Abstract: Coastal ecosystems in general and intertidal systems in particular, are likely to be pro- foundly affected by climate change. On temperate rocky shores, the responses of foun- dational seaweed species to climate change can potentially profoundly change the entire intertidal ecosystem. The overall objective of our work is to identify the impact of climate change on seaweeds meadows in the North-Atlantic rocky intertidal with focus on the brown canopy-forming macroalga Fucus serratus. More specifically, we aimed to identify the impact of climate change on 1) the geographical distribution, 2) the thermal tolerance, and 3) the genetic changes, of the brown alga F. serratus. Ecological Niche Model pre- dictions until year 2200 suggest that habitat-provisioning key species on North-Atlantic rocky shores will shift northwards as an assemblage, and while disappearing from warm- temperate regions, they may establish on Arctic shores. These predictions, however, do not take plastic and adaptive responsiveness into account. Thus, we further assessed the plastic response of F. serratus in four populations along its distribution range in West Europe in common garden heat stress experiments. The four subpopulations showed differences in photosynthetic performance and heat shock protein gene up-regulation. Phenotypic plasticity was found to be likely insufficient to prevent the predicted extinc- tion of F. serratus from the North-Iberian Peninsula. A temporal and spatial genome scan approach was used to assess the adaptive responsibility and revealed that the sub- populations differed in genetic diversity and that the most southerly subpopulations (i.e. in Spain) suffered a significant loss of allelic richness and an abundance decline of about 90 % during the past decade. The thesis predicts that temperate seaweeds will most Page 1 likely face extinction from rocky shores in southern Europe and this will likely trigger major ecosystem changes in the entire associated ecosystem.