Integrating morphological and molecular approaches to assess photosynthetic species diversity in the Venice Lagoon

The Venice Lagoon is the largest transitional environment in the Mediterranean Sea and an important biodiversity hotspot. However, it is under increasing climate and anthropogenic pressures, highlighting the need for long-term monitoring of its biodiversity. Three monitoring campaigns were conducted between 2023 and 2025, sampling seaweeds, seagrasses and halophytes. Samples were identified using an integrated morphological and molecular approach, with morphological traits guiding the selection of the most appropriate DNA-barcoding markers. High-quality sequences were obtained for 50% of the samples and compared with reference databases. To improve identification reliability, phylogenetic trees were reconstructed for each major taxonomic group to assess clustering patterns and sequence relationships. Two independent species-delimitation methods, ASAP and GMYC, were also applied. Overall, 81 taxa were identified, 13 belonging to Chlorophyta, 11 to Heterokontophyta, 28 to Rhodophyta, and 29 to Tracheophyta. We further conducted a targeted survey on Sporobolus species using the trnL-trnF marker and genome-size estimates to distinguish between native and non-indigenous ones. The presence of S. x townsendii was confirmed in the central and southern lagoon, while the non-indigenous S. anglicus was more prevalent than the native S. maritimus.