Interactive effects of shading and proximity to the seafloor on the development of subtidal epibiotic assemblages

A manipulative experiment was done to test hypotheses about the interactive effects of shading and proximity to the seafloor on the development of subtidal epibiotic assemblages. It was pro posed that differences in these 2 factors could explain differences described previously between assemblages of sessile organisms on rocky reefs and pier pilings in estuaries. Sandstone settlement plates were deployed in 2 positions (close to and far from the seafloor) and sets of plates were experimentally shaded in each position. Effects of shading and proximity to the seafloor were apparent after 12 wk and many persisted for all 33 wk of the experiment. Largest differences in assemblages were due to position relative to the seafloor (which greatly affected the cover of spirorbid polychaetes, barnacles, bryozoans and sponges) and there was a significant multivariate effect of shading on assemblages close to the seafloor. Shading influenced the cover of colonial ascidians, algae and cyanobacteria in both positions, but many other taxa in only 1 position. Possible reasons for these differences are discussed. Results suggested that differences in assemblages between rocky reefs and pilings could largely be due to differences in shading and proximity to the seafloor.