Coral reef restoration has emerged as a central strategy to address the widespread degradation of reef ecosystems caused by a combination of anthropogenic (e.g., overfishing), biological (e.g., disease and predation), and physical (e.g., hurricanes, elevated seawater temperatures) stressors. Restoration initiatives aim to recover both the structural complexity and biological diversity of coral reef communities through interventions designed to enhance coral cover and population resilience. One widely adopted technique is human-assisted propagation, which leverages the natural asexual fragmentation ability of many coral species to produce clones for outplanting (Bowden-Kirby, 2001; Hernández-Delgado et al., 2014; Boström-Einarsson et al., 2020). In this approach, small coral fragments are cultivated in ex situ or in situ nurseries, where they can grow under reduced environmental stress and minimal predation or competition. Once sufficiently developed (i.e., reaching an ecologically relevant size), these fragments are outplanted onto degraded reefs to increase coral cover, promote habitat complexity, and support self-sustaining population growth. While this method has been implemented globally, its long-term ecological benefits and motivations have been critically examined. Some authors argue that, in some contexts, reef restoration efforts may be motivated less by demonstrated ecological outcomes and more by a desire to act-informed by emotional, socio-cultural, or ethical considerations-alongside scientific reasoning (Streit et al., 2024; Suggett et al., 2024).

The effectiveness of coral reef restoration lies in the scientific understanding and intentional, informed design of methods, grounded in knowledge of how corals grow, survive, and reproduce within their environments. In this sense, over the past decade, coral demographic studies have advanced substantially, highlighting the value of population-level data in developing more effective and context-sensitive restoration strategies. However, a major gap remains in our understanding of how coral outplants respond demographically to acute and chronic disturbances, particularly under varying site-specific environmental conditions. Environmental stressors such as light intensity (Torres et al., 2008), sedimentation (Fourney and Figueiredo, 2017), temperature anomalies (Hernández-Delgado et al., 2018), and hydrodynamic forces (Nakamura and van Woesik, 2001; Lentz et al., 2024) can strongly influence key demographic processes like survival and growth. Thus, understanding how these factors interact with stressful events is critical to assessing the long-term success of outplanting efforts.

In 2023, tropical and subtropical oceans experienced one of the most intense marine heatwaves on record, with Degree Heating Weeks (DHWs) exceeding 25 °C-weeks across parts of the Caribbean (Miller et al., 2024). DHW, a standard measure of accumulated thermal stress over a 12-week period, reached thresholds known to cause widespread coral bleaching and mortality (Liu et al., 2003). As a result, severe bleaching was reported across the Caribbean basin (Mejias-Rivera and Courtney, 2024; López-Pérez et al., 2024; Hernández-Delgado and Rodríguez-González, 2025), including Puerto Rican reef systems, where active coral reef restoration efforts using the staghorn coral Acropora cervicornis were underway. This extreme event provided an opportunity to evaluate the demographic consequences of bleaching in restored populations of this threatened coral. Although the demographic effects of bleaching on A. cervicornis have been previously studied (Drury et al., 2017), few investigations have examined how vital rates respond to bleaching across reefs with contrasting environmental conditions, particularly under natural, large-scale disturbance events, like marine heatwaves (Brown et al., 2023; Doropoulos et al., 2025).

Leveraging ongoing restoration activities across three reefs differing in light intensity, sedimentation, hydrodynamics, and temperature in Culebra, Puerto Rico, this study addressed three main questions through a focus on Acropora cervicornis, a keystone Caribbean coral whose rapid growth rate, complex morphological properties, demographic sensitivity to even modest variations in environmental conditions, and extensively explored demographic patterns make it particularly suitable for revealing links between performance outcomes and environmental variability. Specifically, we asked: (1) How does bleaching susceptibility vary among environmentally distinct reef sites? (2) How do key demographic responses, specifically survival and growth, differ following bleaching across the studied sites? (3) Which environmental variables best explain observed spatiotemporal patterns of bleaching and demographic performance? By doing so, this study aims to improve our understanding of how local reef conditions influence demographic resilience and to inform the design of more effective, site-adapted restoration strategies in the face of future climate-driven disturbances.

To develop effective strategies for managing and conserving natural resources, it is essential to understand how the living world responds to ongoing climate change. In the case of coral reef ecosystems, increased seawater temperature is considered one of the major disturbances shaping overall ecosystem dynamics, primarily due to its role in triggering coral bleaching (Lesser, 2024). While the relationship between elevated temperatures and coral bleaching is well-established, it is important to recognize that temperature does not act in isolation (Anthony et al., 2007). Other environmental factors, such as hydrodynamics (Anthony, 2007; Lentz et al., 2024), light intensity (Anthony et al., 2007; Torres et al., 2008), and sedimentation (Anthony et al., 2007; Zweifler, 2024), may also play significant roles, and their interactions could further complicate our understanding of coral population responses to stress. These factors can vary significantly across small spatial scales, underscoring the need for a more nuanced understanding of how site-specific environmental conditions shape coral health and bleaching susceptibility.

To address this, we monitored restored populations of the coral Acropora cervicornis across three reefs in Culebra, Puerto Rico, which differed in temperature, light intensity, sedimentation rates, and hydrodynamics during a widespread Caribbean bleaching event. Across all sites, approximately 45% of colonies bleached, reflecting the high susceptibility of A. cervicornis. However, patterns of bleaching and demographic responses were unexpected in several respects. First, bleaching susceptibility showed spatial variability among closely situated reefs, even though these differences were not statistically significant. Second, statistical analyses suggest that bleaching was not the primary driver of mortality. Third, coral demographics (i.e., survival and growth) were not negatively affected by increased seawater temperatures. Instead, site-specific sedimentation and wave energy conditions appeared to exert stronger effects on coral performance, emphasizing the need to incorporate local environmental drivers into assessments of bleaching dynamics and population resilience.

Not surprisingly, the incidence and prevalence of coral bleaching peaked during the warmest months of the year (September–October), following the pattern observed across the entire Caribbean through 2023. An intriguing finding of this study was the unexpectedly poor performance of outplants at the Maguey reef site. Maguey was characterized by lower temperatures and light intensity but higher wave energy, environmental conditions generally assumed to reduce bleaching risk; thermal stress is typically exacerbated by high irradiance (Brown, 1997; Torres et al., 2008) and mitigated by increased water movement (Nakamura et al., 2003; Finelli et al., 2006; Fifer et al., 2021). Despite these seemingly favorable conditions, colonies at Maguey exhibited the highest bleaching incidence, being approximately 1.15 times more likely to bleach than those at Melones and nearly 1.3 times more likely than those at Tamarindo (~15% and ~30% higher incidence, respectively), although these differences were not statistically significant. Given the well-documented sensitivity of Acropora cervicornis to environmental variation, even modest differences in local conditions may translate into disproportionate population-level effects.

The results of the Bayesian models indicate that sedimentation and wave energy, rather than temperature and light, were the primary predictors of coral bleaching. This finding complements previous work by Drury et al. (2017), who also found no significant relationship between temperature or light intensity and bleaching probability in A. cervicornis. Instead, sediment load emerged as a very strong positive predictor of bleaching probability, with a relatively large effect size and narrow credible interval, a result that is not traditionally emphasized in bleaching frameworks dominated by thermal stress. High sediment loads are typically associated with reduced light availability, which has often been assumed to mitigate bleaching risk by lowering photosynthetic stress (Fabricius, 2005, Zweifler, 2024). However, sediment exposure can impose multiple, non-mutually exclusive physiological costs that may instead increase bleaching susceptibility (Wooldridge, 2009; Zweifler, 2024). These include chronic tissue abrasion, increased energetic expenditure for mucus production and sediment rejection, as well as impaired gas exchange at the tissue-water interface, all of which can reduce the energetic capacity of corals to cope with thermal stress (Rogers, 1990; Erftemeijer et al., 2012; Junjie et al., 2014).

Importantly, the strong sediment effect observed in our study occurred in combination with elevated wave energy, suggesting that sediment stress may be amplified under high hydrodynamic conditions. Rather than settling passively, sediments in wave-exposed environments are frequently resuspended, increasing the frequency and intensity of physical contact with coral tissue (Storlazzi et al., 2004). This repeated disturbance may exacerbate oxidative stress and compromise coral internal physiological balance, thereby lowering the threshold at which bleaching is triggered. Under such conditions, sediment may act not as a light-attenuating buffer, but as a chronic mechanical and metabolic stressor that predisposes colonies to bleaching.

While increased water movement is generally considered beneficial, as it may enhance circulation, dissipate heat, and reduce sediment and nutrient accumulation, our results showed the opposite pattern. When exposed to higher wave energy, colonies exhibited the highest bleaching susceptibility and lowest survival. This counterintuitive result aligns with observations by McClanahan et al. (2005) and Lentz et al. (2024), who also reported that strong hydrodynamic forces can exacerbate physiological stress. We therefore hypothesize that, under stressful conditions, mechanical stress from wave action may directly contribute to both bleaching and mortality, potentially overriding the protective effects of otherwise favorable conditions.

Together, these results indicate that sedimentation should be considered an active contributor to bleaching risk, particularly in dynamic, wave-exposed reef environments. This finding challenges the assumption that reduced irradiance and increased wave motion necessarily confer protection against bleaching and highlights the need to account for interacting physical stressors when predicting coral responses to thermal extremes.

The immediate effect of coral bleaching is the loss of up to 95% of the energy typically provided to corals by photosynthetic microalgae (Muscatine et al., 1981). This energy deprivation generally leads to reduced survival in corals during the bleaching period. For example, in Jamaica, Quinn and Kojis (2008) reported that 90% of bleached A. cervicornis colonies died within four months, while Drury et al. (2017) through their work with A. cervicornis in the Florida Keys attributed a bleaching event to all subsequent observed mortality. These findings underscore the commonly assumed link between bleaching and mortality. In contrast, in this study it was found that the survival of bleached colonies was 42% at Maguey, 71% at Melones, and 65% at Tamarindo. The relatively high survival (compared to the previous studies) of bleached coral colonies is consistent with recent findings. For example, Page et al. (2023) reported that 94% of the colonies affected by bleaching in the Keppel Islands, Australia, during 2020 survived.

It could be argued that regional differences in survival patterns of bleached colonies may be linked to site-specific genetic pools (Lohr and Patterson, 2017). Nevertheless, the relationship between genotype and thermal tolerance, at least for A. cervicornis, remains debated as no clear pattern has been established (Drury et al., 2017; O’donnell et al., 2018). Given such genetic-specific uncertainty, it is plausible that the observed survival patterns in this study could be influenced by factors beyond genetics. Specifically, the colonies used in our study have survived major bleaching events and other environmental disturbances, such as hurricanes, which might have led to local acclimatization to withstand seawater temperatures (and other stressors) above the normal range (Fox et al., 2021).

Although coral coloration is a widely used indicator of bleaching and physiological stress, our results show that it is not always a reliable predictor of survival. Bleached colonies exhibited survival rates comparable to, and in some cases higher than (Melones and Maguey), those of visually healthy colonies. A similar contradiction between expected stress and demographic outcome was reported by Santiago-Padua et al. (2023) at Melones, where A. cervicornis outplants exposed to predation by corallivore fireworms, an interaction traditionally assumed to reduce demographic performance, paradoxically showed higher survival than non-predated colonies. Together, these findings suggest that stressors typically interpreted as detrimental based on appearance or conventional ecological assumptions do not necessarily translate into reduced survival. Instead, such stress signals may reflect prior exposure, selective filtering, or physiological conditioning that enhances resilience under subsequent disturbance.

It is plausible that non-bleached colonies also experience thermal stress but do not expel their symbionts and therefore do not exhibit visible signs of stress (i.e., bleaching), instead managing oxidative stress internally. Under the Oxidative Theory of Coral Bleaching, the expulsion of zooxanthellae can function as a protective mechanism, reducing reactive oxygen species (ROS) -induced cellular damage during periods of stress (Lesser, 2024). In contrast, colonies that retain their symbionts and remain unbleached may still experience comparable oxidative stress, but at levels just below the bleaching threshold, relying primarily on internal antioxidant defenses (Saragosti et al., 2010).

From this perspective, bleaching may represent one of several physiological strategies for coping with oxidative stress, rather than a direct determinant of survival probability. Alternatively, non-bleached A. cervicornis colonies may delay the activation of survival mechanisms by prioritizing growth under stressful conditions (see below). By the time severe stress is detected, physiological damage may already be irreversible, resulting in more rapid mortality compared with colonies that activated stress-response pathways earlier. Under this framework, colonies that appear visually healthy may, under certain conditions, be less stress-adapted than bleached colonies that have already reallocated energy toward survival.

The unexpected finding suggests two key possibilities: 1) A. cervicornis outplants may be reallocating their physiological energy toward survival strategies during stressful events (e.g., loss of the endosymbiotic algae), and 2) factors at the local scale may be more significant contributors to coral mortality than regional thermal anomalies. Supporting this last argument is the fact that the higher level of mortality recorded at Melones during the study occurred before the bleaching period (i.e. early September), coinciding with significant sedimentation when heavy rainfall caused localized excess sediment runoff from coastal construction. Survival rates increased again to previous levels following this event. In contrast, Maguey and Tamarindo demonstrated a gradual decline in survival rates over the course of the study year, consistent with expected outplant performance from general outplanting stress (Becker and Mueller, 2001; Schopmeyer et al., 2017; Fogarty, 2012).

Contrary to conventional expectations, higher wave energy was associated with lower survival of A. cervicornis colonies in this study. Wave action is often considered beneficial for corals, enhancing circulation, dissipating heat, reducing sediment accumulation, and supporting nutrient exchange (Nakamura et al., 2003; Finelli et al., 2006; Fifer et al., 2021). However, under certain conditions, strong wave action may impose significant mechanical stress, particularly on branching species. Repeated bending, breakage, and abrasion can compromise tissue integrity, reduce feeding efficiency, and divert energy from growth and reproduction toward repair (Okubo et al., 2007).

Additionally, high-hydrodynamic environments can exacerbate the impacts of other stressors. For example, resuspended sediments are more likely to scour coral surfaces under strong water movement, increasing oxidative stress and reducing the energetic capacity to cope with thermal anomalies (see above, Rogers, 1990; Fabricius, 2005; Weber et al., 2012). In this way, wave energy may shift from being a protective factor under moderate conditions to a net stressor when combined with environmental challenges, effectively lowering survival probability. These findings highlight that the ecological effects of hydrodynamics are context-dependent and may interact with local environmental and physiological factors to influence coral performance.

The theory of energy redistribution posits that an organism’s energetic budget is finite, and that investment in one demographic function, such as survival, necessarily limits investment in others, such as growth. This energetic trade-off proposal may be supported by the observed reduction in coral growth rates during the bleaching event across all three sites, suggesting a physiological shift from growth toward survival. Notably, the site with the highest survivorship, Tamarindo, exhibited the lowest overall growth rates, further reinforcing the idea of energy reallocation under stress. This aligns with findings by Bay and Palumbi (2017), who documented a fitness trade-off in corals: colonies with higher survivorship following transplantation generally exhibited slower growth, particularly in more environmentally stable locations. Likewise, in this study, unbleached coral colonies grew faster but exhibited lower survival probabilities than their bleached counter parts, which survived better. Although differences were not significant, the finding supports the negative relationship between growth and survival.

By the end of the study, 63% and 67% of corals survived at Melones and Tamarindo, respectively, despite being subject to bleaching. These rates are similar to those observed by Mercado-Molina et al. (2015a) at two reefs also located in Culebra with depth and topographic relief comparable to those of the present study sites. Such rates also fall within the range reported across the region (Becker and Mueller, 2001; Schopmeyer et al., 2017; Van Woesik et al., 2021). However, these broadly comparable survival rates mask spatial differences in demographic performance, as colony survival was significantly lower at Maguey. Spatial differences in the demographic performance of Acropora cervicornis (i.e., survival), including within Culebra, have also been reported by Mercado-Molina et al. (2015a, b). Together, these findings support the need for restoration strategies targeting A. cervicornis to be implemented across multiple locations, rather than concentrating all the effort into a single location. Likewise, careful examination of the prevailing environmental conditions across selected locations for restoration is essential.

Although bleaching was observed at all three sites, it did not emerge as the primary driver of mortality. Instead, our results suggest that environmental variability, particularly in sedimentation and hydrodynamics, better accounted for the observed patterns in A. cervicornis performance. These site-specific factors likely underlie the spatial variation in bleaching intensity, survival, and growth documented in this study.

Maguey Reef presents a paradox: despite environmental conditions typically associated with stress relief, i.e., cooler temperatures, reduced light, lower sedimentation, and higher water motion, it exhibited the highest bleaching and mortality. This unexpected pattern may reflect a “selective sieve” effect, in which reefs experiencing greater environmental variability filter out sensitive genotypes, thereby enhancing population resilience (Bay and Palumbi, 2017). Conversely, Maguey’s relative environmental stability could limit such selection, producing colonies less acclimated to temperature anomalies. This interpretation aligns with observations by Lentz et al. (2024) and McClanahan et al. (2005), who found that high-flow conditions can intensify bleaching despite thermal buffering. Moreover, bleaching under low light may increase reactive nitrogen species (RNS) production (Lesser, 2024), potentially explaining similar mortality between bleached and unbleached colonies. Finally, Maguey’s intermediate growth rate may reflect metabolic costs associated with high-flow, lower light conditions, which elevate dark respiration and particle feeding while limiting photosynthesis (Sebens et al., 2003).

Alternatively, Melones Reef was characterized by the highest levels and variability of temperature, light intensity, and sedimentation rate, coupled with the lowest wave energy. This site exhibited intermediate bleaching and survival, alongside rapid growth. According to the “selective sieve” hypothesis, environmental variability may enhance acclimation capacity, resulting in improved survival (Bay and Palumbi, 2017). However, excessive variability may exceed a “beneficial” stress threshold, leading to heightened stress, bleaching, and mortality, which could explain the intermediate bleaching and survival rates observed at this site. As noted above, the decrease in survival at Melones coincided with a specific sedimentation event prior to bleaching, triggered by intense rainfall and terrestrial runoff from adjacent coastal construction. This interpretation is supported by modeling results: the environment-only model (Model 3, Table 3) identified sediment load as a strong negative predictor of survival (estimate = –0.65, 95% CrI = –1.16 to –0.15), corresponding to an approximately 48% reduction in survival odds per unit increase in sediment load, highlighting sedimentation as a dominant driver of colony mortality.

In contrast, Tamarindo Reef exhibited the lowest bleaching prevalence, highest survival, and slowest growth rates. This site experienced moderate environmental conditions, including lower sediment loads and reduced wave energy compared to Melones, despite having the highest nutrient concentrations (Casariego-Rodríguez et al., unpublished data). Survival models indicated that both sediment load and wave energy were negatively associated with survival (Model 3, Table 3), suggesting that the absence of extreme physical stressors likely contributed to the favorable survival outcomes at Tamarindo. Although wave energy consistently showed a negative relationship with survival in our models, previous work suggests that intermediate flow regimes can enhance coral performance by balancing mass transfer and mechanical stress, potentially explaining the positive outcomes observed at this site (Lentz et al., 2024).

Considering the theory of energetic trade-offs between growth and survival, the relatively high survival at Tamarindo may reflect an allocation of energy toward maintenance and stress resistance rather than rapid growth. However, this mechanism does not act alone: at Melones, where growth rates were highest, survival was intermediate rather than lowest, illustrating the complex and context-dependent nature of A. cervicornis demographic responses. Moreover, the reduced growth at Tamarindo cannot be explained by bleaching alone, as this site also experienced the lowest bleaching prevalence, further decoupling bleaching intensity from patterns of growth and survival.

The slower growth rates at Tamarindo may also be linked to elevated nutrient concentrations, including phosphorus and nitrogen, which have been associated with suppressed calcification and skeletal extension in corals (Hallock and Schlager, 1986; Stambler et al., 1991; Ferrier-Pagès et al., 2000). We hypothesize that runoff from a nearby waste facility contributes to elevated nutrient levels at the reef. Future studies should include direct quantification and identification of nutrients through water column and sediment profiling, stable isotope tracing, and analysis of coral tissue and symbiont composition to further evaluate their influence on outplant performance.

Wave energy emerged as a particularly influential variable, both in isolation and in interaction with other environmental factors (Nakamura, 2010). Previous studies have shown that hydrodynamic regimes can drive fine-scale heterogeneity in bleaching outcomes across reefs separated by only a few kilometers (Nakamura and van Woesik, 2001). Nevertheless, hydrodynamic patterns, including wave energy, remain underreported in coral restoration and conservation research, with over 95% of bleaching studies ignoring water motion entirely (Grottoli et al., 2021; McLachlan et al., 2020). This oversight may reflect the inherently complex and interactive role of water movement in modulating temperature, light, sediment, oxygen, and nutrient dynamics-interactions that are beginning to be explored in this study.

Yamano et al. (2003) described hydrodynamic and other environmental parameters as a consequence of reef orientation on the leeward or windward sides of an island. Similar to their study, Maguey lies on the windward side, exposed to high wave energy and greater stress. Melones is situated on the leeward side, completely protected from prevailing winds by Luis Peña Cay. Leeward reefs, sheltered from open-ocean swells, may be subject to greater sedimentation (Yamano et al., 2003), a pattern consistent with observations at Melones. Tamarindo occupies a semi-exposed leeward orientation, which has been suggested to be optimal for coral survival. We have begun preliminary collaborations with a geological partner to explore high-resolution bathymetric data, paired with wind and current data, to further map the role of hydrodynamics in conjunction with reef structure in driving the observed reef-specific outcomes.

Overall, our results reveal a complex interplay among environmental variables, coral bleaching, survival, and growth. No single variable or mechanism can fully explain the observed demographic patterns across the three reef sites. These findings highlight the limitations of relying solely on simplified metrics, such as growth rate or bleaching intensity, to assess reef resilience or restoration success. While informative, such metrics do not capture the full complexity of coral physiological responses to increasingly variable environments. Instead, multiple physical and biological processes likely act in concert to determine coral survival and resilience under stress. Furthermore, genotypes that have persisted through prior bleaching events and chronic environmental stress may possess novel adaptive mechanisms, such as enhanced thermal tolerance, shifts in symbiont partnerships, or improved cellular stress responses, beyond those previously described.

Addressing gaps in our understanding of coral outplant demographics within their reef and ecosystem contexts is essential for effective restoration and conservation. By identifying key stressors and relief factors, as well as understanding how corals respond to them, it may be possible to manage systems in ways that promote outplant persistence. This study also underscores the importance of incorporating hydrodynamic patterns into coral restoration and conservation research. The limited understanding of how water movement influences coral demography and restoration outcomes may partly explain discrepancies observed in the existing literature.

Finally, it is worth emphasizing that coral reef field studies are inherently constrained by logistics and cost, with intensive repeated surveys of hundreds of colonies requiring a trade-off between spatial replication and both temporal and individual-level resolution. Our Bayesian mixed-effects models explicitly account for repeated measurements on the same colonies and allow uncertainty to be quantified through posterior distributions. However, inference is still limited by the structure of the data. In particular, the small number of study sites (n = 3) limits our ability to fully separate spatial from environmental effects, reduces statistical power for detecting site-level patterns, and constrains the generalizability of spatial and environmental relationships beyond the studied reefs. Although Bayesian models can improve parameter estimation under limited replication through partial pooling, they cannot compensate for the lack of true spatial replication. Consequently, results related to environmental drivers should be interpreted cautiously and within the ecological context of the study system.