After the mass bleaching events in 2016 and 2017 on the Great Barrier Reef, recruitment of new coral larvae fell by 89% according to a study published online in Nature this week. Given the projected increase in the occurrence of extreme climate events over the next two decades, the extent to which the Great Barrier Reef will be able to recover is unclear.

The Great Barrier Reef experienced mass bleaching events four times in the past 20 years and, under a business-as-usual scenario for greenhouse gas emissions, projections suggest that it will bleach twice a decade from 2035 and annually after 2044.

To investigate the capacity of the Great Barrier Reef to recover after extreme climate events, Terry Hughes and colleagues examined the relationships between the adult stock and larval recruitment of corals before and after the mass bleaching events in 2016 and 2017. The authors found that the loss of adult coral was associated with a collapse in larval recruitment and there was a shift in the composition of the coral species that were recruited. Brooding species of coral release fertilized larvae that typically settle within a day, whereas spawners release eggs and sperm that are externally fertilized and larval settlement takes place four to seven days later. In the depleted recruitment pools, they observed that locally sourced larval recruits of brooders became the dominant population rather than the more widely dispersed and diverse larvae of spawners.

The authors conclude that the shift in larval recruitment will influence the composition of recovering coral assemblages and could affect their ability to cope with bleaching events in the future.