By Dr. Nora von Xylander

The cultivation of sugar kelp (S. latissima) and blue mussel (M. edulis) species is rapidly expanding worldwide, largely driven by alignment with several United Nations Sustainable Development Goals and the European Union’s Water Framework Directive aiming for “good ecological status” in all coastal waters. This is a promising approach contributing to sustainable marine ecosystem management.

Unlike traditional agricultural practices, kelp and mussel ocean farming does not compete for arable land, requires no freshwater resources, and contributes to the active removal of excess nutrients from marine ecosystems. By integrating these two native species into regenerative aquaculture systems, Algapelago’s Blue Forest project in the U.K. aims to address key ecological challenges, including nutrient cycling, eutrophication mitigation, carbon extraction and marine habitat restoration.

Algapelago’s initiative will address the pressing need for sustainable, large-scale aquaculture systems that restore marine ecosystems by deploying an advanced modular cultivation system at Algapelago’s licensed site off the coast of North Devon this year. This 5 hectares (ha) pilot system, uniquely engineered by Arctic Seaweed for high energy conditions and offshore scalability, incorporates automated seeding and harvesting technologies, with the capacity to produce up to 40 tonnes of fresh sugar kelp annually. The licensed site offers the potential to scale up operations to 116 ha.

Beyond its ecological objectives, the Blue Forest project also evaluates the natural capital value of regenerative ocean farming, offering vital insights into its long-term potential for fostering resilient marine ecosystems and supporting a sustainable blue economy.

Nutrient and Carbon Extraction by Sugar Kelp

In Europe, sugar kelp is the most extensively cultivated kelp species, with numerous research and development initiatives focused on scaling up its production. This species, having seasonal growth patterns, has shown remarkable potential for nutrient mitigation, effectively assimilating nitrogen (N), phosphorus (P) and carbon dioxide (CO₂) directly from seawater during its growth cycle. Elevated nutrient concentrations, particularly nitrate (N) and phosphate (P), are primary contributors to eutrophication in coastal waters, causing detrimental ecological, economical, and societal impacts.

By acting as a nutrient sink, S. latissima offers a natural solution to these challenges. Studies report nutrient extraction rates ranging from approximately 50 to 100 kg N and approximately 0.2 to 6 kg P per hectare (ha^-¹) per year (yr^-¹), respectively. For carbon extraction, S. latissima cultivation has demonstrated removal potentials of 2.4 t CO₂ ha^-¹ yr^-¹, with optimized cultivation methods yielding up to 5.0 t C ha^-¹ yr^-¹. Innovations in cultivation infrastructure, such as high-density line systems and multi-layered designs, have proven effective in increasing yields and nutrient extraction capacity. The optimization of net cultivation systems can achieve yields of 91.3 tonnes (t) fresh weight ha^-¹ yr^-¹, extracting 110 kg N ha^-¹ and 13 kg P ha^-¹ per year. The integration of sugar kelp farming in nutrient-rich environments, i.e., where there is increased dissolved inorganic nitrogen, has been previously shown to significantly enhance biomass production and nutrient bio-extraction. Large-scale cultivation in these conditions can yield between 220 to 340 t wet weight (WW) ha^-¹ per growing season, corresponding to nitrogen removal rates of approximately 1.2 t N ha^-¹ yr^-¹.

Blue Mussels: Nutrient Cycling, Water Quality

The blue mussel, another integral species in the Blue Forest project, contributes to nutrient and carbon cycling through its filter-feeding and shell production capabilities. Mussels remove particulate organic matter, including phytoplankton and detritus, from the water column, thereby reducing nutrient levels and improving water clarity, promoting benthic primary production and overall ecosystem resilience. Studies have shown that mussel cultivation can significantly contribute to nutrient removal, with estimates indicating the extraction of approximately 700 kg N ha^-1 yr^-¹ and 6,600 kg C ha^-1 yr^-¹ under optimal conditions. Further research has demonstrated that mussel farming can achieve a high area-specific biomass of 60 t WW ha^-¹ yr^-¹, corresponding to nitrogen and phosphate removal of approximately 600 to 900 kg N ha^-¹ and 30 to 40 kg P ha^-¹ per full one-year production cycle. Additionally, long-line mussel farming has been proposed as a cost-effective mitigation tool, with nutrient removal costs lower than many land-based measures.

However, mussel farming can also negatively influence local nutrient cycling through the sedimentation of bio-deposits and the regeneration of nutrients back into the water column. Mussels produce feces and other waste, which, upon decomposition, release nutrients back into the local marine environment. Previous research found that within three weeks of deposition, up to 13.1 percent of available nitrogen in the form of dissolved inorganic nitrogen and up to 8.7 percent of available phosphate can be regenerated back into the water column.

To maximize the nutrient removal benefits of mussel farming while minimizing potential nutrient regeneration, studies suggest harvesting within the first year of the production cycle, before bio-deposit accumulation leads to hypoxic conditions and the initial net sink of nitrogen becomes a net source. Although mussel farming can help mitigate eutrophication and improve coastal water quality while generating food, more research is needed to assess the environmental impact of mussel farming, particularly looking at the prevention of nutrient regeneration and waste (i.e., empty shells and sludge) management.

Increasing awareness of the potential of synergistic cultivation approaches and conducting environmental life cycle assessments of regenerative aquaculture will provide valuable insights for the improvement of sustainable aquaculture practices.

 

 

Synergies

The co-cultivation of sugar kelp and blue mussels in integrated aquaculture systems presents a highly promising strategy for enhancing nutrient mitigation in marine ecosystems. As extractive species, mussels and kelp contribute to nutrient cycling in complementary ways. Mussels filter feed on suspended organic matter from the water column, while kelp absorbs dissolved inorganic nutrients, such as nitrogen and phosphate, directly from seawater. This coupled interaction helps mitigate the regeneration of nutrients from mussel feces into the water column, thus maximizing nutrient uptake efficiency and overall productivity.

The integration of M. edulis with S. latissima in aquaculture systems offers multiple ecological benefits beyond nutrient removal. Mussel filtration enhances water clarity, reducing competition for dissolved nutrients and optimizing conditions for kelp growth by improving light penetration. Meanwhile, kelp canopies provide structural habitat, enhancing biodiversity and supporting the settlement of mussel larvae. Beyond nutrient mitigation and biodiversity enhancement, the co-cultivation of these two native species has potential applications for carbon sequestration and climate change mitigation. Both sugar kelp and blue mussel production present an opportunity to contribute to “blue carbon” sequestration by directly absorbing CO₂ from seawater for photosynthesis and shell formation, respectively.

Site selection plays a critical role in the success of integrated cultivation. Research suggests that mussels perform better in sheltered, nutrient-rich inner coastal areas, whereas kelp thrives in more exposed outer coastal waters with high dissolved nutrient concentrations, less resuspended particulate organic matter, and sufficient light penetration. Understanding these environmental requirements is essential for optimizing co-cultivation strategies and ensuring maximum nutrient mitigation efficiency while maximizing ecological and economic benefits.

Full Site Deployment Potential

Bideford Bay, situated on the north coast of Devon, hosts a diverse array of marine life, including fish, invertebrates, migratory birds and salt marsh habitats. This location has been selected as Algapelago’s farm site to deploy a modular integrated kelp and mussel cultivation system over a total area of 116 ha.

Nutrient sources for the bay enter primarily from the Taw and Torridge Rivers, which transport a mix of agricultural runoff and urban sewage. Algapelago’s Blue Forest Project will assess the nutrient cycling, carbon extracting, and ecological and economic potential of co-cultivating S. latissima and M. edulis at large scale.

The current kelp cultivation system setup at Bideford Bay comprises 4-by-200-m long-lines at approximately 2-m cultivation depth, seeded using twine, with a total seed line length of 0.8 km. This setup yields approximately 6 tonnes of sugar kelp at a density of 8 kg/m.

To enhance scalability and efficiency, the Blue Forest project will deploy a modular kelp cultivation system in collaboration with Arctic Seaweed (AS), whose offshore-engineered cultivation system is designed for large-scale production. The AS system features automation for rapid seeding and harvesting, a high-density seeding approach, and adjustable cultivation depths. It currently comprises 10 seeding modules, utilizing direct seeding, with a total seed line length of 4 km and an expected yield of 20 tonnes at 5 kg/m density. As the Blue Forest project evolves, the goal is to integrate mussel cultivation onto the AS rig. The mussel cultivation system will employ a dropper-based setup, comprising 2-by-100-m long-lines, with droppers spaced at 1-m intervals and extending 10 m in length. With a total seeded dropper length of 2 km, the system will rely on natural settlement and operate at 3- to 13-m depths, targeting a predicted yield of 20 tonnes at a density of 10 kg/m.

At full production potential, the integrated cultivation rig covering 116 ha will comprise 256 km of sugar kelp seed lines yielding 6 kg/m and a total of 197 km of mussel droppers yielding 8 kg/m. This large-scale production system is projected to yield 1,536 tonnes of sugar kelp and 1,576 tonnes of blue mussels per year.

Based on scientific literature values for optimized large-scale cultivation systems, 1 t of sugar kelp is estimated to extract approximately 1.2 to 3.5 kg N, 0.14 kg P and 15.0 to 54.8 kg C per year. This results in a total nutrient and carbon extraction potential of approximately 1,851 to 5,421 kg N; 219 kg P; and 22,588 to 84,118 kg C per year for 1,536 t of sugar kelp.

Meanwhile, 1 t of blue mussels is expected to extract approximately 10 to 15 kg N, 0.5 to 0.7 kg P, and 110 kg C per year. This will yield a total nutrient and carbon extraction potential of approximately 15,760 to 23,640 kg N; 788 to 1,051 kg P; and 173,360 kg C per year for 1,576 t of blue mussel cultivation.

In total, Algapelago’s Blue Forest integrated cultivation system is estimated to produce 3,112 tonnes of sugar kelp and blue mussel biomass and extract approximately 17.6 to 21.2 t N, 1.0 to 1.3 t P, and 195.9 to 257.5 t C per year for the full farm layout.

Ecosystem Services

Valuation of ecosystem services is being increasingly used as a financial tool to support aquaculture practices and sustainable marine management. The Economics of Ecosystems and Biodiversity initiative has been instrumental in quantifying the economic benefits of ecosystem services, estimating the total value of coastal and marine ecosystem services at $50 trillion per year.

A cost-benefit analysis of S. latissima cultivation also revealed promising potential for profitability, demonstrating a positive net present value of approximately €446,000 per hectare over a 10-year period.

Proper accounting of ecosystem services provided by regenerative aquaculture, such as those delivered by the Blue Forest Project, incentivizes greater investment in regenerative aquaculture through blue financing, carbon credit schemes and marine biodiversity-linked funding mechanisms.

Conclusion

As Algapelago prepares to deploy its modular cultivation system, it will test the scalability and ecological impacts of co-cultivating S. latissima and M. edulis. By demonstrating the nutrient mitigation potential and broader ecological benefits of this integrated approach, the project will position regenerative aquaculture as a cornerstone of sustainable marine resource management and a critical tool for combating climate change.

Overall, this integrated scalable and environmentally conscious approach will not only remove excess nutrients from coastal waters but also provide opportunities for food and biomass production while enhancing marine biodiversity and ecosystem resilience. The Blue Forest project also provides significant potential for natural capital value, enhancing ecosystem services that support biodiversity, water quality and climate regulation. By integrating economic viability with ecological benefits, this project exemplifies how regenerative aquaculture can serve as a scalable and sustainable solution to environmental challenges, providing both tangible financial returns and invaluable natural capital gains. By recognizing and quantifying these benefits, Algapelago’s Blue Forest project advances sustainable aquaculture practices while aligning with global conservation and climate goals.

Acknowledgments

The author would like to thank Dr. Alejandra Zazueta Lopez, a biodiversity scientist at Tunley Environmental, for reviewing this article.

 

 

Dr. Nora von Xylander is a marine biodiversity and sustainability scientist at Tunley Environmental.