Due to the combined effects of overfishing, hurricane damage and the 1983 die-off of the black spiny urchin Diadema antillarum, the reef composition of the North Shore of Jamaica has become characterized by a low percent cover of living coral and a high abundance of macroalgae in the past two decades. The importance and frequency of coral-macroalgal competition has increased as a result, yet remains poorly studied.

In this study I examined the effect of the macroalga Sargassum hystrix on the growth rate of the branching coral Porites porites. Corals placed in algal treatments, where they were fully exposed to macroalgae (with shading, abrasion and possible allelochemical/nutrient effects), grew at significantly reduced(~60%) rates compared to controls. Corals in caged algal treatments, where macroalgae shaded corals but were prevented from brushing against colonies directly, experienced no growth rate reduction. In algal-mimic treatments (using clear plastic strips), where corals were abraded at an intermediate level but were not shaded, 25% growth rate reduction occurred.

These results suggest that competition with macroalgae can significantly reduce the growth rate of corals, and that physical contact with corals is an important mechanism by which this reduction occurs. The strength and frequency of this interaction suggests that, unless this competition is mediated by increased herbivory, macroalgae may continue to outcompete corals for reef substratum, leaving Jamaican reefs with high abundances of algal species and low living coral cover.