A train of marine heat waves affected benthic local beta but not alpha diversity

Over the past decades, ocean temperatures have risen substantially, with far-reaching consequences for marine biodiversity and ecosystem functioning. Marine heat waves (MHWs), have increased in both duration and frequency, leading to shifts in species composition and ecosystem processes. We analysed the effects of repeated MHWs on recruitment of benthic communities in a Marine Protected Area (MPA) in Southern Sardinia. We used standardized sampling units, Autonomous Reef Monitoring Structures (ARMS), combined with high-throughput sequencing of mitochondrial cytochrome c oxidase subunit I. This allowed us to measure short-term variations in benthic biodiversity at two sites, Berni and Santa Caterina, which initially had different temperatures. We found 241 eukaryotic families belonging to 22 different phyla recruited on ARMS. Both before and after MHWs, Porifera were the most abundant phylum (21-37 %), followed by Bryozoa (27-16 %) and Cnidaria (15-18 %). Before MHWs, the two sites exhibited different taxa richness and taxonomic composition, with significantly higher alpha diversity in the warmer site. After the MHWs train, alpha diversity did not change significantly. However, the taxonomic composition of the two sites tended to homogenise, resulting in a significant decrease in beta diversity. In both sites, the variation in the benthic assemblage structure after the MHWs train was driven by the decreased abundance of slow-moving and less heat-tolerant taxa (e.g., Bryozoa, Nudibranchia, Rhodophyta). This decrease was counterbalanced by an increase in mobile and more heat-tolerant ones (e.g., Decapoda). The short-term outcome of repeated MHWs is a homogenization of the benthic recruits' community, without any significant change in the number of taxa. Because biodiversity and ecosystem functioning are generally positively linked, the short-term effects of MHWs may have limited impacts on ecosystem functioning. However, they could reshape the benthic community composition, altering the ratio of 'winners' to 'losers' taxa contributing to ecosystem functions.