Delayed bleaching-induced coral mortality limits reef recovery after a marine heatwave

Abstract

Coral reefs worldwide are increasingly exposed to marine heatwaves, which cause recurrent coral bleaching and mass mortality events. While the immediate impact of bleaching has been extensively studied, the longer-term consequences and recovery trajectories remain poorly understood. By integrating two long-term monitoring datasets, we quantified how fine-scale variation in heat stress exposure shaped post-bleaching trajectories. We examined patterns of coral mortality and recovery across 13 sites in Mo'orea (French Polynesia) after the most severe marine heatwave recorded on the island in 2019. The maximum accumulated heat stress (AHSmax) varied between sites (from 5.76 to 6.26 {degrees}C weeks), allowing the assessment of heat stress-related mortality between coral genera. Pocillopora mortality increased with AHSmax, and mean coral cover collapsed from 35% to 17% within one year. Acropora experienced consistently high mortality rates across all sites (median: 84%), reducing mean cover from 5% to 1%. Six years of tracking individual Pocillopora colonies revealed strong size-dependent bleaching responses. The heatwave increased the probability of complete and partial mortality more in large than in small Pocillopora colonies. Remarkably, elevated partial mortality persisted for at least three years after the thermal stress had subsided, indicating substantial delayed impacts. Contrary to the rapid recovery historically observed on Mo'orea's reefs following other disturbances, coral cover continued to decline for four years after the bleaching event, with no evidence of recovery. Our findings reveal that bleaching impacts extend far beyond the heatwave itself. Persistent delayed mortality of surviving colonies can limit reef recovery for years, indicating that post-bleaching demographic processes may be a major, yet underappreciated, determinant of coral reefs' resilience.