Algal turf accrues both costs and benefits to growth via association with adjacent stands of Turbinaria ornata, a rapidly expanding macroalga in the South Pacific

Algal turf on coral reefs can transition from short productive forms to longer less productive forms that often form a persistent degraded state. We evaluated whether adjacent communities, such as stands of the macroalga Turbinaria ornata, can drive these transitions in Moorea, French Polynesia. We first found that T. ornata stands that overhang turf reduced growth as restricting canopy movement increased turf height compared to controls, implying an overhanging T. ornata canopy shades or abrades adjacent turf. Second, we manipulated T. ornata presence (intact/completely removed) and herbivore access (uncaged/caged) and found that, in the presence of herbivores, removal of stands reduced the height of adjacent turf compared to plots adjacent to intact Turbinaria stands. The presence of intact stands also deterred grazing, demonstrating they provide a refuge for adjacent turf. Thus, algal turf derives both benefits and costs of proximity to macroalgal stands. It is imperative to explore the relative strengths of these costs and benefits as these interactions may drive local transitions to degraded algal states.