability of arable land and freshwater,
unlike land-based agriculture.

51

Fig. 10. Major strengths (internal and helpful factors) for seaweed farming relating to the potential for climate benefits,environmental co-benefits and risks, socioeconomic co-benefits and risks, and upscaling production.

52

5.2 Weaknesses

propagation and selective breeding,susceptibility to diseases and pests.

canincrease

Several weaknesses of seaweed farming have been
identified (Fig. 11). Regarding the climate, the fraction of
the carbon built into seaweed biomass that is exported tolong-term oceanic carbon sinks is unknown, and thisinformation will be needed to quantify the climate benefitsof seaweed farming through natural carbon sequestration.
Moreover, commercial scale implementation of various
seaweed end-uses for climate mitigation, such as seaweed
biofuels, are limited by the high cost of seaweed
production, which cannot currently compete with low-costcarbon-intensive commodities. Biofuel production is also
limited by the efficiency of seaweed biomass conversion
to energy, and the seaweed feed pathway for methane
mitigation is limited by the number of ruminant livestock
nourished by feeds. Moreover, the viability of intentional
deep ocean sinking of seaweed is constrained by the sizeof carbon markets, uncertainty in the veracity of long-term
carbon storage, and the unknown environmental
ramifications.
Regarding the environment, seaweed farms thatsupersede existing complex habitats tend to reduce
biodiversity, and farmed seaweeds use sunlight andnutrients for growth, which may displace native
photosynthetic organisms. Moreover, the effects ofseaweed farms on ocean waves and currents are not wellunderstood. There is also a lack of understanding on therisks of pathogen spillover and genetic pollution fromseaweed farms to the environment. The highenvironmental tolerance and fast growth of farmed
seaweeds can facilitate species invasions when nonnative seaweeds species/strains are farmed, and theinfrastructure needed for seaweed farming can entangle
marine megafauna or be a source of plastic pollution. The
environmental risks of seaweed farming may be mostpronounced in the tropics but are poorly studied in thoseregions.
Regarding socioeconomic weaknesses, seaweed farmers
can experience increased incidence of adverse healthconditions and may live under the poverty or extreme
poverty line. Moreover, low farming income can lead tonegative feedback loops that further decrease farming
income. In addition, seaweed farming faces resistancewhen it conflicts with other spatial uses by humans.
Finally, regarding upscaling, weaknesses of seaweedfarming include a lack of research and understanding on
the carrying capacity of environments for seaweedfarming. Additionally, the low production value of some
farmed seaweeds is exacerbated by the susceptibility ofcrops to outbreaks of diseases and pests; and low genetic
diversity of some farmed seaweed crops, due to vegetative53

Fig. 11. Major weaknesses (internal and harmful factors) for seaweed farming relating to the potential for climate benefits,environmental co-benefits and risks, socioeconomic co-benefits and risks, and upscaling production.

54

5.3 Opportunities
Several opportunities have been identified for seaweedfarming (Fig. 12). In terms of the climate, seaweed farming
could provide carbon sequestration and GHG emissions
reduction that is proportional to industry scale, althoughsome of these benefits are likely to be spatially andtemporally limited. Technological improvements could
increase the energy return on investment for seaweed
biofuels. Innovation towards a biorefinery concept,
wherein multiple seaweed products are co-produced withzero waste, could increase the cost efficiency of various
seaweed commercial use pathways. Additionally, policy
interventions could incentivize a shift from consumption of
low-cost carbon-intensive commodities to seaweed-based
commodities. Finally, improved understanding of thecycling of seaweed carbon could improve accounting of
the climate benefits.

improvements have the potential to increase cost
efficiency of seaweed production. There are also
opportunities for exposure of new markets to seaweedbased foods and for national regulations to support theseaweed farming industry. Establishment of nutrient
trading credits and expansion of the carbon market also
represent opportunities to upscale seaweed farming.

Regarding the environment, seaweed farming could
enhance local habitats for marine species if farms are notplaced over existing complex habitats, such as seagrassbeds or coral reefs. Seaweed farming could locallymitigate ocean acidification and deoxygenation when
farms are placed in acidified waters and areas with highchemical oxygen demand, respectively. Seaweed farmingin polluted waters could locally mitigate coastal pollution
from nitrogen, phosphorus, and heavy metals, but the latter
collides with the safe use of seaweeds for consumption.
There is also potential for seaweed farms to provide
coastal protection if sufficient research is carried out tounderstand effects of farms on waves and currents.
Regarding socioeconomic opportunities, seaweed farming
could contribute to raising communities in coastal
developing countries out of poverty. Seaweed-based foodproducts could contribute to global food security, andseaweed farming could contribute to cultural heritage andsocial cohesion in coastal communities. Diversification ofseaweed products through value addition could increase
income for seaweed farmers, and government orcommunity management of seaweed farming could
increase incomes.
Regarding upscaling, seaweed farms could be co-locatedwith offshore wind or as part of IMTA to share space andresources. Crop innovations and enhancement of global
biosecurity, including control of diseases and pests, also
could increase production. Collaboration between
developed countries, with high rates of scientificinnovation, and developing countries, with high rates ofseaweed farming, can help facilitate seaweed farming
expansion. In addition, technological and infrastructural55

Fig. 12. Major opportunities (external and helpful factors) for seaweed farming relating to the potential for climate benefits,environmental co-benefits and risks, socioeconomic co-benefits and risks, and upscaling production.

56

5.4 Threats
Several threats have been identified for seaweed farming(Fig. 13). Regarding the climate, rapid upscaling of
seaweed commercial use pathways for carbon
sequestration and GHG emissions reduction could have
unforeseen environmental and social risks. Regarding theenvironment, habitat competition with seaweed farms may
degrade sensitive ecosystems, such as seagrass beds andcoral reefs, when farms are placed directly over thesehabitats. The high dispersal of biological material in theocean also may lead to the spread of pathogens andinvasive species and genetic pollution from seaweedfarms to the environment, particularly when appropriate
mitigation measures are not in place. Infrastructure to
support seaweed farming may compromise marine
megafauna conservation and worsen plastic pollution
when farms are not appropriately sited and managed.
Socioeconomic threats of seaweed farming include theloss of seaweed farming livelihoods and degradation ofcultural services in coastal regions if the industry is notproperly managed. Coastal communities in developing
countries could be exploited when seaweed farming yields
adverse health effects and low income, and seaweedfarming projects could be stalled when local stakeholders
are not consulted early and often in the farm development
process. Accumulation of heavy metals in seaweeds posea risk to human health if seaweed farms are notappropriately sited and seaweed food products are notproperly monitored and regulated. Finally, emerging
diseases and pests and ongoing climate change could
threaten the upscaling of seaweed farming production.

57

Fig. 13. Major threats (external and harmful factors) for seaweed farming relating to the potential for climate benefits, environmental
co-benefits and risks, socioeconomic co-benefits and risks, and upscaling production.

58

PART 6: CONCLUSIONS© Unsplash / Rosie Steggles

59

The seaweed farming industry has seen rapid growth inrecent decades, and while this growth has been
maintained to some degree in top-producing countries
with strong biotechnology markets and research outputs,
it has slowed in top-producing countries with limitedaccess to and control of these resources by women andmen. Research is needed to overcome major
environmental and socioeconomic barriers to seaweedfarming production, including outbreaks of crop diseases
and pests. In countries where seaweed farming is a
nascent industry, research and innovation can be strong,
yet expansion of the industry is limited by factors such associal license to operate and a lack of regulatory support.

and collaboration on gender, social inclusion, and adoptionof gender responsive laws and policies will be important incontributing to the development of a sustainable andequitable seaweed farming industry.

Various approaches have been identified in the literature to
overcome the limitations to global seaweed farming
production, including an MSP approach, crop innovations,advanced technology and infrastructure, global
biosecurity, market and product development, and new
economic incentives and regulatory support. It is important
to note that the adoption of such measures should occur
only under careful consideration for the environmental andsocioeconomic benefits and risks of an upscaled seaweedfarming industry. By proceeding carefully and taking swift
action to fill the gaps that exist in scientific understanding
across many environmental and socioeconomic benefits
and risks, as identified in this report, it may be possible toopen a sustainable pathway for the expansion of seaweedfarming. Additionally, interactive participatory engagement

water quality, regulation of hazards and extreme events,
energy, food and feed, materials and assistance, learningand inspiration, physical and psychological experiences,
supporting identities, and maintenance of options (IPBES
2019) (Appendix A, Table A1–A3, A5). Dedicated research
towards identifying the contexts wherein sustainable
expansion of seaweed farming can contribute to society,particularly towards solutions to the urgent climate crisis,
is strongly warranted.

Seaweed farming has the potential to advance many (13 of18) of the beneficial contributions of nature to people
identified by the Intergovernmental Science-Policy
Platform on Biodiversity and Ecosystem Services, as partof a circular bioeconomy, if the risks are appropriately
mitigated (Fig. 14). These benefits encompass: habitat
creation and maintenance, regulation of climate, regulationof ocean acidification, regulation of freshwater and coastal

Fig. 14. Seaweed farming as part of a circular bioeconomy (wherein carbon and nutrients are recycled through seaweed commercial
uses such as feed, fertilizer, food, and fuels) could yield carbon sequestration and GHG emissions reduction. The potential climate
benefits of seaweed farming and associated environmental and socioeconomic co-benefits and risks require further research, butwhere the benefits are deemed to outweigh the risks, approaches exist to upscale production. Based on Duarte et al. (2022a).

60

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APPENDIX A: Tables
Table A1. Quantitative data relating to the potential for natural carbon sequestration as a climate benefit of seaweedfarming.

Location

Farmed Species

Lidao town,
China

Saccharina japonica and Carbon dioxide
Gracilaria
absorption / flux oflemaneiformis
CO2 into sea from air

286.44 vs. 307.25 μmol L-1 −34.85 vs. −24.17 mmol Jiang et al.
aqueous pCO2 at seaweed m-2 d-1 sea-air CO2 flux in 2013
farms vs. reference area seaweed farms vs.
reference area

Long Island
Sound/ Bronx
River Estuary,
USA
Long Island
Sound/ Bronx
River Estuary,USA
Republic of
Korea
China

Gracilaria tikvahiae

Carbon built into
biomass

Up to 300–727 kg C ha-1
held

Kim et al. 2014

Saccharina latissima

Carbon built into
biomass

Up to 1,200–1,800 kg C
ha-1 held

Kim et al. 2015

Ecklonia cava andEcklonia stolonifera
Various species

GlobalGlobalSungo Bay,
China
NorwaySaccharina japonica

GlobalGlobalGlobalGlobalGlobalGlobalChung et al.
2013
Zheng et al.
2019
Hughes et al.
2012
Zhang et al.
2012
Fieler et al.
2021
Turan and Neori
2010
Duarte et al.
2017
Kim et al. 2017

GlobalGlobalAcross China

Various species

GlobalGlobalGlobalGlobalCarbon built into
~10 t CO2 ha-1 y-1 held
biomass
Carbon built into
421.78 t C km-2 y-1 held
biomass
Refractory component 0.36t C ha-1 seaweed farmof DOCPOC export
58 per cent ww seaweed 61 per cent gross C
releasedproduction released
POC export
8–49 per cent dw
63–88 g C m-2 y-1
seaweed released
releasedUpper-bound carbon 0.67 Tg C y-1 removed
sequestration
Upper-bound carbon 0.68 Tg C y-1 removed
sequestration
Upper-bound carbon 0.76 Tg C y-1 removed
sequestration
Upper-bound carbon 0.78 Tg C y-1 removed
sequestration
Carbon built into
605,830 t C y-1 held
344,128 t C y-1
biomass and naturalsequesteredsequestration
Natural carbon
0–205 kg CO2 Mt-1 dw
sequestration
seaweed y-1 sequestered
Seaweed farming-62–287 kg CO2 Mt-1 dw
carbon footprint (pre- seaweed
processing)

Saccharina latissima

Climate Benefit

Metric 1

Metric 2

SourceSondak et al.
2017
Gao et al. 2022
emLab 2019emLab 201971

Table A2. Quantitative data on the potential commercial use pathways for climate mitigation.
Location

FarmedSpeciesCommercial
Pathway

GlobalVarious

Seaweedbiofuels

GlobalVariousGlobalVariousGlobalVariousGlobalVarious

In vitro

Asparagopsis
taxiformis

Australia

Asparagopsis
taxiformis

Australia

Asparagopsis
armataAustralia

Asparagopsis
taxiformis

GlobalAsparagopsis

GlobalBrown

AcrossChina
GlobalVariousAcross
China

VariousGlobalBrown

GlobalVarious

Various

Metric 1

Metric 2

Metric 3

1,500 t CO2 km-2 y-1 in potential
avoidance of fossil fuelemissions
Seaweed biogas 0.78 t C ha-1 in potential
sequestration
Seaweed ethanol Up to 2.9 per cent global
emissions reduced comparedto gasoline
Seaweed10 kg CO2 eq ha-1 in net climate
bioethanolchange reduction
Seaweed biogas 1870 kg CO2 eq ha-1 in net
climate change reduction
Seaweed in steer 99 per cent reduction infeedmethane at 2 per cent addition
rate
Seaweed in
80 per cent reduction in
sheep feedmethane at 3 per cent addition
rate
Seaweed in dairy 26 per cent reduction in
67 per centcattle feedmethane at 0.5 per centreduction in
addition ratemethane at 1 percentaddition rate
Seaweed in beef 40 per centreduction in98 per centcattle feedmethane at 0.1 per centreduction in
addition ratemethane at 0.2 percent addition rate
Seaweed in cow Up to 2.3 per cent of global
feedemissions reduced
Land crop
12 kg CO2 eq kg-1 of seaweed
displacement
protein produced
Land crop
62,492 ha saved in land
displacement
resources
Land crop
4 million L freshwater saved
displacement
per ton of food produced atsea
Land crop
16,554 t N saved in fertilizer 5503 t P2O5 saved 7255 t K2O
displacementuse
in fertilizer use
saved infertilizer use
Land crop
1 kg CO2 eq kg-1 of inorganics
displacement
produced
Deep ocean
Up to 0.43 per cent of global
sinkingemissions reduced
considering carbon market

SourceDuarte et al.
2017
Hughes et al.
2012
emLab 2019
Thomsen andZhang 2020
Thomsen andZhang 2020
Machado et al.
2016
Li et al. 2016
Roque et al. 2019

Kinley et al. 2020

emLab 2019
Sadhukhan et al.
2019
Zheng et al. 2019
Radulovich 2011
Zheng et al. 2019
Sadhukhan et al.
2019
emLab 201972

Table A3. Quantitative data on the potential for environmental co-benefits of seaweed farming.
Location

Farmed Species

Environmental Co- Metric 1
BenefitWest of Ireland

Laminaria digitata

Costa Rica

Various species

Biodiversity
provisioning
Biodiversity
provisioning

China

Various species

Great Barrier Reef Modelled Chlorophyta
and Phaeophyta
Across China
Various species
Across China

Various species

Buffering against
acidification andoxygenation

SourceWalls et al.
2016
Radulovich etal. 2015a~5 vs. 100 fish
individuals incontrol vs seaweed
plot
0.02–0.35 mg L-1 Xiao et al. 2021
higher dO2

0.03–0.1 units higher pH,
27.3–113.9 μatm lower
pCO2, and 0–0.29 units
higher ΩaBuffering against
0.1 increase in Ωa over a 24
Mongin et al.
ocean acidification
km2 area
2016
Oxygenation and
1,440,612 t O2 y-1 produced 59.07 t N and 7.50 Zheng et al.
nutrient removalt P km-2 y-1
2019
removedOxygenation and
2,533,221 t O2 y-1 produced 70,615 t N and
Gao et al. 2022
nutrient removal
8,515 t P y-1
removedUpper-bound nutrient 28–94 kg N ha-1 removed
Kim et al. 2014
removalLong Island Sound/ Gracilaria tikvahiae
Bronx River
Estuary, USA
Long Island Sound/ Saccharina latissima
Upper-bound nutrient
Bronx River
removalEstuary, USA
Jiangsu Province, Pyropia yezoensis
Nutrient removal
China
Jiangsu Province, Ulva
Nutrient removal
China
Across China
Saccharina japonica,
Nutrient removal
Gracilariopsis and others
Across China
Various species
Nutrient removal

38–180 kg N ha-1 removed

Kim et al. 2015

3688 t N season-1 removed 106 t P season-1
removed77 t N season-1 removed
3 t P season-1
removed75,000 t N y-1 removed
9,500 t P y-1
removed75,000 t N y-1 removed
9,500 t P y-1
removedNutrient removal
70,615 t N y-1 removed
8,515 t P y-1
removedUpper-bound nutrient 65,000 t N y-1 removed
removalWu et al. 201754 per cent gross N
production released

Zhang et al.
2012

Across China

Various species

GlobalPyropia/Porphyra,
Gracilaria,
Kappaphycus/Eucheuma,
kelp, and Sargassum
Saccharina japonica
Nutrient removal

Sungo Bay, China

42 species in farmed
holdfasts
3 vs. 14 fish species incontrol vs seaweed plot

Metric 2

Wu et al. 2017
Xiao et al. 2017
Zheng et al.
2019
Gao et al. 2022
Kim et al. 201773

Table A4. Quantitative data on the potential for environmental risks of seaweed farming.
Location

Farmed Species

Environmental Risk

SwedenSaccharina latissima

Light attenuation

Zanzibar
China
PanamaChina
Tropics
Tropics
GlobalMetric 1

Source40 per cent attenuation at 5 m
Visch et al. 2020
depth beneath farm
Eucheuma andSeagrass biomass decrease 40 per cent lower under farm than Eklöf et al. 2006b
Kappaphycus
in control plots
Saccharina japonica
Reduction in surface
Average 40 per cent reduction by Shi et al. 2011
currents
farm
Kappaphycus alvarezii
Farmed seaweed species
Up to >30 per cent cover bySellers et al. 2015
invasioninvasive algae in wild habitats
Saccharina japonica
Sedimentary acid volatile
1.22 vs. 0.14 mg g-1 dw in farm vs. Zhou 2012
sulfide content
control, respectively
Various species (primarily Current halocarbon
2 per cent of wild macroalgae
Leedham et al. 2013
rhodophytes)
emissionsemissions
Various species (primarily Increase in halocarbon
12–20 per cent of wild macroalgae Leedham et al. 2013
rhodophytes)
emissionsemissions at future farming area
GlobalIncrease in halocarbon
1 per cent more global emissions Duarte et al. 2022a
emissionsat 50 times greater farming area

Table A5. Quantitative data on the potential for socioeconomic co-benefits of seaweed farming.
Location

Farmed Species

Socioeconomic Risk

MetricSourceIndonesia

Various species

267,000 households

Langford et al. 2021

Philippines

Various species
Various species

100,000–150,000
people30,000 people

Hurtado 2013

The United
Republic of
Tanzania
India

Estimated number of households
supported
Estimated number of people
supported
Minimum number of people
supported

200,000 households

Kenya

Eucheuma and Kappaphycus

Estimated number of near-future
households supported
Percentage of women farmers

Krishnan and Kumar
2010
Mirera et al. 2020

ZanzibarEucheuma and Kappaphycus

Percentage of women farmers

90 per centAcross China

Various species

Production of iodine

4,954 kg I km yZheng et al. 2019

Netherlands

Saccharina latissima

Iodine intake

300 μg d intake

Vellinga et al. 2022

Portugal

Saccharina latissima

Iodine intake

208 μg d intake

Vellinga et al. 2022

Various species

75.2 per centEklöf et al. 2012
-2

-1

-1
-1

Msuya et al. 2022

74

Table A6. Quantitative data on the potential for socioeconomic risks of seaweed farming.
Location

Farmed Species

Socioeconomic Risk

MetricSpain

Various species

Heavy metal exposure

Spain

Hizikia fusiforme

Heavy metal exposure

0.025–4.82 mg kg-1 dw cadmium
Besada et al. 2009
concentration
32.1–69.5 mg kg-1 dw inorganic
Besada et al. 2009arsenic concentration
67–96 mg kg-1 inorganic arsenic
Rose et al. 2007
concentration in unpreparedseaweed5.1–23 mg kg-1 inorganic arsenic
Rose et al. 2007
concentration in prepared seaweed
1.02 μg kg-1 bw d-1 total arsenic
Vellinga et al. 2022
concentration
1.67 μg kg-1 bw d-1 total arsenic
Vellinga et al. 2022
concentration

United Kingdom Hijiki seaweed (scientific Heavy metal exposurename not given)
United Kingdom Hijiki seaweed (scientific Heavy metal exposurename not given)
Netherlands
Saccharina latissima
Heavy metal exposure
Portugal

Saccharina latissima

Heavy metal exposure

Source75

UNITS AND SYMBOLS
bw

Body Weightmmol

MillimoleC

CarbonMt

MegatonCO2

Carbon Dioxide

N

Nitrogen

d

Day

O2

OxygendO2

Dissolved Oxygen

P

Phosphorus

DOC

Dissolved Organic Carbon

P2O5

Phosphorus Pentoxidedw

Dry Weight

pCO2

Partial Pressure of Aqueous Carbon Dioxideeq

Equivalent

POC

Particulate Organic Carbon

g

Gram

s

secondGt

Gigaton

t

Ton

ha

HectareTg

Terragram

I

Iodine

wk

Week

K 2O

Potassium Oxideww

Wet Weight

kCal

Kilocalorie

y

Year

kg

Kilogram

μatm

Microatmospherekm

Kilometerμg

Microgram

L

Liter

μmol

Micromole

m

Meter

Ωa

Aragonite Saturation State

mg

Milligram76