Bushy or smooth, high or low; importance of habitat architecture and vertical position for distribution of fauna on kelp

The attraction of kelp fauna to different habitat architectures and vertical positions on kelp (Laminaria hyperborea) was tested by colonisation experiments using three artificial habitats (bushy, rough, smooth) at five different vertical positions in a kelp forest. The fauna on epiphytic algae of different structures and at different vertical positions on the kelp was sampled for comparison. Both abundance and number of species were generally highest on bushy substrates, and numbers were significantly highest at the lowest level. The number of species was lowest on smooth substrate, and numbers of individuals increased towards the upper levels on this substrate. The reverse was true on bushy and rough substrates. Amphipods and gastropods were the most numerous groups. Amphipods colonised bushy substrates in higher numbers than smooth. Of the two dominating gastropod species, Rissoa parva was abundant on bushy substrates, mainly in low vertical positions, while Lacuna vincta showed opposite preferences in both habitat selection and vertical distribution. Faunal structure varied along a vertical gradient on natural kelp, and between single epiphytic algal species of different architectures, supporting the results obtained from artificial habitats. Habitat heterogeneity along a vertical gradient within a kelp forest is important for fauna diversity and abundance. The habitat preferences of the fauna may be related to feeding and predator avoidance strategies.