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The worldwide decline of coral reefs has renewed interest in coral communities at the edge of environmental limits because they have the potential to serve as resilience hotspots and climate change refugia, and can provide insights into how coral reefs might function in future ocean conditions. These coral communities are often referred to as marginal or extreme but few definitions exist and usage of these terms has therefore been inconsistent. This creates significant challenges for categorising these often poorly studied communities and synthesising data across locations. Furthermore, this impedes our understanding of how coral communities can persist at the edge of their environmental limits and the lessons they provide for future coral reef survival. Here, we propose that marginal and extreme coral communities are related but distinct and provide a novel conceptual framework to redefine them. Specifically, we define coral reef extremeness solely based on environmental conditions (i.e., large deviations from optimal conditions in terms of mean and/or variance) and marginality solely based on ecological criteria (i.e., altered community composition and/or ecosystem functioning). This joint but independent assessment of environmental and ecological criteria is critical to avoid common pitfalls where coral communities existing outside the presumed optimal conditions for coral reef development are automatically considered inferior to coral reefs in more traditional settings. We further evaluate the differential potential of marginal and extreme coral communities to serve as natural laboratories, resilience hotspots and climate change refugia, and discuss strategies for their conservation and management as well as priorities for future research. Our new classification framework provides an important tool to improve our understanding of how corals can persist at the edge of their environmental limits and how we can leverage this knowledge to optimise strategies for coral reef conservation, restoration and management in a rapidly changing ocean.

Coral reefs are enduring decline due to the intensifying impacts of anthropogenic global change. This widespread decline has resulted in increased efforts to identify resilient coral populations and develop novel restoration strategies. Paramount in these efforts is the need to understand how environmental variation and thermal history affect coral physiology and resilience. Here, we assess the acclimatization capacity of Siderastrea siderea and Pseudodiploria strigosa corals via a 17-month reciprocal transplant experiment between nearshore and offshore reefs on the Belize Mesoamerican Barrier Reef System. These nearshore reefs are more turbid, eutrophic, warm, and thermally variable than offshore reefs. All corals exhibited some evidence of acclimatization after transplantation. Corals transplanted from nearshore to offshore calcified slower than in their native habitat, especially S. siderea corals which exhibited 60% mortality and little to no net growth over the duration of the 17-month study. Corals transplanted from offshore to nearshore calcified faster than in their native habitat with 96% survival. Higher host tissue δ15N in nearshore corals indicated that increased heterotrophic opportunity or nitrogen sources between nearshore and offshore reefs likely promoted elevated calcification rates nearshore and may facilitate adaptation in nearshore populations to such conditions over time. These results demonstrate that offshore populations of S. siderea and P. strigosa possess the acclimatization capacity to survive in warmer and more turbid nearshore conditions, but that local adaptation to native nearshore conditions may hinder the plasticity of nearshore populations, thereby limiting their utility in coral restoration activities outside of their native habitat in the short term.

Remote coral reefs are thought to be more resilient to climate change due to their isolation from local stressors like fishing and pollution. We tested this hypothesis by measuring the relationship between local human influence and coral community resilience. Surprisingly, we found no relationship between human influence and resistance to disturbance and some evidence that areas with greater human development may recover from disturbance faster than their more isolated counterparts. Our results suggest remote coral reefs are imperiled by climate change, like so many other geographically isolated ecosystems, and are unlikely to serve as effective biodiversity arks. Only drastic and rapid cuts in greenhouse gas emissions will ensure coral survival. Our results also indicate that some reefs close to large human populations were relatively resilient. Focusing research and conservation resources on these more accessible locations has the potential to provide new insights and maximize conservation outcomes.

Anthropogenic global change and local stressors are impacting coral growth and survival worldwide, altering the structure and function of coral reef ecosystems. Here, we show that skeletal extension rates of nearshore colonies of two abundant and widespread Caribbean corals (Siderastrea siderea, Pseudodiploria strigosa) declined across the Belize Mesoamerican Barrier Reef System (MBRS) over the past century, while offshore coral conspecifics exhibited relatively stable extension rates over the same temporal interval. This decline has caused nearshore coral extension rates to converge with those of their historically slower growing offshore coral counterparts. For both species, individual mass coral bleaching events were correlated with low rates of skeletal extension within specific reef environments, but no single bleaching event was correlated with low skeletal extension rates across all reef environments. We postulate that the decline in skeletal extension rates for nearshore corals is driven primarily by the combined effects of long-term ocean warming and increasing exposure to higher levels of land-based anthropogenic stressors, with acute thermally induced bleaching events playing a lesser role. If these declining trends in skeletal growth of nearshore S. siderea and P. strigosa continue into the future, the structure and function of these critical nearshore MBRS coral reef systems is likely to be severely impaired.

Ocean plastic pollution is a global problem that causes ecosystem degradation. Crucial knowledge gaps exist concerning patterns in microfiber abundance across regions and ecosystems, as well as the role of these pollutants within the environment. Here, we quantified the abundance of microfibers in coral samples collected from the Belize Mesoamerican Barrier Reef System (MBRS) using a polarized light microscope and identified a subsample of these to the polymer level using an Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy microscope. Microfibers were found in all coral samples with rayon being identified as the most common microfiber, comprising 85% of quantified pollutants. We found a greater average abundance of microfibers in coral samples from the Sapodilla Cayes (296 ± SE 89) than in samples from the Drowned Cayes (75 ± SE 14), indicating spatial variation in microfiber abundance within coral tissue along the MBRS. These results demonstrate that corals on the Belize MBRS interact with microfibers and that microfiber abundance on reefs varies spatially due to point sources of pollution and local oceanography. As rayon from clothing typically enters the ocean through wastewater effluent, alterations to waste water infrastructure may prove useful in decreasing rayon pollution in coastal waters.

Mass coral bleaching events are increasing in frequency and intensity and are predicted to occur annually in the coming decades. However, it remains poorly understood how quickly Caribbean corals can recover from bleaching. To explore the responses to heat stress and subsequent recovery in Caribbean corals, three species ( Porites divaricata , Porites astreoides , and Orbicella faveolata ) were experimentally bleached in outdoor flow-through tanks for 15 days then allowed to recover on the reef for 1.5 and 11 months. At each interval on the reef, endosymbiont concentrations, energy reserves (i.e., total soluble lipid, soluble animal carbohydrate, soluble animal protein), calcification, and stable carbon and nitrogen isotope values of the animal host (δ 13 C h , δ 15 N h ) and endosymbiotic algal fractions (δ 13 C e , δ 15 N e ) were measured in treatment and control fragments of each species. Despite variations in bleaching recovery strategies among the coral species, all corals recovered within one year. Specifically, bleached P. divaricata catabolized lipids and decreased calcification in response to lower endosymbiont concentrations. In contrast, both P. astreoides and O. faveolata maintained energy reserves despite lower endosymbiont concentrations, yet both decreased calcification rates after bleaching. Overall, these findings indicate that these corals are capable of surviving and recovering from a mild bleaching event within one year. Though these finding indicate that P. astreoides and O. faveolata may be resilient through single isolated bleaching events under annual bleaching, many Caribbean coral reefs may still experience a decline over the coming decades.

Two primary methods—the buoyant weight (BW) and alkalinity anomaly (AA) techniques—are currently used to quantify net calcification rates (G) in scleractinian corals. However, it remains unclear whether they are directly comparable since the few method comparisons conducted to date have produced inconsistent results. Further, such a comparison has not been made for tropical corals. We directly compared G BW and G AA in four tropical and one temperate coral species cultured under various pCO2, temperature, and nutrient conditions. A range of protocols for conducting alkalinity depletion incubations was assessed. For the tropical corals, open-top incubations with manual stirring produced G AA that were highly correlated with and not significantly different from G BW. Similarly, G AA of the temperate coral was not significantly different from G BW when incubations provided water motion using a pump, but were significantly lower than G BW by 16% when water motion was primarily created by aeration. This shows that the two techniques can produce comparable calcification rates in corals but only when alkalinity depletion incubations are conducted under specific conditions. General recommendations for incubation protocols are made, especially regarding adequate water motion and incubation times. Further, the re-analysis of published data highlights the importance of using appropriate regression statistics when both variables are random and measured with error. Overall, we recommend the AA technique for investigations of community and short-term day versus night calcification, and the BW technique to measure organism calcification rates integrated over longer timescales due to practical limitations of both methods. Our findings will facilitate the direct comparison of studies measuring coral calcification using either method and thus have important implications for the fields of ocean acidification research and coral biology in general.

Anthropogenic increases in atmospheric carbon dioxide concentration have caused global average sea surface temperature (SST) to increase by approximately 0.11°C per decade between 1971 and 2010 – a trend that is projected to continue through the 21st century. A multitude of research studies have demonstrated that increased SSTs compromise the coral holobiont (cnidarian host and its symbiotic algae) by reducing both host calcification and symbiont density, among other variables. However, we still do not fully understand the role of heterotrophy in the response of the coral holobiont to elevated temperature, particularly for temperate corals. Here, we conducted a pair of independent experiments to investigate the influence of heterotrophy on the response of the temperate scleractinian coral Oculina arbuscula to thermal stress. Colonies of O. arbuscula from Radio Island, North Carolina, were exposed to four feeding treatments (zero, low, moderate, and high concentrations of newly hatched Artemia sp. nauplii) across two independent temperature experiments (average annual SST (20°C) and average summer temperature (28°C) for the interval 2005–2012) to quantify the effects of heterotrophy on coral skeletal growth and symbiont density. Results suggest that heterotrophy mitigated both reduced skeletal growth and decreased symbiont density observed for unfed corals reared at 28°C. This study highlights the importance of heterotrophy in maintaining coral holobiont fitness under thermal stress and has important implications for the interpretation of coral response to climate change.

Reliably predicting how coral calcification may respond to ocean acidification and warming depends on our understanding of coral calcification mechanisms. However, the concentration and speciation of dissolved inorganic carbon (DIC) inside corals remain unclear, as only pH has been measured while a necessary second parameter to constrain carbonate chemistry has been missing. Here we report the first carbonate ion concentration ([CO32−]) measurements together with pH inside corals during the light period. We observe sharp increases in [CO32−] and pH from the gastric cavity to the calcifying fluid, confirming the existence of a proton (H+) pumping mechanism. We also show that corals can achieve a high aragonite saturation state (Ωarag) in the calcifying fluid by elevating pH while at the same time keeping [DIC] low. Such a mechanism may require less H+-pumping and energy for upregulating pH compared with the high [DIC] scenario and thus may allow corals to be more resistant to climate change related stressors., Predicting coral response to ocean acidification is dependent on our understanding of their internal carbonate chemistry. Here, using microelectrodes, the authors show that corals elevate pH and carbonate ion concentration in their calcifying fluid, but keep total dissolved inorganic carbon low.

Through the continuous growth of their carbonate skeletons, corals record invaluable information about past environmental conditions and their effect on colony fitness. Here, we characterize century-scale growth records of inner and outer reef corals across ~200 km of the Florida Keys Reef Tract (FKRT) using skeletal cores extracted from two ubiquitous reef-building species, Siderastrea siderea and Pseudodiploria strigosa. We find that corals across the FKRT have sustained extension and calcification rates over the past century but have experienced a long-term reduction in skeletal density, regardless of reef zone. Notably, P. strigosa colonies exhibit temporary reef zone-dependent reductions in extension rate corresponding to two known extreme temperature events in 1969-70 and 1997-98. We propose that the subtropical climate of the FKRT may buffer corals from chronic growth declines associated with climate warming, though the significant reduction in skeletal density may indicate underlying vulnerability to present and future trends in ocean acidification.

Through the continuous growth of their carbonate skeletons, corals record information about past environmental conditions and their effect on colony fitness. Here, we characterize century-scale growth records of inner and outer reef corals across ~200 km of the Florida Keys Reef Tract (FKRT) using skeletal cores extracted from two ubiquitous reef-building species, Siderastrea siderea and Pseudodiploria strigosa. We find that corals across the FKRT have sustained extension and calcification rates over the past century but have experienced a long-term reduction in skeletal density, regardless of reef zone. Notably, P. strigosa colonies exhibit temporary reef zone-dependent reductions in extension rate corresponding to two known extreme temperature events in 1969-1970 and 1997-1998. We propose that the subtropical climate of the FKRT may buffer corals from chronic growth declines associated with climate warming, though the significant reduction in skeletal density may indicate underlying vulnerability to present and future trends in ocean acidification.

Anthropogenic global change and local anthropogenic stressors are decreasing coral growth and survival globally, thus altering the structure and function of coral reef ecosystems. We show that skeletal extension rates of nearshore colonies of Siderastrea siderea and Pseudodiploria strigosa across the Belize Mesoamerican Barrier Reef System (MBRS) have declined at average rates of 0.01 and 0.08 mm/yr, respectively, over approximately the past century, while offshore conspecifics exhibited no significant trend in extension with time. This caused extension rates of nearshore colonies to converge with their historically slower-growing offshore conspecifics. Bleaching events negatively impacted extension rates in S. siderea but not in P. strigosa. The more negative trend in linear extension for nearshore versus offshore colonies may arise from ocean warming combined with stronger land-based anthropogenic stressors within nearshore environments. Extrapolating these trends in linear extension into the future suggests that nearshore P. strigosa and S. siderea will cease growing by years 2110 and 2370, respectively.

Annual coral bleaching events due to increasing sea surface temperatures are predicted to occur globally by the mid-century and as early as 2025 in the Caribbean, and severely impact coral reefs. We hypothesize that heterotrophic carbon (C) in the form of zooplankton and dissolved organic carbon (DOC) is a significant source of C to bleached corals. Thus, the ability to utilize multiple pools of fixed carbon and/or increase the amount of fixed carbon acquired from one or more pools of fixed carbon (defined here as heterotrophic plasticity) could underlie coral acclimatization and persistence under future ocean-warming scenarios. Here, three species of Caribbean coral—Porites divaricata, P. astreoides, and Orbicella faveolata—were experimentally bleached for 2.5 weeks in two successive years and allowed to recover in the field. Zooplankton feeding was assessed after single and repeat bleaching, while DOC fluxes and the contribution of DOC to the total C budget were determined after single bleaching, 11 months on the reef, and repeat bleaching. Zooplankton was a large C source for P. astreoides, but only following single bleaching. DOC was a source of C for single-bleached corals and accounted for 11–36 % of daily metabolic demand (CHARDOC), but represented a net loss of C in repeat-bleached corals. In repeat-bleached corals, DOC loss exacerbated the negative C budgets in all three species. Thus, the capacity for heterotrophic plasticity in corals is compromised under annual bleaching, and heterotrophic uptake of DOC and zooplankton does not mitigate C budget deficits in annually bleached corals. Overall, these findings suggest that some Caribbean corals may be more susceptible to repeat bleaching than to single bleaching due to a lack of heterotrophic plasticity, and coral persistence under increasing bleaching frequency may ultimately depend on other factors such as energy reserves and symbiont shuffling.

Increasing ocean acidification (OA) and seawater temperatures pose significant threats to coral reefs globally. While the combined impacts of OA and seawater temperature on coral biology and calcification in corals have received significant study, research to date has largely neglected the individual and combined effects of OA and seawater temperature on coral- mediated organic carbon (OC) fluxes. This is of particular concern as dissolved and particulate OC (DOC and POC, respectively) represent large pools of fixed OC on coral reefs. In the present study, coral-mediated POC and DOC, and the sum of these coral-mediated flux rates (total OC, TOC = DOC + POC) as well as the relative contributions of each to coral metabolic demand were determined for 2 species of coral, Acropora millepora and Turbinaria reniformis, at 2 levels of pCO2 (382 and 741 μatm) and seawater temperatures (26.5 and 31.0°C). Independent of tempera- ture, DOC fluxes decreased significantly with increases in pCO2 in both species, resulting in more DOC being retained by the corals and only representing between 19 and 6% of TOC fluxes for A. millepora and T. reniformis. At the same time, POC and TOC fluxes were unaffected by elevated temperature and/or pCO2. These findings add to a growing body of evidence that certain species of coral may be less at risk to the impacts of OA and temperature than previously thought.

The authors regret that acknowledgment for Dr. Adrian Marchetti was omitted from the manuscript. The correct acknowledgment is written below.

Reef-building corals maintain a symbiotic relationship with dinoflagellate algae of the genus Symbiodinium, and this symbiosis is vital for the survival of the coral holobiont. Symbiodinium community composition within the coral host has been shown to influence a coral's ability to resist and recover from stress. A multitude of stressors including ocean warming, ocean acidification, and eutrophication have been linked to global scale decline in coral health and cover in recent decades. Three distinct thermal regimes (highTP, modTP, and lowTP) following an inshore-offshore gradient of declining average temperatures and thermal variation were identified on the Belize Mesoamerican Barrier Reef System (MBRS). Quantitative metabarcoding of the ITS-2 locus was employed to investigate differences and similarities in Symbiodinium genetic diversity of the Caribbean corals Siderastrea siderea, S. radians, and Pseudodiploria strigosa between the three thermal regimes. A total of ten Symbiodinium lineages were identified across the three coral host species. S. siderea was associated with distinct Symbiodinium communities; however, Symbiodinium communities of its congener, S. radians and P. strigosa, were more similar to one another. Thermal regime played a role in defining Symbiodinium communities in S. siderea but not S. radians or P. strigosa. Against expectations, Symbiodinium trenchii, a symbiont known to confer thermal tolerance, was dominant only in S. siderea at one sampled offshore site and was rare inshore, suggesting that coral thermal tolerance in more thermally variable inshore habitats is achieved through alternative mechanisms. Overall, thermal parameters alone were likely not the only primary drivers of Symbiodinium community composition, suggesting that environmental variables unrelated to temperature (i.e., light availability or nutrients) may play key roles in structuring coral-algal communities in Belize and that the relative importance of these environmental variables may vary by coral host species.

Lipids are key biomolecules within the coral holobiont, serving as energy reserves and promoting bleaching tolerance. To better understand the physiological implications of bleaching and recovery on lipid acquisition and storage, an isotope pulse-chase labeling experiment was conducted on treatment (bleached) and control (non-bleached) Hawaiian corals Porites compressa and Montipora capitata after 1 and 11 months on the reef. After 1 month on the reef, 75% of the carbon in newly acquired lipids in treatment corals of both species was heterotrophic in origin, which is two times higher than that in control corals. However, stored lipids in both treatment and controls were hugely dominated by heterotrophic C in P. compressa (i.e., 75–100%) and by photoautotrophic C in M. capitata (i.e., 90–100%). After 11 months on the reef, elevated levels of heterotrophic C input in lipid acquisition and storage were only seen in treatment P. compressa . Many previously measured physiological parameters are recovered in P. compressa after 8 months, suggesting that the need for increased heterotrophic C input for lipids appears to last beyond recovery of all other parameters, indicating that P. compressa is not yet fully recovered even after 11 months. In M. capitata , lipid acquisition and storage were 28–52% heterotrophic in origin and 48–72% photoautotrophic and, along with many previously measured physiological parameters, were fully recovered after 11 months on the reef. However, the evidence suggests that carbohydrate and possibly protein acquisition was not fully recovered even after 11 months. These species-specific differences in lipid acquisition and storage mechanisms, and response patterns to temperature stress indicate that corals may take more than 11 months to recover from bleaching and that heterotrophic carbon is critical to coral lipids, especially when bleached.

Mass coral bleaching events caused by elevated seawater temperatures result in extensive coral loss throughout the tropics, and are projected to increase in frequency and severity. If bleaching becomes an annual event later in this century, more than 90% of coral reefs worldwide may be at risk of long-term degradation. While corals can recover from single isolated bleaching and can acclimate to recurring bleaching events that are separated by multiple years, it is currently unknown if and how they will survive and possibly acclimatize to annual coral bleaching. Here, we demonstrate for the first time that annual coral bleaching can dramatically alter thermal tolerance in Caribbean corals. We found that high coral energy reserves and changes in the dominant algal endosymbiont type (Symbiodinium spp.) facilitated rapid acclimation in Porites divaricata, whereas low energy reserves and a lack of algal phenotypic plasticity significantly increased susceptibility in Porites astreoides to bleaching the following year. Phenotypic plasticity in the dominant endosymbiont type of Orbicella faveolata did not prevent repeat bleaching, but may have facilitated rapid recovery. Thus, coral holobiont response to an isolated single bleaching event is not an accurate predictor of its response to bleaching the following year. Rather, the cumulative impact of annual coral bleaching can turn some coral species 'winners' into 'losers', and can also facilitate acclimation and turn some coral species 'losers' into 'winners'. Overall, these findings indicate that cumulative impact of annual coral bleaching could result in some species becoming increasingly susceptible to bleaching and face a long-term decline, while phenotypically plastic coral species will acclimatize and persist. Thus, annual coral bleaching and recovery could contribute to the selective loss of coral diversity as well as the overall decline of coral reefs in the Caribbean.

The dinoflagellate Symbiodinium trenchii associates with a wide array of host corals throughout the world, and its thermal tolerance has made it of particular interest within the context of reef coral resilience to a warming climate. However, future reefs are increasingly likely to face combined environmental stressors, further complicating our understanding of how S. trenchii will possibly acclimatize to future climate scenarios. Over a 33-day period, we characterized the individual and combined affects of high temperature (26.5 vs. 31.5 °C), pCO2 (400 vs. 760 µatm), and elevated nutrients (0.4 and 0.2 vs. 3.5 and 0.3 µmol of NO3/NO2 and $${\text{PO}}_{4}^{3 - }$$ , respectively) on S. trenchii within the host coral species Turbinaria reniformis. Global analysis across all treatments found temperature to be the largest driver of physiological change. However, exposure to elevated temperature led to changes in symbiont physiology that differed across pCO2 concentrations. Net photosynthesis and cellular chlorophyll a increased with temperature under ambient pCO2, whereas temperature-related differences in cellular volume and its affect on pigment packaging were more pronounced under elevated pCO2. Furthermore, increased nutrients mitigated the physiological response to high temperature under both ambient and elevated pCO2 conditions and represented a significant interaction between all three physical parameters. Individual responses to temperature and pCO2 were also observed as cellular density declined with elevated temperature and calcification along with respiration rates declined with increased pCO2. Symbiodinium trenchii remained the dominant symbiont population within the host across all treatment combinations. Our results reveal distinct physiological changes in response to high temperature within the S. trenchii/T. reniformis symbioses that are dependent on pCO2 and nutrient concentration, and represent important interactive effects to consider as we consider how corals will respond under future climate change scenarios.

Anthropogenic increases in atmospheric carbon dioxide concentration have caused global average sea surface temperature (SST) to increase by approximately 0.11°C per decade between 1971 and 2010 – a trend that is projected to continue through the 21st century. A multitude of research studies have demonstrated that increased SSTs compromise the coral holobiont (cnidarian host and its symbiotic algae) by reducing both host calcification and symbiont density, among other variables. However, we still do not fully understand the role of heterotrophy in the response of the coral holobiont to elevated temperature, particularly for temperate corals. Here, we conducted a pair of independent experiments to investigate the influence of heterotrophy on the response of the temperate scleractinian coral Oculina arbuscula to thermal stress. Colonies of O. arbuscula from Radio Island, North Carolina, were exposed to four feeding treatments (zero, low, moderate, and high concentrations of newly hatched Artemia sp. nauplii) across two independent temperature experiments (average annual SST (20°C) and average summer temperature (28°C) for the interval 2005–2012) to quantify the effects of heterotrophy on coral skeletal growth and symbiont density. Results suggest that heterotrophy mitigated both reduced skeletal growth and decreased symbiont density observed for unfed corals reared at 28°C. This study highlights the importance of heterotrophy in maintaining coral holobiont fitness under thermal stress and has important implications for the interpretation of coral response to climate change.

Coral reefs are increasingly threatened by global and local anthropogenic stressors such as rising seawater temperature, nutrient enrichment, sedimentation, and overfishing. Although many studies have investigated the impacts of local and global stressors on coral reefs, we still do not fully understand how these stressors influence coral community structure, particularly across environmental gradients on a reef system. Here, we investigate coral community composition across three different temperature and productivity regimes along a nearshore-offshore gradient on lagoonal reefs of the Belize Mesoamerican Barrier Reef System (MBRS). A novel metric was developed using ultra-high-resolution satellite-derived estimates of sea surface temperatures (SST) to classify reefs as exposed to low (lowTP), moderate (modTP), or high (highTP) temperature parameters over 10 years (2003 to 2012). Coral species richness, abundance, diversity, density, and percent cover were lower at highTP sites relative to lowTP and modTP sites, but these coral community traits did not differ significantly between lowTP and modTP sites. Analysis of coral life history strategies revealed that highTP sites were dominated by hardy stress tolerant and fast-growing weedy coral species, while lowTP and modTP sites consisted of competitive, generalist, weedy, and stress-tolerant coral species. Satellite-derived estimates of Chlorophyll-a (chl-a) were obtained for 13-years (2003-2015) as a proxy for primary production. Chl-a concentrations were highest at highTP sites, medial at modTP sites, and lowest at lowTP sites. Notably, thermal parameters correlated better with coral community traits between site types than productivity, suggesting that temperature (specifically number of days above the thermal bleaching threshold) played a greater role in defining coral community structure than productivity on the MBRS. Dominance of weedy and stress-tolerant genera at highTP sites suggests that corals utilizing these two life history strategies may be better suited to cope with warmer oceans and thus may warrant protective status under climate change.

The physiological response to individual and combined stressors of elevated temperature and pCO2 were measured over a 24-day period in four Pacific corals and their respective symbionts (Acropora millepora/Symbiodinium C21a, Pocillopora damicornis/Symbiodinium C1c-d-t, Montipora monasteriata/Symbiodinium C15 and Turbinaria reniformis/Symbiodinium trenchii). Multivariate analyses indicated that elevated temperature played a greater role in altering physiological response, with the greatest degree of change occurring within M. monasteriata and T. reniformis. Algal cellular volume, protein and lipid content all increased for M. monasteriata. Likewise, S. trenchii volume and protein content in T. reniformis also increased with temperature. Despite decreases in maximal photochemical efficiency, few changes in biochemical composition (i.e. lipids, proteins and carbohydrates) or cellular volume occurred at high temperature in the two thermally sensitive symbionts C21a and C1c-d-t. Intracellular carbonic anhydrase transcript abundance increased with temperature in A. millepora but not in P. damicornis, possibly reflecting differences in host mitigated carbon supply during thermal stress. Importantly, our results show that the host and symbiont response to climate change differs considerably across species and that greater physiological plasticity in response to elevated temperature may be an important strategy distinguishing thermally tolerant vs. thermally sensitive species.

Mass bleaching events are predicted to occur annually later this century. Nevertheless, it remains unknown whether corals will be able to recover between annual bleaching events. Using a combined tank and field experiment, we simulated annual bleaching by exposing three Caribbean coral species ( Porites divaricata , Porites astreoides and Orbicella faveolata ) to elevated temperatures for 2.5 weeks in 2 consecutive years. The impact of annual bleaching stress on chlorophyll a , energy reserves, calcification, and tissue C and N isotopes was assessed immediately after the second bleaching and after both short- and long-term recovery on the reef (1.5 and 11 months, respectively). While P. divaricata and O. faveolata were able to recover from repeat bleaching within 1 year, P. astreoides experienced cumulative damage that prevented full recovery within this time frame, suggesting that repeat bleaching had diminished its recovery capacity. Specifically, P. astreoides was not able to recover protein and carbohydrate concentrations. As energy reserves promote bleaching resistance, failure to recover from annual bleaching within 1 year will likely result in the future demise of heat-sensitive coral species.