The Effects of anthropogenic activities on the Epiphytic Communities of Seagrasses in the Venice Lagoon.

The Venice Lagoon, one of the biggest and most studied areas in the Mediterranean Sea due to its dynamic ecosystem, greatly suffers from anthropogenic pressures, like urbanization, industrial activities, and maritime traffic. These pressures have seriously impacted many of the ecosystems present in the lagoon, as seagrass meadows and their associated epibiotic communities which play a key role in nutrient cycling and ecosystem functioning (e.g., carbon cycling). Therefore, it is crucial to understand how urbanization and related human activities impact them. Here, we focused on the epiphyte communities associated with the three dominant seagrasses of the Venice Lagoon: Zostera marina, Zostera nolteii, Cymodosa nodosa. Leaf and Rhizome samples were collected from six sites along an urban gradient (natural, peri-urban, urban), scattered across the entire lagoon basin, on three occasions (April, June, and August) to measure leaf morphology and biomass, as well as epiphyte species richness and biomass abundance. In natural sites, the three seagrass species supported higher epiphyte biomass and richness through the seasons, while in urban sites seagrass leaves had lower epiphyte richness, dominated by opportunistic species (e.g., Ceramium sp. and Polysiphonia sp.) but had higher biomass. Peri-urban sites had intermediate values of epiphyte biomass and richness. Overall, epiphyte biomass increased from April to June in all the seagrass species, and remained stable through August; while richness increased from June to August. Interestingly, C. nodosa supported the highest epiphyte biomass, followed by Z. marina and then Z. nolteii. In contrast, epiphyte richness is highest in Z.nolteii, followed by Z. marina and then C. nodosa. Furthermore, C. nodosa leaves dominated by encrusting and filamentous algae, while Z. nolteii leaves showed lower diversity, colonized by opportunistic taxa (e.g., Ceramium sp., Polysiphonia sp., Ulva spp..). This study advances our understanding of epiphyte-seagrass interactions, emphasizing the role that urbanization and human activities have on this system. Specifically, this study shows that urbanization negatively impacts the richness of the epiphyte community, yet, it increases their overall biomass which can have repercussions on the cycling of nutrients and carbon in the system.