Bleaching Patterns Research Articles

High coral heat tolerance at local-scale thermal refugia

Marine heatwaves and mass bleaching have devastated coral populations globally, yet bleaching severity often varies among reefs. To what extent a reef’s past exposure to heat stress influences coral bleaching and mortality remains uncertain. Here we identify persistent local-scale hotspots and thermal refugia among the reefs of Palau, Micronesia, based on 36 years of satellite-derived cumulative heat stress (degree heating weeks–DHW, units: °C-weeks). One possibility is that hotspots may harbour more heat tolerant corals due to acclimatisation, directional selection, and/or loss of tolerant genotypes. Historic patterns of assemblage-wide mass bleaching and marine heatwaves align with this hypothesis, with DHW-bleaching responses of hotspots occurring at 1.7°C-weeks greater heat stress than thermal refugia. This trend was consistent yet weaker for Acropora and corymbose Acropora, with severe bleaching risk reduced by 4–10% at hotspots. However, we find a contrasting pattern for Acropora digitifera exposed to a simulated marine heatwave. Fragments of 174 colonies were collected from replicate hotspot and thermal refugium outer reefs with comparable wave exposure and depth. Higher heat tolerance at thermal refugia (+0.7°C-weeks) and a correlation with tissue biomass suggests that factors other than DHW may overwhelm any spatially varying effects of past DHW exposure. Further, we found considerable A. digitifera heat tolerance variability across sites; compared to the least-tolerant 10% of colonies, the most-tolerant 10% could withstand additional heat stresses of 5.2 and 4.1°C-weeks for thermal refugia and hotspots, respectively. Our study demonstrates that hotspot reefs do not necessarily harbour more heat tolerant corals than nearby thermal refugia, and that mass bleaching patterns do not necessarily predict species responses. This nuance has important implications for designing climate-smart initiatives; for instance, in the search for heat tolerant corals, our results suggest that investing effort into identifying the most tolerant colonies within individual reefs may be warranted.

Cumulative risk of future bleaching for the world's coral reefs.

Spatial and temporal patterns of future coral bleaching are uncertain, hampering global conservation efforts to protect coral reefs against climate change. Our analysis of daily projections of ocean warming establishes the severity, annual duration, and onset of severe bleaching risk for global coral reefs this century, pinpointing vital climatic refugia. We show that low-latitude coral regions are most vulnerable to thermal stress and will experience little reprieve from climate mitigation. By 2080, coral bleaching is likely to start on most reefs in spring, rather than late summer, with year-round bleaching risk anticipated to be high for some low-latitude reefs regardless of global efforts to mitigate harmful greenhouse gasses. By identifying Earth's reef regions that are at lowest risk of accelerated bleaching, our results will prioritize efforts to limit future loss of coral reef biodiversity.

Hot spots of bleaching in massive Porites coral colonies

Coral bleaching events have become more frequent and severe due to ocean warming. While the large-scale impacts of bleaching events are well-known, there is growing recognition of the importance of small-scale spatial variation in bleaching and survival probability of individual coral colonies. By quantifying bleaching in 108 massive Porites colonies spread across Lizard Island, Great Barrier Reef, during the 2016 bleaching event, we investigated how hydrodynamic exposure levels and colony size contribute to local variability in bleaching prevalence and extent. Our results revealed that exposed locations were the least impacted by bleaching, while lagoonal areas exhibited the highest prevalence of bleaching and colony-level bleaching extents. Such patterns of bleaching could be due to prolonged exposure to warm water in the lagoon. These findings highlight the importance of considering location-specific factors when assessing coral health and emphasize the vulnerability of corals in lagoonal habitats to rapid and/or prolonged elevated temperatures.

Fine-scale variability in coral bleaching and mortality during a marine heatwave

Coral bleaching and mortality can show significant spatial and taxonomic heterogeneity at local scales, highlighting the need to understand the fine-scale drivers and impacts of thermal stress. In this study, we used structure-from-motion photogrammetry to track coral bleaching, mortality, and changes in community composition during the 2019 marine heatwave in Kāneʻohe Bay, Hawaiʻi. We surveyed 30 shallow reef patches every 3 weeks for the duration of the bleaching event (August-December) and one year after, resulting in a total of 210 large-area, high-resolution photomosaics that enabled us to follow the fate of thousands of coral colonies through time. We also measured environmental variables such as temperature, sedimentation, depth, and wave velocity at each of these sites, and extracted estimates of habitat complexity (rugosity R and fractal dimension D) from digital elevation models to better understand their effects on patterns of bleaching and mortality. We found that up to 80% of corals experienced moderate to severe bleaching in this period, with peak bleaching occurring in October when heat stress (Degree Heating Weeks) reached its maximum. Mortality continued to accumulate as bleaching levels dropped, driving large declines in more heat-susceptible species (77% loss of Pocillopora cover) and moderate declines in heat-tolerant species (19% and 23% for Porites compressa and Montipora capitata, respectively). Declines in live coral were accompanied by a rapid increase in algal cover across the survey sites. Spatial differences in bleaching were significantly linked to habitat complexity and coral species composition, with reefs that were dominated by Pocillopora experiencing the most severe bleaching. Mortality was also influenced by species composition, fractal dimension, and site-level differences in thermal stress. Our results show that spatial heterogeneity in the impacts of bleaching are driven by a mix of environmental variation, habitat complexity, and differences in assemblage composition.

Hydrodynamic and atmospheric drivers create distinct thermal environments within a coral reef atoll

Within coral reefs, different thermal environments can be found at locations separated by less than 100 s of meters and can generate fine-scale patterns of thermal stress and subsequent bleaching. In this study, we use an 11-month record of in situ temperature measurements, coupled with oceanographic and atmospheric data to examine the role of surface and advective heat fluxes in driving spatial patterns of temperature variability across several reef zones (i.e., fore-reef, reef flat, channel and lagoon) within an individual coral reef atoll. We show that advection of heat (driven by a combination of wave and tidal flows) was dominant across all sites and surface heating was more important across shallow areas or areas of low net exchange (i.e., reef flat and lagoon zones). Tidal flows were important in driving short term variability in the transport of heat across the atoll, but their contribution to the net transport of heat (cooling vs heating) was less significant over the longer timescales (days to weeks) that are typically used to assess thermal stress experienced by coral reef communities (e.g., Degree Heating Weeks). Conversely, although the wave-driven advection of heat contributed minimally to reef temperature changes over short timescales, the net transport of heat over daily to weekly timescales had a significant influence on persistent temperature anomalies. By parameterising the mechanisms driving temperature variability across the reef flat and lagoon zones, we demonstrate how satellite measurements of sea surface temperatures can be corrected to provide robust temperature estimates at the reef scale.

A global analysis of coral bleaching patterns in association with mangrove environments under global warming

Marine heatwaves caused by global warming are progressively degrading coral reefs worldwide via the process of coral bleaching – the expulsion of photosynthetic endosymbionts. However, coral bleaching is not spatially homogeneous, but varies across environmental gradients in association with local conditions and taxonomic composition. Emerging evidence suggests that mangrove habitats are recurrent ‘'microenvironments' for reef corals, providing key biogeochemical services believed to influence their resilience, and housing co‐tolerant coral taxa. This close association between coral reefs and mangroves has led to the hypothesis that the extent of coral bleaching is reduced by the proximity of reef building corals to mangroves. Here, we present the first global‐scale test addressing this hypothesis. Using Bayesian generalised linear mixed effect modelling, we show that coral bleaching tends to increase under higher thermal stress when corals are further away from mangroves. When comparing conditional effects, corals furthest away from mangroves also show the highest levels of bleaching, suggesting a phenomenon of reduced bleaching for corals residing closer to mangroves under thermal stress. The drivers of this pattern may be numerous, but most likely include the presence of co‐tolerant coral species which are known to be associated with mangroves. Future research should focus on disentangling the mechanisms underlying these coral bleaching patterns in association with mangroves, specifically focusing on in‐situ environmental and taxonomic data collection. These endeavours will most likely require high‐resolution analyses of coral reefs at more local‐scales. Ultimately, our findings add to the ever‐growing importance of mangroves for near‐shore coral reef environments, and suggest a potentially important benefit for coral reefs associated with mangroves in the Anthropocene.

Appraisal of coral bleaching thresholds and thermal projections for the northern Red Sea refugia

Corals in the northern Red Sea exhibit high thermal tolerance despite the increasing heat stress. It is assumed that corals throughout the Red Sea have similar bleaching thresholds (32°C or higher), and hence greater bleaching tolerance of corals in the northern Red Sea region is likely due to lower ambient water temperatures (25–28°C) that remain well below the corals’ physiological maxima. Whether bleaching patterns across the Red Sea are independent of the local maximum monthly mean of seawater temperature and aligned with an assumed 32°C threshold has yet to be determined. Here, we used remotely sensed surface sea temperature data spanning 1982–2020 to model spatial distributions of Degree Heat Weeks across the Red Sea in relation to assumed coral thermal threshold values of 30, 31, and 32°C. We also used the Coupled Model Intercomparison Project Phase 5 model outputs to predict warming trends in the Red Sea under different greenhouse gas representative concentration pathways (RCPs). We show that applying 32°C thresholds dramatically reduces effective Degree Heat Weeks in the north, but not in central or southern Red Sea regions, a finding that is consistent with historical bleaching observations (1998–2020) throughout the Red Sea. Further, model predictions under the most extreme RCP8.5 scenario exhibited ~3°C warming by the end of the 21st century throughout the Red Sea with less pronounced warming for the northern Red Sea (2–2.5°C) compared to the central and southern regions (2.7–3.1°C).This warming rate will remain below the assumed thermal threshold for the northern Red Sea which should help this region to serve as refugia (i.e., maintaining favorable temperatures) for corals to persist for decades ahead. Together, our results support the notion that corals have similar thresholds throughout the Red Sea; hence, coral bleaching thresholds are independent of the local maximum monthly mean. Consequently, where regional warming projections suggest the northern Red Sea will not reach assumed bleaching thresholds (32°C) before the end of the 21st century, coral reefs in the northern region may be among the last standing against climate change.

Photo-physiology of healthy and bleached corals from the Mascarene Plateau

This study presents the first report of variable photo-physiology of healthy-looking and bleached corals from the upper mesophotic waters of the Mascarene Plateau. In May 2018, during the FAO EAF-Nansen research expedition cruise, coral bleaching was visually observed. Five coral species from Saya de Malha Bank, namely Heliopora coerulea, Favites sp. and Porites sp. from 27 m and Acropora sp. and Lithophyllon repanda from 30 m, and three coral species from the Nazareth Bank, namely Acropora sp. and Galaxea fascicularis from 36 m and Stylophora-like species from 58 m were studied using the Video-Assisted Multi-Sampler (VAMS) and collected using a Van Veen grab. Chlorophyll a fluorescence parameters such as effective quantum yield at photosystem II (ΦPSII), relative maximum electron transport rate (rETRm), photosynthetic efficiency (α), photoinhibition (β), saturating light level (Ek), and maximum non-photochemical quenching (NPQm) were measured using a Diving-Pulse-Amplitude-Modulated (D-PAM) fluorometer to study variable photo-physiology in bleached and non-bleached corals. All photo-physiological parameters varied significantly among coral species tested and between coral conditions, except for β. The interaction between species and coral conditions was only significant in the case of β, but generally not significant. A two-way ANOVA indicated significant effects of depth and coral conditions in Acropora sp. on almost all photo-physiological parameters, except for β, and the effect of depth on rETRmax and α, and the effect of depth along with its interaction with coral conditions on Ek. ΦPSII did not differ in bleached and healthy-looking coral parts of Porites and Lithophyllon from 27 m, Galaxea and Acropora from 36 m while it decreased significantly in Heliopora and Favites at 27 m, Acropora from 30 m, and Stylophora-like at 58 m. NPQm did not change for Porites, Acropora (30 m) and Galaxea but it tended to increase for Heliopora, Acropora (36 m), Lithophyllon, Galaxea, and decrease for Favities, Acropora (30 m) and Stylophora-like. The thermally tolerant coral Porites exhibited normal photo-physiology even in bleached conditions while the bleached parts of Favites, Acropora (30 m) and Stylophora-like corals exhibited photo-physiological dysfunctioning. This study revealed that the seven studied corals from the upper mesophotic waters of the Mascarene Plateau are not spared from the bleaching phenomenon and exhibit variable photo-physiology in bleached and non-bleached conditions. Further studies are warranted to thoroughly understand the coral bleaching patterns and severity during summer periods at the Saya de Malha and Nazareth Banks.

Ocean acidification elicits differential bleaching and gene expression patterns in larval reef coral Pocillopora damicornis under heat stress

The successful dispersal of coral larvae is vital to the population replenishment and reef recovery and resilience. Despite that this critical early stage is susceptible to ocean warming and acidification, little is known about the responses of coral larvae to warming and acidification across different biological scales. This study explored the influences of elevated temperature (29 °C versus 33 °C) and pCO2 (500 μatm versus 1000 μatm) on brooded larvae of Pocillopora damicornis at the organismal, cellular and gene expression levels. Heat stress caused bleaching, depressed light-enhanced dark respiration, photosynthesis and autotrophy, whereas high pCO2 stimulated photosynthesis. Although survival was unaffected, larvae at 33 °C were ten-times more likely to settle than those at 29 °C, suggesting reduced capacity to disperse and differentiate suitable substrate. Remarkably, heat stress induced greater symbiont loss at ambient pCO2 than at high pCO2, while cell-specific pigment concentrations of symbionts at 33 °C increased twofold under ambient pCO2 relative to high pCO2, suggesting pCO2-dependent bleaching patterns. Considerable increases in activities of host antioxidants superoxide dismutase (SOD) and catalase (CAT) at 33 °C indicated oxidative stress, whereas lipid peroxidation and caspase activities were contained, thereby restraining larval mortality at 33 °C. Furthermore, the coral host mounted stronger transcriptional responses than symbionts. High pCO2 stimulated host metabolic pathways, possibly because of the boosted algal productivity. In contrast, host metabolic processes and symbiont photosystem genes were downregulated at 33 °C. Interestingly, the upregulation of extracellular matrix genes and glycosaminoglycan degradation pathway at 33 °C was more evident under ambient pCO2 than high pCO2, suggesting compromised host tissue integrity that could have facilitated symbiont expulsion and bleaching. Our results provide insights into how coral larvae respond to warming and acidification at different levels of biological organization, and demonstrate that ocean acidification can mediate thermal bleaching and gene expression in coral larvae under heat stress.

Size-dependent mortality of corals during marine heatwave erodes recovery capacity of a coral reef.

For many long-lived taxa, such as trees and corals, older, and larger individuals often have the lowest mortality and highest fecundity. However, climate change-driven disturbances such as droughts and heatwaves may fundamentally alter typical size-dependent patterns of mortality and reproduction in these important foundation taxa. Working in Moorea, French Polynesia, we investigated how a marine heatwave in 2019, one of the most intense marine heatwaves at our sites over the past 30years, drove patterns of coral bleaching and mortality. The marine heatwave drove island-wide mass coral bleaching that killed up to 76% and 65% of the largest individuals of the two dominant coral genera, Pocillopora and Acropora, respectively. Colonies of Pocillopora and Acropora ≥30cm diameter were ~3.5× and ~1.3×, respectively, more likely to die than colonies <30-cm diameter. Typically, annual mortality in these corals is concentrated on the smallest size classes. Yet, this heatwave dramatically reshaped this pattern, with heat stress disproportionately killing larger coral colonies and equalizing annual mortality rates across the size spectrum. This shift in the size-mortality relationship reduced the overall fecundity of these genera by >60% because big corals are disproportionately important for reproduction on reefs. Additionally, the survivorship of microscopic coral recruits, critical for the recovery of corals following disturbances, declined to 2%, over an order of magnitude lower compared to a year without elevated thermal stress, where 33% of coral recruits survived. While other research has shown that larger corals can bleach more frequently than smaller corals, we show the severe impact this phenomenon can have at the reef-wide scale. As marine heatwaves become more frequent and intense, disproportionate mortality of the largest, most fecund corals and near-complete loss of entire cohorts of newly-settled coral recruits will likely reduce the recovery capacity of these iconic ecosystems.

Global decline in capacity of coral reefs to provide ecosystem services

Global decline in capacity of coral reefs to provide ecosystem services

Rebuilding relationships on coral reefs: Coral bleaching knowledge-sharing to aid adaptation planning for reef users: Bleaching emergence on reefs demonstrates the need to consider reef scale and accessibility when preparing for, and responding to, coral bleaching.

Coral bleaching has impacted reefs worldwide and the predictions of near-annual bleaching from over two decades ago have now been realized. While technology currently provides the means to predict large-scale bleaching, predicting reef-scale and within-reef patterns in real-time for all reef users is limited. In 2020, heat stress across the Great Barrier Reef underpinned the region's third bleaching event in 5 years. Here we review the heterogeneous emergence of bleaching across Heron Island reef habitats and discuss the oceanographic drivers that underpinned variable bleaching emergence. We do so as a case study to highlight how reef end-user groups who engage with coral reefs in different ways require targeted guidance for how, and when, to alter their use of coral reefs in response to bleaching events. Our case study of coral bleaching emergence demonstrates how within-reef scale nowcasting of coral bleaching could aid the development of accessible and equitable bleaching response strategies on coral reefs. Also see the video abstract here: https://youtu.be/N9Tgb8N-vN0.

The role of mechanical stratigraphy on CO2 migration along faults – examples from Entrada Sandstone, Humbug Flats, Utah, USA

Bleaching patterns along a faulted succession provides new insight into fluid migration pathways controlled by the fault zone and mechanical stratigraphy. In order to investigate this effect, we logged stratigraphic layers in Humbug Flats, Utah, USA both in host rocks and faulted rocks, where a 40 m offset normal fault cuts through the Entrada Sandstone. We measured petrophysical and mechanical properties using TinyPerm II, Schmidt Hammer and laboratory core testing for selected layers. Bleaching has been identified as a proxy for fluid migration parallel to bedding, which fades out with distance from faults. A new observation is that fracturing of bed interfaces contributes to horizontal flow out from the fault zone. Longer-distance comprehensive bleaching dominates the most permeable beds (units), hosting porosity-controlled deformation band associated to fault zones. Fractures prevail in low permeability and stiffer units, where isolated fracture parallel bleaching haloes are observed within fault damage and process zones. Strong correlation is observed between permeability/porosity and stiffness/strength properties for the tested samples of different layers along the stratigraphic logs. For the studied fault, fractures provide important conduits for vertical flow across low permeability and stiff layers, whereas the more porous layers provide bedding-parallel flow paths out of the fault zone.

The combined use of systemic analgesic/anti-inflammatory drugs and a bioactive topical desensitizer for reduced in-office bleaching sensitivity without jeopardizing the hydrogen peroxide efficacy: a randomized, triple blinded, split-mouth clinical trial.

To evaluate the effect of combined systemic administration of paracetamol 500mg/codeine phosphate 30mg (PACO) and postoperative topical application of a bioactive desensitizer on in-office bleaching sensitivity and tooth color change. A randomized, triple-blind, split-mouth clinical trial was conducted. Forty volunteers ingested PACO (n = 20) or placebo (PLA) (n = 20). Their left/right hemiarches received topical application of a bioactive desensitizer [Nano-P™(NP)] and prophylactic paste (PAS), generating four treatment approaches: PACO/NP, PACO/PAS, PLA/NP, and PLA/PAS. Two bleaching sessions (35% hydrogen peroxide) were performed, and the PAS/NP were applied after the procedure. Sensitivity was obtained since the first bleaching session up to 7days post-bleaching. The color change was evaluated using CIEDE2000 and whiteness index parameters up to 7days post-bleaching. Data were analyzed using one- and two-way ANOVA/Tukey post hoc tests (p < 0.05). The PLA/PAS showed a sensitivity average of at least two times higher than the PACO/NP. The treatment approaches promoted statistically similar bleaching patterns (p > 0.05). The combined approach of systemic administration of PACO and postoperative topical application of NP reduced the level of in-office bleaching sensitivity without jeopardizing hydrogen peroxide efficacy. Professionals can adopt the combined approach of systemic administration of analgesic/anti-inflammatory drugs and topical application of a bioactive desensitizer for decreased bleaching sensitivity caused by 35% hydrogen peroxide in-office.

A preliminary study of skin bleaching and factors associated with skin bleaching among women living in Zimbabwe.

Skin bleaching was reported to be commonly practiced among women and Africa was reported to be one of the most affected yet the subject is not given much attention in public health research in Zimbabwe despite the adverse effects of skin bleaching on health. This study was an exploratory cross-sectional survey to explore skin bleaching, skin bleaching patterns and factors associated with skin bleaching among women living in Zimbabwe. An online self-administered questionnaire was sent out to women on social network i.e. WhatsApp, Facebook, LinkedIn and Twitter. A total number of 260 respondents, mean age 31.69 (SD, 8.12) years participated in the survey. The prevalence of skin bleaching among the participants was 31.15%. The major reason reported for skin bleaching was to have smooth and healthy skin alongside other factors such as beauty, gaining social favours for example getting married and good jobs. Occupation, complexion and marital status were associated with skin bleaching. The odds of skin bleaching for participants who were employed was 1.45(95% confidence interval [CI],0.32-1.91);p-value 0.02, dark skinned participants 2.56(95% CI, 0.76-2.87);p-value 0.01 and unmarried participants 2.87(95% CI,0.29-3.58);p-value 0.03. Evidence from the research suggests skin bleaching might be common among women living in Zimbabwe and possibly poses serious health threats to the women. Skin bleaching seems to be deep rooted in colourism. The colourism seems to be taken advantage of by the cosmetic industry which produce the potentially hazardous products which promise the revered light skin to women but which comes with a price. However, the study provides a base for future studies to explore more on skin bleaching practices among women living in Zimbabwe.

The past century of coral bleaching in the Saudi Arabian central Red Sea.

Accurate knowledge of the spatial and temporal patterns of coral bleaching is essential both for understanding how coral reef ecosystems are changing today and forecasting their future states. Yet, in many regions of the world, the history of bleaching is poorly known, especially prior to the late 20th century. Here, I use the information preserved within skeleton cores of long-lived Porites corals to reconstruct the past century of bleaching events in the Saudi Arabian central Red Sea. In these cores, skeletal “stress bands”—indicative of past bleaching—captured known bleaching events that occurred in 1998 and 2010, but also revealed evidence of previously unknown bleaching events in 1931, 1978, and 1982. However, these earlier events affected a significantly lesser proportion of corals than 1998 and 2010. Therefore, coral bleaching may have occurred in the central Red Sea earlier than previously recognized, but the frequency and severity of bleaching events since 1998 on nearshore reefs is unprecedented over the past century. Conversely, corals living on mid- to outer-shelf reefs have not been equally susceptible to bleaching as their nearshore counterparts, which was evident in that stress bands were five times more prevalent nearshore. Whether this pattern of susceptible nearshore reefs and resistant outer-shelf reefs continues in the future remains a key question in forecasting coral reef futures in this region.

Local acclimatisation-driven differential gene and protein expression patterns of Hsp70 in Acropora muricata: Implications for coral tolerance to bleaching.

Corals show spatial acclimatisation to local environment conditions. However, the various cellular mechanisms involved in local acclimatisation and variable bleaching patterns in corals remain to be thoroughly understood. In this study, the modulation of a protein implicated in cellular heat stress tolerance, the heat shock protein 70, was compared at both gene (hsp70) and protein (Hsp70) expression level in bleaching tolerant near-coast Acropora muricata colonies and bleaching susceptible reef colonies, in the lagoon of Belle Mare (Mauritius). The relative Hsp70 levels varied significantly between colonies from the two different locations, colonies having different health conditions and the year of collection. Before the bleaching event of 2016, near-coast colonies had higher basal levels of both Hsp70 gene and protein compared to reef colonies. During the bleaching event, the near-coast colonies did not bleach and had significantly higher relative levels of both Hsp70 gene and protein compared to bleached reef colonies. No significant genetic differentiation between the two studied coral populations was observed and all the colonies analysed were associated with Symbiodiniaceae of the genus Symbiodinium (Clade A) irrespective of location and sampling period. These findings provide further evidence of the involvement of Hsp70 in conferring bleaching tolerance to corals. Moreover, the consistent expression differences of Hsp70 gene and protein between the near-coast and reef coral populations in a natural setting indicate that the modulation of this Hsp is involved in local acclimatisation of corals to their environments.

Coral community resilience to successive years of bleaching in Kāne‘ohe Bay, Hawai‘i

The Hawaiian Islands are at the northern edge of coral reef distributions, and corals found there are exposed to large seasonal temperature changes. Historically, coral bleaching in the Hawaiian Islands was extremely rare and had only occurred in 1996. However, in the summers of both 2014 and 2015, successive bleaching events occurred in Kāne‘ohe Bay, O‘ahu. Seawater temperatures were above 28 °C for approximately 1 month in 2014 and 3 months in 2015 and peaked above 30 °C in both years. Patterns of bleaching did not vary among the three sites within Kāne‘ohe Bay. Severe bleaching and paling covered 77 and 55% of reefs in 2014 and 2015, respectively. Different species showed a range of susceptibility with 80–100% of Pocillopora spp. bleaching in both years, but less than 50% bleaching of Porites compressa and Montipora capitata in Kāne‘ohe Bay. Less than 1% of the encrusting coral Leptastrea purpurea colonies bleached in both years. Sixty individual colonies of P. compressa and M. capitata and 28 colonies of Pocillopora damicornis were tagged and monitored for rates of bleaching, recovery and mortality throughout the two-year period. Most of the colonies that bleached recovered their symbionts within 3–4 months, though P. compressa visually recovered more rapidly than M. capitata and P. damicornis. Cumulatively, 19% of P. damicornis, 10% of M. capitata and no P. compressa died by May 2016. Partial mortality within a colony did not occur in 2014, but impacted 13% of the colonies in 2015, with P. damicornis and M. capitata having higher rates of partial mortality than P. compressa. Relatively, low susceptibility in the dominant species and low rates of mortality combined with rapid rates of recovery show coral resilience to anomalously high temperatures in Kāne‘ohe Bay, O‘ahu.

Fine-scale delineation of Symbiodiniaceae genotypes on a previously bleached central Red Sea reef system demonstrates a prevalence of coral host-specific associations

Widespread coral bleaching occurred in the central Red Sea in 2010 and 2015. During both events, a cross-shelf and depth gradient of bleaching severity was identified within the Thuwal reef system, central Red Sea, Saudi Arabia. While bleaching and survival of coral taxa were monitored, neither in situ reef temperatures nor coral-associated algal communities (family Symbiodiniaceae) were characterized. Here, we determined coral host-associated Symbiodiniaceae communities and monitored temperatures along the same cross-shelf and depth gradient on six reefs in 2017 to better understand the role of these factors in the observed bleaching patterns and to generate a baseline for further studies. We characterized > 600 coral–algal associations across winter and summer in six genera of scleractinian coral (Pocillopora, Stylophora, Seriatopora, Galaxea, Gardineroseris, and Porites) and one fire coral (Family Milleporidae) using ITS2 next-generation sequencing in conjunction with the SymPortal analytical framework. We show that previous bleaching patterns correlate poorly with the largely coral host-specific structure of the 2017 Symbiodiniaceae community and are in better agreement with absolute and intraday sea water temperature variations monitored on the reefs. We demonstrate a greater distinctiveness of Symbiodiniaceae communities at the more severely bleached inshore reefs compared to those reefs further offshore. However, the potential Symbiodiniaceae community changes at these reefs prior to our sampling prevent us from evaluating this distinctiveness as determinative of the differences in bleaching severities. Based on our analyses, we discuss how fine-scale delineation of algal genotypes, including host-specific putative genotypes of Durusdinium trenchii that represent alluring targets for further taxonomic identification, corroborate a niche-adapted rather than generalist character of many coral–Symbiodiniaceae associations. In conclusion, as studies such as this one continue to build the global catalogue of coral–Symbiodiniaceae associations, we may be afforded a better oversight of how specialized coral–algal associations really are and how restricted their modification may be, both of which are critical considerations in predicting the adaptive potential of corals and the reef ecosystems they build.

Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape

Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that heterogeneity in nitrogen pollution can shape spatial patterns of coral bleaching across a seascape. Using island-wide surveys of coral bleaching and nitrogen availability within a Bayesian hierarchical modeling framework, we tested the hypothesis that excess nitrogen interacts with temperature anomalies to alter coral bleaching for the two dominant genera of branching corals in Moorea, French Polynesia. For both coral genera, Pocillopora and Acropora, heat stress primarily drove bleaching prevalence (i.e., the proportion of colonies on a reef that bleached). In contrast, the severity of bleaching (i.e., the proportion of an individual colony that bleached) was positively associated with both heat stress and nitrogen availability for both genera. Importantly, nitrogen interacted with heat stress to increase bleaching severity up to twofold when nitrogen was high and heat stress was relatively low. Our finding that excess nitrogen can trigger severe bleaching even under relatively low heat stress implies that mitigating nutrient pollution may enhance the resilience of coral communities in the face of mounting stresses from global climate change.