Award: OCE-1737311

Coral reefs globally are under threat from ocean warming and the increase in frequency and intensity of heatwaves. In the central equatorial Pacific, coral reefs have been exposed to El Nino-induced heatwaves hundreds if not thousands of years, yet maintain relatively high coral cover and diverse coral populations. Understanding the factors that enable these reefs to respond to, and recover from, frequent heat exposure will inform management strategies for reefs under global warming and improve model projections of reef futures that do not currently consider adaptation. Historical bleaching data is key to understanding the reef response yet observational data in this remote region is sparse: only two direct observations of coral bleaching have been made, both during the 2015/16 El Nino. To fill these vast spatial and temporal gaps in observational data, we developed a bleaching proxy based on high-density skeletal stress bands accreted by massive corals. Under this award, we developed an automated code to identify and quantify bleaching stress bands in 3-D CT-scan images of coral skeletons. We established a robust link between bleaching and skeletal stress bands by showing that prolonged bleached causes starvation, a reduction in coral tissue biomass, and cessation of skeletal extension (upward growth) that leads to formation of anomalously high density "stress" bands. Using skeletal cores and bleaching observations from 11 Pacific and Atlantic coral reefs, we established a robust link between the proportion of Porites (Pacific)/Orbicella, Siderastrea (Caribbean) that form stress bands during a heatwave, and the observed percent bleaching on the reef during that heatwave. This means we can use the stress band proxy to identify bleaching events that were not directly observed, and to estimate the severity of bleaching on different parts of the reef. Applying the stress band proxy in the central equatorial Pacific, we demonstrated a clear partitioning of thermal tolerance amongst different reefs linked to the history of thermal stress exposure of each island. Coral communities exposed to a history of frequent strong heatwaves had higher heat tolerance than those exposed to less frequent and severe heatwaves. Using long skeletal cores extracted from old, massive coral colonies, we reconstructed a 100 year-long history of bleaching in the central equatorial Pacific, spanning 1920 to 2020. We uncovered 20 regional scale bleaching events during this time period, of which the 2015/16 event stands out as the most severe by far. This award supported two post-doctoral fellows, one PhD thesis, one Master?s thesis, three undergraduate senior theses, and a high school science fair project. Results were disseminated through two documentary films posted on YouTube, multiple public speaking engagements including the UN General Assembly, the Polynesian Leaders Group Summit and the UN Ocean Decade. 3-D meter-scale hydrodynamic models of two coral reef islands were developed. Nine manuscripts have been published, submitted or prepared for publication in international peer reviewed journals. All published data and codes are publically available on BCO-DMO. Last Modified: 02/13/2021 Submitted by: Anne Cohen