Biofouling assemblages on anthropogenic structures reflect seafloor access for benthic predators

The design of anthropogenic marine structures can have unintended consequences for ecological communities. We describe differences in biofouling assemblages between two primary anthropogenic habitats—pontoons and pilings—in a marina in British Columbia, Canada, using multiple measures of diversity (i.e., richness, evenness, and Shannon indices) and multiple metrics of variability in taxonomic composition (i.e., Jaccard and modified Gower methods) using two complementary surveys. First, a video-transect survey revealed abundances of crawling benthic predators to be 19 times greater on pilings than on pontoons. Second, a photo-quadrat survey of sessile invertebrates revealed moderate differences in diversity but differences of 86% in taxonomic composition and 88% in non-indigenous species (NIS), owing largely to the dominance of mussels (Mytilus species complex) on pontoons and their absence on pilings. We discuss several environmental factors associated with the design of anthropogenic infrastructure and propose that contact with the seafloor is the key driver of observed differences between the biofouling assemblages at this site. Contact with the seafloor permits access to crawling predators and thereby drives the abundances of ecosystem-engineering taxa. Patterns of taxonomic composition and assemblage dispersion indicate that biofouling organisms are affected by these phenomena according to their roles either as prey or as competitors with mussels and not whether they are native or NIS. Finally, we consider how the growing body of ecological studies on biofouling communities such as ours can inform the implementation of infrastructure for the purposes of enhancing biotic diversity and resilience.