Seaweed-based blue carbon potential of eco-engineered coastal structures

Natural coastal ecosystems hold substantial capacity for blue carbon storage, but habitat replacement with harbour and shoreline infrastructure has severely diminished their potential to mitigate climate change. Enhancement of topographic complexity on coastal infrastructure through eco-engineering can increase biota abundance, such as the standing stock of long-lived seaweeds, thus positively impacting carbon capture and mitigating the carbon footprint of concrete-based infrastructure. However, ecosystem services of eco-engineered structures remain largely unquantified. Here, we assessed the seaweed biomass on a large-scale, eco-engineered seawall and show that topographically complex panels supported up to 19-fold greater blue carbon storage than flat surfaces. However, estimated panel CO₂ emissions exceeded CO₂ captures by 3- to 64-fold, after two years of seaweed growth. No retrofitted panel was realistically expected to achieve carbon neutrality within its 20-year design life, despite seaweed turnover and sequestration supporting long-term blue carbon potential. Predictive calculations, using six topographic designs, four concrete mixtures, two installation scenarios, and two detritus sequestration rates, suggested that the most efficient topography for seaweed growth, produced from low-carbon concrete and integrated during the construction phase, may achieve carbon neutrality within 15-32 years. Less effective or less sustainable scenarios were expected to require 52-277 years until carbon neutrality. An active transition towards low-carbon, integrated designs is necessary to align biodiversity-focused thinking in green infrastructure design with broader principles of sustainability and climate change mitigation.