Scaling the influence of topographic heterogeneity on intertidal benthic communities: alternate trajectories mediated by hydrodynamics and shading

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-depende nt influence of topographic heterogeneity on benthic communities. Twelve co ncrete reefs (cylinders) of 3 different sizes (52, 76 and 115 cm) were plac ed on randomly selected sites along a rocky intertidal platform. The area a round 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 cylinder s. Flow velocity was simulated around reefs using a finite-element hydrodyn amic model. Substratum temperature was also measured. The biomass and densi ty 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 e dulis 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 f low velocity on the downstream side of large reefs. Substratum temperature was lower closer to reefs, and shaded areas increased with reef size. Maxim um MI. edulis biomass was around large reefs, while the biomasses of other dominant species were not positively influenced by reef size. Biomass and d ensity 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 ( R-2 = 0.27), while mussel growth was best correlated with substratum temper ature close to the medium reefs (R-2 = 0.66). Our study shows that the spat ial structure of the benthic community studied and its scaling with topogra phic heterogeneity depends on dominant mediating physical factors (i.e. hyd rodynamic processes or substratum temperature).