Coral bleaching: causes and consequences
TL;DR: Evaluated data on temperature and irradiance-induced bleaching, including long-term data sets which suggest that repeated bleaching events may be the consequence of a steadily rising background sea temperature that will in the future expose corals to an increasingly hostile environment, are evaluated.
Abstract: It has been over 10 years since the phenomenon of extensive coral bleaching was first described. In most cases bleaching has been attributed to elevated temperature, but other instances involving high solar irradiance, and sometimes disease, have also been documented. It is timely, in view of our concern about worldwide reef condition, to review knowledge of physical and biological factors involved in bleaching, the mechanisms of zooxanthellae and pigment loss, and the ecological consequences for coral communities. Here we evaluate recently acquired data on temperature and irradiance-induced bleaching, including long-term data sets which suggest that repeated bleaching events may be the consequence of a steadily rising background sea temperature that will in the future expose corals to an increasingly hostile environment. Cellular mechanisms of bleaching involve a variety of processes that include the degeneration of zooxanthellae in situ, release of zooxanthellae from mesenterial filaments and release of algae within host cells which become detached from the endoderm. Photo-protective defences (particularly carotenoid pigments) in zooxanthellae are likely to play an important role in limiting the bleaching response which is probably elicited by a combination of elevated temperature and irradiance in the field. The ability of corals to respond adaptively to recurrent bleaching episodes is not known, but preliminary evidence suggests that phenotypic responses of both corals and zooxanthellae may be significant.
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TL;DR: A review of the ecological impacts of recent climate change exposes a coherent pattern of ecological change across systems, from polar terrestrial to tropical marine environments.
Abstract: There is now ample evidence of the ecological impacts of recent climate change, from polar terrestrial to tropical marine environments. The responses of both flora and fauna span an array of ecosystems and organizational hierarchies, from the species to the community levels. Despite continued uncertainty as to community and ecosystem trajectories under global change, our review exposes a coherent pattern of ecological change across systems. Although we are only at an early stage in the projected trends of global warming, ecological responses to recent climate change are already clearly visible.
9,369 citations
Cites background from "Coral bleaching: causes and consequ..."
...The impact of thermal stress on reefs can be dramatic, with the almost total removal of corals in some instance...
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TL;DR: The results suggest that the thermal tolerances of reef-building corals are likely to be exceeded every year within the next few decades, and suggests that unrestrained warming cannot occur without the loss and degradation of coral reefs on a global scale.
Abstract: Sea temperatures in many tropical regions have increased by almost 1 degrees C over the past 100 years, and are currently increasing at similar to 1-2 degrees C per century. Coral bleaching occurs when the thermal tolerance of corals and their photosynthetic symbionts (zooxanthellae) is exceeded. Mass coral bleaching has occurred in association with episodes of elevated sea temperatures over the past 20 years and involves the loss of the zooxanthellae following chronic photoinhibition. Mass bleaching has resulted in significant losses of live coral in many parts of the world. This paper considers the biochemical, physiological and ecological perspectives of coral bleaching. It also uses the outputs of four runs from three models of global climate change which simulate changes in sea temperature and hence how the frequency and intensity of bleaching events will change over the next 100 years. The results suggest that the thermal tolerances of reef-building corals are likely to be exceeded every year within the next few decades. Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years. Most information suggests that the capacity for acclimation by corals has already been exceeded, and that adaptation will be too slow to avert a decline in the quality of the world's reefs. The rapidity of the changes that are predicted indicates a major problem for tropical marine ecosystems and suggests that unrestrained warming cannot occur without the loss and degradation of coral reefs on a global scale.
3,627 citations
TL;DR: A community-structural shift occurred on Okinawan reefs, resulting in an increase in the relative abundance of massive and encrusting coral species, and two hypotheses whose synergistic effect may partially explain observed mortality patterns are suggested.
Abstract: Sea surface temperatures were warmer throughout 1998 at Sesoko Island, Japan, than in the 10 preceding years. Temperatures peaked at 2.8 °C above average, resulting in extensive coral bleaching and subsequent coral mortality. Using random quadrat surveys, we quantitatively documented the coral community structure one year before and one year after the bleaching event. The 1998 bleaching event reduced coral species richness by 61% and reduced coral cover by 85%. Colony morphology affected bleaching vulnerability and subsequent coral mortality. Finely branched corals were most susceptible, while massive and encrusting colonies survived. Most heavily impacted were the branched Acropora and pocilloporid corals, some of which showed local extinction. We suggest two hypotheses whose synergistic effect may partially explain observed mortality patterns (i.e. preferential survival of thick-tissued species, and shape-dependent differences in colony mass-transfer efficiency). A community-structural shift occurred on Okinawan reefs, resulting in an increase in the relative abundance of massive and encrusting coral species.
1,327 citations
TL;DR: The short- and long-term ecological impacts of coral bleaching on reef ecosystems are reviewed, and recovery data worldwide is quantitatively synthesized to maintain ecosystem resilience by restoring healthy levels of herbivory, macroalgal cover, and coral recruitment.
Abstract: Since the early 1980s, episodes of coral reef bleaching and mortality, due primarily to climate-induced ocean warming, have occurred almost annually in one or more of the world's tropical or subtropical seas. Bleaching is episodic, with the most severe events typically accompanying coupled ocean–atmosphere phenomena, such as the El Nino-Southern Oscillation (ENSO), which result in sustained regional elevations of ocean temperature. Using this extended dataset (25+ years), we review the short- and long-term ecological impacts of coral bleaching on reef ecosystems, and quantitatively synthesize recovery data worldwide. Bleaching episodes have resulted in catastrophic loss of coral cover in some locations, and have changed coral community structure in many others, with a potentially critical influence on the maintenance of biodiversity in the marine tropics. Bleaching has also set the stage for other declines in reef health, such as increases in coral diseases, the breakdown of reef framework by bioeroders, and the loss of critical habitat for associated reef fishes and other biota. Secondary ecological effects, such as the concentration of predators on remnant surviving coral populations, have also accelerated the pace of decline in some areas. Although bleaching severity and recovery have been variable across all spatial scales, some reefs have experienced relatively rapid recovery from severe bleaching impacts. There has been a significant overall recovery of coral cover in the Indian Ocean, where many reefs were devastated by a single large bleaching event in 1998. In contrast, coral cover on western Atlantic reefs has generally continued to decline in response to multiple smaller bleaching events and a diverse set of chronic secondary stressors. No clear trends are apparent in the eastern Pacific, the central-southern-western Pacific or the Arabian Gulf, where some reefs are recovering and others are not. The majority of survivors and new recruits on regenerating and recovering coral reefs have originated from broadcast spawning taxa with a potential for asexual growth, relatively long distance dispersal, successful settlement, rapid growth and a capacity for framework construction. Whether or not affected reefs can continue to function as before will depend on: (1) how much coral cover is lost, and which species are locally extirpated; (2) the ability of remnant and recovering coral communities to adapt or acclimatize to higher temperatures and other climatic factors such as reductions in aragonite saturation state; (3) the changing balance between reef accumulation and bioerosion; and (4) our ability to maintain ecosystem resilience by restoring healthy levels of herbivory, macroalgal cover, and coral recruitment. Bleaching disturbances are likely to become a chronic stress in many reef areas in the coming decades, and coral communities, if they cannot recover quickly enough, are likely to be reduced to their most hardy or adaptable constituents. Some degraded reefs may already be approaching this ecological asymptote, although to date there have not been any global extinctions of individual coral species as a result of bleaching events. Since human populations inhabiting tropical coastal areas derive great value from coral reefs, the degradation of these ecosystems as a result of coral bleaching and its associated impacts is of considerable societal, as well as biological concern. Coral reef conservation strategies now recognize climate change as a principal threat, and are engaged in efforts to allocate conservation activity according to geographic-, taxonomic-, and habitat-specific priorities to maximize coral reef survival. Efforts to forecast and monitor bleaching, involving both remote sensed observations and coupled ocean–atmosphere climate models, are also underway. In addition to these efforts, attempts to minimize and mitigate bleaching impacts on reefs are immediately required. If significant reductions in greenhouse gas emissions can be achieved within the next two to three decades, maximizing coral survivorship during this time may be critical to ensuring healthy reefs can recover in the long term.
1,098 citations
Cites background from "Coral bleaching: causes and consequ..."
...A variety of other stressors have also been documented to result in bleaching (Glynn, 1993; Brown, 1997a; Coles and Brown, 2003; Lesser, 2004), but the physiological and cellular mechanisms by which these stressors cause bleaching are not as well understood (Douglas, 2003)....
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...…during the past three decades, and several compilations are available in the published literature (Williams and Bunkley-Williams, 1990; Glynn, 1996; Brown, 1997a; Wilkinson, 2000, 2004; Coles and Brown, 2003; Wilkinson and Souter, 2008), as well as online in various databases maintained by…...
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...In the Java Sea, Brown (1997a) observed that community composition changed from sensitive Acropora to resistant Porites....
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...…the spectrum, have also been variably implicated in mass coral bleaching (Hoegh-Guldberg and Smith, 1989; Brown et al., 1994; Fitt and Warner, 1995; Shick et al., 1996; Brown, 1997a,b; Lesser, 1997; Brown et al., 1999; Dunne and Brown, 2001; Coles and Brown, 2003; Lesser, 2004; Smith et al., 2005)....
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TL;DR: Unusual symbionts normally found only in larval stages, marginal environments, uncommon host taxa, or at latitudinal extremes may prove critical in understanding the long-term resilience of coral reef ecosystems to environmental perturbation.
Abstract: ▪ Abstract Reef corals (and other marine invertebrates and protists) are hosts to a group of exceptionally diverse dinoflagellate symbionts in the genus Symbiodinium. These symbionts are critical components of coral reef ecosystems whose loss during stress-related “bleaching” events can lead to mass mortality of coral hosts and associated collapse of reef ecosystems. Molecular studies have shown these partnerships to be more flexible than previously thought, with different hosts and symbionts showing varying degrees of specificity in their associations. Further studies are beginning to reveal the systematic, ecological, and biogeographic underpinnings of this flexibility. Unusual symbionts normally found only in larval stages, marginal environments, uncommon host taxa, or at latitudinal extremes may prove critical in understanding the long-term resilience of coral reef ecosystems to environmental perturbation. The persistence of bleaching-resistant symbiont types in affected ecosystems, and the possibilit...
995 citations
Additional excerpts
...Increasingly frequent and severe episodes of mass coral bleaching and mortality over the past two decades as a result of warmer baseline temperatures and increasingly severe temperature anomalies such as El Niño (Brown 1997, Glynn 1993) suggest that reef ecosystems may be fast approaching a critical survival threshold (Hoegh-Guldberg 1999)....
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...Glynn et al. (2001) reported that colonies ofPocilloporacontainingSymbiodinium Dwere unaffected by bleaching during the severe 1997–1998 El Ni˜no event in the far eastern Pacific....
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...Coral bleaching and mortality in Panam´a and Ecuador during the 1997– 1998 El Niño-Southern Oscillation event: spatial/temporal patterns and comparisons with the 1982–1983 event....
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...…mass coral bleaching and mortality over the past two decades as a result of warmer baseline temperatures and increasingly severe temperature anomalies such as El Niño (Brown 1997, Glynn 1993) suggest that reef ecosystems may be fast approaching a critical survival threshold (Hoegh-Guldberg 1999)....
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References
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TL;DR: A dramatic phase shift has occurred in Jamaica, producing a system dominated by fleshy macroalgae (more than 90 percent cover), and immediate implementation of management procedures is necessary to avoid further catastrophic damage.
Abstract: Many coral reefs have been degraded over the past two to three decades through a combination of human and natural disturbances. In Jamaica, the effects of overfishing, hurricane damage, and disease have combined to destroy most corals, whose abundance has declined from more than 50 percent in the late 1970s to less than 5 percent today. A dramatic phase shift has occurred, producing a system dominated by fleshy macroalgae (more than 90 percent cover). Immediate implementation of management procedures is necessary to avoid further catastrophic damage.
2,815 citations
"Coral bleaching: causes and consequ..." refers background in this paper
...Bleaching was recorded in Jamaica in 1987, 1988, 1990 (Hughes 1994) and 1995 (Woodley in press), and at Lee Stocking Island in the Bahamas in 1987, 1990, 1993 and 1995 (J....
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...Bleaching was recorded in Jamaica in 1987, 1988, 1990 (Hughes 1994) and 1995 (Woodley in press), and at Lee Stocking Island in the Bahamas in 1987, 1990, 1993 and 1995 (J. Lang personal communication) while in the Indian Ocean, corals in the Andaman Sea bleached in 1991 and again in 1995 (Brown et…...
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TL;DR: An effort must be made to understand the impact of bleaching on the remainder of the reef community and the long-term effects on competition, predation, symbioses, bioerosion and substrate condition, all factors that can influence coral recruitment and reef recovery.
Abstract: Coral reef bleaching, the whitening of diverse invertebrate taxa, results from the loss of symbiotic zooxanthellae and/or a reduction in photosynthetic pigment concentrations in zooxanthellae residing within the gastrodermal tissues of host animals. Of particular concern are the consequences of bleaching of large numbers of reef-building scleractinian corals and hydrocorals. Published records of coral reef bleaching events from 1870 to the present suggest that the frequency (60 major events from 1979 to 1990), scale (co-occurrence in many coral reef regions and often over the bathymetric depth range of corals) and severity (>95% mortality in some areas) of recent bleaching disturbances are unprecedented in the scientific literature. The causes of small scale, isolated bleaching events can often be explained by particular stressors (e.g., temperature, salinity, light, sedimentation, aerial exposure and pollutants), but attempts to explain large scale bleaching events in terms of possible global change (e.g., greenhouse warming, increased UV radiation flux, deteriorating ecosystem health, or some combination of the above) have not been convincing. Attempts to relate the severity and extent of large scale coral reef bleaching events to particular causes have been hampered by a lack of (a) standardized methods to assess bleaching and (b) continuous, long-term data bases of environmental conditions over the periods of interest. An effort must be made to understand the impact of bleaching on the remainder of the reef community and the long-term effects on competition, predation, symbioses, bioerosion and substrate condition, all factors that can influence coral recruitment and reef recovery. If projected rates of sea warming are realized by mid to late AD 2000, i.e. a 2°C increase in high latitude coral seas, the upper thermal tolerance limits of many reef-building corals could be exceeded. Present evidence suggests that many corals would be unable to adapt physiologically or genetically to such marked and rapid temperature increases.
954 citations
TL;DR: This article considers only the phenomenon of algal loss, the loss of pigment associated with their symbiotic algae (zooxanthellae) in organisms such as hard and soft corals, giant clams, and sea anemones.
Abstract: leaching in organisms such as hard and soft corals, giant clams, and sea anemones is the loss of pigment associated with their symbiotic algae (zooxanthellae). Bleaching can be the result of loss of the pigmented algae from the host or of loss of pigment from the algae. This article considers only the phenomenon of algal loss. Most attention has been directed to bleaching in scleractinian (hard) corals in the tropics (e.g., Brown 1990, Glynn 1990, Williams and Bunkley-Williams 1990). It is a subject of concern because of mortality and local extinctions associated with large-scale bleaching episodes (Glynn and de Weert 1991; but see Glynn and Feingold 1992), the widespread belief that bleaching episodes have increased dramatically in frequency and intensity within the past decade (D'Elia et al. 1991), and speculation about possible links to global warming (Glynn
612 citations
"Coral bleaching: causes and consequ..." refers background in this paper
...Buddemeier and Fautin (1993) and Ware et al. (1996) have hypothesised that bleaching is an adaptive mechanism which allows the coral to be repopulated with a different type of alga, possibly conferring greater stress resistance....
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...Buddemeier and Fautin (1993) and Ware et al. (1996) have hypothesised that bleaching is an adaptive mechanism which allows the coral to be repopulated with a di⁄erent type of alga, possibly conferring greater stress resistance....
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TL;DR: Corals in both tropical and subtropical locations live at temperatures close to their lethal limits during the summer months, and any factor that increases respiration (such as high incident light) accelerates bleaching at higher temperatures.
Abstract: Loss of symbiotic zooxanthellae, or “bleaching” is one of the first visible signs of thermal stress. Critical threshold temperatures for coral bleaching vary geographically, but can be expressed universally as fixed increments relative to the historical mean local summer maximum. Bleaching can be induced by short-term exposure (i.e. 1–2 days) at temperature elevations of 3°C to 4°C above normal summer ambient or by long-term exposure (i.e. several weeks) at elevations of 1°C to 2°C. Corals in both tropical and subtropical locations live at temperatures close to their lethal limits during the summer months. Temperature elevations above summer ambient, but still below the bleaching threshold, can impair growth and reproduction. Temperature and light interact synergistically; high light accelerates bleaching caused by elevated temperature. Bleaching susceptibility is correlated with respiration rate. Any factor that increases respiration (such as high incident light) accelerates bleaching at higher temperatures. Ultraviolet (UV) radiation is a detrimental factor associated with solar radiation. Increased UV due to thinning of the earth's protective ozone layer may aggravate bleaching and mortality caused by global warming. A warming trend in Hawaiian waters has been observed over the past decade. In 1986, 1987 and 1988 Hawaiian corals were perilously close to their bleaching threshold during the summer months, and localized bleachings did occur. In some cases, local warming of surface water on shallow reef flats exceeded this threshold temperature and caused localized coral bleaching. In other cases, heating of large mesoscale eddies in the lee of the larger islands apparently caused wide-scale bleaching of the most sensitive coral species (Pocillopora meandrina) to depths of 20 m. A continuation of the warming trend in Hawaii would lead to mass bleachings similar to those observed recently in other geographic locations.
608 citations
TL;DR: Results from this study explore the conditions that induce bleaching in two species of reef coral-zooxanthellae associations from Lizard Island, Great Barrier Reef, Australia and suggest that closer inspection of the underlying reasons for the pale color of bleached corals is warranted.
550 citations
"Coral bleaching: causes and consequ..." refers background in this paper
...Bleaching has also been observed in the coral Stylophora pistillata by sudden exposure of colonies, previously grown at 25% radiance, to full sunlight (Hoegh-Guldberg and Smith 1989)....
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...Observed laboratory responses to high temperature are the paling of the coral through either loss of zooxanthellae (Hoegh-Guldberg and Smith 1989; Glynn and D’Croz 1990; Lesser et al. 1990; Sharp 1995; Fitt and Warner 1995) and/or the loss of photosynthetic pigment per zooxanthella (Sharp 1995; Fitt and Warner 1995)....
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...Observed laboratory responses to high temperature are the paling of the coral through either loss of zooxanthellae (Hoegh-Guldberg and Smith 1989; Glynn and D’Croz 1990; Lesser et al. 1990; Sharp 1995; Fitt and Warner 1995) and/or the loss of photosynthetic pigment per zooxanthella (Sharp 1995;…...
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