Mechanisms of interaction between macroalgae and scleractinians on a coralreef in Jamaica

After several decades of disturbance, many coral reefs in the Caribbean are dominated by macroalgae, One process affecting this transition is coral-ma croalgal competition, yet few studies have addressed the mechanisms involve d. In this study, we investigated competition between the tall and bushy ma croalga Sargassum hystrix (J. Agardh) and the branching coral Porites porit es (Pallas) on a shallow reef in Jamaica. Experiments were designed to expo se coral branches to different treatments to test the role of shading and a brasion by Sargassum on coral growth and polyp expansion. Corals exposed to Sargassum grew significantly more slowly (80% reduction) than controls, bu t this effect was absent when corals were caged to prevent physical contact with macroalgae. Light levels were reduced in both the algal and cage trea tments, but shading apparently had little effect on the growth of corals in cages. Short-term measurements of integrated net water flow did not detect variation among treatments. In algal-mimic treatments, where clear plastic strips could touch but not shade the corals, growth rates were 25% lower t han controls, but this effect was not statistically significant. Thus, the growth of corals in contact with Sargassum was reduced by abrasion and, to a lesser extent, by factors unique to living macroalgae. Analysis of polyp expansion showed that polyps were more frequently retracted when corals wer e in contact, with macroalgae or algal-mimics compared to controls or cage treatment; the frequency of polyp contraction was correlated positively wit h growth. Together, these results suggest that abrasion-mediated polyp retr action is one of the primary mechanisms of competition utilized by tall (ca . 17 cm) macroalgae against scleractinian corals. (C) 2001 Elsevier Science B.V. All rights reserved.