Assessment of the colonization and dispersal success of non-indigenous species introduced in recreational marinas along the estuarine gradient

The introduction of non-indigenous species (NIS) is considered as one of the main causes associated with bio-diversity loss. The number of NIS has increased significantly emphasizing the need to know and manage the processes of biological invasions. Recreational marinas are points of entry for potential colonizers and can act as stepping-stones for the spread of these species. The main objectives of this work were to assess the distribution patterns of the fouling communities within recreational marinas in the Tagus estuary and to evaluate NIS dispersion ability. The fouling communities were assessed at four different recreational marinas and neighbouring hard substrate areas along the estuarine gradient of the Tagus estuary. The native communities of those recreational marinas showed a spatial gradient consistent with the estuarine gradient, with higher similarity between marinas with higher marine influence. A total of 14 NIS were identified within the recreational marinas but none occurred at the marina with higher freshwater influence. Although NIS distribution pattern reflected, to some extent, the estuarine gradient, there was a lower heterogeneity between locations when compared to native communities, evidencing a greater tolerance of NIS to a wider range of environmental conditions. Nine of the NIS identified at recreational marinas were also found in the neighbouring areas, suggesting their dispersal ability. NIS that seem to be able to disperse outside the marinas have planktonic larval stages and higher environmental tolerance, which seems to have contributed to the successful spread. These results emphasize the importance of monitoring NIS occurrence in recreational marinas in order to have an early warning on the arrival of species with higher invasion risks and to prevent its dispersal to sensitive ecosystems.