Data from: Curation: heat stress responses and population genetics of the kelp Laminaria digitata (Phaeophyceae) across latitudes reveal differentiation among North Atlantic populations

We aim to understand the thermal plasticity of a coastal foundation species across its latitudinal distribution by assessing physiological responses to high temperature stress in the kelp Laminaria digitata in combination with population genetic characteristics. We hypothesize that Arctic and cold-temperate populations are less heat resilient than warm-temperate populations. Using meristems of natural L. digitata populations from six locations ranging between Kongsfjorden, Spitsbergen (79°N), and Quiberon, France (47°N), we performed a common-garden heat stress experiment applying 15°C to 23°C over eight days. We assessed growth, quantum yield, carbon and nitrogen storage, and xanthophyll pigment contents as response traits. Population connectivity and genetic diversity were analysed with microsatellite markers to relate heat resilience to genetic features and phylogeography. Microsatellite genotyping revealed all sampled populations to be genetically distinct, underlying strong hierarchical structuring between and within southern and northern clades. Genetic diversity was lowest in the isolated population of the North Sea island of Helgoland and highest in Roscoff in the English Channel. Results from the heat stress experiment suggest that the upper temperature limit of L. digitata is nearly identical across its distribution range, but subtle differences were revealed for the two populations currently at their warm limits. They respectively show a significant advantage in growth at 19°C and 21°C (Quiberon) and a lack of stress responses in photosynthetic quantum yield and xanthophyll pigments at 23°C (Helgoland). In addition, quantum yield indicated the highest heat sensitivity in L. digitata from the northernmost population in Spitsbergen. All together, these results support the hypothesis of moderate local differentiation across L. digitata's European distribution, whereas effects are likely too weak to ameliorate the species' capacity to withstand ocean warming and marine heatwaves at the southern range edge.