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Reef-scale Climate Protection – screening coral skeletal records for proxy Traces of natural bleaching mitigation by internal waves (ReefCPTrace)

Ongoing global warming and increased prevalence and magnitude of heatwaves threaten coral reef persistence in many tropical regions wordwide by causing bleaching and mass mortality. The aim of this project is to obtain an improved understanding of the future fate of coral reefs by learning from the past. Cold water, supplied by wind-driven mixing, upwelling or breaking internal waves, has been shown to considerably enhance coral survival during marine heat waves, both at local and regional scales. However, we do not know whether cooling events occur regularly enough at specific locations to serve as a protection mechanism for reefs. A better understanding of beneficial reef cooling dynamics through time can be critical for improved reef management. Studying the coral skeletal record of past bleaching events in regions with known occurrence of cooling events will allow me to address this question. This project will focus on internal waves (IWs) in the Andaman Sea, Thailand. Here, reefs exposed to IWs have been shown to be protected from severe bleaching and mortality, in contrast to reefs sheltered from IW influence in the same region. The Andaman Sea represents an ideal natural laboratory to address the project goal, especially as multiple replicated IW-sheltered vs. IW-exposed reef sites are available. My main aims are to use the coral skeletal record to a) identify a suite of proxies that can unequivocally trace thermal bleaching stress, b) identify how internal wave forcing modulates these proxies and c) ultimately apply these proxies to coral cores that cover bleaching events during the last three decades to decipher whether IWs mitigated bleaching responses during these events. Combined with natural recovery rates from the 2010 severe bleaching (by repeating previous field monitoring of IW-sheltered as well as IW-exposed reefs) and linked with projections of future marine heat wave events, my project will enable more robust forecasts of future reef-persistence and predictions regarding the role of IW exposed reef locations as prime conservation targets.

Principle Investigator

Marlene Wall (GEOMAR, Kiel)