An experimental assessment of herbivory and nutrient effects on a small-scale in a coral reef macroalgal community

• Manipulations of herbivores and nutrients were conducted 20 m and 12 days apart.
• Change in biomass varied spatially with alterations in nutrient supply and herbivore access.
• Differences were most pronounced in opportunistic macroalgal species.

Understanding spatial variation in top–down vs. bottom–up control may help resolve some of the conflicts among research findings on the controlling processes of marine plant communities. In this study, I manipulate herbivory and nutrient limitation on assembled communities of coral reef macroalgae. I conducted two identical 10-day manipulative experiments 20 m and 12 days apart and measured change in biomass of the 4 assembled species, Padina boryana (Thivy), Halimeda opuntia (L. J.V.Lamouroux), Galaxaura divaricata (L. Huisman and R.A.Townsend), and Dictyota bartayresiana (J.V.Lamouroux). I found that the response of all individual species varied markedly between the two sites. Between the two sites, P. boryana and D. bartayresiana responded differently to manipulations whereas H. opuntia and G. divaricata differed in growth magnitude and variance. The difference in final biomass between the two sites was most pronounced for the opportunistic algal species, suggesting small-scale spatial variation might be more important for these types of macroalgae. The constant environmental conditions during the study period suggest that small-scale spatial differences may affect the processes controlling community structure.