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Coral reefs globally are threatened by climate change, but reef assemblages at high latitudes may serve as refugia. Marginal coral communities located in the subtropical Southwestern Atlantic are poorly studied, but were subject to an unprecedented heatwave and associated coral bleaching in 2019. Record values of 18.5 and 20.5 °C-weeks were registered for coastal and insular sites, which are the highest ever documented for a reef in the South Atlantic. As a consequence, approximately 80% and 20% of the population of the reef-building coral Mussismilia hispida (1116 colonies surveyed) underwent bleaching in coastal and insular sites, respectively. However, mortality (2%) was far lower than for episodes of similar magnitude in other regions, particularly in the Indo-Pacific. Therefore, the subtropical Southwestern Atlantic MCC displayed low mortality and remarkable tolerance when exposed to the most intense bleaching episode ever recorded for the region, reinforcing its potential as a refugium.

Annual coral bleaching events, which are predicted to occur as early as the next decade in the Florida Keys, are expected to cause catastrophic coral mortality. Despite this, there is little field data on how Caribbean coral communities respond to annual thermal stress events. At Cheeca Rocks, an inshore patch reef near Islamorada, FL, the condition of 4234 coral colonies was followed over 2 yr of subsequent bleaching in 2014 and 2015, the two hottest summers on record for the Florida Keys. In 2014, this site experienced 7.7 degree heating weeks (DHW) and as a result 38.0% of corals bleached and an additional 36.6% were pale or partially bleached. In situ temperatures in summer of 2015 were even warmer, with the site experiencing 9.5 DHW. Despite the increased thermal stress in 2015, only 12.1% of corals were bleached in 2015, which was 3.1 times less than 2014. Partial mortality dropped from 17.6% of surveyed corals to 4.3% between 2014 and 2015, and total colony mortality declined from 3.4 to 1.9% between years. Total colony mortality was low over both years of coral bleaching with 94.7% of colonies surviving from 2014 to 2016. The reduction in bleaching severity and coral mortality associated with a second stronger thermal anomaly provides evidence that the response of Caribbean coral communities to annual bleaching is not strictly temperature dose dependent and that acclimatization responses may be possible even with short recovery periods. Whether the results from Cheeca Rocks represent an aberration or a true resilience potential is the subject of ongoing research.

Mass bleaching is the most significant threat to coral reefs. The United States National Oceanic and Atmospheric Administration monitors world sea surface temperature (SST) and releases warnings for bleaching based on degree heating weeks (DHW), which is the accumulation of temperature anomalies exceeding the monthly maximum mean SST for a given region. DHW values >4.0 °C-weeks are thought to induce bleaching, and those >8.0 °C-weeks are thought to result in widespread bleaching and some mortality. This study validates the effectiveness of DHW as a mass bleaching index by on-site historical observation at eight sites in the northwestern Pacific Ocean. The mass bleaching events occurred during different years at different sites. The recorded years of the bleaching events matched well with DHW values >8 °C-weeks, and the logistically projected probability of bleaching against DHW showed a positive relationship. DHW provides a reasonable threshold for bleaching.

The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) program has been providing resource managers, scientific researchers, and other coral reef ecosystem stakeholders with coral bleaching heat stress products for more than 20 years. The development of the CoralTemp sea surface temperature (SST) dataset has allowed CRW to produce the Coral Bleaching Heat Stress product suite with climatologies and daily SST measurements from within the same SST dataset, significantly improving data quality. Previously, the Monthly Mean (MM) SST and Maximum Monthly Mean (MMM) SST climatologies were derived using a different dataset from the near real-time SST. Here we provide an up-to-date description of how each product within the Coral Reef Watch Coral Bleaching Heat Stress product suite version 3.1 is derived, including descriptions of the MM, MMM, SST Anomaly, Coral Bleaching HotSpot and Degree Heating Week (DHW) products.

The U.S. National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) program has developed a daily global 5-km product suite based on satellite observations to monitor thermal stress on coral reefs. These products fulfill requests from coral reef managers and researchers for higher resolution products by taking advantage of new satellites, sensors and algorithms. Improvements of the 5-km products over CRW’s heritage global 50-km products are derived from: (1) the higher resolution and greater data density of NOAA’s next-generation operational daily global 5-km geo-polar blended sea surface temperature (SST) analysis; and (2) implementation of a new SST climatology derived from the Pathfinder SST climate data record. The new products increase near-shore coverage and now allow direct monitoring of 95% of coral reefs and significantly reduce data gaps caused by cloud cover. The 5-km product suite includes SST Anomaly, Coral Bleaching HotSpots, Degree Heating Weeks and Bleaching Alert Area, matching existing CRW products. When compared with the 50-km products and in situ bleaching observations for 2013–2014, the 5-km products identified known thermal stress events and matched bleaching observations. These near reef-scale products significantly advance the ability of coral reef researchers and managers to monitor coral thermal stress in near-real-time.

Despite rapidly rising sea surface temperatures and recurrent positive temperature anomalies, corals in the Gulf of Aqaba (GoA) rarely experience thermal bleaching. Elsewhere, mass coral bleaching has been observed in corals when the water temperature exceeds 1–2 °C above the local maximum monthly mean (MMM). This threshold value or “bleaching rule” has been used to create predictive models of bleaching from satellite sea surface temperature observations, namely the “degree heating week” index. This study aimed to characterize the physiological changes of dominant reef building corals from the GoA in response to a temperature and light stress gradient. Coral collection and experiments began after a period of 14 consecutive days above MMM in the field. Stylophora pistillata showed negligible changes in symbiont and host physiology parameters after accumulating up to 9.4 degree heating weeks during peak summer temperatures, for which the index predicts widespread bleaching and some mortality. This result demonstrates acute thermal tolerance in S. pistillata from the GoA and deviation from the bleaching rule. In a second experiment after 4 weeks at 4 °C above peak summer temperatures, S. pistillata and Acropora eurystoma in the high-light treatment visibly paled and suffered greater midday and afternoon photoinhibition compared to corals under low-light conditions (35% of high-light treatment). However, light, not temperature (alone or in synergy with light), was the dominant factor in causing paling and the effective quantum yield of corals at 4 °C above ambient was indistinguishable from those in the ambient control. This result highlights the exceptional, atypical thermal tolerance of dominant GoA branching corals. Concomitantly, it validates the efficacy of protecting GoA reefs from local stressors if they are to serve as a coral refuge in the face of global sea temperature rise.

Tropical coral reefs are a critical ecosystem in global peril as a result of anthropogenic climate change, and effective conservation efforts require reliable methods for identifying and predicting coral bleaching events. To this end, temperature threshold-based models such as the National Oceanic and Atmospheric Administration’s (NOAA) degree-heating week (DHW) metric are useful for forecasting coral bleaching as a function of heat stress accumulation. DHW does not adequately account for regional variation in coral stress responses, however, and the current definition consistently underpredicts coral bleaching occurrence. Using a weather forecasting skill-based framework, our analysis cross-tested 1080 variations of the DHW-based bleaching occurrence (presence/absence) model against 22 years of contemporary coral bleaching observations (1998–2019) in order to optimize bleaching forecast skill at different levels of geographic specificity. On a global basis and relative to the current definition, reducing the current 1 °C warming cutoff to 0.4 °C, adjusting the accumulation window to 11 weeks, and defining a bleaching threshold of 3 DHW improved forecast skill by 70%. Allowing our new DHW definitions to vary across regions and ocean basins further doubled model skill. Our results also suggest that the most effective bleaching forecast models change over time as coral reef systems respond to a shifting climate. Since 1998, the coral bleaching threshold for the globally optimized forecast model has risen at a significant rate of 0.19 DHW/year, matching the pace of ocean warming. The bleaching threshold trajectory for each ocean basin varies. Though further work is necessary to parse the mechanism behind this trend, the dynamic nature of coral stress responses demands that our forecasting tools be continuously refined if they are to adequately inform marine conservation efforts.

The summer 2023 marine heatwave was the most severe on record for Florida’s Coral Reef, with unprecedented water temperatures and cumulative heat stress leading to 100% coral bleaching. An existing fate-tracking program of over 4200 brain and boulder coral colonies across five offshore and four inshore reefs allowed for analyses of bleaching-related mortality and diseases through the event. Across the vast majority of assessed corals, there was no partial or full mortality as a result of the 2023 bleaching event. At seven of the nine sites, only 0 – 2% of fate-tracked colonies experienced any mortality. The other two sites, both inshore, had the highest cumulative heat stress and did experience substantial bleaching-related mortality. However, acute mortality at one of them began at relatively low cumulative heat stress, suggesting death was the result of exceeding thermal maxima rather than bleaching-related resource depletion. At the two most impacted sites, 43% and 30% of all monitored corals died, but mortality varied among species: brain corals fared worse than boulder corals. The health status of corals before the bleaching event had little impact on whether they exhibited mortality during the event. At three sites, we observed unusual lesions on Orbicella faveolata colonies shortly after color returned to the corals; these were only present for a few months, but on some colonies led to substantial tissue loss. Though not part of the monitoring program, we also observed local extinctions of Acroporid corals at most sites, as well as local extinctions of octocorals at three inshore reefs. Though most reef-building corals survived the 2023 marine heatwave in the Florida Keys, continually rising temperatures are likely to make these temperature regimes more common. We encourage future research on why the brain and boulder corals fared differently at highly-impacted sites, and on what the unusual O. faveolata lesions are. Our results also provide perspective on how restoration strategies, particularly those focused on species likely to die under current and future climate regimes, might prioritize species likely to survive. Finally, these results highlight the importance of fate-tracking individuals of different species and in different geographies and habitat types through disturbance events.

Climate-induced warming events increasingly threaten coral reefs, heightening the need for accurate quantification of baseline temperatures and thermal stress in these ecosystems. To assess the strengths and weaknesses of NOAA satellite sea surface temperature and in situ measurements, we compared 5 yr of these data on Kiritimati atoll, in the central equatorial Pacific. We find that (1) satellite measurements were similar to in situ measurements (~ 10 m depth), albeit slightly warmer, with measurements converging once above Kiritimati’s maximum monthly mean; (2) in situ loggers detected subsurface cooling events missed by satellites; (3) thermal baselines and anomalies were consistent around the island; and (4) in situ degree heating week (DHW) calculations were most comparable to NOAA DHWs when calculated using NOAA’s climatology. These results suggest that NOAA’s satellite products accurately reflect conditions on central Pacific reefs, but that in situ measurements can identify localized events, such as subsurface upwelling, that may be ecologically relevant for corals.

The Great Barrier Reef Expedition (1928–1929) observed two of the earliest known examples of coral bleaching during a 13-month stay on Low Isles, northern Great Barrier Reef, Australia. The first was during austral summer in February 1929 in a shallow moat and the second during austral winter in June 1929 on a deeper seaward slope. Using Expedition records and modern analytical techniques, we geolocated previously undocumented photographs of bleaching and built a detailed picture of environmental conditions which show that the two episodes were driven by different proximate causes. The February bleaching coincided with highest annual seawater temperatures and Degree Heating Weeks, coupled with early afternoon aerial exposure leading to intense heating in tide pools and corals subjected to high solar insolation. Zooxanthellae densities would also have been at a seasonal low and the corals particularly susceptible to bleaching stress. In contrast in June, the seaward slope, normally submerged during the daytime, was aerially exposed to the afternoon sun and although seawater temperatures were 5 °C cooler than in the summer and despite relatively low sun altitudes and insolation in the winter, bleaching and mortality due to solar radiation was clearly visible on faces of exposed corals oriented towards the sun. Whilst reports of widespread temperature driven coral bleaching are commonplace today in an era of global warming, this paper demonstrates how similar environmental cues were involved a century ago and highlights the additional complexity of coral survival in the intertidal zone.