Comparative sustainability assessment of net and longline nearshore cultivation systems for Saccharina latissima in Danish waters

This study presents a comparative sustainability assessment of two nearshore seaweed cultivation systems (net and longline setups) used for the cultivation of Saccharina latissima in Danish waters. It integrates a process-based Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA). The analysis focuses on greenhouse gas (GHG) emissions and nutrient indicators, including marine and freshwater eutrophication. Economic viability was assessed using the Net Present Value (NPV) approach. Empirical data from two pilot cultivation sites were scaled to 18.75-ha seaweed cultivation areas and used to build the life cycle inventory, including infrastructure, energy use, biomass yield, and labor. Results showed that the net setup achieved yields of 8.6–10.1 kg fresh weight per meter of pipe, while the longline setup yielded 5.1 kg fresh weight per meter of main line. However, when scaled to 18.75 ha, the longline setup showed up to 1.5 times higher total yield than the net setup due to differences in the modelled spatial arrangement and design principles between the two cultivation setups. The net setup incurred higher environmental burdens (67–86 t CO₂-eq./18.75 ha/yr), whereas the longline setup showed considerably lower GHG emissions (14 t CO₂-eq./18.75 ha/yr). The longline setup also showed the highest NPV of €1.35 million, while the net setup achieved lower NPV values of €0.3–0.5 million. These findings highlight trade-offs between yield, carbon footprint, and economic return, and provide decision-support insights for seaweed farmers and policymakers. By situating the analysis within nearshore cultivation conditions, this study contributes to the development of sustainable seaweed aquaculture.