Physical Disturbance to Foundational Macroalgal Communities on Coral Reefs Causes Rapid Changes in Use by Herbivorous Fish Communities

Many coral reefs have shifted to alternative community states following disturbance. In some cases, shifts are to macroalgae that inhibit coral recovery, motivating research into potential changes in the ecosystem services they provide. After storm disturbances in French Polynesia, the macroalga Turbinaria ornata can form a self-replicating alternative state and dominate shallow hard-bottomed habitats, thereby providing trophic support to some herbivorous fishes.

 

To examine how physical disturbance from storms of differing intensities affects trophic support to herbivorous fishes, we mimicked effects on Turbinaria-dominated patches from four levels of simulated wave power by experimentally removing different size classes of T. ornata: none removed (No Disturbance), only large adult thalli (>10cm) removed (Storm), adult and juvenile thalli removed (> 5cm; Large Storm), and all size classes removed (Strong Cyclone). We quantified the species and feeding activities of herbivorous fishes in each treatment immediately (3 hours) and five days after disturbance. NMDS was used to visualize use by the herbivore community, measured as the species-specific number of bites taken in each experimental unit.

 

The fish community in the No Disturbance treatment did not differ between the two time periods. By contrast, after a Storm, we found a substantial increase in the number of species foraging and their foraging activity that persisted to day 5. Further increases in simulated wave power (Large Storm and Cyclone Treatments) initially reduced both the number of species and foraging activity below the level observed in the No Disturbance Treatment; however, the number of herbivore species that used intensely disturbed plots subsequently increased by day 5. These results show that the magnitude of disturbance to patches of T. ornata influences the composition of herbivore assemblage that target disturbance-exposed algal resources. Although this may represent an ephemeral food source to the herbivorous fish community, our results provide insight into how emergent alternative communities states on coral reefs may function in an increasingly stormy Anthropocene.