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

Abstract

Significant Conclusions: “Our results suggest that the structure of shell mound communities is strongly influenced by the presence of the platform structure and the food subsidies provided by the clumps of mussels and associated organisms that continually slough from the platform to the seafloor. Predatory and omnivorous seastars (Pisaster spp., Asterina miniata), in particular, were much less abundant and of a smaller size at the shell mound-only sites. However, the relative abundance (as CPUE) of commercially important crabs, Cancer antennarius and C. anthonyi, did not differ among shell mound sites. These crabs are predators and carnivorous scavengers likely consume components of faunal litterfall when available, but have greater mobility than other invertebrate taxa of shell mounds, and can forage over a larger area both on and off the mounds. Populations of two macroinvertebrate detritivores on shell mounds, Parastichopus parvimensis and P. californicus, also appeared to be little affected by the removal of the platform structure. We hypothesize that the food subsidy provided by the dislodged organisms that fall from platform structures to the shell mounds exerts bottom up control upon populations of predatory and omnivorous sea stars. The platform structure could also affect the abundance and size structure of benthic organisms through the provision of recruitment habitat. There was no overlap in species composition of mobile macroinvertebrates between benthic communities on the shell mound under Platform Gina and the adjacent soft bottom locations. Thus, our results also suggest that the presence or absence of hard substrate was an important factor in determining the distribution of shell mound associated taxa.

The relative effect of the platform on the benthic community appeared to vary among mound species in relation to trophic level, degree of mobility, and substrate preference. Mobile crabs (e.g., Cancer antennarius) and the sea cucumber, Parastichopus parvimensis, which prefer hard substrate, were seemingly least affected, but given the estimated sedimentation rate at mound-only sites (Hazel and Hilda, 0.9 to 1.4 mm year-1; Heidi and Hope, 1.5 to 2.0 mm year-1), these sites may no longer provide suitable habitat in a few years. Thus, while the presence of the platform structure may enhance secondary productivity in the benthic community, this effect may disappear rapidly (<5 yr in this case) after the platform is removed."