Role of food subsidies and habitat structure in influencing benthic communities of shell mounds at sites of existing and former offshore oil platforms

Mussels (Mytilus californianus, M. galloprovincialis) and other organisms sloughed from offshore oil platforms provide a food subsidy to benthic consumers and alter underlying soft bottom habitat by creating hard substrate. The removal of overlying platforms eliminates this food subsidy, but large shell mounds remain. The distribution, abundance, and population characteristics of mobile macroinvertebrates differed among shell mounds beneath existing offshore oil platforms, shell mounds at the former sites of offshore oil platforms, and soft bottom. Predatory and omnivorous echinoderm and mollusk species were more abundant and generally larger on shell mounds under platforms than on shell mounds without platforms. Omnivorous and deposit feeding echinoderms were the most abundant macroinvertebrate taxa sampled on mound-only sites. The brown rock crab (Cancer antennarius), known to have a strong preference for hard substrate, was significantly more abundant on shell mounds, with or without platforms, than adjacent soft bottom sites. Results suggest that the effects of platform removal differed among benthic species according to trophic level, degree of mobility, and substrate preference. Although the shell mound habitat persists after removal of platform structures, species abundance and the composition of the associated benthic community is altered by removal of the platform structure.Communicated by P.W. Sammarco, Chauvin