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Abstract Ocean warming is increasing the frequency, extent, and severity of tropical-coral bleaching and mortality. During 2014–2017, marine heatwaves caused the Third Global Coral Bleaching Event. We analyze data from 15,066 reef surveys globally during 2014–2017. Across all surveyed reefs, 80% and 35% experienced moderate or greater (affecting >10% of corals) bleaching and mortality, respectively. We assess the global extent of coral bleaching and mortality by applying bleaching response curves calibrated from surveyed reefs to predict bleaching globally, based on comprehensive remote-sensing of heat stress. These models predict that 51% and 15% of the world’s coral reefs suffered moderate or greater bleaching and mortality, respectively, during one or multiple years, surpassing damage from any prior global coral bleaching event. Our findings demonstrate that the impacts of ocean warming on coral reefs are accelerating, with the near certainty that ongoing warming will cause large-scale, possibly irreversible, degradation of these essential ecosystems. With heat stress levels during this event surpassing those observed previously, the National Oceanic and Atmospheric Administration developed more extreme Bleaching Alert levels that are now being used during the ongoing Fourth Global Coral Bleaching Event. 

Abstract Many reef‐building tropical corals are becoming rare. We considered the meaning of rarity in corals and highlighted taxa that have reached low abundances in the last few decades. The difficulties of quantifying rarity in the marine environment arise from the sheer scale and 3‐dimensional nature of the biome and the inherent challenges therein of ecological surveys with scuba. To meet the demands of coral conservation biology in the 21st century, we suggest that contemporary studies of coral communities will require enhanced capacity to identify species and a species‐specific focus on corals occurring at low abundances, which traditional ecological approaches to quantifying populations of benthic marine organisms have a limited capacity to address. Now is the time to revise scientific approaches to respond to the challenges posed by the need to understand and protect rare tropical corals. 

Abstract The utility of passive vs. active coral restoration continues to be debated as reefs decline worldwide. Here, we evaluated the efficacy of coral transplantation into the degraded East and Center lagoons of Palmyra Atoll. Corals have failed to recover in these sites over the eight decades since disturbances associated with World War II, despite high coral cover elsewhere around the atoll that could theoretically provide source propagules. We explicitly compared (i) species beginning to recolonize the lagoons with species common elsewhere at Palmyra, (ii) performance between the East and Center lagoons, and (iii) coral growth at sites near vs. far from causeway inlets as a proxy for the benefits of flow. We found that six common coral species were all physiologically capable of growing in the lagoon, but there were: i) large among-species difference in survival, ii) less, but still significant, differences in growth among species, and iii) localized differences in growth and survival across the eight test locations. Many of these differences appeared to be driven by patterns in fish predation on corals. Survival was greatest forPoritesandPavonaspecies, neither of which have substantially colonized the lagoons. Their superior performance relative toAcroporaandPocilloporaspecies that have begun to recolonize the East Lagoon suggests that transplantation of hardierPoritesandPavonaspecies may accelerate recovery. Coral reef restoration efforts often focus on more threatened and fragile corals likeAcroporaandPocillopora. Prioritizing the initial planting of hardier corals likePoritesandPavonamay help establish foundational reef functions before introducing more fragile species. 

ABSTRACT Identifying the drivers and timescales of population synchrony is critical for understanding metapopulation resilience. Using wavelet analyses of a 19‐year coral community timeseries from Moorea, French Polynesia, we quantified timescale‐specific population synchrony in four common coral genera and evaluated the predictors of spatial portfolio effects. We detected synchrony within genera associated with synchrony in degree heating days, diurnal temperature range (DTR) and macroalgal cover at different timescales. Synchrony in DTR and macroalgal cover was associated with lower synchrony ofPocilloporaandPorites populations, respectively. Population (for three of four genera) and environmental synchrony were stronger within than among habitats across timescales, underscoring the role of habitat‐specific conditions in driving spatial synchrony and spatial portfolios. These results describe how the spatial and temporal scales of heterogeneity in environmental and ecological conditions determine synchrony in coral population dynamics and support a spatial portfolio effect, which may buffer coral metapopulations from island‐scale collapse. 

Abstract Ecosystems around the world have experienced shifts in community state as a result of anthropogenic activities that alter important ecological feedbacks. On coral reefs, transitions to a state where fleshy macroalgae are a major space holder have often been associated with the loss of top‐down control, which frequently has been attributed to overharvesting of herbivorous fishes. However, nutrients could also facilitate the proliferation of macroalgae, but evidence for the role of this mechanism in state shifts on coral reefs is inconsistent. Here, we combine a common garden nutrient enrichment experiment with surveys of algal traits to assess potential mechanisms by which nutrient subsidies could trigger and/or maintain a regime shift to abundant macroalgae on reefs. We explored how nutrients influenced individual performance at three life stages of the frondose brown algaTurbinaria ornata, which is often a dominant fleshy macroalga on coral reefs throughout the Indo‐Pacific. Our enrichment experiment revealed that nutrients greatly increased growth and survival rates of youngTurbinariarecruits during this brief life stage when they are most vulnerable to herbivory. For the juvenile stage, we found strong correlations between nutrient availability and the expression of physical defenses that deter herbivory. Additionally, reproductive investment by adults in natural populations was positively correlated with nutrient availability. Taken together, our findings provide several mechanisms by which nutrients could facilitate a persistent shift from coral to fleshy macroalgae by allowing more vulnerable recruit and juvenile life stages to escape herbivore control and by enhancing the reproductive output of adults that produce germlings that recruit locally. These results lend novel insights into bottom‐up mechanisms by which increases in nutrients can bolster the establishment and subsequent replenishment of populations of fleshy macroalgae, which can inform management strategies for conserving or restoring coral reefs. 

Abstract On reefs, interaction between the flow and complex bottom topography results in drag forces on currents, dissipation of wave energy, and generation of turbulence. Here, field observations on a shallow backreef were used to investigate wave and current interactions with the bottom at scales of individual colonies across a coral reef patch. Wave direction was aligned with current direction, and the ratio of wave orbital velocities to current () was less than 0.5. The time‐averaged flow was a network of wakes behind colonies. Wake signatures were also observed for wave orbital velocities associated with longer period (13–32 s) waves but were absent for shorter period (3–5 s) waves. This pattern was explained by a modified Keulegan‐Carpenter number representing the ratio of wave period (T) to time scale for advection of water past an obstacle with length scaleLby the current (L/). Turbulent dissipation rates were elevated in obstacle wakes. For examples whereKC_c > 1, time‐averaged dissipation varied in proportion to the mean of the cubed total (wave plus current) velocity, consistent with parameterization as work done by a quadratic drag force that varied with incident velocity during the wave cycle. Bulk friction coefficients estimated from volume‐integrated dissipation in colony wakes together with topography measurements were similar to previous estimates from the reef‐scale momentum budget. These results illustrate that, although uncertainties are large, a quadratic drag law in conjunction with spatial averaging is a reasonable approach for scaling up colony to reef‐scale drag and dissipation. 

Abstract Persistent shifts to undesired ecological states, such as shifts from coral to macroalgae, are becoming more common. This highlights the need to understand processes that can help restore affected ecosystems. Herbivory on coral reefs is widely recognized as a key interaction that can keep macroalgae from outcompeting coral. Most attention has been on the role ‘grazing’ herbivores play in preventing the establishment of macroalgae, while less research has focused on the role of ‘browsers’ in extirpating macroalgae. Here we explored patterns, environmental correlates and state shift consequences of spatial co-variation in grazing and browsing functions of herbivorous fishes. Grazing and browsing rates were not highly correlated across 20 lagoon sites in Moorea, French Polynesia, but did cluster into 3 (of 4) combinations of high and low consumption rates (no site had low grazing but high browsing). Consumption rates were not correlated with grazer or browser fish biomass, but both were predicted by specific environmental variables. Experiments revealed that reversibility of a macroalgal state shift was strongly related to spatial variation in browsing intensity. Our findings provide insights and simple diagnostic tools regarding heterogeneity in top-down forcing that influences the vulnerability to and reversibility of shifts to macroalgae on coral reefs. 

Abstract Human activities drive changes in freshwater ecosystems by altering biogeochemical cycles. Freshwater networks provide important ecosystem services to human societies by purifying water and serving as an intermediary between terrestrial and marine systems. On high volcanic tropical islands, human activities are compartmentalized by steep terrain that delineates watershed boundaries. Patterns of land use affect adjacent stream ecosystems through runoff of sediment and nutrients, which fluctuates in the tropics as a result of seasonal rainfall. Here, we sought to reveal human impacts on nutrient and sediment regimes of tropical rivers by tracking patterns of river chemistry across a series of watersheds on Moorea, French Polynesia, between 2018 and 2019. Repeated sampling of rivers across a gradient of human activities revealed that water chemistry varied seasonally and with respect to rainfall and land use. In particular, dissolved inorganic nitrogen was more concentrated in rivers of watersheds with higher amounts of land clearing during the rainy season, and total suspended solids and phosphate were higher when recent rainfall was high. Importantly, the water quality of the rivers on Moorea repeatedly exceeded safe water quality standards established for similar high tropical islands in the Pacific. Our results show that differential land use across the landscape can have a substantial impact on the amounts of nutrients and sediment that tropical rivers transport, which on tropical islands could facilitate movement of materials from land to sea as precipitation increases with intensifying climate change. 

Abstract On tropical reefs, environmental conditions and biological interactions are fundamental drivers of the spatial distribution of corals, which in turn influences community attributes and ecosystem rate processes. Here we focus on a major habitat-providing species of coral, the thicket-forming staghornAcropora pulchra, in Moorea, French Polynesia, to explore environmental attributes that shape its local distribution. At the island scale,A. pulchradecreased in abundance with increasing distance from the shoreline and was inversely related to nitrogen enrichment. To investigate growth-predation risk tradeoffs, we quantified growth and corallivory rates across major abiotic (nutrients, sedimentation) and biotic (corallivorous fish biomass) gradients. At 20 sites divided between fringing reef and mid-lagoon habitats, we quantified colony growth of transplantedA. pulchrafragments (nubbins) that were either exposed or protected from predators after 83 days. Nubbins protected from corallivores grew more in the mid-lagoon than on the fringing reef, whereas mid-lagoon nubbins exposed to predators only achieved 30% of the growth of exposed fringing reef nubbins. These findings suggest that a growth-predation risk tradeoff exists forA. pulchra, with predation a major constraint on the local distribution ofA. pulchrathickets that hinders the ability of staghorn to proliferate further offshore from the fringing reef. 

ABSTRACT Globally, corals face an increased frequency of mass mortality events (MMEs) as populations experience repeated marine heatwaves which disrupt their obligate algal symbiosis. Despite greater occurrences of MMEs, the relative roles of the environment, host, and symbiont genetic variation in survival, subsequent recovery, and carry‐over effects to the next generation remain unresolved. High‐resolution temporal and spatial whole genome sequencing of corals before, after, and several years following an MME reveal that host genetics have an impact on bleaching and mortality and that selected alleles important for adaptation persist through the next generation, demonstrating rapid evolution in this coral population. Bleaching resistance and survival following the bleaching event were highly polygenic, and allele frequency shifts show reef habitat specificity, emphasizing the spatial complexity of environmental selection and how it shapes population recovery following an MME. This study reveals how MMEs reshape the genomic landscape and the spatial and temporal distribution of genomic diversity within coral populations facing severe threats from global change.