Abstract

Corals are increasingly threatened by the impacts of global climate change and associated increasing seawater temperatures that result in dysbiosis, known as bleaching, and potential subsequent mortality. The frequency and severity of thermal anomalies has increased significantly over the last several decades with 2023 being the warmest year on record. In the Cayman Islands, shallow seawater temperatures exceeded 31oC for an extended period, reaching an unprecedented 19.5 Degree Heating Weeks (DHW). In September 2023, during the 13th DHW, we conducted a series of vertical transects on the sheer wall in Bloody Bay Marine Park on Little Cayman Island. Transects ran from 10m to 50m with all corals encountered within a 2m span identified and assigned a bleaching status (fully bleached, pale, or healthy), and temperature recorded every second. We found a significant interactive effect of bleaching prevalence with temperature and depth, driven by decreasing temperatures across increasing depths. A marked transition from predominately bleached to healthy colonies occurred at roughly 36m, which coincided with a noticeable thermocline where temperatures dropped from 30.8 to 30.5C. However, refuge potential differed among species with Agaricia spp. showing no difference in bleaching across depth while Stephanocoenia intercepta, Montastraea cavernosa, and Porites astreoides showed significant declines in bleaching with increased depth. Interesting, the opposite trend was found for Orbicella faveolata, suggesting that individuals of this species may be strongly adapted to localized conditions such that shallow individuals have higher temperature tolerances than deeper colonies. Overall, these results suggest that several species capable of living at mesophotic depth may have a higher potential of persisting through climate change.