Marine Heatwaves Overwhelm the Buffering Benefits of Marine Protected Areas: Two Decades of Collapse in a Mediterranean Gorgonian.

Marine heatwaves (MHWs) are driving mass mortalities of coastal foundation species globally, threatening their persistence and the ecosystems they support. However, long-term demographic evidence of these impacts at regional scales is scarce. Accordingly, whether ongoing conservation actions, mainly marine protected areas (MPAs), mitigate the impact of MHWs is still an open question. Here, we address these gaps by analysing over two decades of demographic monitoring data from 49 shallow (< 40 m depth) populations of the foundation gorgonian Paramuricea clavata paired with in situ temperature records across the north-western Mediterranean (38°-43° N, 0°-8° E). Using hierarchical Bayesian models, we estimated regional-scale trends and assessed the combined influence of MHW exposure and protection status. We found that increasingly frequent extreme thermal events have caused widespread mass mortality, resulting in a 45% median decline in biomass at the regional scale over the past two decades. Within this context of extensive decline, protection temporarily buffered MHW impacts by sustaining higher overall biomass, despite protected populations experiencing greater absolute losses. Taken together, these findings indicate that local protection alone cannot offset climate-driven mortality and underscore the urgent need to integrate conservation measures with global climate action to preserve shallow marine ecosystems.