scientific chapter |
Worldwide ecological deterioration of coral reefs is mostly caused by disease-induced mass mortalities linked to thermal anomalies, and aided by local anthropogenic stressors. Mesophotic coral ecosystems (MCEs; 30–150 m) are found deeper where temperatures are cooler, in low light, and mostly offshore. These characteristics are proposed to protect MCEs (“deep reef refugia” hypothesis) from shallow-water threats (e.g., thermal stress and pollution). The most commonly reported mesophotic health problem is thermal-induced bleaching, which is now more widespread due to global climate change (GCC). The oldest deep bleaching report (90 m) is from 1989, in the Caribbean, but recent reports indicate bleached corals below 120 m in Grand Cayman. Cold-water intrusions and turbidity can also cause mesophotic coral bleaching. Not much is known about biotic diseases and potential drivers in MCEs. “White syndromes” (WS) seem to be the most common in Puerto Rico and the US Virgin Islands. Overall, 9 of the 28 common MCE scleractinian species have been observed with disease signs similar to shallow WS and white plague disease. A new disease termed “intercostal mortality syndrome” affected 19% of the colonies of Orbicella, Siderastrea, and Agaricia spp. in Hind Bank. MCE surveys showed coral community disease prevalence in Puerto Rico varied between 0% and 15%, with a mean of 6%. This chapter presents a summary of what is known about disease threats to MCEs and the disruptive potential of GCC-induced changes in seawater thermal dynamics on species susceptibility, and how this could affect the protection these deeper environments may provide.