Recreational marinas are among the fastest-growing types of artificial structures in coastal urban areas. However, their construction and functioning are linked to habitat loss and fragmentation, increased pollutant levels, and the introduction of non-indigenous species (NIS). All these impacts affect native biodiversity and reduce ecosystem services. In line with the recently approved Nature Restoration Law (EU 2024/1991), marinas could be key areas for developing mitigation strategies to mitigate the impacts of these artificial structures/infrastructures on ecosystems. Increasing habitat complexity through the addition of artificial structures has been shown to promote colonization by native habitat-forming species (HFS), thereby reducing NIS and restoring ecosystem services in degraded urban areas such as marinas. However, the effectiveness of these interventions is highly context-dependent, as environmental conditions could influence mitigation outcomes.

To investigate this, a short-term colonization experiment was conducted in four marinas on the NW Iberian Peninsula under two environmental conditions: two in a non-upwelling area in northern Galicia and two in a high-upwelling area in northern Portugal. At each marina, four artificial substrates were submerged for three months, and the colonizing sessile fouling communities were examined. Our results showed that upwelling conditions significantly shaped both the abundance and multivariate structure of colonizing assemblages. Fouling communities under high-upwelling conditions exhibited lower levels of dispersion, suggesting greater homogeneity, likely driven by the dominance of two native HFS: the mussel Mytilus galloprovincialis and the barnacle Perforatus perforatus. In contrast, the abundance of these native HFS was significantly lower in non-upwelling areas, where fouling communities were dominated by the spirorbid Janua heterostropha, along with cryptogenic and NIS bryozoans such as Cryptosulla pallasiana, Tricellaria inopinata, and Watersipora subatra. Overall, our findings demonstrate that the effectiveness of mitigation strategies varies across environments, highlighting the need to adapt interventions to local environmental conditions.