Community structure of infaunal macrobenthos around vestimentiferan thickets at the San Clemente cold seep, NE Pacific

The San Clemente cold seep lies within 100-200 km of other reducing habitats in the NE Pacific, offering an opportunity to compare diversity and species overlap among reducing habitats (i.e. whale-, kelp-, and wood-falls) at similar depths within a single region. Video observations from the research submersible Alvin at the San Clemente seep (1800 m depth) indicated clumps ('thickets') of vestimentiferans distributed as meter-scale patches interspersed with vesicomyid clam beds and black sediments. Sediment-core samples were collected at distances of 0 to 80-200 m along randomly oriented transects radiating outward from vestimentiferan thickets to evaluate changes in macrofaunal community structure from thickets into the background community. Macrofaunal abundance was elevated at distances of 0-1 m compared to 80-200 m (i.e. the 'background' community). The tube-building frenulate worms Siboglinum spp., along with peracarid crustaceans, dominated sediments within 1 m of vestimentiferan thickets. Species diversity was depressed within 1 m of thickets but with high rates of species accumulation, suggesting that seep sites greatly increase sediment heterogeneity and facilitate colonization by non-background macrofaunal species. Stable isotope data indicate chemosynthetic nutrition for some dominant macrofaunal species within 1 m of tubeworm thickets. The macrofaunal community near vestimentiferan thickets in San Clemente seep contains intermediate levels of species richness and diversity compared to other deep-sea seep areas in the northeast Pacific. There was low species overlap between the San Clemente seep macrofauna and communities in reducing habitats near wood-, whale-, and kelp-falls at similar depths within the region, suggesting that seeps harbor a distinct infaunal community.