The effects of two brown alga Sargassum managarevense, and Turbinaria ornata on juvenile colonies of Porites lobata.

Phase shifts from scleractinian coral dominated reefs to algal dominated reefs have become increasingly abundant in the past few decades. These phase shifts cause increased interaction between coral and macroalgae. Coral- algal competition is of central importance to phase shifts on coral reef. This study set out to determine if the physical interactions between juvenile Porites lobata colonies and two coarse brown algae (Turbinaria ornata and Sargassum managarevense) affect coral-algal competition. It was found that Porites in the S.mangarevense treatment grew significantly less (36%) than coral in the Turbinaria treatment and controls. A polyp retraction index (PRI) indicated that decrease in coral growth was due to abrasion by Sargassum.