Exploring the Structure of Seaweed Assemblages Under the Pressure of Non-Indigenous Species (NIS) in the Transitional Water System Mar Piccolo of Taranto (Mediterranean Sea, Southern Italy)

This study characterizes macroalgal assemblage structure in the transitional water system Mar Piccolo of Taranto (eLTER site) from 2012 to 2023, assessing the impact of non-indigenous species (NIS) establishment. Seasonal sampling at three sites evaluated diversity and biomass variation through PERMANOVA, PCoA, PERMDISP and Indicator Value (IndVal) analyses. Results reveal significant spatio-temporal heterogeneity: Site 1 remains dominated by native species (>70% biomass), summer peaks of NIS were recorded at Site 3, whereas Site 4 experienced a substantial NIS expansion, reaching 97% of total biomass by 2021. Statistical clustering identified distinct indicator species for each inlet, such as Amphiroa beauvoisii in the First Inlet and the NIS Hypnea corona in the Second. Water temperature emerged as a primary driver of community shifts. Most species, including both native (Chondracanthus acicularis) and several NIS (Polysiphonia morrowii, Osmundea oederi), exhibited negative correlations with mean thermal values, while Ulva laetevirens showed greater tolerance. These findings highlight the importance of LTER monitoring in demonstrating that the Mar Piccolo’s resistance to NIS pressure is non-uniform across the basin. Under a global warming scenario, thermal forcing is actively reshaping macroalgal assemblages.