Surface composition and orientation interact to affect subtidal epibiota

Settlement panels were used to evaluate the effects of composition of the substratum (sandstone, concrete, wood) and orientation (vertical, horizontal undersides) on subtidal epibiota. It was predicted that both factors would influence the development of epibiotic assemblages, but that differences due to composition would be less marked on horizontal undersides compared to vertical panels. Differences in assemblages among sandstone, concrete and wooden panels orientated vertically were predicted to be similar to those described previously among vertical surfaces of sandstone rocky reefs and concrete and wooden urban structures (pilings and pontoons). Multivariate analyses indicated that assemblages were influenced greatly by orientation, whereas the effects of surface composition differed for the two orientations and among sites. Assemblages on wood were always significantly different from those on sandstone or concrete - patterns between the latter two surfaces depended on the orientation of the panels. The taxa that dominated these surfaces were not similar in identity nor abundance to those on urban structures of the same composition. The covers of most taxa were influenced by orientation alone or by surface composition for just one orientation. This study demonstrates the need for caution in generalizing about effects of orientation and surface composition because they may interact with each other and/or with other factors and they are certainly quite different for different taxa and among sites.