24 - Mesophotic Coral Reefs of the Puerto Rico Shelf

Abstract

Mesophotic coral ecosystems (MCEs) can be defined as light-dependent coral, algal, and sponge communities that occur in the deepest half of the photic zone in tropical and subtropical regions. The distribution of MCEs in the US Caribbean and elsewhere, including ecologically relevant parameters such as percent living coral cover, reef rugosity, incidence of bleaching, and species diversity, are largely unknown. MCEs are known habitats of commercially important fish species and could serve as refugia for coral reef organisms during times of global climate change. MCE habitats in Puerto Rico and the US Virgin Islands (USVI) can be divided into two broad categories: low-gradient platforms composed of insular shelves and banks, and high-gradient slopes. Geomorphology can exert a fundamental control on the occurrence and distribution of MCEs by providing favorable hard substrates for colonization and by directing the downslope transport of sediment. In addition to geomorphology, physical factors such as temperature, light availability, and low sedimentation are determinant factors in the distribution of these reefs. Water turbidity, by limiting the penetration of light in the water column, is a determinant factor in the amount and maximum depth of living coral cover. The benthic imaging capabilities of the Seabed autonomous underwater vehicle have been used since 2002 to map and characterize MCEs throughout the Puerto Rican insular shelf. Some of these reefs are structurally complex with high coral cover and abundant fish and invertebrate fauna.