Combining environmental DNA and traditional sampling to assess the role of kelp aquaculture as an artificial habitat in Norway

Kelp aquaculture is expanding rapidly in the temperate Atlantic, yet its ecological role as an artificial habitat remains poorly understood. This study assessed the biodiversity associated with wave-exposed kelp farms in coastal Norway using both environmental DNA (eDNA) and traditional sampling methods. Fish and invertebrate communities were surveyed at two kelp farms and compared to nearby natural kelp forests and pelagic control sites. Results from gill-net fishing, camera observations, and invertebrate sampling revealed lower species richness and abundance at kelp farms compared to natural habitats. eDNA analyses (12S and CO1 markers) showed few differences in community composition between sites, and no species were identified as indicators of kelp farms. Traditional sampling detected the presence of the amphipod Caprella mutica in the kelp farms, indicating that operators can play roles in alien species spread. Overall, kelp farms supported distinct but less diverse communities than natural kelp forests, being more similar to pelagic control sites. Finally, eDNA provided limited insights compared to traditional methods. These findings suggest that kelp farms currently provide limited habitat provisioning in Norway, likely as a result of how and where kelp aquaculture is currently performed. We suggest the need for population-level assessments for relevant species like Atlantic lumpfish (Cyclopterus lumpus) and locally based assessments in collaboration with authorities to define nature-positive or negative outcomes to balance ecological and economic goals in the growing seaweed aquaculture industries of the Atlantic.