Abstract: Macroalgae forests are suffering important alterations around the world due to climate driven changes. In many cases, canopy-forming species are being replaced by turfing algae, leading to impoverished ecosystems with lower diversity and a simplified food-web complexity. The present study describes the shifts observed since 2005 in the coast of Cantabria, a key transitional region for the distribution of cold-temperate and warm-temperate biological communities of the NE Atlantic, and relates them with changes in the environmental conditions. The study analyses the distribution and abundance of subtidal macroalgae species found in 2005 and 2017/2019 following the same methodological approach. As physical variables sea surface temperature (SST) and significant wave height (Hs) are analysed since 1985, with a special emphasis in the occurrence of extreme events. The results obtained show an almost disappearance of kelps, a notable reduction of other canopy-forming species and an increasing coverage of turf-forming and non-indigenous species. Moreover, significant differences are observed among coastal zones and depth ranges. These changes seem to be related to a combined effect of i) a long-term SST increasing trend observed since the 1980s and ii) a short-term occurrence of several extreme warming events observed since 2003. The loss of resilience produced during the long-term stress could have facilitated the switch in the ecosystem, accelerated by the short-term extreme events. The dramatic consequences that could arise from the disappearance of macroalgae forests calls for the adoption of management measures for the conservation of this valuable component of the coastal ecosystems.
