Herbivory and competition limit the expansion of the macroalga Turbinaria ornata to shallow zones on a fringing reef in the South Pacific

The brown macroalga, Turbinaria ornata, has rapidly expanded across South Pacific reefs, dominating areas once covered by corals. While it is well documented that T. ornata dominates shallow zones of fringing and lagoonal reefs, it is unclear why it is restricted to shallow environments. In a three-factor, fully-crossed field experiment, we tested how the presence of competitors (Dictyota bartayresiana, +/-) and herbivores (caged vs. uncaged) affected height and biomass accumulation of T. ornata across 2 depths (~1m T. ornata dominated vs. ~2.5m D. bartayresiana dominated) along a fringing reef in Moorea, French Polynesia. When transplanted to ~1m, T. ornata (height~3cm) gained 20% more biomass than when transplanted to ~2.5m, indicating that depth-associated abiotic factors may be preventing deeper expansion (p<0.001). In the absence of D. bartayresiana, T. ornata gained 22% more height than with D. bartayresiana (p<0.01), suggesting that competition may also restrict its vertical distribution. However, herbivory had no observable effect. We then tested the effects of depth (~1m vs. 3-4m) and size (>1, 2, 3, and 5cm) of T. ornata on herbivory rate. The smallest two size classes lost significantly more biomass at 3-4m (p<0.001), suggesting that herbivory may prevent the expansion of young T. ornata to deeper depths. Finally, we conducted a two-factor mesocosm experiment to test the effects of shade (+/-) and presence of D. bartayresiana (+/-) on growth. Shaded treatments gained 8.5% biomass without D. bartayresiana compared to a loss of 0.3% with D. bartayresiana over 7 days (p<0.01), suggesting that D. bartayresiana may outcompete T. ornata for light at deeper depths. Combined, these results indicate vertical zonation of T. ornata is limited by herbivory and competition for light with D. bartayresiana. Further research is needed to develop management strategies to limit the impact of T. ornata on shallow fringing reefs and lagoons.