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Urban structures are a conspicuous, yet poorly understood component of the marine environment along urban coastlines. Previous work has shown that different types of structures support different diversities and relative abundances of sessile marine organisms. Studies on the effects of substratum composition, age, orientation and the effects of predation have failed to explain the observed differences in assemblages that develop on different types of structures. We assessed the model that differences in epibiotic assemblages between pontoons and pilings were due to the floating nature of pontoons versus the fixed (relative to the seafloor) nature of pilings, as opposed to other structural differences (colour, shape, surface type, etc.). Two additional (nonmutually exclusive) models were also tested. These were that the presence of a 'swash zone' constantly exposed to wave action and/or attachment to the benthos could cause differences between pontoons and pilings. We hypothesized that purpose-built experimental structures that floated would develop different assemblages from structures that were held fixed relative to the seafloor, regardless of whether they were pontoons or pilings. If swash were important, then structures floating just below the surface would differ from structures floating at the surface. If attachment to the benthos were important, then pilings attached to the benthos would differ from all the other structures. Multivariate analyses supported the hypothesis that both floating and the presence of swash were important in creating a typical pontoon assemblage, while other factors (type of structure, attachment to the benthos) were not. Several taxa contributed to these differences, including the mussel Mytilus edulis, the polychaete tubeworm Hydroides sp. and several algal taxa. Differences between fixed and floating structures have implications for the interpretation of previous studies done on floating docks. More studies of this kind are needed in order to inform the managers of urban waterways about the implications of adding different types of structures to the coastal environment.

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