GEOMORPHOLOGY OF MESOPHOTIC CORAL ECOSYSTEMS IN PUERTO RICO AND US VIRGIN ISLANDS

Diver and ROV surveys and, where available, multibeam bathymetry allow for a general geomorphic characterization and classification of insular mesophotic habitats (depths of ~ 50 to 100 m) around Puerto Rico and the US Virgin Islands. Sites are divided into two broad categories of either platform or slope. Platforms are designated as either insular shelves or isolated banks. Slopes are designated as low gradient (< ~ 30°), steep/high gradient (~ 30-70°) or wall (>70°). Other closely linked factors are also considered including degree of exposure to prevailing seas and storms, occurrence and depth of prominent breaks in slope gradient, proximity to shore and other sediment sources and seafloor roughness/rugosity. More than twenty sites have been examined extending from Mona Island eastward along the south coast of Puerto Rico into the Virgin Islands to Lang Bank off the eastern end of St. Croix. As with the distribution of shallow reefs, mesophotic coral ecosystem (MCE) development is patchy and closely linked to the factors outlined above. Platforms typically have a low-relief, rubble-covered seafloor and limited MCE development dominated by algae and sponges. Low-gradient, rubble-covered slopes are the most common mesophotic habitat encountered and typically associated with more exposed settings. They have limited to moderate MCE development that is concentrated along breaks in slope gradient, which often occur at depths of ~ 50-60 m. High-gradient, high-rugosity slopes are typically associated with more sheltered settings and have the best developed MCEs with dense concentrations of large colonies of Agaricia spp. The geomorphology of these sites is dominated by large buttresses that are the locus of MCE development separated by narrow grooves that serve as the primary conduits for downslope sediment transport. Vertical walls typically display very limited MCE development, likely due to the reduced availability of light. A common feature among sites is the occurrence of a deep escarpment that typically begins at depths of ~ 80-90 m, but at some sites (e.g., NW St. Croix) occurs as shallow as ~ 55 m. The transition to the deep escarpment generally marks the local lower limit of scleractinean coral growth and, thus, MCE development.