Interactions between foundation sessile species and mobile assemblages on artificial hard substrates: a study on...

Within new urban marine environments, the impact of recreational ports on the processes underlying the structure and dynamics of their emerging biodiversity is largely underestimated. The artificial structures that form these marinas represent new hard substrate habitats that can be colonized by diverse sessile taxa, comprising foundational species. These species influence the associated mobile assemblages through the biogenic microhabitats they create. Two artificials structures (pilings and floating pontoons) were monitored across five marinas over a period of two years to evaluate facilitation processes between sessile foundation species and mobile assemblages. Using a mixed-effects modelling approach on key ecological variables (abundance, richness, and diversity and composition) we examine covariance relationships between both assemblages. Additionally, an innovative multivariate approach using redundancy analysis was conducted to assess the individual influences of foundation sessile species and their functional groups on mobile assemblages. Despite a strong site effect (up to 80% of the variance in models), our results partially confirm the stress gradient hypothesis, as the abundance and species richness are more strongly and positively correlated between the two assemblages at the tide exposed piling habitats compared to floating pontoons. Notably, non-indigenous species played a dominant role in shaping community structures and interspecific relationships. These results confirm the anthropogenic pressures these novel ecosystems are facing and raise critical management challenges concerning the influence of biological invasions on port ecosystems in port environments.