Oceanographic and biological influences on recruitment of benthic invertebrates to hard substrata on the Oregon shelf

The number of anthropogenic substrata in the ocean - structures like oil rigs and offshore renewable energy generators - is increasing. These structures provide hard-bottom habitat in areas previously dominated by sand or mud, so they have the potential to alter species distributions or serve as “stepping-stones” between other hard-bottom habitats. It is thus important to understand what factors influence the composition and abundance of benthic fauna recruiting at these sites. We examined recruitment to hard substrata (fouling panels) deployed on sand at various distances from a large rocky reef (∼60 m isobath) on the southern Oregon coast in 2014-2015. Recruitment was dominated by the acorn barnacle Hesperibalanus hesperius. For the majority of the study period in 2014, an anti-cyclonic eddy was present near the deployment sites. However, anomalously high recruitment of H. hesperius during August - early October 2014 coincided with dissipation of the eddy, slower bottom currents, and a positive convergence index, suggesting that H. hesperius larvae from the adjacent area may have been accumulated and retained near our study sites. Other sessile species, including hydroids and bryozoans, recruited to the fouling panels in low abundances, and most of these species have long-range dispersal and fast growth. Mobile invertebrates observed on the fouling panels included gastropods and nudibranchs, most of which also have long-range dispersal and fast growth, and are predators as adults. Thus, a community with two trophic levels assembled on the fouling panels in a relatively short time period (<12 weeks). None of the common hard-bottom species from the adjacent rocky reef recruited to the panels, suggesting that there is a specialized assemblage of species that can exploit hard-bottom habitats surrounded by sandy plains. Our results raise many questions about the influences of dispersal and oceanographic conditions on recruitment to hard substrata.