Surface composition and orientation interact to affect subtidal epibiota

Settlement panels were used to evaluate the effects of composition of the s ubstratum (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 d ue 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 d escribed previously among vertical surfaces of sandstone rocky reefs and co ncrete and wooden urban structures (pilings and pontoons). Multivariate ana lyses indicated that assemblages were influenced greatly by orientation, wh ereas the effects of surface composition differed for the two orientations and among sites. Assemblages on wood were always significantly different fr om those on sandstone or concrete patterns between the latter two surfaces depended on the orientation of the panels. The taxa that dominated these su rfaces were not similar in identity nor abundance to those on urban structu res of the same composition. The covers of most taxa were influenced by ori entation alone or by surface composition for just one orientation. This stu dy demonstrates the need for caution in generalizing about effects of orien tation and surface composition because they may interact with each other an d/or with other factors and they are certainly quite different for differen t taxa and among sites. (C) 2000 Elsevier Science B.V. All rights reserved.