Mixed messages: Eco-engineered tide pools can provide both positive and negative contributions, with the possibility of increasing introduced species diversity.

With marine urbanisation on the rise, decision makers and managers are aiming to mitigate and offset impacts arising from the destruction of natural habitats. Among the potential solutions features marine eco-engineering, which aims to increase the structural complexity of marine infrastructure to enhance bioreceptivity or to diversify ecological niches. However, despite previous scientific efforts showing an increase of biodiversity and functions on such eco-engineered structures, studies rarely consider potential adverse effects such as increased introduced species diversity. In the present study we investigated whether negative contributions (i.e. increased introduced species diversity) were associated with positive ones (i.e. increased total and native epibenthic biodiversity) in two eco-engineering projects involving artificial intertidal concrete pools in two biogeographic provinces. We showed that species diversity and beta diversity were both higher on the eco-engineered sites compared to the unengineered sites, and species diversity was comparable to that of natural intertidal rocky environments. However, introduced species diversity was sometimes higher than in unengineered habitats, with some introduced species being exclusively associated with the artificial pools in one province. Furthermore, community structures on the eco-engineered sites remained distinct from natural ones, with artificial pools containing typical urban fouling species. For these reasons, we make the case that artificial pools should not be considered as offsetting the destruction of natural habitats. Eco-engineering can however be viewed as a means of increasing the attractivity of marine urban habitats, akin to city parks and green rooftops.