Seaweeds use photosynthesis to convert dissolved carbon in the surrounding seawater into organic compounds and living tissue. As dissolved carbon is removed from the ocean, it is replaced by carbon dioxide from the atmosphere. It is through this process that seaweed can remove CO2 from the atmosphere. The ultimate fate of carbon sequestered in living seaweed is varied and includes becoming remineralized, or converted back, to CO2 via grazing, exuded as dissolved organic carbon, and being transported to the deep sea as particulate matter. The rate and proportion of carbon through each of these pathways remains uncertain, though a 2016 study from Krause-Jensen and Duarte sheds some light on possible export efficiencies. What’s more, the rate and proportion of carbon that goes through each of the pathways mentioned are not only uncertain but context-dependent. For an in-depth exploration of the readiness of seaweed as a blue carbon solution (both natural stands and farmed) see Fujita et al. 2023.

Note that this road map will exclusively look at the ability of natural seaweed stands and coastal seaweed farms to passively sequester CO2 (without active human intervention).  For an in-depth look at growing seaweed at large scale to be sequestered using active human intervention (such as deep sea sinking and production of algal biochar), please see the Macroalgae Cultivation and Carbon Sequestration Road Map.

Seaweeds use photosynthesis to convert dissolved carbon in the surrounding seawater into organic compounds and living tissue^[1]Paine, E. R., Schmid, M., Boyd, P. W., Diaz-Pulido, G., & Hurd, C. L. (2021). Rate and fate of dissolved organic carbon release by seaweeds: A missing link in the coastal ocean carbon cycle. Journal of Phycology, 57(5), 1375–1391. doi.org/10.1111/ jpy.13198 . As dissolved carbon is removed from the ocean, it is replaced by carbon dioxide from the atmosphere. It is through this process that seaweed can remove CO2 from the atmosphere. The ultimate fate of carbon sequestered in living seaweed is varied and includes becoming remineralized, or converted back, to CO2 via grazing, exuded as dissolved organic carbon, and being transported to the deep sea as particulate matter. The rate and proportion of carbon through each of these pathways remains uncertain^[2]Macreadie, P. I., Anton, A., Raven, J. A., Beaumont, N., Connolly, R. M., Friess, D. A., Kelleway, J. J. et al. 2019. The future of blue carbon science. Nat. Commun. 10:3998. , though a 2016 study from Krause-Jensen and Duarte sheds some light on possible export efficiencies^[3]Krause-Jensen, D., & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience, 9. nature.com/articles/ngeo2790 . What’s more, the rate and proportion of carbon that goes through each of the pathways mentioned are not only uncertain but context-dependent. For an in-depth exploration of the readiness of seaweed as a blue carbon solution (both natural stands and farmed) see Fujita et al. 2023^[4]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. .

Note that this road map will exclusively look at the ability of natural seaweed stands and coastal seaweed farms to passively sequester CO2 (without active human intervention).  For an in-depth look at growing seaweed at large scale to be sequestered using active human intervention (such as deep sea sinking and production of algal biochar), please see the Macroalgae Cultivation and Carbon Sequestration Road Map.

Seaweeds use photosynthesis to convert dissolved carbon in the surrounding seawater into organic compounds and living tissue^[1]Paine, E. R., Schmid, M., Boyd, P. W., Diaz-Pulido, G., & Hurd, C. L. (2021). Rate and fate of dissolved organic carbon release by seaweeds: A missing link in the coastal ocean carbon cycle. Journal of Phycology, 57(5), 1375–1391. doi.org/10.1111/ jpy.13198 . As dissolved carbon is removed from the ocean, it is replaced by carbon dioxide from the atmosphere. It is through this process that seaweed can remove CO2 from the atmosphere. The ultimate fate of carbon sequestered in living seaweed is varied and includes becoming remineralized, or converted back, to CO2 via grazing, exuded as dissolved organic carbon, and being transported to the deep sea as particulate matter. The rate and proportion of carbon through each of these pathways remains uncertain^[2]Macreadie, P. I., Anton, A., Raven, J. A., Beaumont, N., Connolly, R. M., Friess, D. A., Kelleway, J. J. et al. 2019. The future of blue carbon science. Nat. Commun. 10:3998. , though a 2016 study from Krause-Jensen and Duarte sheds some light on possible export efficiencies^[3]Krause-Jensen, D., & Duarte, C. M. (2016). Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience, 9. nature.com/articles/ngeo2790 . What’s more, the rate and proportion of carbon that goes through each of the pathways mentioned are not only uncertain but context-dependent. For an in-depth exploration of the readiness of seaweed as a blue carbon solution (both natural stands and farmed) see Fujita et al. 2023^[4]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. .

Note that this road map will exclusively look at the ability of natural seaweed stands and coastal seaweed farms to passively sequester CO2 (without active human intervention).  For an in-depth look at growing seaweed at large scale to be sequestered using active human intervention (such as deep sea sinking and production of algal biochar), please see the Macroalgae Cultivation and Carbon Sequestration Road Map.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.. It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported.
    • The Nature Conservancy’s guidance on developing kelp restoration projects
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years.

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification
• Contribution to biodiversity 
• Contribution to habitat provisioning
• Contribution to heavy metal and nutrient pollution removal, 
• Contribution to fishery enhancement, 
• Improved water clarity through the facilitation of settlement of fine sediments
• Nutrient cycling
• Reducing biodiversity loss 
• Restoring the role of marine organisms in the carbon cycle 

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection, providing a refugia from acidified waters that benefits shellfish, mitigation of dead zones by production of dissolved oxygen, or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”. This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity. Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.. It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported.
    • The Nature Conservancy’s guidance on developing kelp restoration projects
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years.

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification
• Contribution to biodiversity 
• Contribution to habitat provisioning
• Contribution to heavy metal and nutrient pollution removal, 
• Contribution to fishery enhancement, 
• Improved water clarity through the facilitation of settlement of fine sediments
• Nutrient cycling
• Reducing biodiversity loss 
• Restoring the role of marine organisms in the carbon cycle 

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection, providing a refugia from acidified waters that benefits shellfish, mitigation of dead zones by production of dissolved oxygen, or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”. This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity. Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.^[1]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. . It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported^[2]A.M. Eger, E.M. Marzinelli, C. Hartvig, C. Fagerli, D. Fujita, Global kelp forest restoration: past lessons, present status, and future directions, in: Biological Reviews, 97, 2022, pp. 1449–1475, https://doi.org/10.1111/brv.12850. .
    • The Nature Conservancy’s guidance on developing kelp restoration projects^[3]Gleason, M., Caselle, J.E., Heady, W.N., Saccomanno, V.R., Zimmerman, J., Anoush McHugh, T. (2021). A Structured Approach for Kelp Restoration and Management Decisions in California. Nature Conservancy and UCSB Marine Science Institute. https://www.scienceforconservation.org/products/structured-decisionmaking-kelp
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year^[4]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[5]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification^[6]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[7]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[8]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[9]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[11]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[12]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.  
• Improved water clarity through the facilitation of settlement of fine sediments^[13]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[14]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Reducing biodiversity loss^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  
• Restoring the role of marine organisms in the carbon cycle^[16]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection^[17]Denny, M. Wave-Energy Dissipation: Seaweeds and Marine Plants Are Ecosystem Engineers. Fluids 2021, 6, 151. https://doi.org/10.3390/fluids6040151 , providing a refugia from acidified waters that benefits shellfish^[18]Edworthy, Carla, et al. “The role of macroalgal habitats as ocean acidification refugia within coastal seascapes.” Cambridge Prisms: Coastal Futures, vol. 1, 2023, https://doi.org/10.1017/cft.2023.9. , mitigation of dead zones by production of dissolved oxygen^[19]Predicted/possible impact , or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves^[20]Predicted/possible impact
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”^[21]Wolske, K. S., K. T. Raimi, V. Campbell-Arvai, and P. S. Hart. 2019. “Public support for carbon dioxide removal strategies: the role of tampering with nature perceptions.” Climatic Change 152 (3-4):345-361. doi:10.1007/s10584-019-02375-z. . This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity^[22]Macreadie, Peter I., et al. “Operationalizing Marketable Blue Carbon.” One Earth, vol. 5, no. 5, 2022, pp. 485–492, https://doi.org/10.1016/j.oneear.2022.04.005. . Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[23]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[24]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.^[1]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. . It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported^[2]A.M. Eger, E.M. Marzinelli, C. Hartvig, C. Fagerli, D. Fujita, Global kelp forest restoration: past lessons, present status, and future directions, in: Biological Reviews, 97, 2022, pp. 1449–1475, https://doi.org/10.1111/brv.12850. .
    • The Nature Conservancy’s guidance on developing kelp restoration projects^[3]Gleason, M., Caselle, J.E., Heady, W.N., Saccomanno, V.R., Zimmerman, J., Anoush McHugh, T. (2021). A Structured Approach for Kelp Restoration and Management Decisions in California. Nature Conservancy and UCSB Marine Science Institute. https://www.scienceforconservation.org/products/structured-decisionmaking-kelp
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year^[4]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[5]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification^[6]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[7]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[8]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[9]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[11]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[12]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.  
• Improved water clarity through the facilitation of settlement of fine sediments^[13]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[14]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Reducing biodiversity loss^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  
• Restoring the role of marine organisms in the carbon cycle^[16]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection^[17]Denny, M. Wave-Energy Dissipation: Seaweeds and Marine Plants Are Ecosystem Engineers. Fluids 2021, 6, 151. https://doi.org/10.3390/fluids6040151 , providing a refugia from acidified waters that benefits shellfish^[18]Edworthy, Carla, et al. “The role of macroalgal habitats as ocean acidification refugia within coastal seascapes.” Cambridge Prisms: Coastal Futures, vol. 1, 2023, https://doi.org/10.1017/cft.2023.9. , mitigation of dead zones by production of dissolved oxygen^[19]Predicted/possible impact , or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves^[20]Predicted/possible impact
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”^[21]Wolske, K. S., K. T. Raimi, V. Campbell-Arvai, and P. S. Hart. 2019. “Public support for carbon dioxide removal strategies: the role of tampering with nature perceptions.” Climatic Change 152 (3-4):345-361. doi:10.1007/s10584-019-02375-z. . This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity^[22]Macreadie, Peter I., et al. “Operationalizing Marketable Blue Carbon.” One Earth, vol. 5, no. 5, 2022, pp. 485–492, https://doi.org/10.1016/j.oneear.2022.04.005. . Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[23]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[24]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.^[1]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. . It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported^[2]A.M. Eger, E.M. Marzinelli, C. Hartvig, C. Fagerli, D. Fujita, Global kelp forest restoration: past lessons, present status, and future directions, in: Biological Reviews, 97, 2022, pp. 1449–1475, https://doi.org/10.1111/brv.12850. .
    • The Nature Conservancy’s guidance on developing kelp restoration projects^[3]Gleason, M., Caselle, J.E., Heady, W.N., Saccomanno, V.R., Zimmerman, J., Anoush McHugh, T. (2021). A Structured Approach for Kelp Restoration and Management Decisions in California. Nature Conservancy and UCSB Marine Science Institute. https://www.scienceforconservation.org/products/structured-decisionmaking-kelp
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year^[4]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[5]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification^[6]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[7]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[8]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[9]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[11]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[12]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.  
• Improved water clarity through the facilitation of settlement of fine sediments^[13]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[14]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Reducing biodiversity loss^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  
• Restoring the role of marine organisms in the carbon cycle^[16]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection^[17]Denny, M. Wave-Energy Dissipation: Seaweeds and Marine Plants Are Ecosystem Engineers. Fluids 2021, 6, 151. https://doi.org/10.3390/fluids6040151 , providing a refugia from acidified waters that benefits shellfish^[18]Edworthy, Carla, et al. “The role of macroalgal habitats as ocean acidification refugia within coastal seascapes.” Cambridge Prisms: Coastal Futures, vol. 1, 2023, https://doi.org/10.1017/cft.2023.9. , mitigation of dead zones by production of dissolved oxygen^[19]Predicted/possible impact , or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves^[20]Predicted/possible impact
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”^[21]Wolske, K. S., K. T. Raimi, V. Campbell-Arvai, and P. S. Hart. 2019. “Public support for carbon dioxide removal strategies: the role of tampering with nature perceptions.” Climatic Change 152 (3-4):345-361. doi:10.1007/s10584-019-02375-z. . This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity^[22]Macreadie, Peter I., et al. “Operationalizing Marketable Blue Carbon.” One Earth, vol. 5, no. 5, 2022, pp. 485–492, https://doi.org/10.1016/j.oneear.2022.04.005. . Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[23]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[24]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.^[1]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. . It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported^[2]A.M. Eger, E.M. Marzinelli, C. Hartvig, C. Fagerli, D. Fujita, Global kelp forest restoration: past lessons, present status, and future directions, in: Biological Reviews, 97, 2022, pp. 1449–1475, https://doi.org/10.1111/brv.12850. .
    • The Nature Conservancy’s guidance on developing kelp restoration projects^[3]Gleason, M., Caselle, J.E., Heady, W.N., Saccomanno, V.R., Zimmerman, J., Anoush McHugh, T. (2021). A Structured Approach for Kelp Restoration and Management Decisions in California. Nature Conservancy and UCSB Marine Science Institute. https://www.scienceforconservation.org/products/structured-decisionmaking-kelp
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year^[4]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[5]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification^[6]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[7]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[8]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[9]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[11]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[12]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.  
• Improved water clarity through the facilitation of settlement of fine sediments^[13]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[14]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Reducing biodiversity loss^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  
• Restoring the role of marine organisms in the carbon cycle^[16]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection^[17]Denny, M. Wave-Energy Dissipation: Seaweeds and Marine Plants Are Ecosystem Engineers. Fluids 2021, 6, 151. https://doi.org/ 10.3390/fluids6040151 , providing a refugia from acidified waters that benefits shellfish^[18]Edworthy, Carla, et al. “The role of macroalgal habitats as ocean acidification refugia within coastal seascapes.” Cambridge Prisms: Coastal Futures, vol. 1, 2023, https://doi.org/10.1017/cft.2023.9. , mitigation of dead zones by production of dissolved oxygen^[19]Predicted/possible impact , or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves^[20]Predicted/possible impact
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”^[21]Wolske, K. S., K. T. Raimi, V. Campbell-Arvai, and P. S. Hart. 2019. “Public support for carbon dioxide removal strategies: the role of tampering with nature perceptions.” Climatic Change 152 (3-4):345-361. doi:10.1007/s10584-019-02375-z. . This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity^[22]Macreadie, Peter I., et al. “Operationalizing Marketable Blue Carbon.” One Earth, vol. 5, no. 5, 2022, pp. 485–492, https://doi.org/10.1016/j.oneear.2022.04.005. . Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[23]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[24]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.^[1]Fujita, Rod, et al. “Seaweed Blue Carbon: Ready? Or Not?” Marine Policy, vol. 155, 2023, p. 105747, https://doi.org/10.1016/j.marpol.2023.105747. . It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported^[2]A.M. Eger, E.M. Marzinelli, C. Hartvig, C. Fagerli, D. Fujita, Global kelp forest restoration: past lessons, present status, and future directions, in: Biological Reviews, 97, 2022, pp. 1449–1475, https://doi.org/10.1111/brv.12850. .
    • The Nature Conservancy’s guidance on developing kelp restoration projects^[3]Gleason, M., Caselle, J.E., Heady, W.N., Saccomanno, V.R., Zimmerman, J., Anoush McHugh, T. (2021). A Structured Approach for Kelp Restoration and Management Decisions in California. Nature Conservancy and UCSB Marine Science Institute. https://www.scienceforconservation.org/products/structured-decisionmaking-kelp
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year^[4]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[5]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification^[6]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[7]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[8]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[9]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[11]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[12]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.  
• Improved water clarity through the facilitation of settlement of fine sediments^[13]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[14]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Reducing biodiversity loss^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  
• Restoring the role of marine organisms in the carbon cycle^[16]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection^[17]Denny, M. Wave-Energy Dissipation: Seaweeds and Marine Plants Are Ecosystem Engineers. Fluids 2021, 6, 151. https://doi.org/ 10.3390/fluids6040151 , providing a refugia from acidified waters that benefits shellfish^[18]Edworthy, Carla, et al. “The role of macroalgal habitats as ocean acidification refugia within coastal seascapes.” Cambridge Prisms: Coastal Futures, vol. 1, 2023, https://doi.org/10.1017/cft.2023.9. , mitigation of dead zones by production of dissolved oxygen^[19]Predicted/possible impact , or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves^[20]Predicted/possible impact
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”^[21]Wolske, K. S., K. T. Raimi, V. Campbell-Arvai, and P. S. Hart. 2019. “Public support for carbon dioxide removal strategies: the role of tampering with nature perceptions.” Climatic Change 152 (3-4):345-361. doi:10.1007/s10584-019-02375-z. . This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity^[22]Macreadie, Peter I., et al. “Operationalizing Marketable Blue Carbon.” One Earth, vol. 5, no. 5, 2022, pp. 485–492, https://doi.org/10.1016/j.oneear.2022.04.005. . Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[23]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[24]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Here, natural seaweed stands denotes seaweed naturally occurring in the ocean, also commonly referred to as kelp forests.

Mechanisms for CDR

• Reforestation of natural seaweed stands is the process of assisting in the recovery of seaweed stands that have been degraded, damaged, or destroyed, and is a type of restoration.
  • The reforestation of natural seaweed stands to increase carbon storage involves many context-specific factors that may complicate such efforts. Factors that will affect the growth and ultimate success of seaweed include climate change, disease, grazers, and predator interactions, etc.. It is also highly species dependent with some species of seaweed better suited than others for CDR. This complex assemblage of factors, unique across geographies, will require restoration efforts to be highly tailored.
  • A 2022 meta-analysis of seaweed restoration projects found a high success rate of restoration projects, however many of the reported projects were small in scale and these findings may not reflect failed restoration projects that were not reported.
    • The Nature Conservancy’s guidance on developing kelp restoration projects
    • One emerging method for restoring degraded seaweed stands is seeding small rocks with juvenile kelp, often termed ‘Green Gravel’. See work by the Green Gravel Action Group.
    • Another proposed technique actively removes urchins from seaweed stands to allow for the settlement of juvenile seaweed and subsequent regrowth. See Urchinomics for one approach.
• Increasing productivity of existing seaweed stands via genetic manipulation
  • There is current research on improving strain selection for enhanced carbon sequestration, productivity, or resiliency of seaweed. See ongoing research by Charles Yarish and lab at the University of Connecticut.

CDR Potential

Estimated Sequestration Potential: 0 - 0.018 Gt CO2e /year

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years.

Environmental Co-Benefits

Restoration of natural seaweed stands may generate many benefits for surrounding marine ecosystems. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant that these may in fact be the main benefits of restoration. (Note that there are significantly more co-benefits in blue carbon than any of the other marine CDR Road Maps.)

• Potential amelioration of ocean acidification
• Contribution to biodiversity 
• Contribution to habitat provisioning
• Contribution to heavy metal and nutrient pollution removal, 
• Contribution to fishery enhancement, 
• Improved water clarity through the facilitation of settlement of fine sediments
• Nutrient cycling
• Reducing biodiversity loss 
• Restoring the role of marine organisms in the carbon cycle 

Environmental Risks

In contrast to other proposed marine CDR pathways, there are not as many apparent environmental risks to reforesting natural seaweed stands for carbon dioxide removal. However, proper restoration techniques which take context-specific factors into consideration are critical to ensuring no or limited negative environmental impacts. As with any CDR pathway, incremental scale-up with careful monitoring and verification of impacts will be critical to mitigate risks. Possible environmental risks may include:

• Species entanglement
• Nutrient competition
• Smothering of adjacent sensitive ecosystems
• Negative effects of organic exudates on neighboring ecosystems
• If hard bottom substrates are needed (for seaweed to attach to) this could disturb soft bottom habitats. See examples of artificial reef creation to mitigate the loss of natural reef and restore seaweed.

Social Co-Benefits

Reforestation of natural seaweed stands may generate benefits for surrounding communities and economies. While we are calling these “co-benefits” here, it should be noted that some of these benefits may be so significant (while carbon sequestration feasibility and efficacy remain unknown) that these may in fact be the main benefits of restoration. Many of the environmental benefits relate directly to social benefits, such as fisheries enhancement and coastal protection. (Note that there are significantly more co-benefits in blue carbon than any of the other CDR Road Maps.)

• Increased adaptive capacity for communities to handle climate change and natural disasters via mechanisms such as wave attenuation for coastal protection, providing a refugia from acidified waters that benefits shellfish, mitigation of dead zones by production of dissolved oxygen, or possibly providing shading from seaweed blades to create a thermal refugia during marine heat waves
• Job creation and economic stimulation from enhanced fisheries and tourism (of particular value to coastal communities that are dependent on marine-based livelihoods)
• Cultural / intrinsic / recreational value
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

Efforts around restoring natural ecosystems are often met with social acceptance and public support, especially when compared to other climate solutions seen as “tampering with nature”. This may be in part due to the minimal risks (real and perceived) associated with restoring natural ecosystems. While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for inequity in benefits: Macreadie et al. (2022) points to concerns around the distribution of benefits from any given blue carbon project and whether benefits are evenly distributed or whether activities reinforce or add to social inequity. Similarly, there may be confusion around who “owns” the blue carbon and who has rights to control transactions of credits resulting from blue carbon projects. This may be particularly challenging in marine spaces where the movement of carbon must be taken into account.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining however, there may be synergies for some industries, like utilizing infrastructure from nearby renewable energy for monitoring.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by natural seaweed stands?
• Which of the carbon fluxes between natural seaweed stands and their environment are well quantified, and which are not?
• How does carbon sequestered by natural seaweed stands compare with other ocean-based carbon sequestration pathways?
• Is the restoration of natural seaweed stands a durable and feasible enough pathway to help stabilize the climate?

*Note that all of these are highly dependent upon what species of seaweed is being considered.

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed

• Conduct research to better understand and characterize carbon fluxes in natural seaweed stands
  • Determine the best way to measure carbon sequestration rates by natural seaweed stands*
  • Determine which of the carbon fluxes relevant to carbon sequestration by seaweeds are well quantified and which are not. This includes carbon fluxes from the atmosphere to ocean, ocean to seaweed, seaweed to microbial and other food webs, and seaweed to deep water*.
  • Characterize how the potential carbon sequestration by natural seaweed stands compares with other ocean-based carbon sequestration pathways*
  • Determine the durability of carbon sequestered via natural seaweed stands and if it is long enough to help stabilize the climate*
• Accelerate the innovation and testing of new technology that can aid in restoration projects
  • Conduct further field testing of techniques such as green gravel for afforestation of kelp forests.
  • Conduct more research on the efficacy of utilizing genetic manipulation and biotechnology to enhance seaweed carbon capture efficiencies.

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years.

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification
• Contribution to biodiversity 
• Contribution to habitat provisioning
• Contribution to heavy metal and nutrient pollution removal, 
• Contribution to fishery enhancement,
• Improved water clarity through the facilitation of settlement of fine sediments
• Nutrient cycling
• Restoring the role of marine organisms in the carbon cycle 

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measuring Carbon
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

*Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms*

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measuring Carbon
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years.

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification
• Contribution to biodiversity 
• Contribution to habitat provisioning
• Contribution to heavy metal and nutrient pollution removal, 
• Contribution to fishery enhancement,
• Improved water clarity through the facilitation of settlement of fine sediments
• Nutrient cycling
• Restoring the role of marine organisms in the carbon cycle 

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measuring Carbon
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms*

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measuring Carbon
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measuring Carbon
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. https://www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling:
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measurement:
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *

Mechanism for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur.

CDR Potential

Estimated Sequestration Potential: 0.00144 Gt CO2e/year – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling:
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measurement:
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *

Mechanisms for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur

CDR Potential

Estimated Sequestration Potential: 0.00144 Gt CO2e/year – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling:
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measurement:
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First-Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *

Mechanisms for CDR

Expansion of seaweed farming

• Possibly the most promising pathway to increase carbon sequestration by seaweed due to rapid advances in infrastructure, farm operations, and monitoring.
• Achieving additional, passive, carbon sequestration would require biomass to be left in the ocean long enough for fragmentation, transport, and burial to occur

CDR Potential

Estimated Sequestration Potential: 0.00144 Gt CO2e/year – 0.0079 Gt CO2e/year^[1]Hoegh-Guldberg, O., Northrop, E. et al. 2023. "The ocean as a solution to climate change: Updated opportunities for action." Special Report. Washington, DC: World Resources Institute. Available online at https://oceanpanel.org/publication/ocean-solutions-to-climate-change

Sequestration Durability: Potentially hundreds of years, however, more studies are needed to understand how much seaweed becomes “refractory”, or resistant to degradation on the timescale of hundreds of years^[2]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. URL .

Environmental Co-Benefits

The creation or expansion of seaweed farms may generate benefits for surrounding marine ecosystems, including but not limited to:  

• Potential amelioration of ocean acidification^[3]Mongin, M., Baird, M. E., Hadley, S., & Lenton, A. (2016). Optimising reef-scale CO₂ removal by seaweed to buffer ocean acidification. IOPscience. iopscience.iop.org/ article/10.1088/1748-9326/11/3/034023
• Contribution to biodiversity^[4]Theuerkauf, S. J., Barrett, L. T., Alleway, H. K., Costa-Pierce, B. A., St. Gelais, A., & Jones, R. C. (2021). Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps. Reviews in Aquaculture. onlinelibrary.wiley.com/doi/10.1111/raq.12584  
• Contribution to habitat provisioning^[5]Langton, R., Augyte, S., Price, N., Forster, J., Noji, T., & Grebe, G. (2019). An Ecosystem Approach to the Culture of Seaweed. NOAA, 30.
• Contribution to heavy metal and nutrient pollution removal^[6]Zheng, Y., Jin, R., Zhang, X., & Wang, Q. (2019). The considerable environmental benefits of seaweed aquaculture in China. Stochastic Environmental Research and Risk Assessment, 33. doi.org/10.1007/s00477-019-01685-z ,^[7]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561  
• Contribution to fishery enhancement^[8]Rimmer MA, Larson S, Lapong I, Purnomo AH, Pong-Masak PR, Swanepoel L, Paul NA. Seaweed Aquaculture in Indonesia Contributes to Social and Economic Aspects of Livelihoods and Community Wellbeing. Sustainability. 2021; 13(19):10946. https://doi.org/10.3390/su131910946 ,^[9]Theuerkauf, Seth J., et al. “Habitat value of bivalve shellfish and seaweed aquaculture for fish and invertebrates: Pathways, synthesis and next steps.” Reviews in Aquaculture, vol. 14, no. 1, 2021, pp. 54–72, https://doi.org/10.1111/raq.12584.
• Improved water clarity through the facilitation of settlement of fine sediments^[10]Jiang, Z., Liu, J., Li, S., Chen, Y., Du, P., Zhu, Y., Liao, Y., Chen, Q., Shou, L., Yan, X., Zeng, J., & Chen, J. (2020). Kelp cultivation effectively improves water quality and regulates phytoplankton community in a turbid, highly eutrophic bay. The Science of the Total Environment, 707, 135561. doi. org/10.1016/j.scitotenv.2019.135561
• Nutrient cycling^[11]Cotas, João, et al. “Ecosystem Services provided by seaweeds.” Hydrobiology, vol. 2, no. 1, 2023, pp. 75–96, https://doi.org/10.3390/hydrobiology2010006.
• Restoring the role of marine organisms in the carbon cycle^[12]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  

Environmental Risks

• Burning of fossil fuels associated with production activities (harvest, transport, processing)
• Production of CH4, N2O, and other potentially hazardous gases by the macroalgae
• Impacts to biodiversity and ecosystem function from farming operations may include: enhanced disease and parasite risk, alteration of population genetics, introduction of non-native species into new environments
• Enhancement in epiphytic calcifiers that could offset carbon sequestration through calcification-induced CO2 release.
• Reduced phytoplankton production in and around seaweed farms due to competition for nutrients and light
• Changes in light and nutrient availability (including possible changes in ocean albedo due to canopy cover)
• Entanglement of marine megafauna
• Addition of noise pollution due to vessel traffic and machinery

Social Co-Benefits

Seaweed farming may generate many benefits for surrounding communities and economies.

• Basis for circular marine bio-economies, generating multiple benefits^[13]Zhang, Xueqian, and Marianne Thomsen. 2019. "Biomolecular Composition and Revenue Explained by Interactions between Extrinsic Factors and Endogenous Rhythms of Saccharina latissima" Marine Drugs 17, no. 2: 107. https://doi.org/10.3390/md17020107  including integrated multi-trophic farming efforts to collocate multiple species (i.e., seaweed and shellfish)
• Education and research – the pursuit of restoring blue carbon for CDR will naturally add value to the existing body of research and can provide valuable educational experiences for the upcoming generation of scientists and conservationists.

Social Risks

While some risks, outlined below, do exist, there may be a higher tolerance for such risks given the long list of potential benefits and co-benefits (both environmentally and socially).

• Potential for conflict with other industries (fishing, tourism etc.)^[14]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
• Potential for conflict with other uses such as commercial fisheries, shipping, marine renewable energy, and mining^[15]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.  (However, note there are also possible synergies that may exist such as co-location of seaweed farms with compatible marine infrastructure types such as the moorings for wind energy, which could reduce costs for both industries.)

Challenges

• Scaling:
  • Rough seas, light and nutrient availability, and the availability of space are all constraints.
• Measurement:
  • Difficulties in measuring and verifying how much carbon is being sequestered passively by the presence of seaweed farms.

Key Knowledge Gaps

Adapted from EDF 2022 Carbon Sequestered by Seaweed

• What is the best way to measure rates of carbon sequestration by seaweed farms?
• Which of the carbon fluxes between seaweed farms and their environment are well quantified and which are not?
• How does carbon sequestered by seaweed farms compare with other ocean-based carbon sequestration pathways?

First Order Priorities

*Adapted from EDF 2022 Carbon Sequestered by Seaweed^[16]Fujita, R. M., Collins, J. R., Kleisner, K. M., Rader, D. N., Mejaes, A., Augyte, S., and Brittingham, P. A., 2022. Carbon sequestration by seaweed: background paper for the Bezos Earth Fund - EDF workshop on seaweed carbon sequestration. Environmental Defense Fund, New York, NY. www.edf.org/sites/default/ files/2022-10/Carbon%20Sequestration%20by%20Seaweed.pdf

• Investigate and characterize carbon fluxes in coastal seaweed farms
  • Determine the best ways to measure rates of carbon sequestered by seaweed farms*
  • Investigate which carbon flows relevant to quantifying carbon sequestration by seaweed farms are well documented and which remain uncertain*
  • Estimate total greenhouse gas emissions (including N2O and CH4) and carbon sequestration associated with seaweed farms^[17]Duarte, Carlos M., et al. Carbon Burial in Sediments below Seaweed Farms, 2023, https://doi.org/10.1101/2023.01.02.522332. *