Size matters in competition between corals and macroalgae

Corals and macroalgae compete for space, but the influence of species and size on the competitive outcome is poorly understood. Using a manipulative experiment, we evaluated the effect of macroalgal competition on the growth rate of corals with an emphasis on the colony size, species identity and the intensity of competition. Coral-macroalgal competition was studied among 3 Caribbean coral species (Porites astreoides, Agaricia agaricites and Colpophyllia natans) and 2 macroalgal species (Lobophora variegata and Halimeda opuntia) for 1 yr. Two coral colony sizes were used and, for the smaller size class, 2 levels of intensity for macroalgal competition (25 and 100% contact with the coral perimeter). Coral size had the greatest effect on competitive outcome; 2 species of large corals under competition grew as much as controls and a third species did not lose tissue. All small colonies lost between 18 and 22% of their original area after a year of competing with macroalgae, and the competitive outcome was insensitive to algal species. Coral colony size is a critical factor for the competitive outcome with algae and the intensity of contact between competitors is not important in smaller corals. In general, our results support the theory that algal blooms can inhibit coral population dynamics by causing a bottleneck in the survivorship of smaller size classes.