| Vertical and undercut rock surfaces in the subtidal typically support invertebrate-dominated communities, while nearby horizontal rock is dominated by algae. A number of hypotheses have been proposed to explain why invertebrates dominate vertical surfaces, including (1) shading by walls excludes competitively superior algae, (2) reduced predation on walls, (3) reduced sedimentation on walls and (4) increased water flow on walls. The sessile invertebrates that make up these communities, moreover, may have evolved to preferentially select these habitats via negatively phototactic larval settlement behavior. I investigated this pattern in the Gulf of Maine by quantifying species diversity and abundance of sessile invertebrates at 8 sites. Species diversity and abundance was higher on vertical walls compared to horizontal rock at all sites. Abundance of predators and grazers was also investigated with respect to substrate angle, and results are discussed. To evaluate processes responsible for the pattern, I experimentally tested whether light levels or predation on horizontal rock surfaces shift the community towards invertebrates. Granite slabs were shaded with tinted plexiglas and caged to exclude predators on horizontal rock surfaces at 10m depth off Nahant, Massachusetts, and Damariscotta, Maine, in the Gulf of Maine. Results from two years of community development are presented. Shading resulted in increased invertebrate abundance, while predation level had no effect on sessile invertebrates. Unshaded plots had high algal cover, although invertebrates are not altogether excluded. It appears that shading, leading to either differential settlement or competitive release from algae, is more important than predation in shaping this pattern, at least where sea urchin predation is not intense. Physical disturbance, in the form of sedimentation and scour, is also likely important in structuring these communities. |
