MEPS 217:27-41 (2001)  -  doi:10.3354/meps217027

ABSTRACT: Scale-dependent influence of environmental complexity has become a central issue in ecology. We quantified the impact of artificial reefs on community characteristics (biomass, density) and on individual mussel growth, and we tested the relative importance of physical processes (i.e. flow velocity, substratum temperature) as intermediate factors mediating the scale-dependent influence of topographic heterogeneity on benthic communities. Twelve concrete reefs (cylinders) of 3 different sizes (52, 76 and 115 cm) were placed on randomly selected sites along a rocky intertidal platform. The area around each reef and 4 control sites were divided into 24 sampling cells (6 orientation and 4 distance categories). Hydrodynamic patterns around reefs and control sites were determined using the dissolution of plaster cylinders. Flow velocity was simulated around reefs using a finite-element hydrodynamic model. Substratum temperature was also measured. The biomass and density of benthic community adjacent to the reefs was sampled using 10 x 10 cm quadrats before and 1 yr after installation. Growth of individual Mytilus edulis attached to experimental panels was measured. A flow index revealed a strong scale-dependent gradient of decreasing water motion intensity with distance from the reefs, and the hydrodynamic model showed a reduction of flow velocity on the downstream side of large reefs. Substratum temperature was lower closer to reefs, and shaded areas increased with reef size. Maximum M. edulis biomass was around large reefs, while the biomasses of other dominant species were not positively influenced by reef size. Biomass and density patterns close to the reefs were significant only around large reefs, with the downstream side having the lowest M. edulis biomass. Growth of M. edulis decreased significantly with distance away from the reefs. Biomass patterns were best explained by the flow velocity around the large reefs (R² = 0.27), while mussel growth was best correlated with substratum temperature close to the medium reefs (R² = 0.66). Our study shows that the spatial structure of the benthic community studied and its scaling with topographic heterogeneity depends on dominant mediating physical factors (i.e. hydrodynamic processes or substratum temperature).

KEY WORDS: Benthic community · Spatial scale · Intertidal · Environmental complexity · Finite element model