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Corals are a major habitat-building life-form on tropical reefs that support a quarter of all species in the ocean and provide ecosystem services to millions of people. Marine heat waves continue to threaten and shape reef ecosystems by killing individual coral colonies and reducing their diversity. However, marine heat waves are spatially and temporally heterogeneous, and so too are the environmental and biological factors mediating coral resilience during and following thermal events. This combination results in highly variable outcomes at both the coral bleaching and mortality stages of every event. This, in turn, impedes the assessment of changing reef-scale patterns of thermal tolerance or places of resistance known as reef refugia. We developed a largescale, high-resolution coral mortality monitoring capability based on airborne imaging spectroscopy and applied it to a major marine heat wave in the Hawaiian Islands. While water depth and thermal stress strongly mediated coral mortality, relative coral loss was also inversely correlated with preheat-wave coral cover, suggesting the existence of coral refugia. Subsequent mapping analyses indicated that potential reef refugia underwent up to 40% lower coral mortality compared with neighboring reefs, despite similar thermal stress. A combination of human and environmental factors, particularly coastal development and sedimentation levels, differentiated resilient reefs from other more vulnerable reefs. Our findings highlight the role that coral mortality mapping, rather than bleaching monitoring, can play for targeted conservation that protects more surviving corals in our changing climate.

Caribbean reef corals have experienced unprecedented declines from climate change, anthropogenic stressors and infectious diseases in recent decades. Since 2014, a highly lethal, new disease, called stony coral tissue loss disease, has impacted many reef-coral species in Florida. During the summer of 2018, we noticed an anomalously high disease prevalence affecting different coral species in the northern portion of the Mexican Caribbean. We assessed the severity of this outbreak in 2018/2019 using the AGRRA coral protocol to survey 82 reef sites across the Mexican Caribbean. Then, using a subset of 14 sites, we detailed information from before the outbreak (2016/2017) to explore the consequences of the disease on the condition and composition of coral communities. Our findings show that the disease outbreak has already spread across the entire region by affecting similar species (with similar disease patterns) to those previously described for Florida. However, we observed a great variability in prevalence and tissue mortality that was not attributable to any geographical gradient. Using long-term data, we determined that there is no evidence of such high coral disease prevalence anywhere in the region before 2018, which suggests that the entire Mexican Caribbean was afflicted by the disease within a few months. The analysis of sites that contained pre-outbreak information showed that this event considerably increased coral mortality and severely changed the structure of coral communities in the region. Given the high prevalence and lethality of this disease, and the high number of susceptible species, we encourage reef researchers, managers and stakeholders across the Western Atlantic to accord it the highest priority for the near future.

Ng, T.Y.; Chui, A.P.Y.; Tsang, R.H.L.; Yeung, C.W., and Ang, P., 2024. Hong Kong coral recruits do not gain advantage by being flexible in establishing symbiosis under simulated climate change conditions. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 936-940. Charlotte (North Carolina), ISSN 0749-0208. Symbiosis establishment in juvenile scleractinian corals appears to be flexible and is possibly affected by various environment factors. How the onset of coral-algal symbiosis could be affected by changes in environmental conditions due to global warming was examined in this study using recruits of the coral Acropora tumida from Hong Kong, a marginal environment for coral growth. Different Symbiodinium subclades C1, C15 and D8-12 were provided to A. tumida recruits under different temperature [24, 27 (ambient), 30, 32°C] and salinity [32 (ambient), 27, 22 psu] treatments in a factorial experimental design. Successful uptake of the symbionts by A. tumida recruits was assessed visually under a microscope 14, 21 and 31 days after the first symbiont inoculation. Infection percentage and differences in mortality between recruits infected by different symbiont types under different conditions were assessed. The results indicated that recruits of A. tumida could be infected by all three subclades. However, elevated temperature impaired infection by C1 and C15 but not D8-12. Lowered salinities showed limited effect. No mortality difference was found among recruits infected by different symbiont types. Coral recruits did not seem to gain any advantage by forming symbiosis with D8-12 symbiont, higher infection rate may be more related to the aggressive behavior of subclade D symbionts. However, subclade D symbionts are reported to be less photosynthetically efficient, leading to lower growth rate in coral juveniles. Increasing seawater temperature may therefore drive the coral recruits to form symbiosis with less favorable symbionts. The implications of this on the future of coral community should be more seriously considered.

Studying scleractinian coral bleaching and recovery dynamics in remote, isolated reef systems offers an opportunity to examine impacts of global reef stressors in the absence of local human threats. Reefs in the Chagos Archipelago, central Indian Ocean, suffered severe bleaching and mortality in 2015 following a 7.5 maximum degree heating weeks (DHWs) thermal anomaly, causing a 60% coral cover decrease from 30% cover in 2012 to 12% in April 2016. Mortality was taxon specific, with Porites becoming the dominant coral genus post-bleaching because of an 86% decline in Acropora from 14 to 2% cover. Spatial heterogeneity in Acropora mortality across the Archipelago was significantly negatively correlated with variation in DHWs and with chlorophyll-a concentrations. In 2016, a 17.6 maximum DHWs thermal anomaly caused further damage, with 68% of remaining corals bleaching in May 2016, and coral cover further declining by 29% at Peros Banhos Atoll (northern Chagos Archipelago) from 14% in March 2016 to 10% in April 2017. We therefore document back-to-back coral bleaching and mortality events for two successive years in the remote central Indian Ocean. Our results indicate lower coral mortality in 2016 than 2015 despite a more severe thermal anomaly event in 2016. This could be caused by increased thermal resistance and resilience within corals surviving the 2015 thermal anomaly; however, high bleaching prevalence in 2016 suggests there remained a high sensitivity to bleaching. Similar coral mortality and community change were seen in the Chagos Archipelago following the 1998 global bleaching event, from which recovery took 10 yr. This relatively rapid recovery suggests high reef resiliency and indicates that the Archipelago’s lack of local disturbances will increase the probability that the reefs will again recover over time. However, as the return time between thermal anomaly events becomes shorter, this ability to recover will become increasingly compromised.

Anthropogenic and natural disturbances have modified coral reef ecosystems over the last decades, ultimately, exerting negative impacts on the persistence of the carbonated matrix and the physical function. In 2014, the Caribbean region saw the onset of a new deadly coral disease, often known as the stony coral tissue loss disease. In summer of 2018, the outbreak was first reported in the Mexican Caribbean in a diving site named ‘Fish Market’. From July 2018 to March 2019, surveys were made to assess the impacts of this new outbreak in the stony coral community in terms of composition and the effect on the coral community calcification. This disease outbreak had severe consequences for this site. Several colonies of susceptible species were lost, which resulted in significant changes in the coral community composition and reductions in the gross carbonate production in a period of only 8 months.

Globally coral reefs have been declining at alarming rates as a result of anthropogenic stressors, leading to increased frequency and severity of widespread bleaching and disease events. These events are often associated with increased water temperatures due to climate change as well as regional and local stress from nutrient enrichment through runoff and sedimentation from coastal development. In late 2014, a white syndrome disease outbreak was reported off the coast of southeast Florida and was subsequently documented spreading throughout the region. This study examined the regional impacts of the disease event on the southeast Florida stony coral population utilizing stony coral demographic data from the Southeast Florida Coral Reef Evaluation and Monitoring Project (SECREMP). SECREMP is a long-term monitoring project examining 22 sites distributed from Miami-Dade County north to Martin County, Florida. The results revealed significant region-wide declines in stony coral diversity, density, and live tissue area corresponding with increased disease prevalence, which reached its maximum for the study period in 2016. Regional declines in coral density approached 30% loss and live tissue was upwards of 60% as a result of the disease outbreak. Additionally, multiple species were severely impacted, especially the reef building, complexity-contributing species Montastraea cavernosa, Meandrina meandrites, and Siderastrea siderea. The disease outbreak resulted in acute mortality and altered the ecosystem function to a point such that recovery is uncertain. This multiyear, region-wide disease outbreak has been indiscriminate relative to coral species impacted and was arguably the most devastating disturbance event documented on the Southeast Florida Reef Tract.

Knowledge of the degree to which corals undergo physiological acclimatization or genetic adaptation in response to changes in their thermal environment is crucial to the success of coral reef conservation strategies. The potential of corals to acclimatize to temperatures exceeding historical thermal regimes was investigated by reciprocal transplantation of Acropora millepora colonies between the warm central and cool southern regions of the Great Barrier Reef (GBR) for a duration of 14 months. Colony fragments retained at native sites remained healthy, whereas transplanted fragments, although healthy over initial months when temperatures remained within native thermal regimes, subsequently bleached and suffered mortality during seasonal temperature extremes. Corals hosting Symbiodinium D transplanted to the southern GBR bleached in winter and the majority suffered whole (40%; n = 20 colonies) or partial (50%) mortality at temperatures 1.1°C below their 15-year native minimum. In contrast, corals hosting Symbiodinium C2 transplanted to the central GBR bleached in summer and suffered whole (50%; n = 10 colonies) or partial (42%) mortality at temperatures 2.5°C above their 15-year native maximum. During summer bleaching, the dominant Symbiodinium type changed from C2 to D within corals transplanted to the central GBR. Corals transplanted to the cooler, southern GBR grew 74-80% slower than corals at their native site, and only 50% of surviving colonies reproduced, at least partially because of cold water bleaching of transplants. Despite the absence of any visual signs of stress, corals transplanted to the warmer, central GBR grew 52-59% more slowly than corals at their native site before the summer bleaching (i.e., from autumn to spring). Allocation of energy to initial acclimatization or reproduction may explain this pattern, as the majority (65%) of transplants reproduced one month earlier than portions of the same colonies retained at the southern native site. All parameters investigated (bleaching, mortality, Symbiodinium type fidelity, reproductive timing) demonstrated strong interactions between genotype and environment, indicating that the acclimatization potential of A. millepora populations may be limited by adaptation of the holobiont to native thermal regimes.

Coral cover is declining worldwide due to multiple interacting threats. We compared the effects of elevated nutrients and temperature on three Caribbean corals: Acropora cervicornis, Orbicella faveolata, and Siderastrea siderea. Colonies hosting different algal symbionts were exposed to either ambient nutrients (A), elevated NH4 (N), or elevated NH4 + PO4 (N + P) at control temperatures (26 °C) for > 2 months, followed by a 3-week thermal challenge (31.5 °C). A. cervicornis hosted Symbiodinium (S. fitti) and was highly susceptible to the combination of elevated nutrients and temperature. During heat stress, A. cervicornis pre-exposed to elevated nutrients experienced 84%–100% mortality and photochemical efficiency (Fv/Fm) declines of 41–50%. In comparison, no mortality and lower Fv/Fm declines (11–20%) occurred in A. cervicornis that were heat-stressed but not pre-exposed to nutrients. O. faveolata and S. siderea response to heat stress was determined by their algal symbiont community and was not affected by nutrients. O. faveolata predominantly hosted Durusdinium trenchii or Breviolum, but only corals hosting Breviolum were susceptible to heat, experiencing 100% mortality, regardless of nutrient treatment. S. siderea colonies predominantly hosted Cladocopium C1 (C. goreaui), Cladocopium C3, D. trenchii, or variable proportions of Cladocopium C1 and D. trenchii. This species was resilient to elevated nutrients and temperature, with no significant mortality in any of the treatments. However, during heat stress, S. siderea hosting Cladocopium C3 suffered higher reductions in Fv/Fm (41–56%) compared to S. siderea hosting Cladocopium C1 and D. trenchii (17–26% and 10–16%, respectively). These differences in holobiont susceptibility to elevated nutrients and heat may help explain historical declines in A. cervicornis starting decades earlier than other Caribbean corals. Our results suggest that tackling only warming temperatures may be insufficient to ensure the continued persistence of Caribbean corals, especially A. cervicornis. Reducing nutrient inputs to reefs may also be necessary for these iconic coral species to survive.

Coral reefs have undergone drastic declines due to anthropogenic and natural disturbances. In response, restoration efforts were developed to recover lost ecosystem services. Restoration in the Caribbean has focused almost exclusively on branching Acropora but declines of corals with massive morphologies highlight the need to develop a multi-species approach. Recent studies have reported high mortality rates (> 50%) of outplanted fragments of massive coral taxa because of predation by fish, creating a restoration bottleneck. We conducted targeted experiments aimed at mitigating predation and understanding factors driving fish predation impacts on fragments of Orbicella faveolata. Fragment mortality due to predation was extremely high (> 80% average across all experiments). By limiting physical access to newly outplanted corals, Acropora cervicornis proved to be an effective predator deterrent, providing a predation refuge for susceptible species like O. faveolata. Protecting outplanted corals using metal spikes and cages was effective at reducing predation impacts while protection remained in place, but benefits declined immediately after barrier removal, with predation causing an average of 84% mortality one week after removal. We identified a size threshold where larger colonies (25 cm2) are less susceptible to predation than smaller corals (5 cm2). Coral genotype influenced predation impacts, with the most susceptible coral genotype experiencing 86% mortality compared to the least susceptible genotype experiencing 29% mortality after 4 weeks. The genotype most impacted by predation had significantly higher lipid and protein content, suggesting prey selectivity by fish may be driven by coral tissue characteristics. Fish impacts were driven by consumption activities and not the removal or targeting of “novel” objects within parrotfish territories as dead coral controls were not impacted while adjacent live corals experienced 100% mortality. These results suggest that outplanting genotypically diverse assemblages of large, massive morphology fragments in combination with fast growing, branching species, and utilizing physical barriers will limit predation impacts and improve the efficiency of reef restoration activities.

Coral reefs are threatened by climate change on a global scale with thermal-stress events and mass coral bleaching being widely reported. The reefs off the east coast of Brazil (and other turbid areas) have, however, historically escaped such thermal-stress events, with relatively low levels of background coral mortality (5-10%). This has recently changed. Here we show that, in 2019, degree heating weeks (DHW) of 19.65 coincided with catastrophic declines in coral cover, especially in the major reef building hydrocoral Millepora alcicornis. The decline was due to bleaching associated with exposure to high temperature stress culminating in DHW values exceeding 15 for a period of 50 days. At two independent sites, surveys showed upwards of 83.5  9.0% and 89.1  3.9% mortality, and at a third site showed relatively lower (albeit still high), mortality rates of 43.3  12.0% were recorded. The mass die-off in 2019 is unprecedented in the South Atlantic reefs and coincides with increased heating events.