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...

No abstract

Abiotic filtering is a major driver of gradients in the structure and functioning of ecosystems from the tropics to the poles. It is thus likely that environmental filtering is an important assembly process at the transition of biogeographical zones where many species occur at their range limits. Shifts in species abundances and association patterns along environmental gradients can be indicative of environmental filtering, which is predicted to be stronger in areas of high abiotic stress and to promote increased similarity of ecological characteristics among co-occurring species. Here we test these hypotheses for scleractinian corals along a broad latitudinal gradient in high-latitude eastern Australia, where corals occur at the margins of their ranges and environmental tolerances. We quantify variation in taxonomic, zoogeographic, and functional patterns combined with null model approaches and demonstrate systematic spatial variation in community structure and significant covariance of species abundance distributions and functional characteristics along the latitudinal gradient. We describe a strong biogeographic transition zone, consistent with patterns expected under abiotic filtering, whereby species are sorted along the latitudinal gradient according to their tolerances for marginal reef conditions. High-latitude coastal reefs are typified by widely distributed, generalist, stress-tolerant coral species with massive and horizontally spreading morphologies and by diminishing influence of tropical taxa at higher latitudes and closer to the mainland. Higher degree of ecological similarity among co-occurring species than expected by chance supports the environmental filtering hypothesis. Among individual traits, the structural traits corallite size and colony morphology were filtered most strongly, suggesting that characteristics linked to energy acquisition and physical stability may be particularly important for coral survival in high-latitude environments. These findings highlight interspecific differences and species interactions with the environment as key drivers of community organization in biogeographic transition zones and support the hypothesis that environmental filters play a stronger role than biotic interactions in structuring ecological communities in areas of high abiotic stress.