The sustainable expansion of seaweed as a restorative aquaculture : using a carrying capacity framework that integrates barriers to expansion and ecosystem services

Seaweed-based lower trophic aquaculture (LTA) offers a promising pathway to meet rising resource demands while restoring degraded coastal ecosystems. Seaweeds provide valuable ecosystem services (ES), including water-quality improvement, nutrient removal, and habitat provision, while supplying biomass for food, feed, cosmetics, and pharmaceuticals. Europe’s seaweed sector is still emerging, making precaution and clear sustainability thresholds essential. This thesis investigates how seaweed aquaculture can expand responsibly by combining desk research with field studies at Ocean Rainforest (Faroe Islands), Ocean Forest (Norway), and Algolesko (France). Three ES, carbon sequestration, nutrient bioextraction, and biodiversity support, were evaluated. While sequestration during growth remains uncertain and relatively small, larger climate benefits may come from product substitution. Nutrient removal proved measurable but highly site-specific. Biodiversity findings showed that multi-year systems, such as ORF’s, significantly enhance biodiversity and resemble natural seaweed communities. Negative impacts remain poorly understood, underscoring the need for systematic monitoring. To guide sustainable scale-up, the thesis develops a carrying capacity (CC), based framework that defines acceptable environmental thresholds and integrates ES into decision-making. Although CC is recognized in the Ecosystem Approach to Aquaculture, it is not yet embedded in policy, and European expansion faces barriers including governance fragmentation and market immaturity. Quantifying and valuing ES could help overcome these barriers by improving financial viability, spatial planning, and social acceptance. The thesis highlights that incorporating CC, ES, and social dimensions is essential for enabling restorative and sustainable seaweed aquaculture in Europe.

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