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With coral cover declining on most tropical reefs, efforts are intensifying to understand the mechanisms by which environmental stressors are driving populations into demographic deficit. Here, the population dynamics of sexual cohorts of small corals (≤ 4 cm diameter) in St. John, US Virgin Islands were used to test the hypothesis that the association of vital rates with environmental conditions has changed over 22 yr. The annual fate of 3999 corals at six sites and 7--9 m depth were used to populate Leslie Matrices and calculate assemblage growth rates (λ, without recruitment), mortality of the smallest corals, and graduation from the largest size class. Vital rates were tested for associations with temperature, the number of days with seawater warmer than 29.4°C (i.e., hot days), and rainfall. Transition probabilities varied among years, and usually more corals remained in their original size class than moved among size classes. Corals in the smallest size class had the highest mortality rate (42% yr-1), λ had a grand mean of 0.688, and there were trends for λ to vary with the number of hot days and rainfall through quadratic relationships. The linear dependence of λ on rainfall over consecutive 3-yr periods declined over two decades, suggesting that this colony size-defined coral assemblage has undergone phenotypic change in their responsiveness to varying environmental conditions. [ABSTRACT FROM AUTHOR]

In this study, fore reef coral communities were exposed to high pCO2 for a year to explore the relationship between net accretion (G net) and community structure (planar area growth). Coral reef communities simulating the fore reef at 17-m depth on Mo'orea, French Polynesia, were assembled in three outdoor flumes (each 500 l) that were maintained at ambient (396 µatm), 782 µatm, and 1434 µatm pCO2, supplied with seawater at 300 l h−1, and exposed to light simulating 17-m depth. The communities were constructed using corals from the fore reef, and the responses of massive Porites spp., Acropora spp., and Pocillopora verrucosa were assessed through monthly measurements of G net and planar area. High pCO2 depressed G net but did not affect colony area by taxon, although the areas of Acropora spp. and P. verrucosa summed to cause multivariate community structure to differ among treatments. These results suggest that skeletal plasticity modulates the effects of reduced G net at high pCO2 on planar growth, at least over a year. The low sensitivity of the planar growth of fore reef corals to the effects of ocean acidification (OA) on net calcification supports the counterintuitive conclusion that coral community structure may not be strongly affected by OA. [ABSTRACT FROM AUTHOR]

Given the severe implications of climate change and ocean acidification (OA) for marine ecosystems, there is an urgent need to quantify ecosystem function in present‐day conditions to determine the impacts of future changes in environmental conditions. For tropical coral reefs that are acutely threatened by these effects, the metabolism of benthic communities provides several metrics suitable for this purpose, but the application of infrastructure to manipulate conditions and measure community responses is not fully realized. To date, most studies of the effects of OA on coral reefs have been conducted ex situ, and while greater ecological relevance can be achieved through free ocean carbon enrichment (FOCE) experiments on undisturbed areas of reef, such approaches have been deterred by technical challenges (e.g., spatial scale and duration, stable maintenance of conditions). In this study, we describe novel experimental infrastructure called shallow coral reef (SCoRe) FOCE to overcome these challenges and present data from a proof of concept application in Mo'orea, French Polynesia. Our objectives were to (1) implement an autonomous system that could be deployed kilometers from shore, (2) regulate the chemical (pCO2) and physical properties of seawater over undisturbed, shallow (∼2–5‐m depth) coral reef over multiple weeks, and (3) measure the metabolic response of the coral community to the treatment conditions. We describe the design, function, and application of the SCoRe FOCE, and present data demonstrating its efficacy. This infrastructure has great potential for advancing ecologically relevant studies of the effects of changing environmental conditions on coral reefs. [ABSTRACT FROM AUTHOR]

Invertebrate ectosymbionts within the coralla of scleractinians enhance host fitness through protection from corallivores and nutrient addition. Here, we explore the ectosymbiotic relationship between the coral Pocillopora verrucosa and the crab Trapezia serenei and the shrimp Alpheus spp., to test for effects on coral calcification under contrasts of seawater temperature (27.7 °C and 29.9 °C) and pH (ambient, 8.0 and reduced, 7.7). Regardless of temperature, ectosymbionts depressed calcification by 55% (vs without ectosymbionts) at ambient pH; however, ectosymbionts only depressed calcification under ambient pH but not at reduced pH. These results suggest that P. verrucosa grows fastest at ambient pH without ectosymbionts, but when ectosymbionts are present, colonies are protected from further declines in calcification at reduced pH. This implies that there may be a change from a currently parasitic ectosymbiont-coral relationship to a commensal relationship that could increase fitness advantages for corals hosting crustacean ectosymbionts under ocean acidification conditions. [ABSTRACT FROM AUTHOR]

Density dependence (DD) controls community recovery following widespread mortality, yet this principle rarely has been applied to coral assemblages. The reefs of Mo'orea, French Polynesia, provide the opportunity to study DD of coral population growth, because coral assemblages in this location responded to declines in abundance with high recruitment and an increase in cover during which recruitment of pocilloporid corals was inversely associated with density. This study tests for DD in this system, first, by describing the context within which it operates: coral cover changed from 46% in 2005, to <1% in 2010 following an outbreak of a corallivorous sea star and a cyclone, and then increased to 74% by 2017, in large part through inverse density‐associated pocilloporid recruitment. Second, a test for DD of recruitment was conducted by decreasing Pocillopora spp. cover from 33% to 19%: one year later, the density of Pocillopora spp. recruits was 1.65‐fold higher in the low vs. high cover treatment. Finally, the effects of DD were investigated by comparing simulated and empirical distributions of pocilloporid colonies: as predicted by DD, small colonies were randomly distributed, while large colonies were uniformly distributed. Together these results demonstrate DD of population regulation for Pocillopora spp. corals, thus revealing the potential importance of this ecological principle in determining the resilience of coral assemblages. [ABSTRACT FROM AUTHOR]

Human activities have led to widespread ecological decline; however, the severity of degradation is spatially heterogeneous due to some locations resisting, escaping, or rebounding from disturbances.We developed a framework for identifying oases within coral reef regions using long‐term monitoring data. We calculated standardised estimates of coral cover (z‐scores) to distinguish sites that deviated positively from regional means. We also used the coefficient of variation (CV) of coral cover to quantify how oases varied temporally, and to distinguish among types of oases. We estimated "coral calcification capacity" (CCC), a measure of the coral community's ability to produce calcium carbonate structures and tested for an association between this metric and z‐scores of coral cover.We illustrated our z‐score approach within a modelling framework by extracting z‐scores and CVs from simulated data based on four generalized trajectories of coral cover. We then applied the approach to time‐series data from long‐term reef monitoring programmes in four focal regions in the Pacific (the main Hawaiian Islands and Mo'orea, French Polynesia) and western Atlantic (the Florida Keys and St. John, US Virgin Islands). Among the 123 sites analysed, 38 had positive z‐scores for median coral cover and were categorised as oases.Synthesis and applications. Our framework provides ecosystem managers with a valuable tool for conservation by identifying "oases" within degraded areas. By evaluating trajectories of change in state (e.g., coral cover) among oases, our approach may help in identifying the mechanisms responsible for spatial variability in ecosystem condition. Increased mechanistic understanding can guide whether management of a particular location should emphasise protection, mitigation or restoration. Analysis of the empirical data suggest that the majority of our coral reef oases originated by either escaping or resisting disturbances, although some sites showed a high capacity for recovery, while others were candidates for restoration. Finally, our measure of reef condition (i.e., median z‐scores of coral cover) correlated positively with coral calcification capacity suggesting that our approach identified oases that are also exceptional for one critical component of ecological function. Our framework provides ecosystem managers with a valuable tool for conservation by identifying "oases" within degraded areas. By evaluating trajectories of change in state (e.g., coral cover) among oases, our approach may help in identifying the mechanisms responsible for spatial variability in ecosystem condition. Increased mechanistic understanding can guide whether management of a particular location should emphasise protection, mitigation or restoration. Analysis of the empirical data suggest that the majority of our coral reef oases originated by either escaping or resisting disturbances, although some sites showed a high capacity for recovery, while others were candidates for restoration. Finally, our measure of reef condition (i.e., median z‐scores of coral cover) correlated positively with coral calcification capacity suggesting that our approach identified oases that are also exceptional for one critical component of ecological function. [ABSTRACT FROM AUTHOR]

An important goal of coral reef science is to understand the roles played by environmental conditions in determining benthic community structure. Pursuit of this goal typically involves testing for associations between community structure and environmental conditions, and in recent years, attention has focused on temperature and seawater pH. Such analyses are helpful in projecting coral reef community structure into the future, but their efficacy depends on the extent to which changes in community structure are correctly associated with causative processes, and whether future conditions are a subset of those experienced in the past. Here, we describe variation from 2014 to 2017 in one environmental factor, photosynthetically active radiation (PAR), at 19-m depth on the reefs of St. John, US Virgin Islands, and evaluate the extent to which this factor could cause changes in benthic community structure. The transmission of downwelling surface PAR to 19-m depth (TPAR-19) varied from 2.0 to 40.0% among days, and from 4.8 to 15.8% among months within a year, and was lower and more variable in the autumn versus other seasons. Monthly TPAR-19 was inversely associated with monthly rainfall, suggesting that runoff affected seawater clarity. The monthly diffuse attenuation coefficient (Kd-PAR) varied 1.7-fold from 0.097 to 0.164 m−1 and was within the range expected for coral reefs. Given the high temporal variation in underwater light intensity, and its importance to foundational reef taxa, understanding of the response of coral reefs to temperature and pH (and other factors) will remain incomplete without closer attention to this physical environmental condition. [ABSTRACT FROM AUTHOR]

The transition of many Caribbean reefs from coral to macroalgal dominance has been a prominent issue in coral reef ecology for more than 20 years. Alternative stable state theory predicts that these changes are reversible but, to date, there is little indication of this having occurred. Here we present evidence of the initiation of such a reversal in Jamaica, where shallow reefs at five sites along 8 km of coastline now are characterized by a sea urchin-grazed zone with a mean width of 60 m. In comparison to the seaward algal zone, macroalgae are rare in the urchin zone, where the density of Diadema antillarum is 10 times higher and the density of juvenile corals is up to 11 times higher. These densities are close to those recorded in the late 1970s and early 1980s and are in striking contrast to the decade-long recruitment failure for both Diadema and scleractinians. If these trends continue and expand spatially, reefs throughout the Caribbean may again become dominated by corals and algal turf.

STEVEN V. VOLLMER [*] Abstract. Although most physiological traits scale allometrically in unitary organisms, it has been hypothesized that modularity allows for isometric scaling in colonial modular taxa. Isometry would [...]

The coral reef crisis has resulted in many reefs stabilizing in a low coral cover state, and this condition is exemplified by the fringing reefs of St. John, US Virgin Islands, where coral cover has remained low for decades. To evaluate the demographic features maintaining this condition, a decade of coral abundance was examined in three domains: recruits, small colonies (≤ 40-mm diameter), and cover. While cover was stable from 2007 to 2016, other aspects of coral community structure changed in taxonomically unique ways differing among domains. Most changes occurred gradually, although the warm year of 2010 was associated with perturbations in growth, mortality, and density that differed among domains and taxa, and summed to a weak signal distinguishing communities before and after ~ 2011/2012. As a result, coral community structure was taxonomically shuffled within each domain, while low cover was maintained. These low coral cover communities may be characterized by functional redundancy among taxa and a weakening of interactions among them that traditionally have supported emergent properties like community calcification. With persistent failure of recruitment to augment coral cover, some corals may already occur at such low abundances that they cannot perform the ecological services with which they were once associated. [ABSTRACT FROM AUTHOR]

Abstract: On Caribbean reefs, colonies of the hydrocoral Millepora alcicornis are capable of pursuing and overgrowing arborescent octocorals (Wahle ). Nearly four decades since these interactions were described, we quantified arborescent octocorals encrusted by Millepora spp. ( Linnaeus 1758) on shallow reefs in St. John, U.S. Virgin Islands, and evaluated encrusted colonies for evidence of an origin through pursuit. In 2014, 8% of octocorals (n=1684 colonies) were encrusted by Millepora spp., and in 2015, 12% were encrusted (n=847 colonies). Millepora spp. encrusted colonies of 10 octocoral genera, and in 2014, the most frequently encrusted were Eunicea spp. (40% of encrusted octocorals); in 2015, the most frequently encrusted were Gorgonia spp. (46% of encrusted colonies). In both years, ≥67% of encrusted octocorals were >21 cm from other colonies of Millepora spp.; 7% of octocorals were >1.3 m from colonies of Millepora spp. in 2014; and 16% of octocorals were >2.0 m from colonies of Millepora spp. in 2015. Relative to the ~5% of octocorals encrusted by Millepora spp. in Jamaica in the late 1970s, a high percentage (~9%) of octocorals were encrusted by Millepora spp. in St. John in 2014 and 2015. The large distances from most encrusted octocorals to the nearest colonies of Millepora spp. in St. John are inconsistent with a hypothesized origin through pursuit, contact, and overgrowth (sensu Wahle ). [ABSTRACT FROM AUTHOR]

For poikilothermic organisms in aquatic environment, temperature strongly affects metabolism and modulates the extent to which it is mass transfer limited. The effects of temperature on metabolism are translated into organism performance depending on the organism size, morphology, and the flow regime to which they are exposed. Environmental forcing of metabolism is particularly germane to tropical reef corals, because they increasingly are exposed to upward temperature perturbations to which they display variable responses that remain incompletely understood. In this study, we tested the effects of colony size and seawater turbulence on the response of the common Indo-Pacific branching coral, Pocillopora verrucosa, to different seawater temperatures. Using whole-colony calcification as a response variable, 12 tanks (each 150 l) were used in two trials lasting 14 days to contrast the effects of seawater turbulence (two levels) and temperature (25.5 vs 29.5 °C) on colonies varying in size from ~ 4 to 13-cm diameter. Turbulence in the tanks was measured as the root mean square (rms) turbulent flow speed ( q ), which quantified the variation in speed of complex and non-linear flow, regardless of direction. Treatments contrasted low ( q = 0.63 cm s) and high ( q = 2.07 cm s) turbulence that were ecologically relevant for shallow coral reefs. The effects of turbulent flow speed and temperature on calcification (mg day) differed among colony sizes: at 25.5 °C, calcification increased with tissue surface area at a slower rate under high, compared to low turbulent flow speeds; at low turbulent flow speeds, calcification increased with tissue surface area more slowly at 29.5 versus 25.5 °C. These results show that the response of branching corals to temperatures depends on colony size and turbulent flow speed. This outcome suggests that spatial heterogeneity in turbulent flow regimes across a coral reef may provide opportunities for branching corals to exploit colony size to modulate susceptibility to rising temperature. [ABSTRACT FROM AUTHOR]

Metabolic scaling is the relationship between organismal metabolic rate and body mass. Understanding the patterns and causes of metabolic scaling provides a powerful foundation for predicting biological processes at the level of individuals, populations, communities, and ecosystems. Despite intense interest in, and debate on, the mechanistic basis of metabolic scaling, relatively little attention has been paid to metabolic scaling in clonal animals with modular construction, such as colonial cnidarians, bryozoans, and colonial ascidians. Unlike unitary animals, modular animals are structural individuals subdivided into repeated morphological units, or modules, each able to acquire, process, and share resources. A modular design allows flexibility in organism size and shape with consequences for metabolic scaling. Furthermore, with careful consideration of the biology of modular animals, the size and shape of individual colonies can be experimentally manipulated to test competing theories pertaining to metabolic scaling. Here, we review metabolic scaling in modular animals and find that a wide range of scaling exponents, rather than a single value, has been reported for a variety of modular animals. We identify factors influencing variation in intraspecific scaling in this group that relate to the general observation that not all modules within a colony are identical. We highlight current gaps in our understanding of metabolic scaling in modular animals, and suggest future research directions, such as manipulating metabolic states and comparisons among species that differ in extent of module integration. [ABSTRACT FROM AUTHOR]

Since about 2000, octocorals on the shallow reefs of St. John, US Virgin Islands, have increased in abundance to form dense stands analogous to terrestrial forests. In this study, the ecology of octocoral forests was explored by testing for an effect of their canopies on the light regime and benthic invertebrate community in the understory habitat. Octocoral forests at five sites (7–13 m depth) were quantified on a spatial scale of meters by the height and density of octocoral colonies. Their effects on the understory habitat were evaluated through measurements of downwelling light, benthic invertebrate community structure, and the photophysiology (i.e., induction time) of the common coral, Porites astreoides (Lamarck, 1816). Octocoral forests composed of dense (> 5 octocorals m−2) and tall (mean height of 42 cm) octocoral colonies created an understory habitat characterized by flashes of bright light (i.e., sunflecking). Within the understory habitat at two of three study sites, the community structure of benthic invertebrates was modified by the octocoral canopy, but it remained unaffected at a third site. A test for the effects of sunflecking on the induction time of P. astreoides found no difference between colonies in the understory versus the open reef habitat. These results do not support the notion that the physiological phenotype of autotrophs in the understory habitat is modified to exploit sunflecking. The analysis of the benthic invertebrates in the understory habitat suggests that octocoral forests can change the community structure of this group of organisms, relative to the open reef. The causes of this change are unclear, but its absence at the most sheltered study site suggests that water motion may be a contributing factor. The strong likelihood that octocoral forests mediate ecologically significant changes in the community structure of invertebrates within their understory habitat warrants further investigation. • Octocoral forests support unique invertebrate communities. • Spectral analysis shows increased high frequency sunflecking within understories. • Octocorals function as ecosystem engineers by modifying light variability. • Photosynthetic induction by hard corals was unaffected by octocoral canopies. • Octocoral density and resilience will likely define future benthic communities. [ABSTRACT FROM AUTHOR]