Hydrodynamics influence coral performance through simultaneous direct and indirect effects

Hydrodynamic conditions can influence the distribution and abundance of aquatic species in many ways, including directly through disturbance and resource delivery, and indirectly by altering environmental cues and species interactions. In coral reef ecosystems, corallivory is an important top-down control of coral populations, while water motion can enhance coral performance via autotrophic and/or heterotrophic pathways. Using a large-scale field assay in Mo'orea, French Polynesia, we measured the extent to which the growth of a branching acroporid coral is influenced by hydrodynamics that deliver nutrients and food to corals but also impede corallivory. We explored two hydrodynamic properties that characterized our study system, mean current velocity and current velocity fluctuation (a measure of waves and turbulence). In treatments where predators were excluded, coral skeletal growth was positively related to mean current velocity, whereas no effect of velocity fluctuations was detected. In the presence of predators, growth was positively related to velocity fluctuations, but no relationship with mean velocity was detected. The major effect of velocity fluctuations was to reduce the attack rate of corallivores by lowering both their per capita bite rate and local density. Corals thus benefited directly from increased currents, probably through enhanced autotrophy and/or heterotrophy, and indirectly from velocity fluctuations through reduced corallivory. Our data indicate that for the coral and locale we examined, the indirect hydrodynamic effect on coral performance via reduction in corallivory was substantial, increasing daily growth rate by 4%. Hydrodynamic enhancement of acroporid coral growth via simultaneous direct and indirect effects can be important for the persistence of coral reefs as well as their recovery following disturbance.