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 portrays macroalgal assemblage structure in the transitional water system Mar Piccolo of Taranto (eLTER site) from 2012 to 2023, assessing the impact of the establishment of non-indigenous species (NIS). 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) and summer peaks of NIS (>70% biomass) were recorded at Site 3, whereas Site 4 experienced a substantial NIS expansion, reaching 97% of the total biomass by 2021. Statistical clustering identified distinct indicator species for each inlet of the basin, such as Amphiroa beauvoisii (IndVal = 100) in the First Inlet and the NIS Hypnea corona (IndVal = 65.6) in the Second. Water temperature emerged as a primary driver of community shifts. Most species, including both native (Chondracanthus acicularis; rs = 0.50, p ≤ 0.05) and several NIS (Polysiphonia morrowii rs = 0.65, p ≤ 0.5; Osmundea oederi rs = 0.70, p ≤ 0.5), exhibited negative correlations with mean thermal values, while Ulva laetevirens (rs = −0.50, p ≤ 0.05) 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.