Species loss and decline in taxonomic diversity of macroalgae in the Gulf of Trieste (Northern Adriatic sea) over the last six decades.

Assessing historical changes in marine biodiversity at regional or local scales is often challenging due to insufficient long-term data for most marine organisms. Yet, these assessments are crucial to understanding potential long-term variation in the species pool in response to complex and interacting local and global environmental changes. Here, we performed a comprehensive review of scientific and grey literature, archival records and floristic data spanning over the last two centuries to reconstruct an updated and revised taxonomic dataset of macroalgae in the Gulf of Trieste (Northern Adriatic Sea), one of the most exposed to human-driven pressures and climatically vulnerable regions in the Mediterranean Sea. The subset of data from 1960 to present, encompassing nearly all available records, was used to assess the contribution of species replacement and gain/loss to temporal β-diversity and to test for changes in the taxonomic distinctness of the species pool over the past six decades. We identified 68 species that have never been recorded again since 1990, indicating their likely local extinction. The major change, however, was due to species replacement and to a reduction in the taxonomic breadth of macroalgal diversity, as highlighted by a significant decrease in the Average Taxonomic Distinctness of the species pool, especially along the Italian coast. The loss of species has mainly affected habitat-formers (e.g., Cystoseira sensu lato) and species with Atlantic/Circumboreal and Mediterranean affinities, which were replaced by turf-formers and species with Pantropical/Cosmopolitan/IndoPacific affinities. While multiple human impacts (e.g., coastal artificialisation, unbalanced N/P ratios) might have contributed to the ongoing change in macroalgal diversity, the observed decline of cold-affinity species in favour of warm-affinity species pointed out a critical role of exacerbating climatic changes. Our study demonstrated that historical reconstructions of species records coupled with effective indicators for the analysis of presence/absence data can help quantify long-term biodiversity changes and provide valuable insights into their possible causes.