Climate change has been increasing at an unprecedented rate in the last decades. Warming has been causing a variety of impacts in marine ecosystems, including shifts in the geographical ranges of species. The North-western Iberian Peninsula coast is particularly interesting to study distribution shifts as it features a strong latitude thermal gradient, establishing a biogeographical transitional region where several cold- and warm-adapted species have their equatorward or poleward distributions. In the early 2000s, it appeared that while warm-water species were already responding to warming, cold-water species did not display a coherent response. It is now necessary to gather fresh data on the distribution of the same group of species to understand if current patterns of change confirm or deny those observed back then, which may give us important clues about the mechanisms setting species limits in the area.

This study aims at providing a fine-scale description of the occurrence of intertidal macroalgae species in the rocky shores of the North-western Iberian coast. Specifically, the spatial distribution and semi-quantitative abundance of 34 native and invasive species were assessed at 70 wave-exposed locations. This included 19 species of cold-water affinity, 10 species of warm-water affinity and 5 neutral species. When contrasted with historical observations, these new data can be used to quantify and map biodiversity change in the region, as well as help understanding the mechanisms constraining species distributions.