Mass Coral Bleaching Research Articles

Current biodiversity of Mandapam group of Islands in Gulf of Mannar Marine Biosphere Reserve, Southeast coast of Tamil Nadu, India

The global climate change is known to involve in the decline of biodiversity from different ecosystems, including coral reefs worldwide. The spatial distribution of reef associated flora and fauna in the Gulf of Mannar (GoM) Marine Biosphere Reserve (GOMMBR) are declined due to damage of coral reefs by three major global problems observed in coral reefs, pertaining to frequent bleaching events, sedimentation and overwhelming algal communities. The occurrence of rich biodiversity in reef areas indicates the healthy condition of an iconic reef ecosystem. In this study, to assess the health status of coral reefs in Mandapam group of Islands under GoM, the diversity and distribution patterns of species composition assembled in reefs at seven Islands were analyzed. In 2019, summer temperature range was exceeded between 32 to 36 °C than usual range 28 to 30 °C, which resulted again massive coral bleaching in GoM. Statistical analysis revealed the high diversity of macroalgae in all the reef sites at Mandapam group Islands. Considerably, dead reefs in Mandapam group Islands represent 90% of algal communities and very less to scarce faunal diversity. While, faunal diversity was well diversified in the live coral cover areas and reefs with less macroalgal assemblage. This assessment provides current reef biodiversity data for gaining further insights into complex spatial distribution patterns of reef biota in GoM, and this will support the authorities to plan conservation efforts and to support livelihood of local fisher folk community.

Massive coral bleaching in the patchy reef of Grande Island, along the eastern Arabian Sea during the 2015/16 global bleaching event

The 2014–2017 mass bleaching event is considered the third massive coral global bleaching event in recorded history. Based on the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch (CRW) product western India was under an alert level 2 (May–June 2015) with ten-Degree Heating Weeks (DHW) followed by level 1 (six-DHW) from October–November 2015. In succession to our regular surveys, we conducted intensive bleaching surveys following the bleaching alerts issued by NOAA to quantify the coral bleaching in Grande Island along the eastern Arabian sea during October–November2015. The in-situ observations of massive bleaching (∼50%) during the predicted bleaching period by NOAA CRW for the study area contrasted with a lower prevalence of bleaching before (6%) and after (8%) the event. Bleaching severity was highest in Porites spp. corals and the least in foliose Turbinaria mesenterina. The long-term effects of bleaching in this area remain to be seen. This study is the first record of massive bleaching and quantitative assessment in patchy reefs along the eastern Arabian Sea and would act as a baseline data for further reef studies.

Keeping pace with marine heatwaves

Marine heatwaves (MHWs) are prolonged extreme oceanic warm water events. They can have devastating impacts on marine ecosystems — for example, causing mass coral bleaching and substantial declines in kelp forests and seagrass meadows — with implications for the provision of ecological goods and services. Effective adaptation and mitigation efforts by marine managers can benefit from improved MHW predictions, which at present are inadequate. In this Perspective, we explore MHW predictability on short-term, interannual to decadal, and centennial timescales, focusing on the physical processes that offer prediction. While there may be potential predictability of MHWs days to years in advance, accuracy will vary dramatically depending on the regions and drivers. Skilful MHW prediction has the potential to provide critical information and guidance for marine conservation, fisheries and aquaculture management. However, to develop effective prediction systems, better understanding is needed of the physical drivers, subsurface MHWs, and predictability limits.

Unravelling the different causes of nitrate and ammonium effects on coral bleaching

Mass coral bleaching represents one of the greatest threats to coral reefs and has mainly been attributed to seawater warming. However, reduced water quality can also interact with warming to increase coral bleaching, but this interaction depends on nutrient ratios and forms. In particular, nitrate (NO3−) enrichment reduces thermal tolerance while ammonium (NH4+) enrichment tends to benefit coral health. The biochemical mechanisms underpinning the different bleaching responses of corals exposed to DIN enrichment still need to be investigated. Here, we demonstrated that the coral Stylophora pistillata underwent a severe oxidative stress condition and reduced aerobic scope when exposed to NO3− enrichment combined with thermal stress. Such condition resulted in increased bleaching intensity compared to a low-nitrogen condition. On the contrary, NH4+ enrichment was able to amend the deleterious effects of thermal stress by favoring the oxidative status and energy metabolism of the coral holobiont. Overall, our results demonstrate that the opposite effects of nitrate and ammonium enrichment on coral bleaching are related to the effects on corals’ energy/redox status. As nitrate loading in coastal waters is predicted to significantly increase in the future due to agriculture and land-based pollution, there is the need for urgent management actions to prevent increases in nitrate levels in seawater. In addition, the maintenance of important fish stocks, which provide corals with recycled nitrogen such as ammonium, should be favoured.

Can Interdisciplinary Insights Encourage a Meaningful Response to the Climate Crisis? Narratives from the Great Barrier Reef, Australia

How we understand the contemporary socio-environmental challenges associated with climate change is shaped by our research disciplines. This paper explores how interdisciplinary collaboration can form new understandings of, and communication about, environmental change. On Australia’s Great Barrier Reef, mass coral bleaching prompts researchers to consider how our work can contribute to environmental stewardship that more effectively addresses the causes and consequences of climate change. Here, we report on one element of an ongoing collaboration involving researchers working in the fields of coastal geomorphology, human geography and art. In the spirit of geohumanities as an interdisciplinary approach to understanding places, we explore the work of narrative as a device for engaging with and communicating about the environmental challenge of coral reef degradation. Through independent and plaited narratives based on field research on the Great Barrier Reef, we undertake a deconstructive practice of “uncommoning” to consider what interdisciplinary collaboration enables, and how it opens up possibilities for engaging with, and communicating about, environmental crises. Sharing a curiosity about the island environment, each participant uses metaphors to evoke landscape features; however, they each work through different imaginative and emotional connections to place, by: reducing observations to measurements, evoking emotional responses, and provoking thinking about the multifarious, often global, processes contributing to the state of the environment.

South Atlantic Coral Reefs Are Major Global Warming Refugia and Less Susceptible to Bleaching

Mass coral bleaching has increased in intensity and frequency and has severely impacted shallow tropical reefs worldwide. Although extensive investigation has been conducted on the resistance and resilience of coral reefs in the Indo-Pacific and Caribbean, the unique reefs of the South Atlantic remain largely unassessed. Here we compiled primary and literature data for reefs from three biogeographical regions: Indo-Pacific, Caribbean and South Atlantic and performed comparative analyses to investigate whether the latter may be more resistant to bleaching. Our findings show that South Atlantic corals display critical features that make them less susceptible to mass coral bleaching: (i) deeper bathymetric distribution, as species have a mean maximum depth of occurrence of 70 m; (ii) higher tolerance to turbidity, as nearly 60% of species are found in turbid conditions; (iii) higher tolerance to nutrient enrichment, as nitrate concentration in the South Atlantic is naturally elevated; (iv) higher morphological resistance, as massive growth forms are dominant and comprise two thirds of species; and (v) more flexible symbiotic associations, as 75% of corals and 60% of symbiont phylotypes are generalists. Such features were associated with occurrence of fewer bleaching episodes with coral mortality in the South Atlantic, approximately 60% less than the Indo-Pacific and 50% less than the Caribbean. In addition, no mass coral mortality episodes associated with the three global mass bleaching events have been reported for the South Atlantic, which suffered considerably less bleaching. These results show that South Atlantic reefs display several remarkable features for withstanding thermal stress. Together with a historic experience of lower heat stress, our findings may explain why climate change impacts in this region have been less intense. Given the large extension and latitudinal distribution of South Atlantic coral reefs and communities, the region may be recognized as a major refugium and likely to resist climate change impacts more effectively than Indo-Pacific and Caribbean reefs.

Climate change impacts on Australia's eucalypt and coral species: Comparing and sharing knowledge across disciplines

AbstractTwo of Australia's most iconic ecosystems have recently sustained heavy damage related to climatic changes: the extensive eucalypt forests from catastrophic bushfires and the Great Barrier Reef from mass coral bleaching. Despite obvious differences, eucalypt trees and reef corals share some similarities in biology and ecology, particularly in relation to climate change impacts and adaptation. Both groups are the focus of an increasing research effort to characterize and respond to climate changes and here we examine how sharing research experiences can benefit both fields. Four key areas of research are considered: (a) modeling current distributions, (b) assessing impacts of climate change on future distributions, (c) using human‐assisted migration to improve survival, and (d) applying genetic enhancement to improve the species’ survival. Examples of each research area are used to examine potential crossovers, limitations of the methods, and future requirements. We conclude that eucalypt and coral researchers, and indeed researchers for many other endangered taxa, can gain much by comparing experiences and methods, despite the apparent differences in their respective taxa.This article is categorized under:Assessing Impacts of Climate Change > Observed Impacts of Climate Change

Coral reef resilience to thermal stress in the Eastern Tropical Pacific.

Coral reefs worldwide are threatened by thermal stress caused by climate change. Especially devastating periods of coral loss frequently occur during El Niño-Southern Oscillation (ENSO) events originating in the Eastern Tropical Pacific (ETP). El Niño-induced thermal stress is considered the primary threat to ETP coral reefs. An increase in the frequency and intensity of ENSO events predicted in the coming decades threatens a pan-tropical collapse of coral reefs. During the 1982-1983 El Niño, most reefs in the Galapagos Islands collapsed, and many more in the region were decimated by massive coral bleaching and mortality. However, after repeated thermal stress disturbances, such as those caused by the 1997-1998 El Niño, ETP corals reefs have demonstrated regional persistence and resiliency. Using a 44year dataset (1970-2014) of live coral cover from the ETP, we assess whether ETP reefs exhibit the same decline as seen globally for other reefs. Also, we compare the ETP live coral cover rate of change with data from the maximum Degree Heating Weeks experienced by these reefs to assess the role of thermal stress on coral reef survival. We find that during the period 1970-2014, ETP coral cover exhibited temporary reductions following major ENSO events, but no overall decline. Further, we find that ETP reef recovery patterns allow coral to persist under these El Niño-stressed conditions, often recovering from these events in 10-15years. Accumulative heat stress explains 31% of the overall annual rate of change of living coral cover in the ETP. This suggests that ETP coral reefs have adapted to thermal extremes to date, and may have the ability to adapt to near-term future climate-change thermal anomalies. These findings for ETP reef resilience may provide general insights for the future of coral reef survival and recovery elsewhere under intensifying El Niño scenarios.

Suppressed coral settlement following mass bleaching in the southern Persian/Arabian Gulf

Coral reefs in the southern Persian/Arabian Gulf have become increasingly degraded in the past two decades, mainly due to recurrent mass coral bleaching events. The recovery of these reefs will be largely contingent upon the arrival and settlement of coral larvae and their post settlement growth and survival. Spatial and temporal patterns of coral settlement were quantified on 10 sites spanning >350 km of the southern Gulf using settlement tiles for two years when consecutive bleaching events occurred. Coral settlement was highly seasonal, with peak settlement occurring in summer each year (>95% of spat), with the remainder of settlement in autumn. Coral settlement was >2-fold greater in the first year (928 spat) compared to the second year (397 spat) representing overall settlement densities of 95 m−2 yr−1 versus 40 m−2 yr−1. The dramatic declines in larval settlement between years suggests bleaching-related impacts on fecundity occurred during the gametogenic cycle late in the first year, as well as impaired survivorship of larvae and/or spat during the second year when severe bleaching coincided with the peak settlement period. Poritids and merulinids (‘others’) comprised 4% and 94% of the spat, respectively, while acroporids were virtually absent (1 recorded spat), suggesting the continued extirpation of this formerly dominant group and a continuing shift towards more stress-tolerant assemblages. Settlement rates in the southern Gulf are low in comparison to other marginal reef environments, and the bleaching-related suppression of settlement observed here suggests that larval supply is unlikely to be sufficient to support recovery of these increasingly degraded habitats. Given the increasing frequency of bleaching events in the southern Gulf the prognosis for the future of regional reefs is grim.

Heat Waves Are a Major Threat to Turbid Coral Reefs in Brazil

Coral reefs are threatened by climate change on a global scale with thermal-stress events and mass coral bleaching being widely reported. The reefs off the east coast of Brazil (and other turbid areas) have, however, historically escaped such thermal-stress events, with relatively low levels of background coral mortality (5-10%). This has recently changed. Here we show that, in 2019, degree heating weeks (DHW) of 19.65 coincided with catastrophic declines in coral cover, especially in the major reef building hydrocoral Millepora alcicornis. The decline was due to bleaching associated with exposure to high temperature stress culminating in DHW values exceeding 15 for a period of 50 days. At two independent sites, surveys showed upwards of 83.5  9.0% and 89.1  3.9% mortality, and at a third site showed relatively lower (albeit still high), mortality rates of 43.3  12.0% were recorded. The mass die-off in 2019 is unprecedented in the South Atlantic reefs and coincides with increased heating events.

Paradise lost: End-of-century warming and acidification under business-as-usual emissions have severe consequences for symbiotic corals.

Despite recent efforts to curtail greenhouse gas emissions, current global emission trajectories are still following the business-as-usual representative concentration pathway (RCP) 8.5 emission pathway. The resulting ocean warming and acidification have transformative impacts on coral reef ecosystems, detrimentally affecting coral physiology and health, and these impacts are predicted to worsen in the near future. In this study, we kept fragments of the symbiotic corals Acropora intermedia (thermally sensitive) and Porites lobata (thermally tolerant) for 7weeks under an orthogonal design of predicted end-of-century RCP8.5 conditions for temperature and pCO2 (3.5°C and 570ppm above present-day, respectively) to unravel how temperature and acidification, individually or interactively, influence metabolic and physiological performance. Our results pinpoint thermal stress as the dominant driver of deteriorating health in both species because of its propensity to destabilize coral-dinoflagellate symbiosis (bleaching). Acidification had no influence on metabolism but had a significant negative effect on skeleton growth, particularly when photosynthesis was absent such as in bleached corals or under dark conditions. Total loss of photosynthesis after bleaching caused an exhaustion of protein and lipid stores and collapse of calcification that ultimately led to A. intermedia mortality. Despite complete loss of symbionts from its tissue, P. lobata maintained small amounts of photosynthesis and experienced a weaker decline in lipid and protein reserves that presumably contributed to higher survival of this species. Our results indicate that ocean warming and acidification under business-as-usual CO2 emission scenarios will likely extirpate thermally sensitive coral species before the end of the century, while slowing the recovery of more thermally tolerant species from increasingly severe mass coral bleaching and mortality. This could ultimately lead to the gradual disappearance of tropical coral reefs globally, and a shift on surviving reefs to only the most resilient coral species.

Abundance, size, and survival of recruits of the reef coral Pocillopora acuta under ocean warming and acidification.

Ocean warming and acidification are among the greatest threats to coral reefs. Massive coral bleaching events are becoming increasingly common and are predicted to be more severe and frequent in the near future, putting corals reefs in danger of ecological collapse. This study quantified the abundance, size, and survival of the coral Pocillopora acuta under future projections of ocean warming and acidification. Flow-through mesocosms were exposed to current and future projections of ocean warming and acidification in a factorial design for 22 months. Neither ocean warming or acidification, nor their combination, influenced the size or abundance of P. acuta recruits, but heating impacted subsequent health and survival of the recruits. During annual maximum temperatures, coral recruits in heated tanks experienced higher levels of bleaching and subsequent mortality. Results of this study indicate that P. acuta is able to recruit under projected levels of ocean warming and acidification but are susceptible to bleaching and mortality during the warmest months.

Changes in the potential stocks of coral reef ecosystem services following coral bleaching in Sekisei Lagoon, southern Japan: implications for the future under global warming

Climate change is projected to have large impacts on natural capital and ecosystem services under scenarios of the IPCC. In the summer of 2016, elevated seawater temperatures triggered mass coral bleaching in Sekisei Lagoon, southern Japan. Based on data from field surveys and relevant websites, we mapped potential stocks of four major ecosystem services (fisheries production, aquarium fish production, recreational diving, and seaweed control) supplied by coral reef fishes before and after the bleaching event and predicted the possible future state of socio-ecological systems in the lagoon under climate change. We also evaluated whether current management options had mitigated the negative effects of extreme heating and coral bleaching on the potential stocks. Before the coral bleaching, the potential stock of recreational diving was estimated as 41.44 billion Japanese yen per year for the entire lagoon, while those of fisheries and aquarium fish productions were 2.10 billion and 15.03 billion yen, respectively. After bleaching, the four potential stocks including seaweed control decreased. The marine park zone and the extermination area of crown of thorns starfish mitigated changes in fish species richness and the value of recreational diving following coral bleaching. Extreme heating and coral bleaching are considered to be representative of a possible future state under global warming, and the results suggest that even management for local disturbances can mitigate the interactive effect of climate change on the potential stocks. Additional integrative management accounting for climate change and local stressors would be necessary for protecting ecosystem services in Sekisei Lagoon.

Bleaching and post-bleaching mortality of Acropora corals on a heat-susceptible reef in 2016.

In 2016, global temperatures were the highest on record, and mass coral bleaching occurred world-wide. However, around Sesoko Island, Okinawa, southwestern Japan, the heat stress assessed by degree heating week (DHW) based on local temperature measurements was moderate in 2016; in 1998, DHW was three times higher than in 2016 (10.6 vs. 3.3 in September in respective years). On a reef flat of Sesoko Island where the effect of severe coral bleaching on coral assemblage was monitored in 1998, significant coral bleaching occurred in 2016. Bleaching of the heat stress sensitive Acropora corals began in July 2016 on the reef flat as seawater temperature rose. We observed the bleaching and post-bleaching mortality status of individual colonies of Acropora spp. in 2016 in fixed plots on the reef flat. In total, 123 Acropora colonies were followed for six months after seawater temperature became normal by multiple surveys. At the beginning of September 2016, 99.2% of colonies, were either completely (92.7%) or partially (6.5%) bleached. Of those, the dominant species or species groups were A. gemmifera (Ag), A. digitifera (Ad), and tabular Acropora (tA). For all Acropora colonies, the overall whole and partial mortality was 41.5% and 11.4%, respectively. Whole mortality rate differed significantly among species; 72.5%, 17.9%, and 27.8% in Ag, Ad, and tA, respectively. Mortality rates at the end of the surveys were similar in smaller (≤10 cm in diameter) and larger Ag, but the former suffered mortality earlier than the latter. Higher survival of smaller colonies was observed only in tA (100%), which may be associated with large morphological differences between smaller and larger colonies. Some of the dominant Acropora colonies had survived without partial mortality including 15.0% survival of the most vulnerable Ag at the end of the surveys. These results suggest that moderate heat stress may have a potential for selecting heat-tolerant genotypes. A longer period of mortality lasting for six months, was observed in Ag in addition to immediate whole mortality after bleaching, due to the continuous loss of living tissue by partial mortality. This highlights the need for multiple surveys at least during several months to accurately assess the impact of thermal stress event to corals. In contrast to DHW based on local measurements, DHW obtained from satellite data were similar between 1998 and 2016. Although satellite-based measurement of sea surface temperature is very useful to reveal variations in heat stress at a large spatial scale, temperature should be measured on site when variations at smaller spatial scales are of interest.

Young fishes persist despite coral loss on the Great Barrier Reef

Unprecedented global bleaching events have led to extensive loss of corals. This is expected to lead to extensive losses of obligate coral-dependent fishes. Here, we use a novel, spatially-matched census approach to examine the nature of fish-coral dependency across two mass coral bleaching events. Despite a >40% loss of coral cover, and the ecological extinction of functionally important habitat-providing Acropora corals, we show that populations of obligate coral-dependent fishes, including Pomacentrus moluccensis, persisted and – critically – recruitment was maintained. Fishes used a wide range of alternate reef habitats, including other coral genera and dead coral substrata. Labile habitat associations of ‘obligate’ coral-dependent fishes suggest that recruitment may be sustained on future reefs that lack Acropora, following devastating climatic disturbances. This persistence without Acropora corals offers grounds for cautious optimism; for coral-dwelling fishes, corals may be a preferred habitat, not an obligate requirement.

Symbiodiniaceae probiotics for use in bleaching recovery

Coral reefs are currently under threat as a consequence of local and global stressors, in particular, mass coral bleaching induced by climate warming. In conjunction with global cuts to carbon emissions, active restoration interventions are being investigated as an additional option to buy time while these stressors are mitigated. One intervention with the potential to improve recovery during or postbleaching involves the addition of probiotic treatments, that is the addition of microorganisms that provide benefits to the host. Fragments of the branching coral, Acropora millepora, were experimentally exposed to a bleaching event coupled with the inoculation of Symbiodiniaceae probiotics (Durusdinium trenchii and Cladocopium goreaui) to determine if these probiotic treatments could ameliorate bleaching related stress and mortality. Fragments inoculated with C. goreaui and exposed to 32°C for 6 days exhibited significantly less mortality (9.1 ± 5%) compared to corals exposed to 32°C without probiotics (66.7 ± 8%) or with D. trenchii (41.7 ± 9%). Fragments in the C. goreaui probiotic treatment also bleached less and exhibited the highest photosynthetic efficiency compared to fragments inoculated with the D. trenchii at 32°C. Internal transcribed spacer‐2 amplicon sequencing did not detect the inoculated D. trenchii and C. goreaui cells within A. millepora tissues at the end of the experiment, suggesting the corals did not reestablish symbiosis but instead used inoculated cells as a nutritional supplement, although other factors such as shuffling conditions may have had an effect. This study highlights that nutritional supplementation can possibly aid coral resilience to temperature stress, though a far more detailed understanding of the factors that influence host regulation during symbiosis establishment is required.

Customized Medicine for Corals

[Extract] Coral reefs are facing unprecedented pressures, on a global scale. Alarming declines in coral abundance and diversity are occurring across the majority of reef systems, driven by the cumulative impacts of local stressors and rapidly changing climate (Hughes et al., 2017, 2018). Increases in the greenhouse gas emission (GGE) rates are linked with marine heatwaves which cause mass coral bleaching events, a phenomenon reported as the main threat to coral reefs into the future (Sweet and Brown, 2016). The recent global coral bleaching event from 2014 to 2017 resulted in extensive mortality impacting reef functioning in ways not previously recorded (Eakin et al., 2019). The knock-on effects of this global environmental extreme are still playing out with recent assessment on the Great Barrier Reef (GBR) demonstrating drastic declines in coral recruitment, which may fundamentally change the ecosystem dynamics across this region into the future (Hughes et al., 2019). In addition, regional stressors such as overfishing, poor water quality, and pollution can also severely affect the health state of corals and reefs (Shaver et al., 2018). Most agree that the reduction of GGE rates is the priority to ensure the persistence of coral reef ecosystems, however under current GGE projections, annual mass coral bleaching events are expected to occur as early as 2050 (National Academies of Science, Engineering and National Academies of Sciences, 2018). Even if GGE emissions are significantly reduced, further decline of reef health appears unavoidable due to the current accumulation of greenhouse gases in the atmosphere and the predicted lag period before any change will be observed (Hoegh-Guldberg et al., 2007; National Academies of Sciences, 2018). This poses a major problem to those caring for and managing reef ecosystems.

The intracellular photopigment and glutathione (GSH) dynamics in symbiodinium natural population during light stress and recovery

It is already proven by scientists, that mass coral bleaching is an early signal for increasing global temperature. Over the last decades, there was a lack of studies aimed at photoinhibition in Symbiodinium during laboratory treatments. It was important to predict the initial adaptation in relation to photo-inhibitory of photosynthesis accompanied by slight photo- protective mechanisms and gradual recovery of Symbiodinium. This information is important to understand the recovery mechanism of mass coral bleaching, which can help to develop sustainable management of the ecosystem. In our manuscript, we applied medium light stress [600 μmol (photon).m−2.s−1] to trigger changes in intracellular photo-pigments (chlorophyll a, chlorophyll c, carotenoid) and glutathione (GSH) contents in Symbiodinium. The result showed that light-induced generated various depletion in chlorophyll (chl) a and chlorophyll (chl) c contents, and more production in intracellular carotenoid and GSH. The total intracellular GSH increased during the first 10 min of culture at high light treatment and followed by a decrease (at 60 min). The GSH recovery finished in Symbiodinium even after 6 h indicated the high ability of photo-protective processes in the species as another photoinhibition effect and its photosynthetic performance in the initial environment.

Nearshore coral growth declining on the Mesoamerican Barrier Reef System

Anthropogenic global change and local stressors are impacting coral growth and survival worldwide, altering the structure and function of coral reef ecosystems. Here, we show that skeletal extension rates of nearshore colonies of two abundant and widespread Caribbean corals (Siderastrea siderea, Pseudodiploria strigosa) declined across the Belize Mesoamerican Barrier Reef System (MBRS) over the past century, while offshore coral conspecifics exhibited relatively stable extension rates over the same temporal interval. This decline has caused nearshore coral extension rates to converge with those of their historically slower growing offshore coral counterparts. For both species, individual mass coral bleaching events were correlated with low rates of skeletal extension within specific reef environments, but no single bleaching event was correlated with low skeletal extension rates across all reef environments. We postulate that the decline in skeletal extension rates for nearshore corals is driven primarily by the combined effects of long-term ocean warming and increasing exposure to higher levels of land-based anthropogenic stressors, with acute thermally induced bleaching events playing a lesser role. If these declining trends in skeletal growth of nearshore S. siderea and P. strigosa continue into the future, the structure and function of these critical nearshore MBRS coral reef systems is likely to be severely impaired.

Thermal stress and tropical reefs: mass coral bleaching in a stable temperature environment?

This study reports on the deepest records (~ 24 m depth) of coral bleaching in a naturally temperature-stable environment (> 26 °C with an intra-annual variability of ~ 2 °C), which was recorded during a mass bleaching event in the locally dominant, massive scleractinian coral Siderastrea stellata in equatorial waters of Brazil (SW Atlantic). An inter-annual analysis (2002–2017) indicated that this bleaching event was related to anomalies in sea surface temperature (SST) that led to the warmest year (2010) in this century (1 to 1.7 °C above average). Such anomalies caused heat stress (28.5–29.5 °C) in this equatorial environment that resulted in a bleaching event. Our results suggest that the increase in SST, low turbidity, and weak winds may have acted together to affect these stress-tolerant corals in marginal reefs. The equatorial coastline of Brazil is characterized by low intra-annual and inter-annual variations in SST, which suggests that the S. stellata corals here may be acclimatized to these stable conditions and, consequently, have a lower bleaching threshold because of lower historical heat stress.