Broadly distributed seaweeds, such as the boreal-temperate kelp species Saccharina latissima, contain a multitude of metabolites supporting acclimation to environmental changes, such as temperature and salinity. In Europe, S. latissima occurs along the coasts from Spitsbergen to Portugal, including the Baltic Sea, exhibiting great morphological plasticity. We investigated the morphological and biochemical traits of field-collected sporophytes from 16 different locations across the species entire European distributional range in relation to local abiotic conditions (sea surface temperature, sea surface salinity, sampling depth). By statistically linking morphological (frond length:width), biochemical (mannitol, phlorotannins, C:N), and genetic data of the mitochondrial cytochrome-c-oxidase I gene (COI-5P) to the geographic abiotic information, we aimed to obtain first insights into the site-specific adaptive features of this species. Mannitol concentrations were analyzed using a HPLC. The molar C:N ratio was analyzed with an elemental analyzer. Phlorotannins were analyzed using the photometric Folin-Ciocalteu method.

COI-5P sequences were deposited in GenBank under the accession no.s ON661800-ON661872 (https://www.ncbi.nlm.nih.gov/nuccore/ON661800 to https://www.ncbi.nlm.nih.gov/nuccore/ON661872).