Degree Heating Weeks Research Articles

Skeletal records of bleaching reveal different thermal thresholds of Pacific coral reef assemblages

Ocean warming is negatively impacting coral reef ecosystems and considerable effort is currently invested in projecting coral reef futures under 21st century climate change. A limiting factor in these projections is lack of quantitative data on the thermal thresholds of different reef communities, due in large part to spatial and temporal gaps in bleaching observations. Here we apply a coral bleaching proxy, skeletal stress bands, to reconstruct the history of bleaching on eight coral reefs in the central equatorial Pacific (CEP) and use this information to constrain the thermal thresholds of their coral communities. First, three genera of massive corals collected on both Pacific and Caribbean reefs are used to derive a calibration between the proportion of corals that form stress bands during a bleaching event, and the total observed bleaching incidence in the community of mixed coral taxa. The correlation is highly significant, indicating that stress bands in massive corals reflect community-level bleaching severity (R2 = 0.945, p < 0.001). We applied the calibration to stress band records from eight Pacific reefs, reconstructing their bleaching histories over the period 1982 to 2015. A percentile-based method of estimating thermal stress (Degree Heating Weeks) for CEP reefs was developed and applied. Comparing the level of thermal stress experienced by each coral community during each event with the reconstructed bleaching response, we characterized the thermal sensitivities of each reef community and quantified the thermal threshold (b½) at which 50% of the coral community bleached. Our analysis reveals a unique non-linear thermal response curve for each reef. The most thermally tolerant reefs in the study (Jarvis and Kanton Islands) experienced 50% bleaching at seven to nine times more thermal stress than did the least resistant reef in the study (Maiana Island). An exploration of the potential drivers of thermal tolerance revealed a strong correlation between b½ and the history of thermal stress events in each reef system. Thermal tolerance was also correlated with concentrations of dissolved inorganic nitrate in the water column and with estimates of coral energetic reserve.

Limited coral mortality following acute thermal stress and widespread bleaching on Palmyra Atoll, central Pacific

During 2015–2016, an El Nino and associated warm water event caused widespread coral bleaching across the equatorial Pacific. Here, we combine 8 yr of benthic monitoring data from permanent photoquadrats with remotely sensed and in situ temperature measurements to assess the impact of the warming event on benthic communities at Palmyra Atoll. We quantified bleaching prevalence across two distinct reef habitats using the best available data. On the fore reef (~ 10 m depth), we quantified bleaching severity within 100–200 m2 large-area plots using the custom visualization and analysis software, Viscore. On the reef terrace (~ 5 m depth), we used 95 focal colonies across three species that have been monitored annually since 2014. The 2015–2016 warm water event was the most extreme such event recorded on Palmyra in the past several decades with a maximum cumulative heat stress (degree heating weeks) of 11.9 °C-weeks. On the fore reef, 90% of live coral cover exhibited some degree of bleaching (32% severe bleaching). On the shallow reef terrace, bleaching was observed in 93% of the focal colonies across all species. Overall, coral cover declined 9% on the fore reef from 2014 to 2017, whereas coral cover did not change on the terrace. These contrasting results may be associated with typical daily temperature ranges on the terrace that are three times greater than on the fore reef. Permanent photoquadrats showed that turf algae initially colonized skeletons of recently dead corals but transitioned to crustose coralline algae within a year. Collectively, our study emphasizes that comprehensive monitoring of benthic communities over time combined with in situ temperature data can provide taxonomically precise trajectories of community change during and following thermal stress.

Relative stability of the Pocillopora acuta microbiome throughout a thermal stress event

Rapid climate change due to anthropogenic greenhouse gas emissions is pushing corals to and past their physiological limits, while their microbiome is being pressed towards dysbiosis. Microbes greatly influence the health and functioning of corals, but thermal anomalies that cause bleaching can affect certain taxa of the host-associated prokaryote and Symbiodiniaceae communities, leading corals towards a disease-prone state. In some coral species, however, even significant thermal stress may not result in visible signs of bleaching. Whether changes in the coral microbiome occur in these resilient species during temperature anomalies is not well described. In the present study, we tagged, visually assessed and sampled 10 colonies of the branching coral Pocillopora acuta on a fringing reef off Orpheus Island in the central Great Barrier Reef for 1 yr, of which the summer coincided with the 2016 mass-bleaching event. No visible signs of bleaching were observed in any of the 10 colonies throughout the study period, despite experiencing two degree heating weeks of thermal stress and observations of bleaching in other coral species on the same reef. Metabarcoding based on the Symbiodiniaceae ITS2 rDNA spacer and the bacterial 16S rRNA gene provided evidence for stability of the overall microbial community structure, although the bacterial community showed increases in a number of potentially beneficial taxa, such as diazotrophs, during the thermal stress event. These findings suggest some flexibility in the microbiome to adjust to higher than average temperatures without disrupting microbiome stability, perhaps contributing to the thermal resilience of P. acuta.

Effects of moderate thermal anomalies on Acropora corals around Sesoko Island, Okinawa.

Over the past several decades, coral reef ecosystems have experienced recurring bleaching events. These events were predominantly caused by thermal anomalies, which vary widely in terms of severity and spatio-temporal distribution. Acropora corals, highly prominent contributors to the structural complexity of Pacific coral reefs, are sensitive to thermal stress. Response of Acropora corals to extremely high temperature has been well documented. However, studies on the effects of moderately high temperature on Acropora corals are limited. In the summer of 2016, a moderate coral bleaching event due to moderately high temperature was observed around Sesoko Island, Okinawa, Japan. The objective of this study was to examine thermal tolerance patterns of Acropora corals, across reefs with low to moderate thermal exposure (degree heating weeks ~2–5°C week). Field surveys on permanent plots were conducted from October 2015 to April 2017 to compare the population dynamics of adult Acropora corals 6 months before and after the bleaching events around Sesoko Island. Variability in thermal stress response was driven primarily by the degree of thermal stress. Wave action and turbidity may have mediated the thermal stress. Tabular and digitate coral morphologies were the most tolerant and susceptible to thermal stress, respectively. Growth inhibition after bleaching was more pronounced in the larger digitate and corymbose coral morphologies. This study indicates that Acropora populations around Sesoko Island can tolerate short-term, moderate thermal challenges.

Causes and consequences of the 2017 coral bleaching event in the southern Persian/Arabian Gulf

Coral reefs of the Persian/Arabian Gulf were the last to succumb to the effects of the global-scale mass coral bleaching event that began in 2015. This study examines the causes and consequences of the 2017 bleaching event on eight reefs located across > 350 km of the southern basin of the Gulf. Using a combination of 5 yr (2013–2017) of reef-based temperature observations, local meteorological data and water column modeling, we show that 2017 was characterized by an extended period of mid-summer calm when winds rarely exceeded breeze conditions, reducing evaporative heat loss and inducing dramatic warming compared with non-bleaching years (2013–2016). Reef-bottom temperatures in the Gulf in 2017 were among the hottest on record, with mean daily maxima averaging 35.9 ± 0.1 °C across sites, with hourly temperatures reaching as high as 37.7 °C. Across the southern Gulf, corals spent nearly 2 months (mean 55.1 ± 3.9 d above bleaching temperatures and nearly 2 weeks above lethal temperatures (11.8 ± 2.4 d), substantially longer than in the non-bleaching years (2013–2016) and equating with 5.5 °C-weeks of thermal stress as degree heating weeks. As a result, 94.3% of corals bleached, and two-thirds of corals were lost to mortality between April and September 2017. Mortality continued after peak bleaching, and by April 2018 coral cover averaged just 7.5% across the southern basin, representing an overall loss of nearly three-quarters of coral (73%) in 1 yr. This mass mortality did not cause dramatic shifts in community composition as earlier bleaching events had removed most sensitive taxa. An exception was the already rare Acropora which were locally extirpated in summer 2017. Given the increasing frequency of mass bleaching in the Gulf and the above global rates of regional warming, the capacity for recovery and the prognosis for the future of Gulf reefs are not optimistic.

High-resolution characterization of the abiotic environment and disturbance regimes on the Great Barrier Reef, 1985-2017.

This data compilation synthesizes 36 static environmental and spatial variables, and temporally explicit modeled estimates of three major disturbances to coral cover on the Great Barrier Reef (GBR): (1) coral bleaching, (2) tropical cyclones, and (3) outbreaks of the coral-eating crown-of-thorns starfish Acanthaster cf. solaris. Data are provided on a standardized grid (0.01°×0.01° ~1×1 km) for reef locations along the GBR, containing 15,928 pixels and excluding the northernmost sections (<12° S) where empirical data were sparse. This compilation provides a consistent and high-resolution characterization of the abiotic environment and disturbance regimes for GBR reef locations at a fine spatial scale to be used in the development of complex ecosystem models. Static estimates of environmental variables (e.g., depth, bed shear stress, average temperature, temperature variation) originally developed by the Commonwealth of Australia's Environment Research Facility (CERF) Marine Biodiversity Hub were provided by Geoscience Australia. Annual (1985-2017) disturbance estimates were either interpolated from empirical data (A. cf. solaris), predicted from proxy indicators (e.g., degree heating weeks [DHW] as a proxy for bleaching severity), or explicitly modeled (e.g., wave height model for each cyclone). This data set synthesizes some of the most recent advances in remote sensing and modeling of environmental conditions on the GBR; yet it is not exhaustive and we highlight areas that should be expanded through future research. The characterization of abiotic and disturbance regimes presented here represent an essential tool for the development of complex regional scale models of the GBR; preventing redundancy between working groups and promoting collaboration, innovation, and consistency. When using the data set, we kindly request that you cite this article and/or the articles cited in the reference section, recognizing the work that went into compiling the data together and the original authors' willingness to make it publicly available.

Back-to-back coral bleaching events on isolated atolls in the Coral Sea

Severe bleaching events caused by marine heat waves over the past four decades have now affected almost every coral reef ecosystem in the world. These recurring events have led to major losses of coral cover, with adverse consequences for tropical reef ecosystems and the people who depend on them. Here, we document two consecutive and widespread coral bleaching events on remote atolls in the Coral Sea in 2016 and 2017. In each year, the proportion of colonies that bleached was strongly related to heat exposure (measured as degree heating weeks, DHW, °C-weeks), depth and coral assemblage structure. Bleaching was more severe at higher DHW exposure and at sites with a higher proportion of susceptible taxa. Bleaching was also lower at 6 m than at 2 m depth. Despite the severe bleaching in 2016 on reefs in the central section of the Coral Sea Marine Park, total coral cover was not significantly reduced by 2017, suggesting that most bleached corals survived. Moreover, bleaching was less severe in 2017 despite a higher exposure to heat stress. These results indicate that while the isolation of these oceanic reefs provides no refuge from bleaching, low nutrient levels, high wave energy and proximity to cooler deeper waters may make coral on these reefs more resistant to bleaching-induced mortality.

Discovery of a recovering climax Acropora community in Kanton Lagoon in the remote Phoenix Islands Protected Area

A thermal stress that started in 2002 and peaked at 21 degree heating weeks resulted in near-100% mortality of corals in Kanton Lagoon. An expedition in 2015 documented a 52.8% recovery of hard corals in the lagoon. Despite temperatures reaching as high as 30.5°C there was almost no bleaching of Acropora stands within the lagoon, suggesting that recovering species are more resistant to thermal stress.

Impacts of coral bleaching on pH and oxygen gradients across the coral concentration boundary layer: a microsensor study

Reef-building corals are surrounded by complex microenvironments (i.e. concentration boundary layers) that partially isolate them from the ambient seawater. Although the presence of such concentration boundary layers (CBLs) could potentially play a role in mitigating the negative impacts of climate change stressors, their role is poorly understood. Furthermore, it is largely unknown how heat stress-induced bleaching affects O2 and pH dynamics across the CBLs of coral, particularly in branching species. We experimentally exposed the common coral species Acropora aspera to heat stress for 13 d and conducted a range of physiological and daytime microsensor measurements to determine the effects of bleaching on O2 and pH gradients across the CBL. Heat stress equivalent to 24 degree heating days (3.4 degree heating weeks) resulted in visible bleaching and significant declines in photochemical efficiency, photosynthesis rates and photosynthesis to respiration (P/R) ratios, whereas dark respiration and calcification rates were not impacted. As a consequence, bleached A. aspera had significantly lower (− 13%) surface O2 concentrations during the day than healthy corals, with concentrations being lower than that of the ambient seawater, thus resulting in O2 uptake from the seawater. Furthermore, we show here that Acropora, and potentially branching corals in general, have among the lowest surface pH elevation of all corals studied to date (0.041 units), which could contribute to their higher sensitivity to ocean acidification. Additionally, bleached A. aspera no longer elevated their surface pH above ambient seawater values and, therefore, had essentially no [H+] CBL. These findings demonstrate that heat stress-induced bleaching has negative effects on pH elevation and [H+] CBL thickness, which may increase the overall susceptibility of coral to the combined impacts of ocean acidification and warming.

Global warming transforms coral reef assemblages.

Global warming is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience of ecosystems and the people who depend on them 1 . Here we show that in the aftermath of the record-breaking marine heatwave on the Great Barrier Reef in 2016 2 , corals began to die immediately on reefs where the accumulated heat exposure exceeded a critical threshold of degree heating weeks, which was 3-4 °C-weeks. After eight months, an exposure of 6 °C-weeks or more drove an unprecedented, regional-scale shift in the composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world's largest coral reef system. Our study bridges the gap between the theory and practice of assessing the risk of ecosystem collapse, under the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems 3 , by rigorously defining both the initial and collapsed states, identifying the major driver of change, and establishing quantitative collapse thresholds. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems.

Marked annual coral bleaching resilience of an inshore patch reef in the Florida Keys: A nugget of hope, aberrance, or last man standing?

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.

Global patterns and impacts of El Niño events on coral reefs: A meta-analysis.

Impacts of global climate change on coral reefs are being amplified by pulse heat stress events, including El Niño, the warm phase of the El Niño Southern Oscillation (ENSO). Despite reports of extensive coral bleaching and up to 97% coral mortality induced by El Niño events, a quantitative synthesis of the nature, intensity, and drivers of El Niño and La Niña impacts on corals is lacking. Herein, we first present a global meta-analysis of studies quantifying the effects of El Niño/La Niña-warming on corals, surveying studies from both the primary literature and International Coral Reef Symposium (ICRS) Proceedings. Overall, the strongest signal for El Niño/La Niña-associated coral bleaching was long-term mean temperature; bleaching decreased with decreasing long-term mean temperature (n = 20 studies). Additionally, coral cover losses during El Niño/La Niña were shaped by localized maximum heat stress and long-term mean temperature (n = 28 studies). Second, we present a method for quantifying coral heat stress which, for any coral reef location in the world, allows extraction of remotely-sensed degree heating weeks (DHW) for any date (since 1982), quantification of the maximum DHW, and the time lag since the maximum DHW. Using this method, we show that the 2015/16 El Niño event instigated unprecedented global coral heat stress across the world's oceans. With El Niño events expected to increase in frequency and severity this century, it is imperative that we gain a clear understanding of how these thermal stress anomalies impact different coral species and coral reef regions. We therefore finish with recommendations for future coral bleaching studies that will foster improved syntheses, as well as predictive and adaptive capacity to extreme warming events.

Pendugaan Kejadian Pemutihan Karang Berdasarkan Analisis Suhu Permukaan Laut (SPL) Tahun 2015-2016 di Perairan Bali

National Ocean Atmospheric Administrations (NOAA) by the program coral reef Watch (CRW) has developed a method to estimate the potential of coral bleaching using Sea Surface Temperature (SST). The products are hot spot (HS) and degree heating week (DHW). HS is the SST 1°C (SSTL?1) above normal and DHW is the length of HS inhabits a place. The CRW product do not provided detail informations because it has a lower resolution. It is need a satellite image with a higher resolution to provide better informations. One of the satellite images that can be used is Moderate Resolution Imaging Spectroradiometer (MODIS) with a spatial resolution of 1 km. The purpose of this study was to know HS and DHW distribution patterns and status of coral bleaching in Bali waters seen from the analysis of HS and DHW. MODIS data is used daily, then do mosaicing process to get a weekly SPL (8 daily) and the monthly SST. Monthly SPL normally used to get maximum montly mean (MMM). HS obtained from the difference between 8 daily weekly SST and SST normal (MMM).).Location bleaching based on data Coral Triangle Center (CTC) and coralwatch.org. SST results revealed difference of SPL in 2015 and 2016 amounted to 1.48°C. Highest DHW in Bali Hai, Nusa Penida is 10 465° C-weeks in April 2016. Based on the value HS and DHW coral reefs in Bali waters threatened bleaching level Alert 1 and Alert level 2.

Marine heatwave causes unprecedented regional mass bleaching of thermally resistant corals in northwestern Australia

In 2015/16, a marine heatwave associated with a record El Niño led to the third global mass bleaching event documented to date. This event impacted coral reefs around the world, including in Western Australia (WA), although WA reefs had largely escaped bleaching during previous strong El Niño years. Coral health surveys were conducted during the austral summer of 2016 in four bioregions along the WA coast (~17 degrees of latitude), ranging from tropical to temperate locations. Here we report the first El Niño-related regional-scale mass bleaching event in WA. The heatwave primarily affected the macrotidal Kimberley region in northwest WA (~16°S), where 4.5–9.3 degree heating weeks (DHW) resulted in 56.6–80.6% bleaching, demonstrating that even heat-tolerant corals from naturally extreme, thermally variable reef environments are threatened by heatwaves. Some heat stress (2.4 DHW) and bleaching (<30%) also occurred at Rottnest Island (32°01’S), whereas coral communities at Ningaloo Reef (23°9’S) and Bremer Bay (34°25’S) were not impacted. The only other major mass bleaching in WA occurred during a strong La Niña event in 2010/11 and primarily affected reefs along the central-to-southern coast. This suggests that WA reefs are now at risk of severe bleaching during both El Niño and La Niña years.

Thermal refugia against coral bleaching throughout the northern Red Sea.

Tropical reefs have been impacted by thermal anomalies caused by global warming that induced coral bleaching and mortality events globally. However, there have only been very few recordings of bleaching within the Red Sea despite covering a latitudinal range of 15° and consequently it has been considered a region that is less sensitive to thermal anomalies. We therefore examined historical patterns of sea surface temperature (SST) and associated anomalies (1982-2012) and compared warming trends with a unique compilation of corresponding coral bleaching records from throughout the region. These data indicated that the northern Red Sea has not experienced mass bleaching despite intensive Degree Heating Weeks (DHW) of >15°C-weeks. Severe bleaching was restricted to the central and southern Red Sea where DHWs have been more frequent, but far less intense (DHWs <4°C-weeks). A similar pattern was observed during the 2015-2016 El Niño event during which time corals in the northern Red Sea did not bleach despite high thermal stress (i.e. DHWs >8°C-weeks), and bleaching was restricted to the central and southern Red Sea despite the lower thermal stress (DHWs<8°C-weeks). Heat stress assays carried out in the northern (Hurghada) and central (Thuwal) Red Sea on four key reef-building species confirmed different regional thermal susceptibility, and that central Red Sea corals are more sensitive to thermal anomalies as compared to those from the north. Together, our data demonstrate that corals in the northern Red Sea have a much higher heat tolerance than their prevailing temperature regime would suggest. In contrast, corals from the central Red Sea are close to their thermal limits, which closely match the maximum annual water temperatures. The northern Red Sea harbours reef-building corals that live well below their bleaching thresholds and thus we propose that the region represents a thermal refuge of global importance.

Unusually high coral recruitment during the 2016 El Niño in Mo'orea, French Polynesia.

The negative implications of the thermal sensitivity of reef corals became clear with coral bleaching throughout the Caribbean in the 1980’s, and later globally, with the severe El Niño of 1998 and extensive seawater warming in 2005. These events have substantially contributed to declines in coral cover, and therefore the El Niño of 2016 raised concerns over the implications for coral reefs; on the Great Barrier Reef these concerns have been realized. A different outcome developed in Mo’orea, French Polynesia, where in situ seawater temperature from 15 March 2016 to 15 April 2016 was an average of 0.4°C above the upper 95% CI of the decadal mean temperature, and the NOAA Degree Heating Weeks (DHW) metric supported a Level 1 bleaching alert (DHW ≥ 4.0). Starting 1 September 2016 and for the rest of the year (122 d), in situ seawater temperature was an average of 0.4°C above the 95% CI of long-term values, although DHW remained at zero. Minor coral bleaching (0.2–2.6% of the coral) occurred on the outer reef (10-m and 17-m depth) in April 2016, by May 2016 it had intensified to affect 1.3–16.8% of the coral, but by August 2016, only 1.4–3.0% of the coral was bleached. Relative to the previous decade, recruitment of scleractinians to settlement tiles on the outer- (10 m) and back- (2 m) reef over 2016/17 was high, both from January 2016 to August 2016, and from August 2016 to January 2017, with increased relative abundances of pocilloporids on the outer reef, and acroporids in the back reef. The 2016 El Niño created a distinctive signature in seawater temperature for Mo’orea, but it did not cause widespread coral bleaching or mortality, rather, it was associated with high coral recruitment. While the 2016 El Niño has negatively affected other coral reefs in the Indo-Pacific, the coral communities of Mo’orea continue to show signs of resilience, thus cautioning against general statements regarding the effects of the 2015/16 El Niño on coral reefs in the region.

Shading as a mitigation tool for coral bleaching in three common Indo-Pacific species

Shading substantially reduced the degree of bleaching in Acropora muricata, Pocillopora damicornis and Porites cylindrica in American Samoa. Experiments were conducted outdoors at two sites on Ofu and Tutuila Islands. An aquarium experiment was set up near some reef-flat pools in the National Park of American Samoa on Ofu Island, using different levels of shading (none, 50% and 75%) early in conditions of cumulative thermal stress corresponding to NOAA's Coral Reef Watch-Bleaching Alert System. We analyzed the effects of cumulative thermal stress regarding coral growth, as well as color changes (evaluated using a standardize reference card) as a proxy for decreases in symbiont density and chlorophyll a content (i.e. bleaching). Thermally stressed corals grew less than controls, but corals without shading experienced a more substantial decrease in growth compared to those under 50% or 75% shade. The analysis of coral color showed that both levels of shading were protective against bleaching in conditions of cumulative thermal stress for all species, but were particularly beneficial for the most sensitive ones: A. muricata and P. cylindrica. Heavier shading (75%) offered better protection than lighter shading (50%) in this experiment, possibly because of the intense light levels corals were subjected to. Although there were limits to the extent shading could mitigate the effects of cumulative heating, it was very effective to at least Degree Heating Week (DHW) 4 and continued to offer some protection until the end of the study (DHW 8). In Tutuila, a shaded/not-shaded platform experiment was carried out in a reef pool in which corals have shown repeated annual summer bleaching for several years. This experiment was designed to investigate if shading could attenuate bleaching in the field and also if there were negative consequences to shading removal. The only factor controlled was light intensity, and our main conclusion was that overall corals on the platform became darker than field colonies in response to shading, but adjusted back to the same color level as field colonies after shade removal. However, the latter results are preliminary and need to be confirmed by future studies under more controlled conditions. As bleaching becomes more frequent and regular due to global warming, we should consider proactively using shading to help mitigate the effects of thermal stress and prolong the survival of at least some coral communities, until solutions to address global climate change become effective.

Mass coral bleaching due to unprecedented marine heatwave in Papahānaumokuākea Marine National Monument (Northwestern Hawaiian Islands).

2014 marked the sixth and most widespread mass bleaching event reported in the Northwestern Hawaiian Islands, home to the Papahānaumokuākea Marine National Monument (PMNM), the world’s second largest marine reserve. This event was associated with an unusual basin-scale warming in the North Pacific Ocean, with an unprecedented peak intensity of around 20°C-weeks of cumulative heat stress at Lisianksi Island. In situ bleaching surveys and satellite data were used to evaluate the relative importance of potential drivers of bleaching patterns in 2014, assess the subsequent morality and its effects on coral communities and 3D complexity, test for signs of regional acclimation, and investigate long-term change in heat stress in PMNM. Surveys conducted at four island/atoll (French Frigate Shoals, Lisianski Island, Pearl and Hermes Atoll, and Midway Atoll) showed that in 2014, percent bleaching varied considerably between islands/atolls and habitats (back reef/fore reef and depth), and was up to 91% in shallow habitats at Lisianski. The percent bleaching during the 2014 event was best explained by a combination of duration of heat stress measured by Coral Reef Watch’s satellite Degree Heating Week, relative community susceptibility (bleaching susceptibility score of each taxon * the taxon’s abundance relative to the total number of colonies), depth and region. Mean coral cover at permanent Lisianski monitoring sites decreased by 68% due to severe losses of Montipora dilatata complex, resulting in rapid reductions in habitat complexity. Spatial distribution of the 2014 bleaching was significantly different from the 2002 and 2004 bleaching events likely due to a combination of differences in heat stress and local acclimatization. Historical satellite data demonstrated heat stress in 2014 was unlike any previous event and that the exposure of corals to the bleaching-level heat stress has increased significantly in the northern PMNM since 1982, highlighting the increasing threat of climate change to reefs.

Beyond peak summer temperatures, branching corals in the Gulf of Aqaba are resilient to thermal stress but sensitive to high light

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.

Common reef-building coral in the Northern Red Sea resistant to elevated temperature and acidification.

Coral reefs are currently experiencing substantial ecological impoverishment as a result of anthropogenic stressors, and the majority of reefs are facing immediate risk. Increasing ocean surface temperatures induce frequent coral mass bleaching events—the breakdown of the nutritional photo-symbiosis with intracellular algae (genus: Symbiodinium). Here, we report that Stylophora pistillata from a highly diverse reef in the Gulf of Aqaba showed no signs of bleaching despite spending 1.5 months at 1–2°C above their long-term summer maximum (amounting to 11 degree heating weeks) and a seawater pH of 7.8. Instead, their symbiotic dinoflagellates exhibited improved photochemistry, higher pigmentation and a doubling in net oxygen production, leading to a 51% increase in primary productivity. Nanoscale secondary ion mass spectrometry imaging revealed subtle cellular-level shifts in carbon and nitrogen metabolism under elevated temperatures, but overall host and symbiont biomass proxies were not significantly affected. Now living well below their thermal threshold in the Gulf of Aqaba, these corals have been evolutionarily selected for heat tolerance during their migration through the warm Southern Red Sea after the last ice age. This may allow them to withstand future warming for a longer period of time, provided that successful environmental conservation measures are enacted across national boundaries in the region.