Ocean Warming and the Reefs OF Palau

After a coral bleaching event in 1998, a comprehensive temperature monitoring program was started in the Pacific Island nation of Palau using vertical arrays of instruments that collected data from near the surface to 90 m depth along outer reef slopes. Data from 1999 to the present show highly variable temperatures at deeper stations, attributed to internal waves/tides, and a correlation between monthly mean temperatures and mean sea levels. A La Nina bleaching event in 2010 recorded temperatures of 29 degrees-30 degrees C throughout the depth range of corals and a breakdown in upwelling mechanisms during the event. No evidence of a deep reef thermal refuge from bleaching conditions was observed. An abortive bleaching event in 2016 provided insight into how reef conditions can change from El Nino to La Nina in only two to three months. Trends from 1999 to 2017 indicate possible temperature increases at all depths. Temperature monitoring networks are important to help inform the relationship between remotely sensed satellite and in situ temperature data and should be included in more coral bleaching studies. New approaches to determining thermal stress on reefs, such as incorporating data on mean sea level, are needed.