Abiotic and biotic controls on coral recovery 16 years after mass bleaching

Abstract

As climate changes increase heat stress on tropical ecosystems, the long-term persistence of coral reefs requires rapid coral recovery following coral bleaching events. Using the extent of coral cover return to a pre-bleaching baseline as a benchmark, recovery of fast-growing and stress-tolerant coral growth forms suggests that reefs can bounce back between repeated disturbances if given adequate time and protection from anthropogenic disturbances.However, long-term recovery dynamics of coral communities following severe bleaching and mass mortality are limited, particularly for fringing reefs along inhabited coastlines where human stressors may compromise recovery potential. Here, we examine the dynamics and drivers of coral recovery in Seychelles, where 12 reefs returned to pre-bleaching coral cover levels after a severe bleaching event caused > 95% coral mortality. Six reefs with initially low cover (< 25%) recovered within 7–12 yr and, after 16 yr, exceeded pre-bleaching cover levels by 132–305%. In contrast, six reefs with initially high cover (20–60%) remained at 48–93% of pre-bleaching levels, with recovery projected to take 17–29 yr. Abiotic and historic conditions constrained recovery rates, with the slowest recovery times observed on deep and wave-exposed reefs with high pre-bleaching coral cover. Reefs with high juvenile coral densities and low nitrogen levels recovered fastest, possibly due to the interplay between nutrient enrichment, algal proliferation, and coral recruitment. Our findings emphasize the importance of understanding small-scale variation in recovery potential, whereby recovery times were governed by natural limits on growth rates and modified by coral recruitment and nutrient enrichment. Ultimately, climate-impacted reefs can recover to moderate coral cover levels, but, if bleaching causes repeated high coral mortality, short recovery windows will prevent a return to historic levels of coral dominance.