COMPOSITION OF LITHIC SUBSTRATES FROM MESOPHOTIC HABITATS IN PUERTO RICO AND US VIRGIN ISLANDS

Mesophotic coral environments (MCEs) are low light deep reef communities (30-150m) of prime interest around the Caribbean because they represent new unexplored habitats. Substrate samples were collected from a variety of mesophotic environments (at depths of 50 – 90m) around Puerto Rico and US Virgin Islands to describe their composition. The collected samples were divided in three broad categories: coral, algal nodule, and carbonate mud. Coral and coralline algae are the main components of the mesophotic substrates, although microcrystalline carbonate appears as a common constituent in the substrate samples. The lithic substrates consists primarily of coral clasts as a central nucleus combined with microcrystalline carbonate infilling skeletal voids, followed by layers of crustose coralline algae associated with encrusting bryozoans. The lithified carbonate has a micritic fabric, very similar to microbialites microfabrics described in other studies. Petrographic analysis of the microcrystalline carbonate reveals two primary morphologies, a dense structureless micrite (leolites) and clotted pelodial fabric. Both microfabrics are common throughout the samples, indicating that these microbialites may be an important component of the mesophotic substrates. The development of microbialites in reef environments has been related to a response of past environmental changes or disturbances. However, we are finding evidence of present development of microbialites in mesophotic environments hard substrates. We associate these microbialites to the contribution of the mesophotic lithic substrates, by encrusting and stabilizing loose sediments, possibly creating new habitat suitable for coral recruitment and colonization. Although coral-microbial dynamics are not well understood, documenting microbialites morphology and development, as well as environmental conditions that promote their growth will contribute to understanding their function in mesophotic reef communities.