Incorporating adaptive responses into future projections of coral bleaching

Climate warming threatens to increase mass coral bleaching events, and several studies have projected the demise of tropical coral reefs this century. However, recent evidence indicates corals may be able to respond to thermal stress though adaptive processes (e.g., genetic adaptation, acclimatization, and symbiont shuffling). How these mechanisms might influence warming‐induced bleaching remains largely unknown. This study compared how different adaptive processes could affect coral bleaching projections. We used the latest bias‐corrected global sea surface temperature (SST) output from the NOAA/GFDL Earth System Model 2 (ESM2M) for the preindustrial period through 2100 to project coral bleaching trajectories. Initial results showed that, in the absence of adaptive processes, application of a preindustrial climatology to the NOAA Coral Reef Watch bleaching prediction method overpredicts the present‐day bleaching frequency. This suggests that corals may have already responded adaptively to some warming over the industrial period. We then modified the prediction method so that the bleaching threshold either permanently increased in response to thermal history (e.g., simulating directional genetic selection) or temporarily increased for 2–10 years in response to a bleaching event (e.g., simulating symbiont shuffling). A bleaching threshold that changes relative to the preceding 60 years of thermal history reduced the frequency of mass bleaching events by 20–80% compared with the ‘no adaptive response’ prediction model by 2100, depending on the emissions scenario. When both types of adaptive responses were applied, up to 14% more reef cells avoided high‐frequency bleaching by 2100. However, temporary increases in bleaching thresholds alone only delayed the occurrence of high‐frequency bleaching by ca. 10 years in all but the lowest emissions scenario. Future research should test the rate and limit of different adaptive responses for coral species across latitudes and ocean basins to determine if and how much corals can respond to increasing thermal stress.     

Forecasted coral reef decline in marine biodiversity hotspots under climate change

Coral bleaching events threaten coral reef habitats globally and cause severe declines of local biodiversity and productivity. Related to high sea surface temperatures (SST), bleaching events are expected to increase as a consequence of future global warming. However, response to climate change is still uncertain as future low‐latitude climatic conditions have no present‐day analogue. Sea surface temperatures during the Eocene epoch were warmer than forecasted changes for the coming century, and distributions of corals during the Eocene may help to inform models forecasting the future of coral reefs. We coupled contemporary and Eocene coral occurrences with information on their respective climatic conditions to model the thermal niche of coral reefs and its potential response to projected climate change. We found that under the RCP8.5 climate change scenario, the global suitability for coral reefs may increase up to 16% by 2100, mostly due to improved suitability of higher latitudes. In contrast, in its current range, coral reef suitability may decrease up to 46% by 2100. Reduction in thermal suitability will be most severe in biodiversity hotspots, especially in the Indo‐Australian Archipelago. Our results suggest that many contemporary hotspots for coral reefs, including those that have been refugia in the past, spatially mismatch with future suitable areas for coral reefs posing challenges to conservation actions under climate change.     

Opposite latitudinal gradients in projected ocean acidification and bleaching impacts on coral reefs

Coral reefs and the services they provide are seriously threatened by ocean acidification and climate change impacts like coral bleaching. Here, we present updated global projections for these key threats to coral reefs based on ensembles of IPCC AR5 climate models using the new Representative Concentration Pathway (RCP) experiments. For all tropical reef locations, we project absolute and percentage changes in aragonite saturation state (Ωarag) for the period between 2006 and the onset of annual severe bleaching (thermal stress >8 degree heating weeks); a point at which it is difficult to believe reefs can persist as we know them. Severe annual bleaching is projected to start 10–15 years later at high‐latitude reefs than for reefs in low latitudes under RCP8.5. In these 10–15 years, Ωarag keeps declining and thus any benefits for high‐latitude reefs of later onset of annual bleaching may be negated by the effects of acidification. There are no long‐term refugia from the effects of both acidification and bleaching. Of all reef locations, 90% are projected to experience severe bleaching annually by 2055. Furthermore, 5% declines in calcification are projected for all reef locations by 2034 under RCP8.5, assuming a 15% decline in calcification per unit of Ωarag. Drastic emissions cuts, such as those represented by RCP6.0, result in an average year for the onset of annual severe bleaching that is ~20 years later (2062 vs. 2044). However, global emissions are tracking above the current worst‐case scenario devised by the scientific community, as has happened in previous generations of emission scenarios. The projections here for conditions on coral reefs are dire, but provide the most up‐to‐date assessment of what the changing climate and ocean acidification mean for the persistence of coral reefs.     

Coping with Commitment: Projected Thermal Stress on Coral Reefs under Different Future Scenarios

Background

Periods of anomalously warm ocean temperatures can lead to mass coral bleaching. Past studies have concluded that anthropogenic climate change may rapidly increase the frequency of these thermal stress events, leading to declines in coral cover, shifts in the composition of corals and other reef-dwelling organisms, and stress on the human populations who depend on coral reef ecosystems for food, income and shoreline protection. The ability of greenhouse gas mitigation to alter the near-term forecast for coral reefs is limited by the time lag between greenhouse gas emissions and the physical climate response.

Methodology/Principal Findings

This study uses observed sea surface temperatures and the results of global climate model forced with five different future emissions scenarios to evaluate the “committed warming” for coral reefs worldwide. The results show that the physical warming commitment from current accumulation of greenhouse gases in the atmosphere could cause over half of the world''s coral reefs to experience harmfully frequent (p≥0.2 year⁻¹) thermal stress by 2080. An additional “societal” warming commitment, caused by the time required to shift from a business-as-usual emissions trajectory to a 550 ppm CO₂ stabilization trajectory, may cause over 80% of the world''s coral reefs to experience harmfully frequent events by 2030. Thermal adaptation of 1.5°C would delay the thermal stress forecast by 50–80 years.

Conclusions/Significance

The results suggest that adaptation – via biological mechanisms, coral community shifts and/or management interventions – could provide time to change the trajectory of greenhouse gas emissions and possibly avoid the recurrence of harmfully frequent events at the majority (97%) of the world''s coral reefs this century. Without any thermal adaptation, atmospheric CO₂ concentrations may need to be stabilized below current levels to avoid the degradation of coral reef ecosystems from frequent thermal stress events.     

Sea surface temperatures and coral reef bleaching off La Parguera, Puerto Rico (northeastern Caribbean Sea)

 Much recent attention has been given to coral reef bleaching because of its widespread occurrence, damage to reefs, and possible connection to global change. There is still debate about the relationship between temperature and widespread bleaching. We compared coral reef bleaching at La Parguera, Puerto Rico to a 30-y (1966–1995) record of sea surface temperature (SST) at the same location. The last eight years of the La Parguera SST record have all had greater than average maximum temperatures; over the past 30 y maximum summer temperature has increased 0.7 °C. Coral reef bleaching has been particularly frequent since the middle 1980s. The years 1969, 1987, 1990, and 1995 were especially noteworthy for the severity of bleaching in Puerto Rico. Seven different annual temperature indices were devised to determine the extent to which they could predict severe coral bleaching episodes. Three of these, maximum daily SST, days >29.5 °C, and days >30 °C predict correctly the four years with severe bleaching. A log-log linear relationship was found between SST and the number of days in a given year above that SST at which severe coral beaching was observed. However, the intra-annual relationship between temperature and the incidence of bleaching suggests that no one simple predictor of the onset of coral bleaching within a year may be applicable. Accepted: 17 March 1998     

Do Clouds Save the Great Barrier Reef? Satellite Imagery Elucidates the Cloud-SST Relationship at the Local Scale

Evidence of global climate change and rising sea surface temperatures (SSTs) is now well documented in the scientific literature. With corals already living close to their thermal maxima, increases in SSTs are of great concern for the survival of coral reefs. Cloud feedback processes may have the potential to constrain SSTs, serving to enforce an “ocean thermostat” and promoting the survival of coral reefs. In this study, it was hypothesized that cloud cover can affect summer SSTs in the tropics. Detailed direct and lagged relationships between cloud cover and SST across the central Great Barrier Reef (GBR) shelf were investigated using data from satellite imagery and in situ temperature and light loggers during two relatively hot summers (2005 and 2006) and two relatively cool summers (2007 and 2008). Across all study summers and shelf positions, SSTs exhibited distinct drops during periods of high cloud cover, and conversely, SST increases during periods of low cloud cover, with a three-day temporal lag between a change in cloud cover and a subsequent change in SST. Cloud cover alone was responsible for up to 32.1% of the variation in SSTs three days later. The relationship was strongest in both El Niño (2005) and La Niña (2008) study summers and at the inner-shelf position in those summers. SST effects on subsequent cloud cover were weaker and more variable among study summers, with rising SSTs explaining up to 21.6% of the increase in cloud cover three days later. This work quantifies the often observed cloud cooling effect on coral reefs. It highlights the importance of incorporating local-scale processes into bleaching forecasting models, and encourages the use of remote sensing imagery to value-add to coral bleaching field studies and to more accurately predict risks to coral reefs.     

Global assessment of coral bleaching and required rates of adaptation under climate change

Elevated ocean temperatures can cause coral bleaching, the loss of colour from reef‐building corals because of a breakdown of the symbiosis with the dinoflagellate Symbiodinium. Recent studies have warned that global climate change could increase the frequency of coral bleaching and threaten the long‐term viability of coral reefs. These assertions are based on projecting the coarse output from atmosphere–ocean general circulation models (GCMs) to the local conditions around representative coral reefs. Here, we conduct the first comprehensive global assessment of coral bleaching under climate change by adapting the NOAA Coral Reef Watch bleaching prediction method to the output of a low‐ and high‐climate sensitivity GCM. First, we develop and test algorithms for predicting mass coral bleaching with GCM‐resolution sea surface temperatures for thousands of coral reefs, using a global coral reef map and 1985–2002 bleaching prediction data. We then use the algorithms to determine the frequency of coral bleaching and required thermal adaptation by corals and their endosymbionts under two different emissions scenarios. The results indicate that bleaching could become an annual or biannual event for the vast majority of the world's coral reefs in the next 30–50 years without an increase in thermal tolerance of 0.2–1.0°C per decade. The geographic variability in required thermal adaptation found in each model and emissions scenario suggests that coral reefs in some regions, like Micronesia and western Polynesia, may be particularly vulnerable to climate change. Advances in modelling and monitoring will refine the forecast for individual reefs, but this assessment concludes that the global prognosis is unlikely to change without an accelerated effort to stabilize atmospheric greenhouse gas concentrations.     

Reduced carbon emissions and fishing pressure are both necessary for equatorial coral reefs to keep up with rising seas

A rapid increase in sea-level rise is generating vertical accommodation space on modern coral reefs. Yet increases in sea-surface temperatures (SSTs) are reducing the capacity of coral reefs to keep up with sea-level rise. We use ensemble species distribution models of four coral species (Porites rus, Porites lobata, Acropora hyacinthus and Acropora digitifera) to gauge potential geographic differences in gross carbonate production. Net carbonate production was estimated by considering erosional rates of ocean acidification, increasing cyclone intensity, local pollution, fishing pressure and the projected burdens of increases in SSTs (under Representative Concentration Pathways (RCPs) 4.5, 6.0 and 8.5) through to the year 2100. Our models predict that only 4 ± 0.1% (~60 000 km²) of Indo-Pacific coral reefs are projected to keep up with sea-level rise by the year 2100 under RCP 8.5 – most of which will be located near the Equator. However, with drastic reductions in emissions (under RCPs 4.5 and 6.0 Wm⁻²), we predict that 15 ± 0.3% (~250 000 km²) (under RCP 4.5 Wm⁻²) and 12 ± 0.7% (~200 000 km²) (under RCP 6.0 Wm⁻²) of Indo-Pacific coral reefs, have the potential to keep up with sea-level rise by the year 2100. Yet the burdens of fishing pressure and its cascading effects are projected to be responsible for substantial reef erosion, nearly halving the number of reefs able to keep up with sea-level rise. If action is taken immediately and emissions are drastically reduced to RCPs 4.5 or 6.0 Wm⁻², and reef management reduces the burdens of local pollution and fishing pressure, then our model predicts that 21–27% (~350 000–470 000 km²) of Indo-Pacific coral reefs – most of which will be located near the Equator – would have the potential to keep up with sea-level rise by the year 2100.     

Species‐specific responses to climate change and community composition determine future calcification rates of Florida Keys reefs

Anthropogenic climate change compromises reef growth as a result of increasing temperatures and ocean acidification. Scleractinian corals vary in their sensitivity to these variables, suggesting species composition will influence how reef communities respond to future climate change. Because data are lacking for many species, most studies that model future reef growth rely on uniform scleractinian calcification sensitivities to temperature and ocean acidification. To address this knowledge gap, calcification of twelve common and understudied Caribbean coral species was measured for two months under crossed temperatures (27, 30.3 °C) and CO₂ partial pressures (pCO₂) (400, 900, 1300 μatm). Mixed‐effects models of calcification for each species were then used to project community‐level scleractinian calcification using Florida Keys reef composition data and IPCC AR5 ensemble climate model data. Three of the four most abundant species, Orbicella faveolata, Montastraea cavernosa, and Porites astreoides, had negative calcification responses to both elevated temperature and pCO₂. In the business‐as‐usual CO₂ emissions scenario, reefs with high abundances of these species had projected end‐of‐century declines in scleractinian calcification of >50% relative to present‐day rates. Siderastrea siderea, the other most common species, was insensitive to both temperature and pCO₂ within the levels tested here. Reefs dominated by this species had the most stable end‐of‐century growth. Under more optimistic scenarios of reduced CO₂ emissions, calcification rates throughout the Florida Keys declined <20% by 2100. Under the most extreme emissions scenario, projected declines were highly variable among reefs, ranging 10–100%. Without considering bleaching, reef growth will likely decline on most reefs, especially where resistant species like S. siderea are not already dominant. This study demonstrates how species composition influences reef community responses to climate change and how reduced CO₂ emissions can limit future declines in reef calcification.     

Boring sponges,an increasing threat for coral reefs affected by bleaching events

Coral bleaching is a stress response of corals induced by a variety of factors, but these events have become more frequent and intense in response to recent climate‐change‐related temperature anomalies. We tested the hypothesis that coral reefs affected by bleaching events are currently heavily infested by boring sponges, which are playing a significant role in the destruction of their physical structure. Seventeen reefs that cover the entire distributional range of corals along the Mexican Pacific coast were studied between 2005/2006, and later between 2009/2010. Most of these coral reefs were previously impacted by bleaching events, which resulted in coral mortalities. Sponge abundance and species richness was used as an indicator of bioerosion, and coral cover was used to describe the present condition of coral reefs. Coral reefs are currently highly invaded (46% of the samples examined) by a very high diversity of boring sponges (20 species); being the coral reef framework the substrate most invaded (56%) followed by the rubbles (45%), and the living colonies (36%). The results also indicated that boring sponges are promoting the dislodgment of live colonies and large fragments from the framework. In summary, the eastern coral reefs affected by bleaching phenomena, mainly provoked by El Niño, present a high diversity and abundance of boring sponges, which are weakening the union of the colony with the reef framework and promoting their dislodgment. These phenomena will probably become even more intense and severe, as temperatures are projected to continue to rise under the scenarios for future climate change, which could place many eastern coral reefs beyond their survival threshold.     

Downscaled projections of Caribbean coral bleaching that can inform conservation planning

Projections of climate change impacts on coral reefs produced at the coarse resolution (~1°) of Global Climate Models (GCMs) have informed debate but have not helped target local management actions. Here, projections of the onset of annual coral bleaching conditions in the Caribbean under Representative Concentration Pathway (RCP) 8.5 are produced using an ensemble of 33 Coupled Model Intercomparison Project phase‐5 models and via dynamical and statistical downscaling. A high‐resolution (~11 km) regional ocean model (MOM4.1) is used for the dynamical downscaling. For statistical downscaling, sea surface temperature (SST) means and annual cycles in all the GCMs are replaced with observed data from the ~4‐km NOAA Pathfinder SST dataset. Spatial patterns in all three projections are broadly similar; the average year for the onset of annual severe bleaching is 2040–2043 for all projections. However, downscaled projections show many locations where the onset of annual severe bleaching (ASB) varies 10 or more years within a single GCM grid cell. Managers in locations where this applies (e.g., Florida, Turks and Caicos, Puerto Rico, and the Dominican Republic, among others) can identify locations that represent relative albeit temporary refugia. Both downscaled projections are different for the Bahamas compared to the GCM projections. The dynamically downscaled projections suggest an earlier onset of ASB linked to projected changes in regional currents, a feature not resolved in GCMs. This result demonstrates the value of dynamical downscaling for this application and means statistically downscaled projections have to be interpreted with caution. However, aside from west of Andros Island, the projections for the two types of downscaling are mostly aligned; projected onset of ASB is within ±10 years for 72% of the reef locations.     

Mass Coral Bleaching in 2010 in the Southern Caribbean

Ocean temperatures are increasing globally and the Caribbean is no exception. An extreme ocean warming event in 2010 placed Tobago''s coral reefs under severe stress resulting in widespread coral bleaching and threatening the livelihoods that rely on them. The bleaching response of four reef building taxa was monitored over a six month period across three major reefs systems in Tobago. By identifying taxa resilient to bleaching we propose to assist local coral reef managers in the decision making process to cope with mass bleaching events. The bleaching signal (length of exposure to high ocean temperatures) varied widely between the Atlantic and Caribbean reefs, but regardless of this variation most taxa bleached. Colpophyllia natans, Montastraea faveolata and Siderastrea siderea were considered the most bleaching vulnerable taxa. Interestingly, reefs with the highest coral cover showed the greatest decline reef building taxa, and conversely, reefs with the lowest coral cover showed the most bleaching but lowest change in coral cover with little algal overgrowth post-bleaching.     

Severe 2010 cold-water event caused unprecedented mortality to corals of the Florida reef tract and reversed previous survivorship patterns

Background

Coral reefs are facing increasing pressure from natural and anthropogenic stressors that have already caused significant worldwide declines. In January 2010, coral reefs of Florida, United States, were impacted by an extreme cold-water anomaly that exposed corals to temperatures well below their reported thresholds (16°C), causing rapid coral mortality unprecedented in spatial extent and severity.

Methodology/Principal Findings

Reef surveys were conducted from Martin County to the Lower Florida Keys within weeks of the anomaly. The impacts recorded were catastrophic and exceeded those of any previous disturbances in the region. Coral mortality patterns were directly correlated to in-situ and satellite-derived cold-temperature metrics. These impacts rival, in spatial extent and intensity, the impacts of the well-publicized warm-water bleaching events around the globe. The mean percent coral mortality recorded for all species and subregions was 11.5% in the 2010 winter, compared to 0.5% recorded in the previous five summers, including years like 2005 where warm-water bleaching was prevalent. Highest mean mortality (15%–39%) was documented for inshore habitats where temperatures were <11°C for prolonged periods. Increases in mortality from previous years were significant for 21 of 25 coral species, and were 1–2 orders of magnitude higher for most species.

Conclusions/Significance

The cold-water anomaly of January 2010 caused the worst coral mortality on record for the Florida Reef Tract, highlighting the potential catastrophic impacts that unusual but extreme climatic events can have on the persistence of coral reefs. Moreover, habitats and species most severely affected were those found in high-coral cover, inshore, shallow reef habitats previously considered the “oases” of the region, having escaped declining patterns observed for more offshore habitats. Thus, the 2010 cold-water anomaly not only caused widespread coral mortality but also reversed prior resistance and resilience patterns that will take decades to recover.     

Satellite observation of Keppel Islands (Great Barrier Reef) 2002 coral bleaching using IKONOS data

An examination of IKONOS satellite imagery of the Keppel Islands (Great Barrier Reef) acquired before and during a coral bleaching event indicates that severe bleaching of reefs can be detected as an increase in brightness in the band 1 (blue) and band 2 (green) IKONOS spectral bands (4-m resolution). The bleaching was not detected in band 3 (red), band 4 (near-infrared), or in the 1-m panchromatic band data. A total of 0.74 km² of bleached coral was identified, with detection occurring in waters as deep as 15 m. The procedure requires that one of the scenes be radiometrically normalized to match the reference scene prior to image differencing. A relative radiometric normalization was used in this case because variable cloud cover present in the image acquired during the bleaching event prevented reliable modeling of atmospheric effects. The success at coral bleaching detection at Keppel Islands represents both a best-case and a cloud-challenged scenario. It was a best-case scenario in that coral cover was extensive (70–90% live coral cover, mostly acroporids) and the bleaching level was extreme (92–95% of coral cover white bleached). It was a cloud-challenged scenario in terms of having extensive and highly variable cloud cover present in the image acquired during the bleaching event. Color difference images reveal extensive areas of bleached coral at sites away from our study area, indicating that this platform and methodology may be a valuable tool for mapping high coral cover areas during bleaching events. Additional studies and technique refinements would be required to test the detection limits of bleaching with IKONOS imagery or to develop a spectrally based bleaching detection index.An erratum to this article can be found at     

An Ecosystem-Level Perspective on the Host and Symbiont Traits Needed to Mitigate Climate Change Impacts on Caribbean Coral Reefs

Caribbean reefs have steadily declined during the past 30 years. Thermal disturbances that elicit coral bleaching have been identified as a major driver of such coral degradation. It has been suggested that either the evolution of more tolerant symbionts, or shifts in the distribution of existing, tolerant symbionts could ameliorate the effect of rising sea temperatures on Caribbean reefs. Using a spatial ecosystem model we describe the characteristics that new tolerant symbionts, ‘super-symbionts’, and their coral hosts, require for coral cover to be maintained. We also quantify the time necessary for such symbionts to become dominant before their potential beneficial effect is lost. Running scenarios under two levels of greenhouse gas emissions, we find that aggressive action to reduce emissions could almost triple the time available for new super-symbionts to become dominant and potentially mitigate the effect of thermal disturbances. The benefits of thermally tolerant super-symbionts depend on the life-history traits of the host, the number of coral species infected and the present coral assemblage. Corals that are strong competitors with macroalgae are likely to become dominant on future reefs if a super-symbiont appears in the next 25–60 years. In principle, super-symbionts could have ecosystem-level benefits in the Caribbean providing that they become dominant in multiple coral hosts with specific life-history traits within the next 60 years. This potential benefit would only be realized if the appearance of the super-symbiont is combined with drastic reductions of greenhouse gas emissions and maintenance of ecosystem processes such as herbivory.     

Coral reef crisis in deep and shallow reefs: 30 years of constancy and change in reefs of Curacao and Bonaire

Coral reefs are thought to be in worldwide decline but available data are practically limited to reefs shallower than 25 m. Zooxanthellate coral communities in deep reefs (30–40 m) are relatively unstudied. Our question is: what is happening in deep reefs in terms of coral cover and coral mortality? We compare changes in species composition, coral mortality, and coral cover at Caribbean (Curacao and Bonaire) deep (30–40 m) and shallow reefs (10–20 m) using long-term (1973–2002) data from permanent photo quadrats. About 20 zooxanthellate coral species are common in the deep-reef communities, dominated by Agaricia sp., with coral cover up to 60%. In contrast with shallow reefs, there is no decrease in coral cover or number of coral colonies in deep reefs over the last 30 years. In deep reefs, non-agaricid species are decreasing but agaricid domination will be interrupted by natural catastrophic mortality such as deep coral bleaching and storms. Temperature is a vastly fluctuating variable in the deep-reef environment with extremely low temperatures possibly related to deep-reef bleaching. An erratum to this article can be found at     

A connection between colony biomass and death in Caribbean reef-building corals

Increased sea-surface temperatures linked to warming climate threaten coral reef ecosystems globally. To better understand how corals and their endosymbiotic dinoflagellates (Symbiodinium spp.) respond to environmental change, tissue biomass and Symbiodinium density of seven coral species were measured on various reefs approximately every four months for up to thirteen years in the Upper Florida Keys, United States (1994-2007), eleven years in the Exuma Cays, Bahamas (1995-2006), and four years in Puerto Morelos, Mexico (2003-2007). For six out of seven coral species, tissue biomass correlated with Symbiodinium density. Within a particular coral species, tissue biomasses and Symbiodinium densities varied regionally according to the following trends: Mexico≥Florida Keys≥Bahamas. Average tissue biomasses and symbiont cell densities were generally higher in shallow habitats (1-4 m) compared to deeper-dwelling conspecifics (12-15 m). Most colonies that were sampled displayed seasonal fluctuations in biomass and endosymbiont density related to annual temperature variations. During the bleaching episodes of 1998 and 2005, five out of seven species that were exposed to unusually high temperatures exhibited significant decreases in symbiotic algae that, in certain cases, preceded further decreases in tissue biomass. Following bleaching, Montastraea spp. colonies with low relative biomass levels died, whereas colonies with higher biomass levels survived. Bleaching- or disease-associated mortality was also observed in Acropora cervicornis colonies; compared to A. palmata, all A. cervicornis colonies experienced low biomass values. Such patterns suggest that Montastraea spp. and possibly other coral species with relatively low biomass experience increased susceptibility to death following bleaching or other stressors than do conspecifics with higher tissue biomass levels.     

Vulnerability of the Great Barrier Reef to climate change and local pressures

Australia's Great Barrier Reef (GBR) is under pressure from a suite of stressors including cyclones, crown‐of‐thorns starfish (COTS), nutrients from river run‐off and warming events that drive mass coral bleaching. Two key questions are: how vulnerable will the GBR be to future environmental scenarios, and to what extent can local management actions lower vulnerability in the face of climate change? To address these questions, we use a simple empirical and mechanistic coral model to explore six scenarios that represent plausible combinations of climate change projections (from four Representative Concentration Pathways, RCPs), cyclones and local stressors. Projections (2017–2050) indicate significant potential for coral recovery in the near‐term, relative to current state, followed by climate‐driven decline. Under a scenario of unmitigated emissions (RCP8.5) and business‐as‐usual management of local stressors, mean coral cover on the GBR is predicted to recover over the next decade and then rapidly decline to only 3% by year 2050. In contrast, a scenario of strong carbon mitigation (RCP2.6) and improved water quality, predicts significant coral recovery over the next two decades, followed by a relatively modest climate‐driven decline that sustained coral cover above 26% by 2050. In an analysis of the impacts of cumulative stressors on coral cover relative to potential coral cover in the absence of such impacts, we found that GBR‐wide reef performance will decline 27%–74% depending on the scenario. Up to 66% of performance loss is attributable to local stressors. The potential for management to reduce vulnerability, measured here as the mean number of years coral cover can be kept above 30%, is spatially variable. Management strategies that alleviate cumulative impacts have the potential to reduce the vulnerability of some midshelf reefs in the central GBR by 83%, but only if combined with strong mitigation of carbon emissions.     

Thermal stress induces persistently altered coral reef fish assemblages

Ecological communities are reorganizing in response to warming temperatures. For continuous ocean habitats this reorganization is characterized by large‐scale species redistribution, but for tropical discontinuous habitats such as coral reefs, spatial isolation coupled with strong habitat dependence of fish species imply that turnover and local extinctions are more significant mechanisms. In these systems, transient marine heatwaves are causing coral bleaching and profoundly altering habitat structure, yet despite severe bleaching events becoming more frequent and projections indicating annual severe bleaching by the 2050s at most reefs, long‐term effects on the diversity and structure of fish assemblages remain unclear. Using a 23‐year time series spanning a thermal stress event, we describe and model structural changes and recovery trajectories of fish communities after mass bleaching. Communities changed fundamentally, with the new emergent communities dominated by herbivores and persisting for >15 years, a period exceeding realized and projected intervals between thermal stress events on coral reefs. Reefs which shifted to macroalgal states had the lowest species richness and highest compositional dissimilarity, whereas reefs where live coral recovered exceeded prebleaching fish richness, but remained dissimilar to prebleaching compositions. Given realized and projected frequencies of bleaching events, our results show that fish communities historically associated with coral reefs will not re‐establish, requiring substantial adaptation by managers and resource users.     

Predicted disappearance of coral-reef ramparts: a direct result of major ecological disturbances

Two coral cays near La Parguera, Puerto Rico, have large, exposed coral ramparts composed almost entirely of loose pieces of elkhorn coral Acropora palmata (88% of horizontal transects, 98% of vertical transects). The total volume of elkhorn coral in the ramparts of the two cays was estimated at 3600 and 12 800 m³. The present volume of living elkhorn coral on these two reefs was estimated at 7 and 14 m³ and previous volumes at 11 000 and 34 900 m³. White-band disease was found on 8.5% of living elkhorn colonies. Lang’s boring sponge Cliona langae covered 10.8% of the total transect area, overgrowing both dead and living corals. White-band disease and coral-reef bleaching have drastically reduced the populations of elkhorn coral, thus, skeletal coral materials to replenish the plate ramparts are severely reduced, disrupting the process of forming and maintaining these coral reef ramparts. We predict that the next series of major storms striking these prominent cay ramparts will remove them. These disappearances will represent a quick, obvious and permanent consequence of global disturbances. Loss of cay ramparts will modify the environments on and around Atlantic coral reefs. Ramparts may be similarly lost from Indo-Pacific reefs. The lack of any other indisputable definitive indicators of long-term, major disturbances on coral reefs makes the distinct loss of coral-reef ramparts an important physical sign.