A Characterization of a Newly Discovered Mesophotic Reef in Pala

An extensive mesophotic reef was discovered in a lagoon on the western side of Palau during an autonomous underwater vehicle mapping mission. Corals within the lagoon experience extremely low light levels due to a transient yet persistent nepheloid layer. Light levels within the lagoon are much lower than equivalent depths on the clear water barrier reef. This study assesses the acclimation strategies between corals in the turbid lagoon and corals on the clear water barrier reef. Ground truthing revealed that coral communities were associated with depth of the lagoon and could be determined using benthic patterns in sidescan sonar images from autonomous vehicles. Stable isotopic analysis showed that corals in the lagoon acclimate to the low light environment by increasing heterotrophic feeding. Deeper corals on the barrier growing in equivalent light levels as the lagoon were found to practice heterotrophy as well. The reliance on autotrophy and heterotrophic feeding varied by species in corals growing at depths on the barrier equivalent to the lagoon. This study challenges the problematic practice of dening mesophotic reefs by depth and rather a denition based on light level is more appropriate as results in this paper suggest that extremely low light, shallow water coral communities exist and corals in these sites acclimate much dierently to corals at similar depths in higher light levels.