Mass Bleaching Research Articles

The Marine Carbon System and Ocean Acidification during Phanerozoic Time

The Marine Carbon System and Ocean Acidification during Phanerozoic Time

Unprecedented mass bleaching and loss of coral across 12° of latitude in Western Australia in 2010-11.

BackgroundGlobally, coral bleaching has been responsible for a significant decline in both coral cover and diversity over the past two decades. During the summer of 2010–11, anomalous large-scale ocean warming induced unprecedented levels of coral bleaching accompanied by substantial storminess across more than 12° of latitude and 1200 kilometers of coastline in Western Australia (WA).Methodology/Principal FindingsExtreme La-Niña conditions caused extensive warming of waters and drove considerable storminess and cyclonic activity across WA from October 2010 to May 2011. Satellite-derived sea surface temperature measurements recorded anomalies of up to 5°C above long-term averages. Benthic surveys quantified the extent of bleaching at 10 locations across four regions from tropical to temperate waters. Bleaching was recorded in all locations across regions and ranged between 17% (±5.5) in the temperate Perth region, to 95% (±3.5) in the Exmouth Gulf of the tropical Ningaloo region. Coincident with high levels of bleaching, three cyclones passed in close proximity to study locations around the time of peak temperatures. Follow-up surveys revealed spatial heterogeneity in coral cover change with four of ten locations recording significant loss of coral cover. Relative decreases ranged between 22%–83.9% of total coral cover, with the greatest losses in the Exmouth Gulf.Conclusions/SignificanceThe anomalous thermal stress of 2010–11 induced mass bleaching of corals along central and southern WA coral reefs. Significant coral bleaching was observed at multiple locations across the tropical-temperate divide spanning more than 1200 km of coastline. Resultant spatially patchy loss of coral cover under widespread and high levels of bleaching and cyclonic activity, suggests a degree of resilience for WA coral communities. However, the spatial extent of bleaching casts some doubt over hypotheses suggesting that future impacts to coral reefs under forecast warming regimes may in part be mitigated by southern thermal refugia.

Variation in the size structure of corals is related to environmental extremes in the Persian Gulf

The size structure of coral populations is the culmination of key demographic events, including recruitment, mortality and growth, thereby providing important insights to recent ecological dynamics. Importantly, the size structure of corals reflects both intrinsic (inherent life-history characteristics) and extrinsic (enhanced mortality due to chronic or acute disturbances) forcing on local populations, enabling post-hoc assessment of spatial and taxonomic differences in susceptibility to disturbance. This study examined the size structure of four locally abundant corals (Acropora downingi, Favia pallida, Platygyra daedalea, and massive Porites spp.) in two regions of the Persian Gulf: the southern Gulf (Dubai and Abu Dhabi) and eastern Gulf (western Musandam). Significant and consistent differences were apparent in mean colony sizes and size-distributions between regions. All corals in the southern Gulf were significantly smaller, and their size structure positively skewed and relatively more leptokurtic (i.e., peaky) compared to corals in the eastern Gulf. Sea surface temperatures, salinity, and the recent frequency of mass bleaching are all higher, in the southern Gulf, suggesting higher mortality rates and/or slower growth in these populations. Differences in size structure between locations were more pronounced than differences between species at each location, suggesting that extreme differences in environmental conditions and disturbance events have a greater influence on population dynamics in the Gulf than inherent differences in their life-history characteristics.

Response of coral assemblages to thermal stress: are bleaching intensity and spatial patterns consistent between events?

Mass bleaching events resulting in coral mortality are among the greatest threats to coral reefs, and are projected to increase in frequency and intensity with global warming. Achieving a better understanding of the consistency of the response of coral assemblages to thermal stress, both spatially and temporally, is essential to determine which reefs are more able to tolerate climate change. We compared variations in spatial and taxonomic patterns between two bleaching events at the scale of an island (Moorea Island, French Polynesia). Despite similar thermal stress and light conditions, bleaching intensity was significantly lower in 2007 (approximately 37 % of colonies showed signs of bleaching) than in 2002, when 55 % of the colonies bleached. Variations in the spatial patterns of bleaching intensity were consistent between the two events. Among nine sampling stations at three locations and three depths, the stations at which the bleaching response was lowest in 2002 were those that showed the lowest levels of bleaching in 2007. The taxonomic patterns of susceptibility to bleaching were also consistent between the two events. These findings have important implications for conservation because they indicate that corals are capable of acclimatization and/or adaptation and that, even at small spatial scales, some areas are consistently more susceptible to bleaching than others.

Limiting global warming to 2 °C is unlikely to save most coral reefs

Comprehensive computer simulations show that coral reefs are likely to suffer extensive long-term degradation resulting from mass bleaching events even if the expected increase in global mean temperature can be kept well below 2 °C. Without major mitigation efforts to limit global warming significantly, the fate of coral reef ecosystems seems to be sealed.

Relationships between temperature, bleaching and white syndrome on the Great Barrier Reef

Coral bleaching and disease have often been hypothesized to be mutually reinforcing or co-occurring, but much of the research supporting this has only drawn an implicit connection through common environmental predictors. In this study, we examine whether an explicit relationship between white syndrome and bleaching exists using assemblage-level monitoring data from up to 112 sites on reef slopes spread throughout the Great Barrier Reef over 11 years of monitoring. None of the temperature metrics commonly used to predict mass bleaching performed strongly when applied to these data. Furthermore, the inclusion of bleaching as a predictor did not improve model skill over baseline models for predicting white syndrome. Similarly, the inclusion of white syndrome as a predictor did not improve models of bleaching. Evidence for spatial co-occurrence of bleaching and white syndrome at the assemblage level in this data set was also very weak. These results suggest the hypothesized relationship between bleaching and disease events may be weaker than previously thought, and more likely to be driven by common responses to environmental stressors, rather than directly facilitating one another.

Regional genetic differentiation among northern high-latitude island populations of a broadcast-spawning coral

Knowledge of genetic connectivity is useful for understanding of the recovery potential of coral populations after various disturbances, such as coral mass bleaching. Population genetic studies in corals are mostly restricted to Australian and Caribbean species; studies in the northern Pacific are relatively limited. Using microsatellite markers, the population genetics of Acropora sp. 1 was examined between two regions in Japan, the Okinawa-Aka and Bonin Islands, which are separated by approximately 1,500 km of open water in a high-latitude area. Statistically significant but small genetic differentiation in Acropora sp. 1 was detected between and within these regions. Genetic diversity was not obviously reduced in populations of the Bonin Islands, which are relatively isolated. Thus, some level of connectivity appears to be maintained between the two regions, likely because of the high dispersal ability of this broadcast spawner.

Preventing coral bleaching, one hurricane at a time

In recent decades, sea surface temperatures and the occurrence of heat stress in coral communities have soared. High surface water temperatures lead coral populations to evict their symbiotic, and colorful, algal residents. The photosynthesizing algae are what feed the coral, and the process—known as bleaching—can eventually kill it, leaving parched white exoskeletons in place of formerly vibrant reefs. However, not all coral reefs seem equally affected by mass bleaching at the hands of global warming. Some processes, like deep water upwelling, are known to offset rising temperatures locally, butCarrigan and Puotinen investigate a novel mechanism that they suggest may be responsible for protecting some susceptible populations.

Assessing the potential for tropical cyclone induced sea surface cooling to reduce thermal stress on the world's coral reefs

[1] Coral reefs face an uncertain future as rising sea surface temperature (SST) continues to lead to increasingly frequent and intense mass bleaching. At broad spatial scales, tropical cyclone (TC) induced cooling of the upper ocean (SST drops up to 6° C persisting for weeks) reduces thermal stress and accelerates recovery of bleached corals - yet the global prevalence and spatial distribution of this effect remains undocumented and unquantified. A global dataset (1985–2009) of TC wind exposure was constructed and examined against existing thermal stress data to address this. Significant correlations were found between TC activity and the severity of thermal stress at various spatial scales, particularly for Caribbean reefs. From this, it is apparent that TCs play a role in bleaching dynamics at a global scale. However, the prevalence and distribution of this interaction varies by region and requires further examination at finer spatial and temporal scales using actual SST data.

Defining the tipping point. A complex cellular life/death balance in corals in response to stress

Apoptotic cell death has been implicated in coral bleaching but the molecules involved and the mechanisms by which apoptosis is regulated are only now being identified. In contrast the mechanisms underlying apoptosis in higher animals are relatively well understood. To better understand the response of corals to thermal stress, the expression of coral homologs of six key regulators of apoptosis was studied in Acropora aspera under conditions simulating those of a mass bleaching event. Significant changes in expression were detected between the daily minimum and maximum temperatures. Maximum daily temperatures from as low as 3°C below the bleaching threshold resulted in significant changes in both pro- and anti-apoptotic gene expression. The results suggest that the control of apoptosis is highly complex in this eukaryote-eukaryote endosymbiosis and that apoptotic cell death cascades potentially play key roles tipping the cellular life/death balance during environmental stress prior to the onset of coral bleaching.

Projecting Coral Reef Futures Under Global Warming and Ocean Acidification

Many physiological responses in present-day coral reefs to climate change are interpreted as consistent with the imminent disappearance of modern reefs globally because of annual mass bleaching events, carbonate dissolution, and insufficient time for substantial evolutionary responses. Emerging evidence for variability in the coral calcification response to acidification, geographical variation in bleaching susceptibility and recovery, responses to past climate change, and potential rates of adaptation to rapid warming supports an alternative scenario in which reef degradation occurs with greater temporal and spatial heterogeneity than current projections suggest. Reducing uncertainty in projecting coral reef futures requires improved understanding of past responses to rapid climate change; physiological responses to interacting factors, such as temperature, acidification, and nutrients; and the costs and constraints imposed by acclimation and adaptation.

Severe, Widespread El Niño–Associated Coral Bleaching in the US Phoenix Islands

The present study provides an assessment of the bleaching response of corals at Baker and Howland Islands (central Pacific) following the 2009-2010 El Nino-Southern Oscillation event. Nearly 35% of colonies belonging to 17 coral genera surveyed within belt transects exhibited bleaching, predominantly along east Howland Island and east and northeast Baker Island, and within the 10-18 m isobaths. Bleaching conditions differed among coral taxa, with Pocillopora, Acropora, and Fungia exhibiting the greatest percentage of affected colonies, followed by Pavona, Porites, and Psammocora; Montipora appeared to be the most resistant to bleaching with < 2.5% of colonies affected. Colony size had direct effects on bleaching responses for Acropora, Pavona, and Pocillopora, with bleached colonies exhibiting greater mean diameters than unbleached corals; mixed effects were observed for Fungia, Porites, and Psammocora. Bleaching extent was also affected by colony size, but only in Acropora; the lack of differences in the other study corals is probably due to the high variability in the extent of bleaching within taxa. In situ subsurface loggers indicated that seawater temperatures increased steadily throughout 2009, peaking at 31.2 °c in early November, and remaining above the coral bleaching threshold (29.7 °c) beyond the time when instruments were recovered on 7 February, 2010. This is the first documented mass bleaching episode at baker and Howland Islands; however, anecdotal evidence suggests recurrent, widespread bleaching may be a principal source of disturbance, affecting coral reef species composition and structural dynamics at Howland and Baker over multi-year timescales.

Drivers of region-wide declines in architectural complexity on Caribbean reefs

Severe declines in the cover of live hard coral on reefs have been reported worldwide, and in the Caribbean region, the architectural complexity of coral reefs has also declined markedly. While the drivers of coral cover loss are relatively well understood, little is known about the drivers of regional-scale declines in architectural complexity. We have used a dataset of 49 time series reporting reef architectural complexity to explore the effect of hurricanes, coral bleaching and fishing on Caribbean-wide annual rates of change in reef complexity. Hurricane impacts greatly influence reef complexity, with the most rapid rates of decline in complexity occurring at sites impacted during their survey period, and with lower rates of loss occurring at unimpacted sites. Reef architectural complexity did not change significantly following mass bleaching events (in a time frame of <5 years) or positive thermal anomalies. Although the rates of change in architectural complexity were similar in and out of marine protected areas (MPAs), significant declines in complexity were observed inside but not outside of MPAs, possibly because reductions in fishing can lead to increased bioerosion by herbivores within MPAs. Our findings suggest that major drivers of coral mortality, such as coral bleaching, do not influence reef architectural complexity in the short term (<5 years). Instead, direct physical impacts and reef bioerosion appear to be important drivers of the widespread loss of architecturally complex reefs in the Caribbean.

Disturbance and the Dynamics of Coral Cover on the Great Barrier Reef (1995–2009)

Coral reef ecosystems worldwide are under pressure from chronic and acute stressors that threaten their continued existence. Most obvious among changes to reefs is loss of hard coral cover, but a precise multi-scale estimate of coral cover dynamics for the Great Barrier Reef (GBR) is currently lacking. Monitoring data collected annually from fixed sites at 47 reefs across 1300 km of the GBR indicate that overall regional coral cover was stable (averaging 29% and ranging from 23% to 33% cover across years) with no net decline between 1995 and 2009. Subregional trends (10–100 km) in hard coral were diverse with some being very dynamic and others changing little. Coral cover increased in six subregions and decreased in seven subregions. Persistent decline of corals occurred in one subregion for hard coral and Acroporidae and in four subregions in non-Acroporidae families. Change in Acroporidae accounted for 68% of change in hard coral. Crown-of-thorns starfish (Acanthaster planci) outbreaks and storm damage were responsible for more coral loss during this period than either bleaching or disease despite two mass bleaching events and an increase in the incidence of coral disease. While the limited data for the GBR prior to the 1980's suggests that coral cover was higher than in our survey, we found no evidence of consistent, system-wide decline in coral cover since 1995. Instead, fluctuations in coral cover at subregional scales (10–100 km), driven mostly by changes in fast-growing Acroporidae, occurred as a result of localized disturbance events and subsequent recovery.

Does Acanthaster planci preferably prey on the reef zoanthid Palythoa tuberculosa?

The crown-of-thorns starfi sh Acanthaster planci is generally known as an intensive predator on hermatypic corals, especially Acropora spp. (Pratchett, 2007). Outbreaks of A. planci cause serious damages to coral reefs. In contrast, it is less known that A. planci also consumes zoanthids. We observed A. planci preying on a reef covering zoanthid, Palythoa tuberculosa at night between August and September 2010, in the outer reef fl at of Mizugama, Okinawa. Several individuals aggregated on P. tuberculosa colonies (Fig. 1). The individuals were relatively small, approximately 5-10 cm in radial length. Acanthaster planci preying on P. tuberculosa were also observed in the daytime in October 2010 in Lyudao, Taiwan. According to Gleibs and Mebs (1999), A. planci predates upon Palythoa in the South Pacifi c, which may indicate this is a usual event. Interestingly, at Mizugama, no individuals were detected on neigh boring Acropora spp. colonies that probably had recruited after the mass bleaching event of 1998. From additional observations in April 2011, neither A. planci individuals nor feeding traces of them were detected on P. tuberculosa colonies, suggesting that preying on P. tuberculosa occurs seasonally. At Mizugama, P. tuberculosa breeds in August (Hirose et al. 2011). In fact, pink-colored mature eggs were observed within colonies damaged by A. planci feeding. It is possible that A. planci preys on P. tuberculosa to obtain a more nutritious food, their eggs.

Demographics of bleaching in a major Caribbean reef-building coral:Montastraea annularis

Mass bleaching events have become a major cause of coral decline at a global scale. In the summer/fall of 2005 the northeastern Caribbean experienced a record-breaking sea surface warming that resulted in a prolonged mass bleaching event and significant percent coral cover decline of the principal Caribbean reef-building coral Montastraea annularis. In this study, we measured changes in the vital rates of a M. annularis population before, during, and after the 2005 mass bleaching event; stochastically projected the population with different bleaching regimes using a 100 year horizon; and quantified the population level effect of the bleaching event using a life table response experiment. Size-based transition matrices from 2001–2009 were constructed following 399 colonies through time in 17 permanent photo-transects located in Culebra Island, Puerto Rico. Temporal variation in the population growth rate indicates the population (1) was in demographic equilibrium before the event (λ ≈ 1.0), (2) suffered a significant decline in growth rate for two consecutive years after the event (λ < 1.0), and (3) demographically recovered three years after the event (λ ≈ 1.0). Partial tissue mortality due to bleaching caused dramatic colony fragmentation that resulted in a population made up almost entirely of small colonies by 2007 (97% were <50 cm2). The stochastic simulation indicates that an annual probability of bleaching in excess of 6% would result in a decreasing population (λs < 1.0) with a reduction of more than 54% in colony abundance after 100 years of projection. The life table response experiments reveal that most of the effect that bleaching had on the population growth rate comes from changes in the survivorship of small colonies. Recent trends in population decline, as well as the life history traits of M. annularis, suggest that recovery of affected populations by sexual recruitment alone is unlikely. Our findings indicate that survival of small colonies will determine the viability of the M. annularis populations within the context of rising sea surface temperatures. We conclude that the demography of M. annularis is highly susceptible to bleaching and that its viability is seriously compromised under the predicted global warming scenarios.

First observed severe mass bleaching in Malaysia, Greater Coral Triangle

[Extract] Malaysia contains approximately 4,000 km2 of reef (Wilkinson 2008) and an estimated 500 hermatypic coral species, which constitutes more than 60% of the worlds described hermatypic coral species (Veron et al. 2009). However, these reefs are exposed to numerous anthropogenic pressures and more than 40% are classified as having a high or very high Integrated Threat Index (Burke et al. 2002). Prior to 2010, severe and widespread coral bleaching had not been observed on Malaysian reefs, including during the 1997/98 global event when bleaching of Malaysian reefs was mild and occurred only in localized patches (Wilkinson 1998; Kushairi 1999).

Coral mass bleaching and reef temperatures at Navassa Island, 2006

Bleaching and associated mortality is an extreme threat to the persistence of coral populations in the projected warming regime of the next few decades. Recent evidence indicates that thermal bleaching thresholds may be affected by water quality gradients. The unexpected encounter of a coral mass bleaching event at a remote, uninhabited Caribbean island (Navassa) during a routine reef assessment cruise in November 2006 provided the opportunity to characterize bleaching responses and thermal exposure in an oceanic area with negligible continental influence or human impact on water quality. The coral taxa most susceptible to bleaching were Agaricia spp. and Montastraea faveolata. Siderastraea siderea, Diploria spp. and Porites porites were intermediately affected, while Porites astreoides and Montastraea cavernosa were minimally affected and negligible bleaching was observed in Acropora palmata. Bleaching prevalence (colonies > 4 cm diameter) ranged from 0.16 to 0.63 among sites. Deeper sites (between 18 and 37 m) had significantly higher prevalence of bleaching than shallow sites (<10 m). This general pattern of more bleaching in deeper sites also occurred within species. Though exposure to high-temperature stress was not greater at deeper sites, water motion, which may bolster bleaching resistance, was likely less. In situ loggers indicated temperatures over 30 °C initiated at shallow sites in mid-August, at deeper sites in early September, and were persistent at all sites until mid-October. Long term (1983–2007) climatologies constructed from AVHRR SSTs suggest that the mass bleaching event observed at Navassa in 2006 corresponded with greater intensity and duration of warm temperature anomalies than occurred in 2005, for which no in situ observations (bleaching nor temperature) are available.

Location and disturbance affect population genetic structure in four coral species of the genus Acropora on the Great Barrier Reef

The impact of a mass bleaching event on temporal and spatial population genetic struc- ture in 4 scleractinian coral species in the Acropora aspera group was studied around the Palm Islands in the central Great Barrier Reef. Species status of sympatric populations of 2 of the 4 species, A. millepora and A. spathulata, was confirmed by the population genetic data; these species have recently been separated based on morphological and breeding characters. Spatial analyses of popu- lation samples from 2004 detected differences in the level of gene flow among locations. No signifi- cant genetic differentiation was inferred between conspecific populations at Orpheus and Pelorus Islands, which are both located in the northern part of the island group and separated by ~1000 m. In contrast, all populations at Fantome Island were genetically differentiated, despite this island being located only 11 km south. Sampling of A. millepora and A. pulchra in the year prior to the 1998 mass bleaching event enabled a temporal comparison across this event. The genetic composition of these populations changed between 1997 and 2004, but patterns of genetic differentiation among locations were similar in 1997 and 2004. Extensive mortality of these species following the 1998 bleaching event did not cause an apparent reduction in genetic diversity and identical multi-locus genotypes were encountered in both temporal samples, suggesting that re-growth of surviving genotypes con- tributed to the recovery of these populations. Comparisons among the 4 study species revealed lower genetic diversity in A. papillare, consistent with its low abundance throughout its distributional range.

Caught in the Net

Ecology![Figure][1] CREDIT: JEFF SHIMA Coral reefs have turned out to be startlingly vulnerable to environmental events. Relatively small temperature rises make corals shed their essential symbionts in mass bleaching events; a small decrease in ocean pH erodes their apatite skeletons;