Beyond Heat: Sediment and Wave Energy Influence Bleaching and Demography in the coral Acropora cervicornis

Isabela T. Cunio^1,2*Alex E. Mercado-Molina^2,3*

• ¹Universidad Ana G Mendez - Recinto de Gurabo, Gurabo, Puerto Rico
• ²Sociedad Ambiente Marino, San Juan, Puerto Rico
• ³University of Puerto Rico at Bayamón, Bayamón, United States

The global coral bleaching event of 2023, driven by record-breaking ocean temperatures, underscores the urgent need to identify factors shaping coral vulnerability and reef resilience. Although elevated temperature is the primary driver of mass bleaching, local environmental conditions can amplify or mitigate thermal stress. Here, we examined how site-specific variation in light, sedimentation, temperature, and wave energy influenced bleaching susceptibility and demographic performance of the threatened coral Acropora cervicornis. The study was conducted 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 assessed by visually classifying colonies as bleached or non-bleached and calculating prevalence and incidence across surveys. Survival was defined as the presence of live tissue, and growth was quantified using net total linear length change and specific growth rates. Bleaching began in mid-September 2023, peaked in November 2023, and affected 45.5% of colonies by the end of the study. Bayesian logistic regression models identified sediment load (β = 6.16; 95% CrI: 4.98–7.37) and wave energy (β = 2.09; 95% CrI: 1.61–2.59) as the strongest predictors of bleaching probability. By the end of the study, survival differed among sites, with 39% of colonies surviving at Maguey, 63% at Melones, and 67% at Tamarindo; Kaplan–Meier analyses confirmed significant spatial differences in survival. Notably, chi-squared tests showed no significant difference in survival between bleached and non-bleached colonies. Instead, Bayesian models indicated that sediment was the strongest predictor of survival (β = –0.65; 95% CrI: –1.16 to –0.15). Growth declined sharply during bleaching but partially recovered within three months and fully recovered six months post-event. Increased sediment (β = –0.340; 95% CrI: –0.400 to –0.270) and, to a lesser extent, wave energy (β = –0.006; 95% CrI: –0.010 to –0.002) significantly reduced growth. Overall, our results demonstrate that bleaching and its demographic consequences are not solely driven by thermal stress but are strongly modulated by local environmental conditions. Incorporating site-specific stressors into restoration and management strategies may enhance the resilience of restored A. cervicornis populations under increasing climate extremes.