• Energy Research

Photoinhibition may constitute an energetic cost for photosynthetic organisms through damage to the photosynthetic apparatus, or by increased metabolism due to damage avoidance or repair. For several species of scleractinian corals, fluorescence techniques have revealed a significant reduction in photochemical efficiency of symbiotic dinoflagellates within coral tissue in response to excess light absorption. To date, it has been unclear whether or not photoinhibition has a negative impact on energy budgets in corals. We simultaneously quantified the effect of exposure to excessive light on net rates of photosynthesis and on fluorescence-derived photochemistry. We acclimated colonies of the reef-building coral Turbinaria mesenterina to 3 different irradiance regimes in the laboratory. The corals were then exposed to light levels up to 10 times higher than their acclimation irradiance and assayed for rates of photosynthesis and photochemical yields. Results indicated that daily costs of photoinhibition are negligible. Reduced net rates of photosynthesis in the afternoon, compared to the morning, were predominantly due to enhanced afternoon rates of dark respiration. However, photoacclimation to high light levels reduces daily energy acquisition in the long term, primarily due to decreased chlorophyll concentrations. Therefore, although changes in the photosynthetic activity of symbiotic dinoflagellates over a diurnal irradiance cycle do not cause a measurable decline in net oxygen evolution for coral colonies, repeated exposure to excessive irradiance can reduce energy acquisition per unit surface area, and hence influence the upper limit of the depth distribution of scleractinian corals.

Distributions of numerical abundance and resource use among species are fundamental aspects of community structure. Here we characterize these patterns for tropical reef fishes and corals across a 10,000-kilometer biodiversity gradient. Numerical abundance and resource-use distributions have similar shapes, but they emerge at markedly different scales. These results are consistent with a controversial null hypothesis regarding community structure, according to which abundance distributions arise from the interplay of multiple stochastic environmental and demographic factors. Our findings underscore the importance of robust conservation strategies that are appropriately scaled to the broad suite of environmental processes that help sustain biodiversity.

The physiological responses of organisms to resources and environmental conditions are important determinants of niche boundaries. In previous work, functional relationships between organism energetics and environment have been limited to energy intakes. However, energetic costs of maintenance may also depend on the supply of resources. In many mixotrophic organisms, two such resource types are light and particle concentration (turbidity). Using two coral species with contrasting abundances along light and turbidity gradients (Acropora valida and Turbinaria mesenterina), we incorporate the dual resource-stressor roles of these variables by calibrating functional responses of energy costs (respiration and loss of organic carbon) as well as energy intake (photosynthesis and particle feeding). This allows us to characterize physiological niche boundaries along light and turbidity gradients, identify species-specific differences in these boundaries, and assess the sensitivity of these differences to interspecific differences in particular functional response parameters. The turbidity-light niche of T. mesenterina was substantially larger than that of A. valida, consistent with its broader ecological distribution. As expected, the responses of photosynthesis, heterotrophic capacity, respiration, and organic carbon loss to light and turbidity varied between species. Niche boundaries were highly sensitive to the functional responses of energy costs to light and turbidity. Moreover, the study species' niche differences were almost entirely attributable to species-specific differences in one functional response: that of respiration to turbidity. These results demonstrate that functional responses of energy-loss processes are important determinants of species-specific physiological limits to growth, and thereby of niche differences in reef corals. Given that many resources can stress organisms when supply rates are high, we propose that the functional responses of energy losses will prove to be important determinants of niche differences in other systems as well.

AbstractConcern is growing about the potential effects of interacting multiple stressors, especially as the global climate changes. We provide a comprehensive review of multiple stressor interactions in coral reef ecosystems, which are widely considered to be one of the most sensitive ecosystems to global change. First, we synthesized coral reef studies that examined interactions of two or more stressors, highlighting stressor interactions (where one stressor directly influences another) and potentially synergistic effects on response variables (where two stressors interact to produce an effect that is greater than purely additive). For stressor‐stressor interactions, we found 176 studies that examined at least 2 of the 13 stressors of interest. Applying network analysis to analyze relationships between stressors, we found that pathogens were exacerbated by more costressors than any other stressor, with ca. 78% of studies reporting an enhancing effect by another stressor. Sedimentation, storms, and water temperature directly affected the largest number of other stressors. Pathogens, nutrients, and crown‐of‐thorns starfish were the most‐influenced stressors. We found 187 studies that examined the effects of two or more stressors on a third dependent variable. The interaction of irradiance and temperature on corals has been the subject of more research (62 studies, 33% of the total) than any other combination of stressors, with many studies reporting a synergistic effect on coral symbiont photosynthetic performance (n = 19). Second, we performed a quantitative meta‐analysis of existing literature on this most‐studied interaction (irradiance and temperature). We found that the mean effect size of combined treatments was statistically indistinguishable from a purely additive interaction, although it should be noted that the sample size was relatively small (n = 26). Overall, although in aggregate a large body of literature examines stressor effects on coral reefs and coral organisms, considerable gaps remain for numerous stressor interactions and effects, and insufficient quantitative evidence exists to suggest that the prevailing type of stressor interaction is synergistic.

Two facets of climate change--increased tropical storm intensity and ocean acidification--are expected to detrimentally affect reef-building organisms by increasing their mortality rates and decreasing their calcification rates. Our current understanding of these effects is largely based on individual organisms' short-term responses to experimental manipulations. However, predicting the ecologically-relevant effects of climate change requires understanding the long-term demographic implications of these organism-level responses. In this study, we investigate how storm intensity and calcification rate interact to affect population dynamics of the table coral Acropora hyacinthus, a dominant and geographically widespread ecosystem engineer on wave-exposed Indo-Pacific reefs. We develop a mechanistic framework based on the responses of individual-level demographic rates to changes in the physical and chemical environment, using a size-structured population model that enables us to rigorously incorporate uncertainty. We find that table coral populations are vulnerable to future collapse, placing in jeopardy many other reef organisms that are dependent upon them for shelter and food. Resistance to collapse is largely insensitive to predicted changes in storm intensity, but is highly dependent on the extent to which calcification influences both the mechanical properties of reef substrate and the colony-level trade-off between growth rate and skeletal strength. This study provides the first rigorous quantitative accounting of the demographic implications of the effects of ocean acidification and changes in storm intensity, and provides a template for further studies of climate-induced shifts in ecosystems, including coral reefs.

Predicting demographic rates is a critical part of forecasting the future of ecosystems under global change. Here, we test if growth rates can be predicted from morphological traits for a highly diverse group of colonial symbiotic organisms: scleractinian corals. We ask whether growth is isometric or allometric among corals, and whether most variation in coral growth rates occurs at the level of the species or morphological group. We estimate growth as change in planar area for 11 species, across five morphological groups and over 5 years. We show that coral growth rates are best predicted from colony size and morphology rather than species. Coral size follows a power scaling law with a constant exponent of 0.91. Despite being colonial organisms, corals have consistent allometric scaling in growth. This consistency simplifies the task of projecting community responses to disturbance and climate change.

AbstractAnthropogenic climate change is a rapidly intensifying selection pressure on biodiversity across the globe and, particularly, on the world's coral reefs. The rate of adaptation to climate change is proportional to the amount of phenotypic variation that can be inherited by subsequent generations (i.e., narrow‐sense heritability, h2). Thus, traits that have higher heritability (e.g., h2 > 0.5) are likely to adapt to future conditions faster than traits with lower heritability (e.g., h2 < 0.1). Here, we synthesize 95 heritability estimates across 19 species of reef‐building corals. Our meta‐analysis reveals low heritability (h2 < 0.25) of gene expression metrics, intermediate heritability (h2 = 0.25–0.50) of photochemistry, growth, and bleaching, and high heritability (h2 > 0.50) for metrics related to survival and immune responses. Some of these values are higher than typically observed in other taxa, such as survival and growth, while others were more comparable, such as gene expression and photochemistry. There was no detectable effect of temperature on heritability, but narrow‐sense heritability estimates were generally lower than broad‐sense estimates, indicative of significant non‐additive genetic variation across traits. Trait heritability also varied depending on coral life stage, with bleaching and growth in juveniles generally having lower heritability compared to bleaching and growth in larvae and adults. These differences may be the result of previous stabilizing selection on juveniles or may be due to constrained evolution resulting from genetic trade‐offs or genetic correlations between growth and thermotolerance. While we find no evidence that heritability decreases under temperature stress, explicit tests of the heritability of thermal tolerance itself—such as coral thermal reaction norm shape—are lacking. Nevertheless, our findings overall reveal high trait heritability for the majority of coral traits, suggesting corals may have a greater potential to adapt to climate change than has been assumed in recent evolutionary models.

Resumen El calentamiento del océano está aumentando la incidencia, la escala y la gravedad del blanqueamiento y la mortalidad de los corales a escala mundial, que culminó en el tercer evento mundial de blanqueamiento de corales que ocurrió durante las olas de calor marinas récord de 2014-2017. Si bien los efectos locales de estos eventos han sido ampliamente reportados, las implicaciones globales siguen siendo desconocidas. El análisis de 15.066 estudios de arrecifes durante 2014-2017 reveló que el 80% de los arrecifes estudiados experimentaron un blanqueamiento significativo de los corales y el 35% experimentó una mortalidad significativa de los corales. El alcance global del blanqueamiento y la mortalidad significativos de los corales se evaluó extrapolando los resultados de los estudios de arrecifes utilizando datos completos de teledetección del estrés por calor regional. Este modelo predijo que el 51% de los arrecifes de coral del mundo sufrieron un blanqueamiento significativo y una mortalidad significativa del 15%, superando el daño de cualquier evento de blanqueamiento global anterior. Estas observaciones demuestran que el daño generalizado del calentamiento global a los arrecifes de coral se está acelerando y subraya la amenaza que el cambio climático antropogénico representa para la transformación irreversible de estos ecosistemas esenciales. Résumé Le réchauffement des océans augmente l'incidence, l'ampleur et la gravité du blanchiment et de la mortalité des coraux à l'échelle mondiale, culminant avec le troisième événement mondial de blanchiment des coraux survenu lors de vagues de chaleur marines record de 2014-2017. Bien que les effets locaux de ces événements aient été largement rapportés, les implications mondiales restent inconnues. L'analyse de 15 066 enquêtes sur les récifs au cours de la période 2014-2017 a révélé que 80 % des récifs étudiés ont connu un blanchissement important des coraux et 35 % ont connu une mortalité importante des coraux. L'étendue mondiale du blanchiment et de la mortalité importants des coraux a été évaluée en extrapolant les résultats des enquêtes sur les récifs à l'aide de données complètes de télédétection du stress thermique régional. Ce modèle prévoyait que 51 % des récifs coralliens du monde souffraient d'un blanchissement important et 15 % d'une mortalité importante, dépassant les dommages causés par tout événement de blanchissement mondial antérieur. Ces observations démontrent que les dommages généralisés causés par le réchauffement climatique aux récifs coralliens s'accélèrent et soulignent la menace que le changement climatique anthropique fait peser sur la transformation irréversible de ces écosystèmes essentiels. Abstract Ocean warming is increasing the incidence, scale, and severity of global-scale coral bleaching and mortality, culminating in the third global coral bleaching event that occurred during record marine heatwaves of 2014-2017. While local effects of these events have been widely reported, the global implications remain unknown. Analysis of 15,066 reef surveys during 2014-2017 revealed that 80% of surveyed reefs experienced significant coral bleaching and 35% experienced significant coral mortality. The global extent of significant coral bleaching and mortality was assessed by extrapolating results from reef surveys using comprehensive remote-sensing data of regional heat stress. This model predicted that 51% of the world's coral reefs suffered significant bleaching and 15% significant mortality, surpassing damage from any prior global bleaching event. These observations demonstrate that global warming's widespread damage to coral reefs is accelerating and underscores the threat anthropogenic climate change poses for the irreversible transformation of these essential ecosystems. يؤدي ارتفاع درجة حرارة المحيطات إلى زيادة حدوث وحجم وشدة تبييض الشعاب المرجانية ونفوقها على نطاق عالمي، وبلغت ذروتها في الحدث العالمي الثالث لتبييض الشعاب المرجانية الذي حدث خلال موجات الحر البحرية القياسية في الفترة 2014-2017. في حين تم الإبلاغ عن الآثار المحلية لهذه الأحداث على نطاق واسع، إلا أن الآثار العالمية لا تزال غير معروفة. كشف تحليل 15,066 مسحًا للشعاب المرجانية خلال الفترة 2014-2017 أن 80 ٪ من الشعاب المرجانية التي تم مسحها عانت من ابيضاض مرجاني كبير و 35 ٪ عانت من وفيات مرجانية كبيرة. تم تقييم المدى العالمي للتبييض والوفيات المرجانية الكبيرة من خلال استقراء النتائج من المسوحات المرجانية باستخدام بيانات شاملة للاستشعار عن بعد للإجهاد الحراري الإقليمي. وتوقع هذا النموذج أن 51 ٪ من الشعاب المرجانية في العالم عانت من تبييض كبير و 15 ٪ من الوفيات الكبيرة، متجاوزة الأضرار الناجمة عن أي حدث تبييض عالمي سابق. تُظهر هذه الملاحظات أن الأضرار الواسعة النطاق للاحترار العالمي التي لحقت بالشعاب المرجانية تتسارع وتؤكد التهديد الذي يشكله تغير المناخ البشري المنشأ على التحول الذي لا رجعة فيه لهذه النظم الإيكولوجية الأساسية.