Invasive ascidians: How predators reduce their dominance in artificial structures in cold temperate areas

Artificial structures can provide suitable space for invasive species through a colonisable surface and refuge against predators. Besides several physical factors, invasive species can present different colonization success and dominance on structures that are connected to the sea floor (i.e. fixed) compared to structures that are suspended, depending on the differential predation pressure that affect both structures. Benthic and nektonic predators can reach fouling communities on fixed structures, while only nektonic predators can affect communities growing on suspended structures, depending on the distance to the sea floor. Only few studies tested both communities of predators at the same time in different artificial structures. In this study, we evaluated the effect of benthic and nektonic predators on the cover, composition and diversity on fouling communities and on the dominance of invasive ascidians on these communities, in two different types of artificial structures. We performed an experiment in the port area of Puerto Madryn (Southwestern Atlantic, Argentina) to compare the fouling community development between fixed and suspended structures and among different predator exclusion treatments. Results showed that benthic predators exerted a higher predation pressure than nektonic predators on the cover, composition and diversity of the fouling communities. In the absence of benthic predators, and even where nektonic predators were not excluded, communities were greatly dominated by two invasive ascidians, Ascidiella aspersa (Muller, 1776) and Ciona robusta Hoshino and Tokioka, 1967. Our results suggest that in this type of cold temperate ports, fouling organisms find refuge from benthic predators growing on suspended structures, where nektonic predators exert low predation pressure. We propose that, since benthic predators in temperate latitudes have an essential role decreasing invasive ascidians dominance and the cover of fouling communities, future studies should consider both predator communities, i.e. nektonic and benthic, in experimental designs performed to test biotic resistance and predation pressure.