The diversity, frequency, and scale of human impacts on coral reefs are increasing to the extent that reefs are threatened globally. Projected increases in carbon dioxide and temperature over the next 50 years exceed the conditions under which coral reefs have flourished over the past half-million years. However, reefs will change rather than disappear entirely, with some species already showing far greater tolerance to climate change and coral bleaching than others. International integration of management strategies that support reef resilience need to be vigorously implemented, and complemented by strong policy decisions to reduce the rate of global warming.

The world's tropical reef ecosystems, and the people who depend on them, are increasingly 60 impacted by climate change [1][2][3][4][5][6][7] Reef, as well as the potential influence of water quality and fishing pressure on the severity of 71 bleaching. 72The geographic footprints of mass bleaching of corals on the Great Barrier Reef have varied 73 strikingly during three major events in 1998 , 2002 and 2016). In 1998, bleaching was 74 primarily coastal and most severe in the central and southern regions. In 2002, bleaching was 75 more widespread, and affected offshore reefs in the central region that had escaped in 1998 8 . 76In 2016, bleaching was even more extensive and much more severe, especially in the 77 northern, and to a lesser extent the central regions, where many coastal, mid-shelf and 78 offshore reefs were affected (Fig. 1a, b). In 2016, the proportion of reefs experiencing 79 extreme bleaching (>60% of corals bleached) was over four times higher compared to 1998 80 or 2002 (Fig. 1f) The severity and distinctive geographic footprints of bleaching in each of the three 88 years can be explained by differences in the magnitude and spatial distribution of sea-surface 89 temperature anomalies (Fig. 1a, b 102The geographic pattern of bleaching also demonstrates how marine heatwaves can be (Fig. 2a) (Fig. 1g). largely escaped bleaching in the two earlier events (Fig. 1a). Thirty-five percent of the reefs (Fig. 1b, e). We conclude that the overlap of disparate geographic bleaching at the scale of both individual reefs and the entire Great Barrier Reef (Fig. 1a, b). 134We found a similar strong relationship between the amount of bleaching measured 135 underwater, and the satellite-based estimates of heat exposure on individual reefs (Fig. 3). 136Low levels of bleaching was observed at some locations when DHW values were only 2-3 137 o C-weeks. Typically, 30-40% of corals bleached on reefs exposed to 4 o C-weeks, whereas an 138 average of 70-90% of corals bleached on reefs that experience 8 o C-weeks or more (Fig. 3). 139Resistance and adaptation to bleaching 140 Once we account for the amount of heat stress experienced on each reef, adding 141 chlorophyll-a, a proxy for water quality, to our statistical model yielded no support for the 142 hypothesis that good water quality confers resistance to bleaching 13 . Rather, the estimated 143 effect of chlorophyll-a was to significantly reduce the DHW threshold for bleaching 144 (Extended Data Table 1). However, despite the statistical significance, the effect in real terms 145 beyond heat stress alone is very small (Extended Data Fig. 1). Similarly, we found no effect 146 of the level of protection (in fished or protected zones) on bleaching (P > 0.1: Extended Data 147 Table 1). These results are consistent with the broad-scale pattern of severe bleaching in the 148 northern Great Barrier Reef, which affected hundreds of reefs across inshore-offshore 149 gradients in water quality, and regardless of their zoning (protection) status (Fig. 1a, b). 150Simila...

Tropical reef systems are transitioning to a new era in which the interval between recurrent bouts of coral bleaching is too short for a full recovery of mature assemblages. We analyzed bleaching records at 100 globally distributed reef locations from 1980 to 2016. The median return time between pairs of severe bleaching events has diminished steadily since 1980 and is now only 6 years. As global warming has progressed, tropical sea surface temperatures are warmer now during current La Niña conditions than they were during El Niño events three decades ago. Consequently, as we transition to the Anthropocene, coral bleaching is occurring more frequently in all El Niño-Southern Oscillation phases, increasing the likelihood of annual bleaching in the coming decades.

Global warming is rapidly emerging as a universal threat to ecological integrity and function, highlighting the urgent need for a better understanding of the impact of heat exposure on the resilience of ecosystems and the people who depend on them . Here we show that in the aftermath of the record-breaking marine heatwave on the Great Barrier Reef in 2016 , corals began to die immediately on reefs where the accumulated heat exposure exceeded a critical threshold of degree heating weeks, which was 3-4 °C-weeks. After eight months, an exposure of 6 °C-weeks or more drove an unprecedented, regional-scale shift in the composition of coral assemblages, reflecting markedly divergent responses to heat stress by different taxa. Fast-growing staghorn and tabular corals suffered a catastrophic die-off, transforming the three-dimensionality and ecological functioning of 29% of the 3,863 reefs comprising the world's largest coral reef system. Our study bridges the gap between the theory and practice of assessing the risk of ecosystem collapse, under the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems , by rigorously defining both the initial and collapsed states, identifying the major driver of change, and establishing quantitative collapse thresholds. The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems.

No abstract

A limited diversity of character states for reproductive traits and a robust phylogeny make scleractinian corals an ideal model organism with which to explore the evolution of life-history traits. Here, we explore systematic and biogeographical patterns in the reproductive biology of the Scleractinia within the context of a new molecular phylogeny and using reproductive traits from nearly 400 species. Our analyses confirm that coral sexuality is highly conserved, and mode of larval development is relatively plastic. An overabundance of species with autotrophic larvae in the eastern Pacific and Atlantic is most likely the result of increased capacity for long-distance dispersal conferred by vertical transmission of symbiotic zooxanthellae. Spawning records from diverse biogeographical regions indicate that multispecies spawning occurs in all speciose coral assemblages. A new quantitative index of spawning synchrony shows peaks at mid-tropical latitudes in the IndoPacific, influenced in part by two spawning seasons in many species on equatorial reefs.

Background Coral bleaching events vary in severity, however, to date, the hierarchy of susceptibility to bleaching among coral taxa has been consistent over a broad geographic range and among bleaching episodes. Here we examine the extent of spatial and temporal variation in thermal tolerance among scleractinian coral taxa and between locations during the 2010 thermally induced, large-scale bleaching event in South East Asia. Methodology/Principal Findings Surveys to estimate the bleaching and mortality indices of coral genera were carried out at three locations with contrasting thermal and bleaching histories. Despite the magnitude of thermal stress being similar among locations in 2010, there was a remarkable contrast in the patterns of bleaching susceptibility. Comparisons of bleaching susceptibility within coral taxa and among locations revealed no significant differences between locations with similar thermal histories, but significant differences between locations with contrasting thermal histories (Friedman = 34.97; p<0.001). Bleaching was much less severe at locations that bleached during 1998, that had greater historical temperature variability and lower rates of warming. Remarkably, Acropora and Pocillopora , taxa that are typically highly susceptible, although among the most susceptible in Pulau Weh (Sumatra, Indonesia) where respectively, 94% and 87% of colonies died, were among the least susceptible in Singapore, where only 5% and 12% of colonies died. Conclusions/Significance The pattern of susceptibility among coral genera documented here is unprecedented. A parsimonious explanation for these results is that coral populations that bleached during the last major warming event in 1998 have adapted and/or acclimatised to thermal stress. These data also lend support to the hypothesis that corals in regions subject to more variable temperature regimes are more resistant to thermal stress than those in less variable environments.

Coral reef recovery from major disturbance is hypothesized to depend on the arrival of propagules from nearby undisturbed reefs. Therefore, reefs isolated by distance or current patterns are thought to be highly vulnerable to catastrophic disturbance. We found that on an isolated reef system in north Western Australia, coral cover increased from 9% to 44% within 12 years of a coral bleaching event, despite a 94% reduction in larval supply for 6 years after the bleaching. The initial increase in coral cover was the result of high rates of growth and survival of remnant colonies, followed by a rapid increase in juvenile recruitment as colonies matured. We show that isolated reefs can recover from major disturbance, and that the benefits of their isolation from chronic anthropogenic pressures can outweigh the costs of limited connectivity.