Annette Bruhn1,2, Michael Bo Rasmussen1,2, Teis Boderskov1,2, Helge Juul Skou1, Georgiana Apetroaei1, Mette Baunsø Kring 1, Freja Christiansen1, Helle Buur1, Marianne Thomsen3, Louise Juul2,4, Signe Hjerrild Nissen2,4, Palle Horn5, Anna Schriver6, Trine Kastrup Dalsgaard2,4,7.
1 Department of Ecoscience, Aarhus University, CF Møllers Alle 3, 8000 Aarhus C, Denmark 2 Centre for Circular Bioeconomy (CBIO), Aarhus University, Denmark 3 Department of Food Science, University of Copenhagen, Rolighedsvej 26, 1958 Frederiksberg Denmark 4 Department of Food Science, Aarhus University, Agro Food Park 48, 8200 Aarhus N, Denmark 5 HedeDanmark, Klostermarken 12, 8800 Viborg, Denmark 6 WSP, Sønderhøj 8, 8260 Viby J, Denmark 7 Centre for Innovative Food Research CiFood, Agro Food Park 48, 8200 Aarhus N, Denmark
Sea lettuce (Ulva spp.) forms massive green tides in nutrient-rich water bodies around the globe. The decomposing of Ulva biomass reduces environmental quality and recreative value. Harvest of Ulva prior to decomposition is of increasing interest as a tool to improve environmental quality and contribute to the circular bioeconomy.
In a science-industrial Danish case-study, the extent of Ulva green tides was documented between 2018 and 2022 using drone and image technologies. Gentle mechanised harvest of the Ulva biomass was tested and documented over three seasons, registering environmental impacts and Ulva biomass quality for food and feed purposes. The biomass abundancy peaked in late summer with biomass densities up to 58 tonnes of wet weight per ha. Harvest was challenged by the Ulva gathering at shallow depths, and by limited options for selective harvest targeting optimal biomass quality. The potential for removal and re-use of nutrients was estimated to 110 kg of nitrogen and 30 kg of phosphorus per ha. The harvested biomass contained up to 14% of epi-fauna animals of relatively low biodiversity, indicating a ‘moderate’ ecological status (Shannon index between 1.04 and 1.71). The cost of harvest was estimated to 540 US$/ton Ulva dry matter (DM). The biomass crude protein content ranged between 6 and 20% of DM, concentrating up to 5-fold by post-harvest processing.
Harvest of Ulva can be used as a tool to improve environmental quality in nutrient-rich water bodies, however improving harvest method and post-harvest processing is needed for utilising the biomass for food and feed.