Beyond heat: sediment and wave energy influence bleaching and demography in the coral Acropora cervicornis

The global coral bleaching event of 2023, fueled by record-breaking ocean temperatures, underscores the urgent need to understand the factors shaping coral vulnerability and reef resilience. While elevated temperatures are widely recognized as the primary driver of mass bleaching, local environmental conditions are increasingly acknowledged for their potential to amplify or mitigate thermal stress. In this study, we examined how site-specific environmental variation in light, sedimentation, temperature, and wave energy influenced bleaching susceptibility and the demographic performance of the threatened coral Acropora cervicornis. The study was carried out from June 2023 to June 2024 in three restored populations (Maguey, Melones, and Tamarindo) along the western coast of Culebra, Puerto Rico. Bleaching susceptibility was quantified by scoring colonies as bleached or non-bleached based on visible paling or whitening of live tissues and calculating prevalence (proportion of colonies bleached at each time point) and incidence (new bleaching events between surveys). Survival was defined as the presence of live tissue on a colony whereas growth was assessed using change in net total linear length and specific growth rates as metrics. Bleaching onset occurred in mid-September 2023, peaking in November 2023, with 45.5% of colonies affected by the end of the study. Bayesian logistic regression models indicated that sediment (β = 6.16; 95% CrI: 4.98 to 7.37) and wave energy (β = 2.09; 95% CrI: 1.61 to 2.59) were the strongest predictors of bleaching probability. At the end of the study, 39% of the colonies survived at Maguey, 63% at Melones, and 67% at Tamarindo. Kaplan-Meier Survival analysis indicated that such spatial difference in survival was statistically different. Surprisingly, chi-squared tests indicated that survival did not differ between colonies that bleached and those that did not. Instead, the results of the Bayesian logistic regression indicated that sediment (β =–0.65; 95% CrI: –1.16 to –0.15) was the strongest predictor of colony survival. Growth declined sharply during bleaching but mostly recovered three months post-event. Bayesian Gaussian generalized linear mixed-effects model showed that increased sediment (β = –0.340; CrI: –0.400 to –0.270) and to lesser extend wave (β = –0.006; CrI: –0.010 to –0.002) led to reduced growth. These results highlight that coral bleaching and its demographic consequences are not solely dictated by temperature anomalies, but are strongly modulated by site-specific environmental stressors. Incorporating these localized dynamics into restoration and management planning could improve the resilience of restored A. cervicornis populations under increasing climate extremes.