Sedimentation and light penetration interact to maintain heterogeneity of subtidal habitats : algal versus invertebrate dominated assemblages

One of the most striking and widespread patterns observed on subtidal rocky reefs is that up-facing surfaces are monopolized by algae, whereas down-facing surfaces are dominated by sessile invertebrates. This study experimentally assessed the model that light and sedimentation interact with surface orientation to maintain this pattern of habitat heterogeneity. We tested the hypothesis that if epibiotic assemblages on down-facing surfaces were rotated to face upward, then the least change in assemblage structure would occur on shaded surfaces with reduced rates of sedimentation. In general, the alternate states of algal vs invertebrate dominated assemblages appeared to be primarily maintained by light intensity, which facilitated the cover of algae on up-facing surfaces (full light) and invertebrates on down-facing surfaces (reduced light). Although sedimentation was only partially responsible for differences between habitat types, it acted as a negative disturbance on the abundance of algae and survivorship of invertebrates. When combined with differences in light intensity, high rates of sediment accumulation had slight negative effects under natural light, but under shaded conditions these negative effects were substantially increased, causing changes tothe structure of the whole assemblage. This result warns that attempts to identify the effects of sedimentation in isolation from light intensity, which depends on factors such as turbidity, may not reveal the true effects of sedimentation on epibiotic assemblages. The ability of invertebrates to withstand high rates of sediment accumulation was related to their morphology, whereby erect forms growing above accumulated sediments had greater rates of survivorship than prostrate growth forms, which tended to be smothered by sediments. To properly understand the physical processes of facilitation (e.g. light intensity) and disturbance (e.g. sedimentation) we need to assess them in meaningful combinations so that explanations of assemblage structure do not create the false impression that such processes, however complex, produce only small effects relative to other processes.