Quality or quantity: small-scale patch structure affects patterns of biodiversity in a sublittoral blue mussel community

It is widely accepted that blue mussel Mytilusedulis L. beds support a high macrofaunal diversity. Nonetheless, sublittoral musselbeds on rocky shoreshave rarely been examined in a patchcontext to determine the processes creating and maintaining the diversity and assemblages of organ- isms occupying them. Whereas the species-area relationship is a well-established theory in ecology, less is known about its effect on faunal compositions in aquatic habitats. Also, the effect of patchshape and relative patchedge are poorly studied within a rocky shore musselframework. The pre- sent study examined the effects of blue mussel patchsize, patchshape, and the presence of algae and sand in mussel patcheson the composition and diversity of associated macrofaunal communities in sublittoral habitats. In line with the species-area relationship, we found that the total number of taxa increased with patchsize up to ca. 500 cm 2 , thereafter declining. Small patcheslargely lacked annelid wormsand showed a lower diversity of gastropod species. We also found that the diversity was further enhanced by a high biomass of blue mussels, the presence of algae and the weight of interstitial sand. The shape of the patchhad no effect on species richness, but affected the species composition. We conclude that structural components of sublittoral blue musselhabitats, such as patchcontent and shape, greatly affect the diversity and faunal assemblage. This has important impli- cations in conservation biology, since habitats are becoming more fragmented. Our study demon- strates that not only habitat quantity, but also quality has a large impact on species communities in marine ecosystems.