Recurrent marine heatwaves threaten the resilience and viability of a key Mediterranean octocoral species

Long-term ecological studies are essential for understanding the resilience of temperate reefs to ocean warming and the response of their habitat-forming species to recurrent marine heatwaves (MHWs).This study evaluated the long-term response (2005-2021) of the habitat-forming octocoral Eunicella singularis (Esper, 1791) to recurrent MHWs in a Mediterranean marine protected area established more than 25 years ago, where restrictions on fishing and diving activities are well enforced.During the study period, there were two mass-mortality events (MMEs), in 2007 and 2018, that dramatically disrupted the local gorgonian population. The MMEs were triggered by different temperature dynamics, which ultimately led to differential patterns in mortality rates.The record-breaking MHW in 2018 prompted one of the largest MMEs ever recorded in the Mediterranean Sea, which affected 75% of the individuals in the gorgonian population and killed 12.5% of colonies.Furthermore, MHWs were detected in all years from 2014 to 2021, preventing injured tissues from fully recovering, leading to significant accumulated damage at the end of the study period (30 times higher than the levels prior to the first MME), and a marked decrease in resilience that threatens the population viability of this long-lived species.The octocoral studied is one of the most important structural and biomass contributors to one of the most diverse Mediterranean communities. These results underscore the great risk that annual MHWs pose to the long-term integrity of the circalittoral rocky bottoms of the Mediterranean coasts.In the face of climate change, MPAs must provide areas of reduced anthropic stress for gorgonian populations, enabling the restoration and maintenance of natural processes. Additionally, MPAs should serve as sentinel areas to test the effectiveness of coral restoration actions, which can help managers and scientists to estimate the usefulness of these techniques.