CORAL MORPHOLOGY AS A CONTROL OF BIOEROSION AND ENCRUSTATION IN HOLOCENE REEF CORALS OF THE WESTERN DOMINICAN REPUBLIC

A Holocene reef outcrop at Cañada Honda in the western Dominican Republic affords continuous exposure spanning nearly 7,000 years of time. Coral-community structure (28 species; dominated by the genera Montastraea and Siderastrea) and colony shape responded to variations in both depth and sedimentation. Where sedimentation was lower, columnar and domal colonies, primarily Montastraea spp. dominated, whereas conical Siderastrea spp. were most common where sediment stress was greater. The dominance of conical and columnar forms over domal colonies is in sharp contrast to what is seen on most high-energy, lower-stress reefs today. A storm-deposited layer of finely branching Madracis sp. that cuts across the seaward 125 m of the outcrop has created the equivalent of an ash layer in volcanic deposits. It thus provides a synchronous surface along which depth-related and onshore-offshore changes in reef character can be examined. Bioerosion and encrustation were quantified in coral colonies just below (and presumably buried by) the Madracis layer by point-counting in quadrats. Conical colonies had significantly greater bioerosion and encrustation (38.0% and 6.6%, respectively) than columnar or domal colonies (BioE=10.6-25.6%; Encr=1.3-2.7%). Because Siderastrea tended toward a conical shape, compared with primarily domes and columns for other corals, species and shape exerted similar effects on bioerosion and encrustation. Post-mortem modifications do not appear to be related to coral density (Siderastrea is more dense but more altered – a counterintuitive relationship). Height (and, by extension, age) of the colony was not significant. It is proposed that higher bioerosion in conical forms is controlled by greater access for encrusters and bioeroders within cryptic spaces beneath them, relative to more-limited space between columns and domes. Where sedimentary infill is lacking between colonies, conical forms will have ample exposed area below the reef surface. In contrast, columns maintain more intimate contact with their nearest neighbors, leaving little or no space for access by encrusting or bioeroding organisms. Coral shape and post-mortem alterations are therefore closely linked, and together may provide valuable indices of sedimentation in ancient coral reefs.