Port environmental gradients and non-indigenous species along the canal-port of Ravenna

Ports are among the main transit points for non-indigenous species worldwide. The large availability of artificial substrates and reduced water quality usually lead to simplified benthic communities that offer limited biotic resistance against non-indigenous species. This study aims to highlight the role of environmental gradients within ports in affecting the settlement, permanence, and diffusion of nonindigenous species. The canal-port of Ravenna (north-western Adriatic Sea), extending 11 km inland, with the high traffic of commercial vessels, represents an ideal case study. This canal-port generates a water pollution and confinement gradient, reflected in a characteristic substitution of intertidal assemblages along the concrete quays. The outer port and the first section of the canal walls are characterized/dominated by beds of the native mussel, Mytilus galloprovincialis, whose assemblages are rich in native species. Towards the innermost areas of the port, the native mussel is replaced by the invasive mussel Xenostrobus securis. After a short transition zone characterized by the coexistence of both species, X. securis dominates the substrate, accompanied by an increase in non-indigenous species. Beds of X. securis could provide ideal conditions for other NIS by creating some facilitating interaction.