Cultivation of sugar kelp for nature restoration

Since 2016, cultivating sugar kelp as a tool for marine nature restoration has been investigated in several national and international projects, with focus on nutrient removal, economy, and ecosystem effects. In 2022-2023, Best Available Technology (BAT) was tested in large-scale (12 hectares) in Sallingsund, Limfjorden. For modelling, small-scale deployments were made in 16 maritime gardens all over Danish waters. The results indicate that kelp production in Limfjorden per m of line can exceed 5 kg fresh kelp with a nutrient removal efficiency of 23 g N and 0.9 g P. Extrapolating here using BAT, the area efficiency can be 24.4 kg of N and 0.9 kg of P per hectare. The impact of sugar kelp cultivation on current velocities and benthic light environment was very limited. Sedimentation of organic matter and related benthic oxygen and nutrient fluxes were not significantly affected. Carbon sequestration via burial or export of dissolved or particulate C was negligible. Modelling confirmed that the nature restoration potential of sugar kelp cultivation is confined to high saline, eutrophic waters, resulting here in removal of N and P after 7 months, and supporting circular bioeconomy.