Coral bleaching due to cold stress on a central Red Sea reef flat

Ocean warming is leading to more frequent coral bleaching events. However,
cold stress can also induce bleaching in corals. Here, we report
observations of a boreal winter bleaching event in Jan 2020 in the central
Red Sea, mainly within a population of the branching
coral Stylophora pistillata on an offshore reef flat.
Sea surface temperatures rarely fall below 24°C in this region, but data
loggers deployed on several nearby reef flats recorded overnight seawater
temperatures as low as 18°C just three days before the observations. The
low temperatures coincided with an extremely low tide and cool air
temperatures, likely resulting in the aerial exposure of the corals during
the nighttime low tide event. The risk of aerial exposure is rare in
winter months, as the Red Sea exhibits seasonal fluctuations in sea level
with winter values typically 0.3-0.4m higher than in summer. These
observations are notable for a region typically characterized as a
high-temperature sea, and highlight the need for long-term monitoring
programs as this rare event may have gone unnoticed.