Subtropical Epibenthos Varies with Location, Reef Type, and Grazing Intensity

Composition of marine epibenthic communities are influenced by both physical and biotic processes. For instance, the larval supply and cues that influence colonization (physical), as well as the growth and mortality of individuals (biotoic), may differ across location and reef type. Determining the relative influence of these processes is important to understanding how epibenthic communities can develop in a region. Using both a partial caging experiment that controlled grazing by urchins and in situ photographic surveys of epibenthic communities, this study examined the relationship between urchin grazing and the composition of epibenthos on natural limestone and artificial reefs in the eastern Gulf of Mexico (eGOM). In the experiment, tiles that were open to urchin grazing had lower percent cover of algae (−12%) and higher cover of crustose coralline algae (CCA) (13%) than those that excluded urchins. Patterns in tile cover were likely the result of CCA either resisting grazing mortality or recolonizing exposed areas after algae were removed. Variation in colonization was observed between inshore and offshore reef groups. Urchin density was positively correlated with the structural complexity of the habitats, which was higher on artificial reefs than natural ones, a factor that potentially had important effects on several observed patterns. Results from photographic surveys indicated that natural reef communities had higher algal cover and lower cover of invertebrates (e.g., corals and hydroids) than artificial reefs. These findings were consistent with previous work conducted in both temperate and tropical ecosystems, and suggested that grazing from urchins plays an important role in shaping epibenthic community structure in the subtropical eGOM.