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.

Mass coral bleaching events caused by elevated seawater temperatures result in extensive coral loss throughout the tropics, and are projected to increase in frequency and severity. If bleaching becomes an annual event later in this century, more than 90% of coral reefs worldwide may be at risk of long-term degradation. While corals can recover from single isolated bleaching and can acclimate to recurring bleaching events that are separated by multiple years, it is currently unknown if and how they will survive and possibly acclimatize to annual coral bleaching. Here, we demonstrate for the first time that annual coral bleaching can dramatically alter thermal tolerance in Caribbean corals. We found that high coral energy reserves and changes in the dominant algal endosymbiont type (Symbiodinium spp.) facilitated rapid acclimation in Porites divaricata, whereas low energy reserves and a lack of algal phenotypic plasticity significantly increased susceptibility in Porites astreoides to bleaching the following year. Phenotypic plasticity in the dominant endosymbiont type of Orbicella faveolata did not prevent repeat bleaching, but may have facilitated rapid recovery. Thus, coral holobiont response to an isolated single bleaching event is not an accurate predictor of its response to bleaching the following year. Rather, the cumulative impact of annual coral bleaching can turn some coral species 'winners' into 'losers', and can also facilitate acclimation and turn some coral species 'losers' into 'winners'. Overall, these findings indicate that cumulative impact of annual coral bleaching could result in some species becoming increasingly susceptible to bleaching and face a long-term decline, while phenotypically plastic coral species will acclimatize and persist. Thus, annual coral bleaching and recovery could contribute to the selective loss of coral diversity as well as the overall decline of coral reefs in the Caribbean.