Particulate and dissolved organic carbon losses in high latitude seaweed farms

The role of macroalgae as natural sinks for carbon dioxide (CO₂) has long been recognized, and interest for climate mitigating solutions from seaweed cultivation is quickly rising. Erosion of biomass provides natural avenues for carbon sequestration at sea, yet data is still lacking for important European cultivars, particularly combining particulate (POC) and dissolved (DOC) organic carbon losses. In this study, data is provided on carbon uptake, lamina growth and erosion over two consecutive seasons for the kelp Saccharina latissima (Phaeophyceae) deployed in Autumn and Winter in Hitra, Norway. A short-term carbon exudation experiment was performed with the same kelp in 2023. By April, the typical harvest time for food applications, average losses to POC and DOC pools amounted to 15 and 34 g C m^-2 yr^-1, respectively, or 9 % and 19 % of the carbon net primary production (C-NPP) of the farm. Combined POC and DOC losses reached 101-247 g C m^-2 yr^-1 (40-47 % of C-NPP) by June. DOC exudation rates reached 4.1-7.6 mg C g^-1 h^-1 after 4 h incubation, reducing significantly after 24 h. On average, 29 % and 12 % of the carbon fixed by S. latissima was released as DOC from Autumn and Winter deployments, respectively, before the progression of bryozoan biofouling. POC and DOC losses provide a continuous source for carbon deposition, burial or further breakdown into RDOC, crucial for environmental impact assessments and carbon accounting methodologies. The study provides valuable data for future research on macroalgae cultivation and its contribution to global carbon mitigation efforts.