Seaweed cultivation and its impacts on biodiversity: a study from the Faroe Islands

Abstract Seaweeds are said to be ecosystem engineers, supporting biodiversity with habitat creation. This study assessed how large-scale seaweed cultivation by the company in the Faroe Islands using three-year cultivation cycle and partial harvesting affects local marine biodiversity. Methods included (a) comparing cultivation site organisms to biodiversity in natural seaweed beds (b) fish stomach content analysis and (c) eDNA samples taken in a transect, gradually distancing from the cultivation site. The dropper line sampling results from the three-year cultivation cycle practice from the seaweed cultivation site showed a significant increase in biodiversity enrichment over time. While single-season seaweed cultivations often provide temporary habitat, our results indicate that long-term cultivation combined with only partially harvesting—leaving holdfast, stem and part of the blade—offer greater ecological advantages. It was further concluded that cultivated Saccharina latissima supports a subset of the taxa also found in near natural seaweed habitats formed in part by S. latissima , indicating that the cultivation does not create entirely novel communities. The key difference is the lack in cultivations of benthic organisms inhabiting natural seaweed forests. Overlap of organisms found within fish stomachs and associated with natural and cultivated seaweed indicates that fish might find their prey also in cultivated seaweed structures. Comparison of results obtained by morphology- and eDNA-based analysis highlights biases of both methods, concluding to a complementary approach. Standardized biodiversity assessment protocols for seaweed cultivations are lacking; questions remain on their impact and to what extent it can be influenced.