Structure of sessile fouling assemblages on pontoons in marinas differs between brackish and marine salinities

The number of recreational marinas has recently increased due to the growing demand for leisure boating. Floating pontoons of marinas provide novel habitats to fouling assemblages, specially to sessile habitat-forming species, that increase heterogeneity creating new habitats for mobile and sessile epibionts. Therefore, sessile species play a crucial role in sustaining biodiversity in urban areas. Despite the relevance of sessile fouling assemblages, the role of environmental factors in shaping the structure of these assemblages has not been comprehensively studied. In the present study, the abundance, richness and multivariate structure of sessile fouling assemblages associated with floating pontoons were studied in marinas under two different environmental conditions (brackish and marine), and the influence of environmental variables (salinity, temperature and dissolved oxygen) on fouling assemblages was tested. Our results showed that univariate variables and assemblage composition significantly differed between environments, with lower values of abundance and richness under brackish conditions. This pattern could result from the influence of freshwater inputs in brackish marinas, that also showed a higher level of multivariate dispersion. Results also pointed out that salinity was the most important factor, explaining 46 %, 56 % and 37 % of abundance, richness and multivariate structure of sessile fouling assemblage, respectively. The sessile assemblage in marine and brackish marinas was dominated in terms of abundance by the native habitat-forming species Mytilus galloprovincialis, which harboured most of the collected sessile epibiont species. These findings highlight the role of floating pontoons to support sessile invertebrates in urban areas where biodiversity may be heavily impacted.