Changes in coral sensitivity to thermal anomalies

The 1998 and 2016 thermal anomalies were among the 2 most severe global-scale anomalies in recent history, with broad-scale impacts on reef condition. In 2 Kenyan fully protected national park reef lagoons, the water flow, light, and temperature exposure severity of these 2 events was grossly similar at 7.3 cm s(-1), similar to 50 Einsteins m(-2) d(-1) and similar to 85 degree-days above summer baseline. Yet, despite similarities in the coral communities' metrics over this time, the bleaching responses were diminished considerably across this 17 yr period. For example, the numbers of pale and bleached colonies declined from 73 to 27% and from 96 to 60% in the low and high thermal exposure reefs, respectively. A metric that weights bleaching by the intensity of the response and the number of individuals of each taxon also found a decline from 35 to 10% and from 65 to 33%. Of the 21 most common coral taxa, 11, including major contributors to coral cover such as Porites and Acropora, showed declines in their sensitivity. Ten taxa, including Montipora and Pocillopora, showed either little or weak evidence for change in sensitivity, and 1 taxon, Acanthastrea, was more sensitive to the exposure in 2016 than in 1998. Sampling limitations and qualitative differences in the pre-peak temperature conditions did not allow separating the influences of genetic adaptation, acclimatization, and community change.