Coral Bleaching Research Articles

Modelling marine heatwaves impact on shallow and upper mesophotic tropical coral reefs

Abstract Coral reefs ecosystems, often compared to rain forests for their high biodiversity, are threatened by ocean warming causing coral bleaching when the symbiotic relationship between dinoflagellates and corals breaks under high ocean temperatures. Thermal stress from marine heatwaves (MHWs) occur both at the surface and subsurface with subsurface MHWs lasting longer with potentially higher cumulative intensities. However, global coral bleaching models generally ignore the differences in thermal stress between surface and sea-bed levels. Here, we define MHWs at sea-bed level to model coral bleaching with daily resolution from 6 May 1993 to 31 October 2023, for 9944 tropical coral reefs between 0 and 60 m depths. We show that deeper reefs experience on average higher thermal stress and bleaching compared to surface reefs. Using surface temperature data to model bleaching for deeper corals underestimates bleaching intensities by an average of 6% ± 9% compared to the subsurface calibrated model. Our study is a starting point for more accurate coral bleaching modelling, providing additional evidence to reshape our perception of deeper coral reefs as potential refugees from climate change.

Climate Change Affects Multiple Coral Reef Fisheries Ecosystem Services

ABSTRACTCoral reef fisheries support livelihoods and provide an affordable source of essential dietary nutrients to coastal people. However, climate‐driven coral bleaching is dramatically altering reef habitats and changing reef fish composition, diversity, and productivity. We used data from fisheries independent surveys and artisanal trap landings to explore how reefs with different responses to coral bleaching differed in habitat configurations, fisheries catch rates, economic returns, and nutritional content. Coral‐associated fishes supported the highest catch rates, greatest species diversity, highest rates of economic returns for fishers, and a wide price range for consumers. Macroalgae‐associated fishes supported the greatest overall fisheries yields, which indicated higher fishery dependence on these habitats. Nutrient content was high in catches across all habitats, but concentrations varied by nutrient, which suggested that fishing across a range of reef habitats should provide the greatest variety of nutrients to coastal communities.

Youthful insight: Nitrogen sequestration in larvae provides clues to coral bleaching.

Impaired nutrient cycling under thermal stress foregoes coral bleaching, the loss of symbiotic algae. A new study in PLOS Biology sheds light on how coral larvae avoid bleaching through nitrogen sequestration to uphold glucose translocation from their algal symbionts.

Reproductive resilience: pathways to gametogenic success in Montipora capitata after bleaching.

Thermal bleaching, or the loss of symbiotic algae that provide most energetic resources for the coral host, is an increasing threat to reefs worldwide and is projected to worsen with climate change. While bleaching is a well-recognized threat, the impact on the process of reproduction in bleaching survivors is not well resolved, despite being central to coral resilience. Montipora capitata can survive bleaching while completing a full gametogenic cycle, offering an ideal system to study gametogenic resilience and physiological tradeoffs. We experimentally bleached fragments of M. capitata colonies and followed their gametogenesis and physiological responses for 10months (six time points). All bleached colonies produced gametes at the same time as controls, suggesting that reproductive processes were energetically prioritized. However, proteomic analysis revealed tradeoffs and delays in activating key physiological processes earlier in gametogenesis in areas such as skeletal growth and reproductive hormone synthesis. Tradeoffs during the gametogenic cycle, likely a direct response to thermal bleaching, resulted in smaller oocytes from bleached colonies, potentially indicating decreased transfer of parental resources to gametes. While gametogenesis is likely to continue in this species, it is unknown how the fecundity, synchrony of spawning, viability and success of future offspring may be impacted by future bleaching events.

Coral Bleaching Detection Using PRISMA Hyperspectral Satellite Imagery in Calatagan, Batangas

Abstract. Coral bleaching arises due to stress from warming seas and pollution. Several studies on bleaching detection utilize multispectral imagery, due to data availability. In this study, the usage of PRISMA hyperspectral data imagery was assessed for coral bleaching detection in Calatagan, Batangas, and sea surface temperature (SST) for confirming bleaching occurrence. In-situ measurements were gathered for spectral data of live and bleached corals, and a collaborative qualitative data interview with a coral bleach patroller and municipal office was conducted to gain further information about bleaching details in the area. Sentinel-2 L2A was also used for comparison during classification. Spectral Angle Mapper (SAM) was utilized for image classification, while using field data and data from related studies for spectral library. A total of 27.12% relative to the coral reef area was classified as bleached corals. T-test statistics were done to analyze the live and bleached corals, which exhibited that only two (2) bands in the green region were found to be not separable. Using box plots, bleached corals showed to have generally lower reflectance values than live corals for RGB region, as compared to Sentinel, with bleached having higher values, which implies that spatial resolution can affect the spectral response of different features. Using ground truth data, 60.6% user’s accuracy for bleached corals, and 63.4% overall accuracy is achieved. Hence, hyperspectral satellite imagery can be utilized in coral bleaching detection, despite its limitations. Depth variables and diverse sample points are recommended for the betterment of coral bleaching detection.

Are we forgetting early observations of coral bleaching?

I argue that the first observed mass coral bleaching event on Australia’s Great Barrier Reef (GBR) occurred in 1982. This claim should not be controversial given the expert testimonies and data collected at the time. However, the GBR mass bleaching of 1982 has been repeatedly and inexplicably absent from descriptions of the GBR bleaching history in recent peer-reviewed literature, government reports, and popular media. This is leading to a paradigm that severe, widespread coral bleaching began on the GBR in 1998, with that event impacting coral communities that had only been previously impacted by minor, localized bleaching. Here, I show that this paradigm is misleading, that mass bleaching during 1982 is consistent with the levels of heat and light stress that have sparked other GBR bleaching events, and that consideration of the full timeline of observed GBR mass bleaching is important for interpretations of community dynamics and potential changes in sensitivity to heat stress over time.

Underlying drivers of coral reef vulnerability to bleaching in the Mesoamerican Reef

Coral bleaching, a consequence of stressed symbiotic relationships between corals and algae, has escalated due to intensified heat stress events driven by climate change. Despite global efforts, current early warning systems lack local precision. Our study, spanning 2015–2017 in the Mesoamerican Reef, revealed prevalent intermediate bleaching, peaking in 2017. By scrutinizing 23 stress exposure and sensitivity metrics, we accurately predicted 75% of bleaching severity variation. Notably, distinct thermal patterns—particularly the climatological seasonal warming rate and various heat stress metrics—emerged as better predictors compared to conventional indices (such as Degree Heating Weeks). Surprisingly, deeper reefs with diverse coral communities showed heightened vulnerability. This study presents a framework for coral reef bleaching vulnerability assessment, leveraging accessible data (including historical and real-time sea surface temperature, habitat variables, and species composition). Its operational potential lies in seamless integration with existing monitoring systems, offering crucial insights for conservation and management.

Flow rates alter the outcome of coral bleaching and growth experiments

It is important to consider flow rate explicitly in coral growth and bleaching studies across multiple species with differing life histories to guide coral conservation, management and captive culture. We quantified growth rates and coral bleaching responses to thermal stress (approx. 18 DHW) in flow-through aquaria with various current velocities to test whether flow conditions alter experimental outcomes. Across natural flow rates (< 1 to over 50 cm/sec), Montipora capitata, Pocillopora acuta, and Pocillopora meandrina showed increased growth and bleaching recovery at intermediate flow rates. Growth rates for all species increased from no flow to intermediate (50–100 turnovers-per-hour, ~ 10–30 cm/s), but then decreased at highest flow (> 190 tph, > 50 cm/s) although this trend was not significant for P. meandrina. The flow treatment with highest recovery from temperature stress differed across species, ranging from 4 tph in the flow-loving P. meandrina to 210 tph in the lagoonal M. capitata, indicating that natural flow regime alone is not predictive. Fragments from the same individual (e.g., P. acuta colony 8) held under identical thermal conditions continue bleaching and die under one flow regime (4 tph), whereas they recover from bleaching (30 tph) or grow fastest (105 tph) under different flow treatments. Flow is rarely reported in the literature, but uncontrolled flow effects may help to explain some of the variation in coral bleaching results reported across the literature. Significant differences among individual colonies, and colony-by-flow interactions, preclude generalizations beyond that flow rates can alter the outcome of both coral growth and bleaching experiments.

Assessment of Bleaching Stress Vulnerability of Lakshadweep Islands using Google Earth Engine (GEE)

Coral reefs, among the Earth’s most diverse and valuable ecosystems, face unprecedented challenges due to climate change. Coral bleaching is a phenomenon wherein corals lose their symbiotic zooxanthellae owing to various stressors, leading to a whitening effect of the coral tissues. In recent decades, climate change has intensified coral bleaching events. Multiple stressors, including elevated Sea Surface Temperature (SST), extreme irradiance levels, and various biotic and abiotic factors trigger bleaching events. Coral bleaching is primarily driven by thermal stress caused by elevated SSTs. Climate change has worsened bleaching’s frequency and intensity. Global bleaching events are often linked to planetary ocean-atmospheric circulation processes such as El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). This study focused on assessing the vulnerability of coral reefs in the Lakshadweep region of India from 2016 to 2023 using National Oceanic and Atmospheric Administration’s Climate Data Record Optimum Interpolation Sea Surface Temperature (NOAA CDR OISST) daily data. GOOGLE EARTH ENGINE (GEE) is a cloud computing platform which is used to collect and generate the base data for this study. The vulnerability assessment utilized two bleaching indices: SST anomaly and Degree Heating Week (DHW). Analysis of DHW data reveals that 2020 experienced the highest SST anomaly residence time due to IOD event of 2019, as compared to 2016 and 2023 which are known El Niño years. All Lakshadweep islands exhibited vulnerability, although in varying degrees across different areas. Based on the magnitude, intensity, and frequency of bleaching stress, the islands are categorized into different categories of vulnerability. This study identifies Baliyapaniyam, Cheriyam-Kalpeni and Suhelipar reefs as very highly vulnerable reefs in Lakshadweep. This study highlights the urgent need for monitoring and management measures to mitigate the impacts of climate change on coral reef ecosystems using spatial vulnerability patterns.

Heat stress and bleaching in corals: a bioenergetic model

AbstractThe coral-dinoflagellate endosymbiosis is based on nutrient exchanges that impact holobiont energetics. Of particular concern is the breakdown or dysbiosis of this partnership that is seen in response to elevated temperatures, where loss of symbionts through coral bleaching can lead to starvation and mortality. Here we extend a dynamic bioenergetic model of coral symbioses to explore the mechanisms by which temperature impacts various processes in the symbiosis and to enable simulational analysis of thermal bleaching. Our model tests the effects of two distinct mechanisms for how increased temperature impacts the symbiosis: 1) accelerated metabolic rates due to thermodynamics and 2) damage to the photosynthetic machinery of the symbiont caused by heat stress. Model simulations show that the model can capture key biological responses to different levels of increased temperatures. Moderately increased temperatures increase metabolic rates and slightly decrease photosynthesis. The slightly decreased photosynthesis rates cause the host to receive less carbon and share more nitrogen with the symbiont. This results in temporarily increased symbiont growth and a higher symbiont/host ratio. In contrast, higher temperatures cause a breakdown of the symbiosis due to escalating feedback that involves further reduction in photosynthesis and insufficient energy supply for $$\hbox {CO}_2$$ CO 2 concentration by the host. This leads to the accumulation of excess light energy and the generation of reactive oxygen species, eventually triggering symbiont expulsion and coral bleaching. Importantly, bleaching does not result from accelerated metabolic rates alone; it only occurs as a result of the photodamage mechanism due to its effect on nutrient cycling. Both higher light intensities and higher levels of DIN render corals more susceptible to heat stress. Conversely, heterotrophic feeding can increase the maximal temperature that can be tolerated by the coral. Collectively these results show that a bioenergetics model can capture many observed patterns of heat stress in corals, such as higher metabolic rates and higher symbiont/host ratios at moderately increased temperatures and symbiont expulsion at strongly increased temperatures.

Loss of Coral Trait Diversity and Impacts on Reef Fish Assemblages on Recovering Reefs.

Understanding patterns of biodiversity change is essential as coral reefs experience recurrent cycles of disturbance and recovery. Shifts in the total cover and species composition of habitat-forming corals can have far-reaching consequences, including shifts in coral functional traits and impacts on local fish assemblages. We surveyed coral and fish assemblages along the southern coast of Hainan Island near Sanya, China, in 2006, 2010, and 2018, during a period with repeated mass bleaching events. We showed that coral biodiversity in this region is in a state of flux, with losses and gains in coral cover and an increase in species richness over time. Despite increasing species diversity, the region suffered a loss of coral trait diversity by 2010, with an incomplete recovery by 2018, owing to declines in species with key habitat-forming traits (e.g., high surface areas and fractal structure) such as corymbose corals. Concurrently, there was an increase in functional redundancy due to the proliferation of the dominant encrusting and massive corals. Coral cover was positively associated with the abundance of reef fish, indicating that the changes observed in coral abundance can impact reef-associated species. These results demonstrate that the slow recovery of coral biodiversity in southern Hainan Island has been hampered by the loss of specific coral traits and highlight the importance of protecting vulnerable coral traits in conservation and management strategies.

Coral Sr/Ca-SST calibration: SST records from Kenyan corals

To better understand the seasonal variability in SST in Kenyan coral reef ecosystems, Sr/Ca ratios have been measured in Kenyan coral skeletons as proxies of SST, and a new Sr/Ca = f(SST) calibration was established: Sr/Ca = − 0.053 (± 0.006) *SST + 10.43 (± 0.16). The slope was within the margins of − 0.04 and − 0.08 mmolmol−1 °C of previously published Sr/Ca-SST calibration equations. A reliability test revealed that the present calibration equation reproduces a previously published calibration equation developed for Kenyan corals. This calibration was applied to corals collected in reef ecosystems under different natural and anthropogenic influences: Tana Delta Ziwayu Island, Watamu National Marine Park, and Shimon Mupunguti Marine Reserve. Using the developed Sr/Ca-thermometry, the reconstructed Sr/Ca-SST revealed that reefs from the southern basin of the Kenyan Indian Ocean are exposed to warmer SSTs than reefs in the northern Basin are. Additionally, the results revealed that the seasonal variability on SST increased moving toward the southern basin of the Kenyan Indian Ocean. We use the local instrument SST as an SST reference, which is aligned against the Sr/Ca seasonal signals recorded in SMR3-18 (the nearest coral site), and the intensity of the third global coral bleaching (April and May 2016) is affirmed.

Degradation of coral reef ecosystems: Mathematical-dynamical modeling approach

Abstract The review delves into the frontier of degradation of coral reef ecosystems, a pressing urgent issue to be addressed in marine ecological environments, and provides a critical assessment of current advances and future perspective on the topic. This study highlights the recent developments in coral bleaching, coral-algal phase shifts, coral disease, and natural enemy bloom, all of which are thoroughly documented and expounded. Special attention focus on the studies employing mathematical-dynamical modeling approaches. This review study also offers some valuable insights, illuminating ideas, and clear perspective into some current, emerging, and promising research directions for future studies, particularly those based on mathematical-biological modeling approaches. These approaches are anticipated to offer a theoretical foundation and provide crucial decision-making reference for the protection, restoration, and management of coral reef ecosystems.

Differential spatio-temporal responses of Red Sea coral reef benthic communities to a mass bleaching event

Understanding how coral reefs respond to disturbances is fundamental to assessing their resistance and resilience, particularly in the context of climate change. Due to the escalating frequency and intensity of coral bleaching events, it is essential to evaluate spatio-temporal responses of coral reef communities to disentangle the mechanisms underlying ecological changes. Here, we used benthic data collected from 59 reefs in the Red Sea over five years (2014–2019), a period that encompasses the 2015/2016 mass bleaching event. Reefs were located within three different geographic regions with different environmental settings: north (Duba; Al Wajh), central (Jeddah; Thuwal), and south (Al Lith; Farasan Banks; Farasan Islands). Coral community responses were region-specific, with communities in the south being more promptly affected than those in the northern and central regions, with hard and soft coral cover dropping drastically in several reefs from around > 40% to < 5% two years after bleaching. Coral bleaching effects were particularly evident in the decrease of cover in branching corals. Overall, we documented a shift towards a dominance of macroalgae, turf algae, and crustose coralline algae (CCA). Using remote sensing data, we analyzed sea surface temperature (SST) regimes at the study sites to infer potential drivers of changes in benthic composition. Both SST and Degree Heating Weeks (DHW) only partially aligned with the responses of benthic communities, highlighting the need for more accurate predictors of coral bleaching in the Red Sea. In times of intense coastal development along Saudi Arabia’s Red Sea coast, our study provides crucial baseline information on developments in coral reef community composition, as well as to guide decision-making, namely restoration efforts.

Unveiling the importance of heterotrophy for coral symbiosis under heat stress.

Global warming endangers reef-building corals as they lose their photosynthetic symbionts, which limits their ability to feed autotrophically. Consequently, heterotrophy, the capture of zooplankton, can become crucial for the energy budget of heat-stressed corals. However, it is difficult to assess the extent of the heterotrophic contribution in corals, as well as the dynamics of nutrient exchange between the host and its symbionts. In this pioneering study, we employed a suite of isotopic markers, including 13C- and 15N bulk tissue isotope measurements, compound-specific isotope analysis of amino acids (CSIA-AAs), and 13C- and 15N-labeled food incubations, to investigate nutrient acquisition and allocation in the coral Stylophora pistillata under controlled and heat-induced bleaching conditions. Bulk isotope values and inorganic carbon assimilation remained unchanged in the bleached corals compared to the control corals, overall indicating undisturbed autotrophic activity of the symbionts under heat stress. However, CSIA-AAs showed an increased dependence on heterotrophy for amino acid synthesis in both the host and the symbionts despite reduced assimilation of 15N-labeled food. Overall, these results suggest that although S. pistillata reduces its assimilation of heterotrophic food under heat stress, the acquisition of amino acids by the coral host and symbionts still relies on heterotrophy. This study emphasizes the importance of using multiple indicators to gain a comprehensive understanding of coral nutrition. It shows that coral dependence on heterotrophy is not only associated with a decline in autotrophic availability. Rather, it demonstrates the ability of S. pistillata to adapt its utilization of food sources to the prevailing environmental conditions.IMPORTANCEThis work highlights that every isotopic marker displays a piece of different information concerning the diet of the model coral S. pistillata. By combining all markers, we observed that although S. pistillata exhibited reduced heterotrophic assimilation under heat stress, amino acid acquisition and synthesis remained dependent on heterotrophy. The findings emphasize the adaptability of corals in utilizing different food sources, which is vital for their resilience and recovery in changing environmental conditions. This research underscores the complexity of coral symbiosis and highlights the need for multiple indicators to understand dietary dynamics comprehensively.

Selective breeding enhances coral heat tolerance to marine heatwaves

Marine heatwaves are becoming more frequent, widespread and severe, causing mass coral bleaching and mortality. Natural adaptation may be insufficient to keep pace with climate warming, leading to calls for selective breeding interventions to enhance the ability of corals to survive such heatwaves, i.e., their heat tolerance. However, the heritability of this trait–a prerequisite for such approaches–remains unknown. We show that selecting parent colonies for high rather than low heat tolerance increased the tolerance of adult offspring (3–4-year-olds). This result held for the response to both 1-week +3.5 °C and 1-month +2.5 °C simulated marine heatwaves. In each case, narrow-sense heritability (h2) estimates are between 0.2 and 0.3, demonstrating a substantial genetic basis of heat tolerance. The phenotypic variability identified in this population could theoretically be leveraged to enhance heat tolerance by up to 1 °C-week within one generation. Concerningly, selective breeding for short-stress tolerance did not improve the ability of offspring to survive the long heat stress exposure. With no genetic correlation detected, these traits may be subject to independent genetic controls. Our finding on the heritability of coral heat tolerance indicates that selective breeding could be a viable tool to improve population resilience. Yet, the moderate levels of enhancement we found suggest that the effectiveness of such interventions also demands urgent climate action.

The meteorological drivers of mass coral bleaching on the central Great Barrier Reef during the 2022 La Niña

The frequencies of marine heatwaves and thermal coral bleaching events (CBEs) over the Great Barrier Reef (GBR) continue to increase with five mass CBEs reported since 2016. While changes in the local meteorology, such as reduced wind speeds and decreased cloud cover, are known to heat the shallow reef waters, little consideration has been given to the overriding synoptic meteorology. The 2022 CBE, occurring under La Niña conditions, saw ocean temperatures at Davies Reef increase 1.9 ∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ \\hbox {C}$$\\end{document} over 19-days and subsequently cool 2.1 ∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ \\hbox {C}$$\\end{document} back to seasonal norms over eight days. This event was found to be triggered by repeated Rossby wave breaking disrupting the local trade winds, thus inhibiting the latent heat flux. Latent heat fluxes, the primary driver of the event, tripled as the trade winds returned via rapid coastal ridging. These same synoptic features are concurrent with the historic Lismore flooding located hundreds of kilometres south of the GBR.

Coral carbonate production across depth: homogenisation after bleaching?

AbstractRecurrent climate-driven warming events, which can induce severe coral bleaching and mortality on tropical reefs, are predicted to cause homogenisation of coral communities and loss of ecosystem functions in shallow reef systems (&lt; 30 m). However, data documenting the variation in coral carbonate production across depth are limited. Here we explore differences in coral cover, community composition, coral colony size structure and carbonate production rates between two depths (10 m and 17.5 m) across four atolls in the remote Chagos Archipelago. We show higher coral carbonate production rates at 10 m (4.82 ± 0.27 G, where G = kg CaCO3 m−2 yr−1) compared to sites at 17.5 m (3.1 ± 0.18 G). The main carbonate producers at 10 m consisted of fast-growing branching and tabular corals (mainly Acroporids) and massive corals (mainly Porites), with high abundances of medium- and large-sized colonies. In contrast, coral carbonate production at 17.5 m was driven by slow-growing encrusting and foliose morphotypes and small colony sizes. Utilising a dataset following 6–7 years of recovery after the 2015–2017 bleaching event, our results show that depth-homogenisation of coral communities was temporary and carbonate production rates at 10 m depth recovered quicker at 3 of 4 studied atolls. The exception is Great Chagos Bank where slower recovery of branching and tabular corals at 10 m has led to a longer-lasting depth-homogenisation of carbonate production rates. The latter example cautions that more frequent bleaching events may drive increasing homogenisation of carbonate production rates across depth gradients, with implications for vital reef geo-ecological functions.

The massive 2016 marine heatwave in the Southwest Pacific: An "El Niño-Madden-Julian Oscillation" compound event.

El Niño typically induces cooling in the Southwest Pacific Ocean during austral summers, usually leading to decreased marine heatwave frequency and severity. However, the 2016 extreme El Niño unexpectedly coincided with the longest and most extensive marine heatwave ever recorded in the region. This heatwave, spanning over 1.7 million square kilometers, persisting for 24 days with a peak intensity of 1.5°C, resulted in massive coral bleaching and fish mortality. This exceptional warming resulted from anomalously strong shortwave radiation and reduced heat loss via latent heat fluxes, owing to low wind speed and increased air humidity. These anomalies are attributed to a rare combined event "Madden-Julian Oscillation and extreme El Niño." Following 10 February, the rapid dissipation of this marine heatwave results from the most intense cyclone ever recorded in the South Pacific. The hazardous ecological impacts of this extreme event highlight the needs for improving our understanding of marine heatwave-driving mechanisms that may result in better seasonal predictions.

Regime shift of skeletal δ13C after 1997/1998 El Nino event in Porites coral from Green Island, Taiwan

The 1997/1998 El Niño event caused mass coral bleaching and mortality in many tropical and subtropical regions, including corals on Green Island, Taiwan, in the northwestern Pacific Ocean. This study analyzed coral carbon isotope ratios (δ13C), oxygen isotope ratios (δ18O), and Sr/Ca ratios for 29 years, including the 1997/1998 El Niño period, to examine how high water temperature events are recorded in coral geochemical indicators. Sr/Ca ratios in coral skeletons from Green Island show the lowest peak, means the highest temperature during the 1997/1998 El Niño period. However, we couldn’t observe high-temperature events on δ18O. Furthermore, a negative δ13C shift was observed after El Niño events. The regime shift of δ13C might have been caused by temporal bleaching and/or a decrease in symbiotic algae due to high water temperature stress under the continuous decrease in δ13C in DIC due to the Suess effect.