Showing papers in "Coral Reefs in 2019"

The 2014–2017 global-scale coral bleaching event: insights and impacts

Abstract: 2014–2017 was an unprecedented period of successive record-breaking hot years, which coincided with the most severe, widespread, and longest-lasting global-scale coral bleaching event ever recorded. The 2014–2017 global-scale coral bleaching event (GCBE) resulted in very high coral mortality on many reefs, rapid deterioration of reef structures, and far-reaching environmental impacts. Through the papers in this special issue of Coral Reefs entitled The 2014–2017 Global Coral Bleaching Event: Drivers, Impacts, and Lessons Learned, as well as papers published elsewhere, we have a good analysis of the 2014–2017 GCBE and its impacts. These studies have provided key insights into how climate change-driven marine heatwaves are destroying coral reef ecosystems: (a) The 2014–2017 GCBE is unique in the satellite record in its spatial scale, duration, intensity, and repetition of bleaching. (b) The impacts have been the most severe ever seen at many reefs. (c) Timing of observations matters and needs to be considered during the analysis of impacts. (d) On both global and local scales, the intensity of heat stress and impacts varied. (e) We continue to see important differences among and within coral taxa, with key roles played by algal symbionts and the microbiome. (f) Heat stress and bleaching both play a role in subsequent disease, which plays a key role in mortality. (g) Impacts ripple far beyond corals, with significant changes to the fish and invertebrate community that may last decades. (h) The structure of both individual coral’s skeletons and entire reefs has been eroded much more quickly than previously realized. (i) The 2014–2017 GCBE provided little support for the proposed “lifeboat” hypothesis, whereby deep or mesophotic reefs serve as a means of coral reef salvation. (j) While marine protected areas (MPAs) provide protection from local stressors, they not only do not protect reefs from global-scale stressors, but also here is also little evidence they provide significant resilience.

The relentless march of mass coral bleaching: a global perspective of changing heat stress

Abstract: The global coral bleaching event of 2014–2017 resulted from the latest in a series of heat stress events that have increased in intensity. We assessed global- and basin-scale variations in sea surface temperature-based heat stress products for 1985–2017 to provide the context for how heat stress during 2014–2017 compared with the past 3 decades. Previously, undefined “Heat Stress Year” periods (used to describe interannual variation in heat stress) were identified for the Northern and Southern Hemispheres, in which heat stress peaks during or shortly after the boreal and austral summers, respectively. The proportion of reef pixels experiencing bleaching-level heat stress increased through the record, accelerating during the last decade. This increase in accumulated heat stress at a bleaching level is a result of longer stress events rather than an increase in the peak stress intensity. Thresholds of heat stress extent for the three tropical ocean basins were established to designate “global” events, and a Global Bleaching Index was defined that relates heat stress extent to that observed in 1998. Notably, during the 2014–2017 global bleaching event, more than three times as many reefs were exposed to bleaching-level heat stress as in the 1998 global bleaching.

Using 3D photogrammetry from ROV video to quantify cold-water coral reef structural complexity and investigate its influence on biodiversity and community assemblage

Abstract: Fine-scale structural complexity created by reef-building coral in shallow-water environments is influential on biodiversity, species assemblage and functional trait expression. Cold-water coral reefs are also hotspots of biodiversity, often attributed to the hard surface and structural complexity provided by the coral. However, that complexity has seldom been quantified on a centimetric scale in cold-water coral reefs, unlike their shallow-water counterparts, and has therefore never been linked in a similar way to the reef inhabitant community. Structure from motion techniques which create high-resolution 3D models of habitats from sequences of photographs is being increasingly utilised, in tandem with 3D spatial analysis to create useful 3D metrics, such as rugosity. Here, we demonstrate the use of ROV video transect data for 3D reconstructions of cold-water coral reefs at depths of nearly 1000 m in the Explorer Canyon, a tributary of Whittard Canyon, NE Atlantic. We constructed 40 3D models of approximately 25-m-length video transects using Agisoft Photoscan software, resulting in sub-centimetre resolution reconstructions. Digital elevation models were utilised to derive rugosity metrics, and orthomosaics were used for coral coverage assessment. We found rugosity values comparable to shallow-water tropical coral reef rugosity. Reef and nearby non-reef communities differed in assemblage composition, which was driven by depth and rugosity. Species richness, epifauna abundance and fish abundance increased with structural complexity, being attributed to an increase in niches, food, shelter and alteration of physical water movement. Biodiversity plateaued at higher rugosity, illustrating the establishment of a specific reef community supported by more than 30% coral cover. The proportion of dead coral to live coral had limited influence on the community structure; instead, within-reef patterns were explained by depth and rugosity, though our results were confounded to a certain extent by multi-collinearity. Fine-scale structural complexity appeared to be integral to local-scale ecological patterns in cold-water coral reef communities.

Causes and consequences of the 2017 coral bleaching event in the southern Persian/Arabian Gulf

Abstract: Coral reefs of the Persian/Arabian Gulf were the last to succumb to the effects of the global-scale mass coral bleaching event that began in 2015. This study examines the causes and consequences of the 2017 bleaching event on eight reefs located across > 350 km of the southern basin of the Gulf. Using a combination of 5 yr (2013–2017) of reef-based temperature observations, local meteorological data and water column modeling, we show that 2017 was characterized by an extended period of mid-summer calm when winds rarely exceeded breeze conditions, reducing evaporative heat loss and inducing dramatic warming compared with non-bleaching years (2013–2016). Reef-bottom temperatures in the Gulf in 2017 were among the hottest on record, with mean daily maxima averaging 35.9 ± 0.1 °C across sites, with hourly temperatures reaching as high as 37.7 °C. Across the southern Gulf, corals spent nearly 2 months (mean 55.1 ± 3.9 d above bleaching temperatures and nearly 2 weeks above lethal temperatures (11.8 ± 2.4 d), substantially longer than in the non-bleaching years (2013–2016) and equating with 5.5 °C-weeks of thermal stress as degree heating weeks. As a result, 94.3% of corals bleached, and two-thirds of corals were lost to mortality between April and September 2017. Mortality continued after peak bleaching, and by April 2018 coral cover averaged just 7.5% across the southern basin, representing an overall loss of nearly three-quarters of coral (73%) in 1 yr. This mass mortality did not cause dramatic shifts in community composition as earlier bleaching events had removed most sensitive taxa. An exception was the already rare Acropora which were locally extirpated in summer 2017. Given the increasing frequency of mass bleaching in the Gulf and the above global rates of regional warming, the capacity for recovery and the prognosis for the future of Gulf reefs are not optimistic.

Oxygen: the universal currency on coral reefs

Abstract: Coral reefs are suffering unprecedented declines worldwide. Most studies focus on stressors such as rising temperatures, nutrient pollution, overfishing, and ocean acidification as drivers of this degradation. However, recent mass mortality events associated with low oxygen on coral reefs indicate that oxygen is a critical factor that can be limiting in reef environments. Here, we present evidence that integrates across disciplines and perspectives to reveal how natural and anthropogenic factors drive variation in oxygen at multiple scales on coral reefs. This variation, in turn, limits essential processes such as productivity, respiration, and calcification on reefs and often plays a role in the outcome of interactions between corals and their competitors, pathogens, and mutualists. Moreover, the apparent effects of temperature, eutrophication, acidification, and other stressors on corals are commonly mediated by oxygen. As a consequence, the imprint of oxygen variation is evident in many patterns including reef biodiversity, coral bleaching, colony morphology, and fish behavior. We suggest that the structure and dynamics of coral reefs can be fully understood only by considering the ubiquitous role of oxygen, and we identify critical areas of future oxygen research to guide the study and management of coral reefs in a changing world.

Coral bleaching impacts from back-to-back 2015–2016 thermal anomalies in the remote central Indian Ocean

Abstract: 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.

High-resolution habitat and bathymetry maps for 65,000 sq. km of Earth’s remotest coral reefs

Abstract: With compelling evidence that half the world’s coral reefs have been lost over the last four decades, there is urgent motivation to understand where reefs are located and their health. Without such basic baseline information, it is challenging to mount a response to the reef crisis on the global scale at which it is occurring. To combat this lack of baseline data, the Khaled bin Sultan Living Oceans Foundation embarked on a 10-yr survey of a broad selection of Earth’s remotest reef sites—the Global Reef Expedition. This paper focuses on one output of this expedition, which is meter-resolution seafloor habitat and bathymetry maps developed from DigitalGlobe satellite imagery and calibrated by field observations. Distributed on an equatorial transect across 11 countries, these maps cover 65,000 sq. km of shallow-water reef-dominated habitat. The study represents an order of magnitude greater area than has been mapped previously at high resolution. We present a workflow demonstrating that DigitalGlobe imagery can be processed to useful products for reef conservation at regional to global scale. We further emphasize that the performance of our mapping workflow does not deteriorate with increasing size of the site mapped. Whereas our workflow can produce regional-scale benthic habitat maps for the morphologically diverse reefs of the Pacific and Indian oceans, as well as the more depauperate reefs of the Atlantic, accuracies are substantially higher for the former than the latter. It is our hope that the map products delivered to the community by the Living Oceans Foundation will be utilized for conservation and act to catalyze new initiatives to chart the status of coral reefs globally.

Sustained mass coral bleaching (2016–2017) in Brazilian turbid-zone reefs: taxonomic, cross-shelf and habitat-related trends

Abstract: Between 2014 and 2017, an unprecedented heat stress accumulated and propagated across the tropical oceans and resulted in the so-called Third Global Bleaching Event (TGBE). Information about the effects of the TGBE in marginal coral reef provinces are still scarce, but can be relevant to understand the trajectories of coral reefs as climate changes intensify. Akin to deep mesophotic reefs and reefs in thermally stressed regions, low diversity, turbid-zone reefs may exhibit high bleaching tolerance due to local adaptations and conditions (e.g., shading by turbidity). Here, we summarize previous bleaching events in the tropical Western South Atlantic Ocean and explore taxonomic, cross-shelf and habitat-related bleaching trends in the Abrolhos reefs in February, May, June and October 2016, and March 2017. Fire corals (Millepora spp.) were the most affected, but all scleractinian species and several octocorals and zoanthids also bleached. Bleaching prevalence was higher in shallow coastal and offshore reef arcs than in deeper mesophotic reefs. All coral species bleached, but there were taxonomic and habitat-related trends in bleaching prevalence. Several species bleached less in the sites and habitats where their abundance was lower. As of March 2017, coral mortality was overall low across the region (< 3% of total coral cover). Our results add to the recent evidence that deep reefs provide partial refugia for a few coral species, and that turbid-zone reefs may be less susceptible to climate stress due to shading, higher heterotrophy levels, and local adaptations.

The state of Western Australia’s coral reefs

Abstract: Western Australia’s coral reefs have largely escaped the chronic pressures affecting other reefs around the world, but are regularly affected by seasonal storms and cyclones, and increasingly by heat stress and coral bleaching. Reef systems north of 18°S have been impacted by heat stress and coral bleaching during strong El Nino phases and those further south during strong La Nina phases. Cumulative heat stress and the extent of bleaching throughout the northern reefs in 2016 were higher than at any other time on record. To assess the changing regime of disturbance to reef systems across Western Australia (WA), we linked their site-specific exposure to damaging waves and heat stress since 1990 with mean changes in coral cover. Since 2010, there has been a noticeable increase in heat stress and coral bleaching across WA. Over half the reef systems have been severely impacted by coral bleaching since 2010, which was further compounded by cyclones at some reefs. For most (75%) reef systems with long-term data (5–26 yrs), mean coral cover is currently at (or near) the lowest on record and a full recovery is unlikely if disturbances continue to intensify with climate change. However, some reefs have not yet experienced severe bleaching and their coral cover has remained relatively stable or increased in recent years. Additionally, within all reef systems the condition of communities and their exposure to disturbances varied spatially. Identifying the communities least susceptible to future disturbances and linking them through networks of protected areas, based on patterns of larval connectivity, are important research and management priorities in coming years while the causes of climate change are addressed.

Boom and bust of keystone structure on coral reefs

Abstract: Repeated bouts of coral bleaching threaten the long-term persistence of coral reefs and associated communities. Here, we document the short- and long-term impacts of heatwave events on coral and fish assemblages, based on regular surveys of 18 reefs of the granitic islands of Seychelles over 23 yr. Extreme heat events in 1998 and 2016 led to bleaching-associated declines in coral cover, whilst between these years there was an interim period of coral recovery on some reefs. Coral decline and recovery were primarily due to changes in the cover of branching coral, particularly those from the families Acroporidae and Pocilloporidae. Surveys during the 2016 bleaching found that 95% of the 468 Acropora and Pocillopora colonies observed were either bleached or recently dead. The extent of bleaching and subsequent mortality were best explained by a priori assessments of community susceptibility to heat stress. One year later (2017), coral cover had fallen by 70% and average coverage across the 18 reefs was at 6%, similar to levels recorded in 2005, 7 yr after the 1998 bleaching. Decline in coral following the 2016 bleaching coincided with reduced abundance of fish < 11 cm TL, particularly corallivores, invertivores and mixed diet feeders. These changes are likely to foreshadow more widespread loss once the habitat structure erodes. Accordingly, 7 yr after the 1998 bleaching, when coral skeletons and reef structure had collapsed on some reefs, abundance of both large- and small-bodied fish had declined. We show that fluctuation in the cover of branching coral is positively associated with changes in the abundance of small-bodied fish which contribute to ecological processes and high diversity, suggesting branching corals are a keystone structure. Increased frequency of bleaching threatens the capacity of branching corals to fully recover after disturbances, reducing the amplitude of boom bust cycles of these corals and the keystone habitat structure they provide reef fish.

Photochemical response of the scleractinian coral Stylophora pistillata to some sunscreen ingredients

Abstract: Ultraviolet (UV) filters and preservatives, which are common constituents of sunscreens and other cosmetics, are reported as a threat for coastal coral reef ecosystems; however, few studies have assessed the effects of these compounds on coral health. This study presents the chronic effects (of measured, long-term and low concentrations) of some preservatives (ethylparaben, butylparaben), mineral UV filter (zinc oxide, ZnO) and organic UV filters (terephthalylidene dicamphor sulfonic acid, drometrizole trisiloxane, ethylhexyltriazone, butylmethoxydibenzoylmethane and 2-ethylhexyl 2-cyano-3,3-diphenylacrylate) on the maximal photosynthetic efficiency (Fv/Fm) of the symbionts associated with the scleractinian coral Stylophora pistillata. It first shows that for many organic filters, measured concentrations were significantly lower than nominal concentrations, due to the lipophilic nature of the compounds. In addition, the Fv/Fm was more sensitive to ZnO than all other sunscreen ingredients, with exposure to 90 µg L−1 ZnO for 35 d, reducing Fv/Fm by 38% compared with controls. The other UV filters tested showed no adverse effect on coral symbionts or animal tissue up to the concentration corresponding to their water solubility limit (and even above). Similarly, no adverse effect was observed in our conditions with the preservative ethylparaben, but the preservative butylparaben decreased the Fv/Fm by 25% at the highest concentration of 100 µg L−1. None of the sunscreen ingredients were as toxic to corals as the reference pollutants tributyltin, diuron and monuron, which significantly inhibited Fv/Fm at 10, 1 and 0.1 µg L−1, respectively. Overall, this study highlights the need to improve our knowledge on the in situ concentrations of UV filters and preservatives as well as their individual and combined effects on corals.

Physiological and reproductive repercussions of consecutive summer bleaching events of the threatened Caribbean coral Orbicella faveolata

Abstract: Thermal stress is a major contributor to loss of coral cover, significantly impacting reefs during the third global bleaching event between 2014 and 2017. The long-term persistence of coral reefs depends on acclimatization and adaptation to changing climate, which are influenced greatly by the interactions between bleaching and reproductive success. We observed a genotypically diverse population of Orbicella faveolata before, during, and after consecutive bleaching events in 2014 and 2015 in the Florida Keys. We documented less bleaching during the second event despite 40% more time above local bleaching thresholds and an association between bleaching severity and subsequent spawning. Approximately 75% of colonies experienced the same or less severe bleaching in the second event despite being metabolically compromised, with a substantial minority (~ 35%) faring better in the second event. The second bleaching event also resulted in smaller decreases in chlorophyll content per symbiont cell and symbiont-to-host cell ratio reef-wide, representing less damage to the coral–algal symbiosis. All colonies that recovered quickly (~ 1 month) or did not bleach in 2014 released gametes in 2015, while only 60% of colonies that recovered more slowly did. Bleaching also impacted the amount of gametes released, with more severe bleaching significantly associated with gamete release from < 50% of the colony surface area. Bleaching and spawning outcomes were supported by dynamic physiological changes during bleaching and recovery. Lipid concentration and symbiont-to-host cell ratios collected from the bottom edge of the colony in the middle of the recovery period (February and April) were most important for predicting spawning the following year, highlighting the dynamic interaction between micro-habitats and time in recovery and gametogenesis. This study finds signals of physiological acclimatization in an important reef-building coral and underscores the importance of recovery post-bleaching and reproduction for the persistence of coral reefs.

Back-to-back coral bleaching events on isolated atolls in the Coral Sea

Abstract: Severe bleaching events caused by marine heat waves over the past four decades have now affected almost every coral reef ecosystem in the world. These recurring events have led to major losses of coral cover, with adverse consequences for tropical reef ecosystems and the people who depend on them. Here, we document two consecutive and widespread coral bleaching events on remote atolls in the Coral Sea in 2016 and 2017. In each year, the proportion of colonies that bleached was strongly related to heat exposure (measured as degree heating weeks, DHW, °C-weeks), depth and coral assemblage structure. Bleaching was more severe at higher DHW exposure and at sites with a higher proportion of susceptible taxa. Bleaching was also lower at 6 m than at 2 m depth. Despite the severe bleaching in 2016 on reefs in the central section of the Coral Sea Marine Park, total coral cover was not significantly reduced by 2017, suggesting that most bleached corals survived. Moreover, bleaching was less severe in 2017 despite a higher exposure to heat stress. These results indicate that while the isolation of these oceanic reefs provides no refuge from bleaching, low nutrient levels, high wave energy and proximity to cooler deeper waters may make coral on these reefs more resistant to bleaching-induced mortality.

Lipidome analysis of Symbiodiniaceae reveals possible mechanisms of heat stress tolerance in reef coral symbionts

Abstract: Climate change-induced global warming threatens the survival of key ecosystems including shallow water coral reefs. Elevated temperatures can disrupt the normal physiological functioning of photosynthetic organisms by altering the fluidity and permeability of chloroplast membranes that is defined and regulated by their lipid composition. Since the habitat-forming reef corals rely on the obligatory symbiosis with dinoflagellates of the family Symbiodiniaceae, their heat stress response can be expected to be strongly influenced by the symbiont's lipid metabolism. However, in contrast to the steady increase in the knowledge of the functioning of coral symbionts at the genomic and transcriptomic level, the understanding of their membrane lipid composition and regulation in response to temperature stress is lagging behind. We have utilised mass spectrometry-based lipidomic analyses to identify the key polar lipids that form the biological membranes of reef coral symbionts, comparing the thermotolerant species Durusdinium trenchii with the thermosensitive taxon Cladocopium C3, both hosted by Acropora valida. Our results indicate that the superior thermotolerance D. trenchii inside the host corals could be achieved through (1) the amount and saturation of sulfoquinovosyldiacylglycerols, in particular through putative photosystem II interactions, (2) the increased digalactosyldiacylglycerol to monogalactosyldiacylglycerol ratio with the potential to stabilise thylakoid membranes and integrated proteins, and (3) the chaperone-like function of lyso-lipids. Thereby, our study provides novel insights into the heat tolerance of coral symbionts, contributing to the understanding of the potential of coral reef ecosystems to respond and adjust to heat stress events that are becoming more frequent due to climate change. Finally, our identification of multiple mechanisms of heat tolerance in Symbiodiniaceae furthers the knowledge of the general stress physiology of photosynthetic organisms.

Successive bleaching events cause mass coral mortality in Guam, Micronesia

Abstract: The reefs of Guam, a high island in the Western Pacific, were impacted by an unprecedented succession of extreme environmental events beginning in 2013. Elevated SSTs induced severe island-wide bleaching in 2013, 2014, 2016, and 2017. Additionally, a major ENSO event triggered extreme low tides beginning in 2014 and extending through 2015, causing additional coral mortality from subaerial exposure on shallow reef flat platforms. Here, we present the results of preliminary analyses of environmental and biological data collected during each of these events. Accumulated heat stress in 2013 was the highest since satellite measurements began, but this record was exceeded in 2017. Overall, live coral cover declined by 37% at shallow reef flat sites along the western coast, and by 34% at shallow seaward slope sites around the island. Staghorn Acropora communities lost an estimated 36% live coral cover by 2017. Shallow seaward slope communities along the eastern windward coast were particularly devastated, with an estimated 60% of live coral cover lost between 2013 and 2017. Preliminary evidence suggests that some coral species are at high risk of extirpation from Guam’s waters. In light of predictions of the near-future onset of severe annual bleaching, and the possibility that the events of 2013–2017 may signal the early arrival of these conditions, the persistence of Guam’s current reef assemblages is in question. Here, we present detailed documentation of ongoing changes to community structure and the status of vulnerable reef taxa, as well as a critical assessment of our response protocol, which evolved annually as bleaching events unfolded. Such documentation and analysis are critical to formulating effective management strategies for the conservation of remaining reef diversity and function.

Relative stability of the Pocillopora acuta microbiome throughout a thermal stress event

Abstract: Rapid climate change due to anthropogenic greenhouse gas emissions is pushing corals to and past their physiological limits, while their microbiome is being pressed towards dysbiosis. Microbes greatly influence the health and functioning of corals, but thermal anomalies that cause bleaching can affect certain taxa of the host-associated prokaryote and Symbiodiniaceae communities, leading corals towards a disease-prone state. In some coral species, however, even significant thermal stress may not result in visible signs of bleaching. Whether changes in the coral microbiome occur in these resilient species during temperature anomalies is not well described. In the present study, we tagged, visually assessed and sampled 10 colonies of the branching coral Pocillopora acuta on a fringing reef off Orpheus Island in the central Great Barrier Reef for 1 yr, of which the summer coincided with the 2016 mass-bleaching event. No visible signs of bleaching were observed in any of the 10 colonies throughout the study period, despite experiencing two degree heating weeks of thermal stress and observations of bleaching in other coral species on the same reef. Metabarcoding based on the Symbiodiniaceae ITS2 rDNA spacer and the bacterial 16S rRNA gene provided evidence for stability of the overall microbial community structure, although the bacterial community showed increases in a number of potentially beneficial taxa, such as diazotrophs, during the thermal stress event. These findings suggest some flexibility in the microbiome to adjust to higher than average temperatures without disrupting microbiome stability, perhaps contributing to the thermal resilience of P. acuta.

Quantifying coral morphology

Abstract: Coral morphology has important implications across scales, from differences in physiology, to the environments they are found, through to their role as ecosystem engineers. However, quantifying morphology across taxa is difficult, and so morphological variation is typically captured via coarse growth form categories (e.g. arborescent and massive). In this study, we develop an approach for quantifying coral morphology by identifying continuous three-dimensional shape variables. To do so, we contrast six variables estimated from 152 laser scans of coral colonies that ranged across seven growth form categories and three orders of magnitude of size. We found that 88% of the variation in shape was captured by two principal components. The main component was variation in volume compactness (cf. convexity), and the second component was a trade-off between surface complexity and top-heaviness. Variation in volume compactness also limited variation along the second axis, where surface complexity and top-heaviness ranged more freely when compactness was low. Traditional growth form categories occupied distinct regions within this morphospace; however, these regions overlapped due to scaling of shape variables with colony size. Nonetheless, with four of the shape variables we were able to predict traditional growth form categories with 70 to 95% accuracy, suggesting that the continuous variables captured most of the qualitative variations implied by these growth forms. Distilling coral morphology into continuous variables that capture shape variation will allow for better tests of the mechanisms that govern coral biology, ecology and ecosystem services such as reef building and provision of habitat.

Morphological and functional diversity of piscivorous fishes on coral reefs

Abstract: Piscivory is a significant ecosystem function on coral reefs, with up to 53% of species on reefs being regarded as piscivorous. Despite this ecological importance, the species that contribute to this function have not been assessed in a broad comparative, morphological context. We therefore conducted a morphological assessment of piscivorous coral reef fishes based on a comparative analysis of 119 species, linking morphology with ecological traits (habitat and activity). After accounting for phylogenetic relationships, we found that head length, premaxilla–maxilla (pmx–mx) length, body depth, and eye size mark the primary axis of variation among species. Pmx–mx length is strongly correlated with both vertical and horizontal gape size. We identify three distinct ecomorphotypes: diurnal benthic, nocturnal, and pelagic piscivores. Benthic diurnal and nocturnal piscivores display a wide array of pmx–mx lengths, potentially reflecting the large array of prey sizes and shapes in benthic habitats. This diversity highlights the potential for niche partitioning based on maximum ingestible prey sizes. By comparison, pmx–mx lengths in pelagic piscivores are more restricted, suggesting limited variance in prey sizes or restrictions associated with their feeding mode. Fin shape was also a primary driver of variation between benthic and pelagic predators. The ecomorphotype of nocturnal piscivores suggests that although they are benthic-associated during daytime, these forms leave the reef at night to feed in more open habitats. When analyzing diurnal benthic piscivores alone, we found a major axis of variation between deep-bodied piscivores with large gapes and large head lengths versus fusiform piscivores with high fin aspect ratio values. This continuum appears to describe the relative strength of benthic associations. Overall, we provide a broad quantitative framework for understanding the morphology and potential functions of piscivorous fishes on coral reefs.

Characterisation of coral-associated bacterial communities in an urbanised marine environment shows strong divergence over small geographic scales

Abstract: The coral holobiont contains a diverse community of bacteria that have been widely acknowledged as a major contributor in the maintenance of host health and in promoting reef resilience under changing environments. However, little is known regarding the spatial distribution of these communities or the processes and mechanisms that are responsible for creating these patterns. Here we show that bacterial communities associated with the reef-building coral, Pocillopora acuta, from nine offshore islands in an urbanised coral reef ecosystem (Singapore) can diverge sharply and are significantly different among sampling locations. We suggest that small-scale environmental factors such as prevailing surface currents and wind direction, even over short distances (< 1 km), are responsible for generating bacterial community structure. Considering the sharp differentiation we observe among bacterial communities from different sites, we recommend that future coral reef restoration projects consider the microbial aspect of the coral holobiont as this may affect the success of coral transplants in recipient populations.

Limited coral mortality following acute thermal stress and widespread bleaching on Palmyra Atoll, central Pacific

Abstract: During 2015–2016, an El Nino and associated warm water event caused widespread coral bleaching across the equatorial Pacific. Here, we combine 8 yr of benthic monitoring data from permanent photoquadrats with remotely sensed and in situ temperature measurements to assess the impact of the warming event on benthic communities at Palmyra Atoll. We quantified bleaching prevalence across two distinct reef habitats using the best available data. On the fore reef (~ 10 m depth), we quantified bleaching severity within 100–200 m2 large-area plots using the custom visualization and analysis software, Viscore. On the reef terrace (~ 5 m depth), we used 95 focal colonies across three species that have been monitored annually since 2014. The 2015–2016 warm water event was the most extreme such event recorded on Palmyra in the past several decades with a maximum cumulative heat stress (degree heating weeks) of 11.9 °C-weeks. On the fore reef, 90% of live coral cover exhibited some degree of bleaching (32% severe bleaching). On the shallow reef terrace, bleaching was observed in 93% of the focal colonies across all species. Overall, coral cover declined 9% on the fore reef from 2014 to 2017, whereas coral cover did not change on the terrace. These contrasting results may be associated with typical daily temperature ranges on the terrace that are three times greater than on the fore reef. Permanent photoquadrats showed that turf algae initially colonized skeletons of recently dead corals but transitioned to crustose coralline algae within a year. Collectively, our study emphasizes that comprehensive monitoring of benthic communities over time combined with in situ temperature data can provide taxonomically precise trajectories of community change during and following thermal stress.

Deep reef fishes in the world's epicenter of marine biodiversity

Abstract: The Philippines is often highlighted as the global epicenter of marine biodiversity, yet surveys of reef-associated fishes in this region rarely extend beyond shallow habitats. Here, we improve the understanding of fish species diversity and distribution patterns in the Philippines by analyzing data from mesophotic coral ecosystems (MCEs; 30–150 m depth) obtained via mixed-gas rebreather diving and baited remote underwater video surveys. A total of 277 fish species from 50 families was documented, which includes thirteen newly discovered and undescribed species. There were 27 new records for the Philippines and 110 depth range extensions, indicating that many reef fishes have a broader geographic distribution and greater depth limits than previously reported. High taxonomic beta-diversity, mainly associated with family and genus turnover with depth, and significant effects of traits such as species body size, mobility and geographic range with maximum recorded depth, were observed. These results suggest that MCEs are characterized by unique assemblages with distinct ecological and biogeographic traits. A high proportion (60.5%) of the fish species are targeted by fishing, suggesting that Philippine MCEs are as vulnerable to overfishing as shallow reefs. Our findings support calls to expand conservation efforts beyond shallow reefs and draw attention to the need to explicitly include deep reefs in marine protected areas to help preserve the unique biodiversity of MCEs in the Philippines.

El Niño-associated catastrophic coral mortality at Jarvis Island, central Equatorial Pacific

Abstract: The 2014–2017 Global Coral Bleaching Event is the longest, most widespread, and impactful on record. Rapid ecological assessment surveys by NOAA’s Pacific Reef Assessment and Monitoring Program reported widespread coral mortality at Jarvis Island in the aftermath of the 2015–2016 super-El Nino warming event; hard coral cover declined from 18.7% in April 2015 (pre-bleaching) to 0.4% in May 2016 (post-bleaching), representing a catastrophic > 98% decline. Between 2015 and 2016, corals at Jarvis experienced maximum heat stress of 22.25 °C-weeks exceeding the bleaching threshold (28.72 °C) for 66 consecutive weeks. Mass coral bleaching was observed in November 2015, which resulted in mass mortality across all coral taxa, depths, and island sectors. The bleaching event altered the benthic community composition including the coral assemblage. In the 2 yrs post-bleaching, the benthic community has transitioned from a short-lived increase of encrusting macroalgae to a more recent near-recovery of crustose coralline algae. Coral cover had not recovered by 2017 and could be potentially delayed by fast-growing turf algae. Within the coral community, the pre-bleaching dominant genus Montipora exhibited extreme mortality and only a handful of colonies of this taxon were enumerated in the 2016 surveys and none in 2017. Some coral taxa have persisted in low densities, including the ESA-threatened Acropora retusa and colonies of encrusting Pavona, Psammocora, and the free living Fungia. As the frequency and intensity of these high-temperature events is projected to increase in coming years, it is essential to track how remote ecosystems normally undisturbed by human influence, such as Jarvis, respond to a climate change.

Recruit symbiosis establishment and Symbiodiniaceae composition influenced by adult corals and reef sediment

Abstract: For most reef-building corals, the establishment of symbiosis occurs via horizontal transmission, where juvenile coral recruits acquire their algal symbionts (family Symbiodiniaceae) from their surrounding environment post-settlement. This transmission strategy allows corals to interact with a diverse array of symbionts, potentially facilitating adaptation to the newly settled environment. We exposed aposymbiotic Pseudodiploria strigosa recruits from the Flower Garden Banks to natal reef sediment (C−S+), symbiotic adult coral fragments (C+S−), sediment and coral fragments (C+S+), or seawater controls (C−S−) and quantified rates of symbiont uptake and Symbiodiniaceae community composition within each recruit using metabarcoding of the ITS2 locus. The most rapid uptake was observed in C+S+ treatments, and this combination also led to the highest symbiont alpha diversity in recruits. While C−S+ treatments exhibited the next highest uptake rate, only one individual recruit successfully established symbiosis in the C+S− treatment, suggesting that sediment both serves as a direct symbiont source for coral recruits and promotes (or, potentially, mediates) transmission from adult coral colonies. In turn, presence of adult corals facilitated uptake from the sediment, perhaps via chemical signaling. Taken together, our results reinforce the key role of sediment in algal symbiont uptake by P. strigosa recruits and suggest that sediment plays a necessary, but perhaps not sufficient, role in the life cycle of algal Symbiodiniaceae symbionts.

Copper enrichment reduces thermal tolerance of the highly resistant Red Sea coral Stylophora pistillata

Abstract: Corals in the Gulf of Aqaba (GoA) in the northern Red Sea show high thermal tolerance. The GoA has therefore been suggested as a coral reef refuge from climate change. However, as a narrow body of water with high residence time and a rapidly growing population, the GoA is prone to anthropogenic stressors, heavy metal pollution being one such stressor. In the present study, the branching coral Stylophora pistillata, extensively studied for its high resistance to elevated seawater temperature in the GoA, was exposed to combinations of thermal treatments (ambient (21–23 °C), + 4 °C and + 8 °C above ambient) and ecologically relevant copper (Cu) concentrations (ambient and + 1 µg L−1 above ambient concentration). Significant interactions were found between elevated temperature and Cu enrichment effects on coral physiology, mainly affecting processes associated with photosynthesis. Contrasting responses were recorded at 72 h and 2 weeks from start of the exposure to Cu enrichment. Cu enrichment caused a decrease in Fv/Fm, maximum rETR, and net photosynthesis in ambient temperature after Cu exposure of 72 h, while at the second sampling point only (2 weeks), a decrease was also recorded at + 4 and + 8 °C. Superoxide dismutase activity was higher in Cu-enriched conditions, suggesting that corals were responding to oxidative stress. Thus, a higher input of Cu in the GoA may be energetically costly and might result in decreased resistance of corals to thermal stress under warming scenarios.

Impact of growing up in a warmer, lower pH future on offspring performance: transgenerational plasticity in a pan-tropical sea urchin

Abstract: Transgenerational plasticity (TGP) may be an important mechanism for marine organisms to acclimate to climate change stressors including ocean warming (OW) and ocean acidification (OA). Conversely, environmental stress experienced by one generation may have detrimental latent effects on subsequent generations. We examined TGP in the embryos and larvae of the pan-tropical sea urchin, Tripneustes gratilla, in response to OA (pH 7.77), OW (+2 °C), or both OA and OW, OAW (+2 °C, pH 7.77) using a parent (F0) generation reared in treatments from the early juvenile to the mature adult, incorporating gonadogenesis and germline differentiation. Embryos and larvae of acclimated parents were reared in all four treatments to the 2-day-old pluteus stage. Larvae from OA and OAW parents were resilient to the effects of acidification, while larvae from OW and OAW parents were more tolerant to warmer temperature (29 °C). Parental acclimation, however, had predominantly negative effects on the size of offspring with reductions in larval arm lengths by as much as 21.4%, while eggs were up to 21.8% smaller in females raised at 29 °C. We highlight the complexity and trade-offs of TGP in this first transgenerational climate change study on a marine macroinvertebrate where the F0 generation was acclimated over their reproductive life.

Physical mechanisms influencing localized patterns of temperature variability and coral bleaching within a system of reef atolls

Abstract: Interactions between oceanic and atmospheric processes within coral reefs can significantly alter local-scale ( 60%) over most of this system; however, the bleaching patterns were not uniform. Little is known about the processes governing thermodynamic variability within atolls, particularly those that are dominated by large amplitude tides. Here, we identify three mechanisms at Scott Reef that alleviated heat stress during the marine heatwave in 2016: (1) the cool wake of a tropical cyclone that induced temperature drops of 1.3 °C over a period of 8 days; (2) air–sea heat fluxes that interacted with the reef morphology during neap tides at one of the atolls to reduce water temperatures by up to 2.9 °C; (3) internal tidal processes that forced deeper and cooler water (up to 2.7 °C) into some sections of the shallow reefs. The latter two processes created localized areas of reduced temperatures that led to lower incidences of coral bleaching for parts of the reef. We predict these processes are likely to occur in other similar tide-dominated reef environments worldwide. Identifying locations where physical processes reduce heat stress will likely be critical for coral reefs in the future, by maintaining communities that can help facilitate local recovery of reefs following bleaching events that are expected to increase in frequency and severity in the coming decades.

Bleaching impacts on carbonate production in the Chagos Archipelago: influence of functional coral groups on carbonate budget trajectories

Abstract: Reefs in the remote Chagos Archipelago (central Indian Ocean) were severely affected by sea surface temperature warming and coral bleaching in 2015–2016. Here we assess the impacts of this event on community composition and reef carbonate production at twelve fore reefs sites across three atolls. Bleaching caused a 69% decline in coral cover, mostly driven by mortality of tabular Acropora spp. and a 77% decline in mean coral carbonate production (2015: 13.1 ± 4.8; 2018: 3.0 ± 1.2 kg CaCO3 m2 yr−1). Changes were accompanied by a major shift from competitive to stress-tolerant coral taxa, with magnitudes of decline comparable to those reported elsewhere in the Indian Ocean, despite inter-site differences in dominant coral species. These trends differ from those on reefs already dominated by stress-tolerant taxa, which experienced minor declines in production post-warming. The study highlights the potential for different suites of functional coral groups to drive divergent post-bleaching budget responses.

Unravelling the links between heat stress, bleaching and disease: fate of tabular corals following a combined disease and bleaching event

Abstract: While links between heat stress and coral bleaching are clear and predictive tools for bleaching risk are well advanced, links between heat stress and outbreaks of coral diseases are less well understood. In this study, the effects of accumulated heat stress on tagged colonies of tabular Acropora were monitored over the 2017 austral summer at Beaver Reef, which is located in the central region of the Great Barrier Reef. The initial surveys in midsummer (21 February) coincided with an accumulated heat stress metric of 4.5 °C-weeks, and documented high coral cover (74.0 ± 6.5%), extensive bleaching (71% of all corals displayed bleaching signs) and an outbreak of white syndromes (WSs) (31% of tabular acroporid corals displayed white syndrome signs). Repeat assessments of the impacts of bleaching and disease on these corals provided real-time information to reef managers by tracking the unfolding reef health incident on 100 colonies of Acropora hyacinthus (Dana, 1846), tagged in mid-March and surveyed intermittently until late October 2017. Heat stress increased rapidly on Beaver Reef, peaking at 8.3 °C-weeks on 31 March, which coincided with the highest prevalence of WS recorded in the study. Of the 85 tagged colonies surviving on 31 March, 41 (~ 48%) displayed WS signs, indicating a link between heat stress and WS. When re-surveyed at eight months (24 October), 68 of 100 tagged colonies had suffered whole-colony mortality and only four colonies had not displayed signs of bleaching or disease (WS) in any of our surveys. Overall, coral cover on Beaver Reef was reduced by more than half to 31.0 ± 11.2%. Significant tissue loss due to severe bleaching was observed with up to 20 times greater tissue loss on severely bleached colonies (i.e. categorised as > 50% bleached) compared to mildly/moderately bleached colonies (< 50% bleached) at the heat stress peak (31 March). This suggests that for Acropora hyacinthus, a threshold of 50% colony bleaching is a good indicator that substantial mortality at both the colony and population level is likely to follow a heat stress event. Across all levels of bleaching, colonies displaying WS signs exhibited up to seven times greater tissue loss than bleached-only colonies. WS caused a threefold increase in accumulated tissue loss (69.6 ± 10.5% tissue lost) in the mildly bleached category, suggesting that disease exacerbated mortality in bleached corals and contributed significantly to the substantial loss of corals on the GBR in 2017.

Abiotic and biotic controls on coral recovery 16 years after mass bleaching

Abstract: As climate changes increase heat stress on tropical ecosystems, the long-term persistence of coral reefs requires rapid coral recovery following coral bleaching events. Using the extent of coral cover return to a pre-bleaching baseline as a benchmark, recovery of fast-growing and stress-tolerant coral growth forms suggests that reefs can bounce back between repeated disturbances if given adequate time and protection from anthropogenic disturbances. However, long-term recovery dynamics of coral communities following severe bleaching and mass mortality are limited, particularly for fringing reefs along inhabited coastlines where human stressors may compromise recovery potential. Here, we examine the dynamics and drivers of coral recovery in Seychelles, where 12 reefs returned to pre-bleaching coral cover levels after a severe bleaching event caused > 95% coral mortality. Six reefs with initially low cover ( < 25%) recovered within 7–12 yr and, after 16 yr, exceeded pre-bleaching cover levels by 132–305%. In contrast, six reefs with initially high cover (20–60%) remained at 48–93% of pre-bleaching levels, with recovery projected to take 17–29 yr. Abiotic and historic conditions constrained recovery rates, with the slowest recovery times observed on deep and wave-exposed reefs with high pre-bleaching coral cover. Reefs with high juvenile coral densities and low nitrogen levels recovered fastest, possibly due to the interplay between nutrient enrichment, algal proliferation, and coral recruitment. Our findings emphasize the importance of understanding small-scale variation in recovery potential, whereby recovery times were governed by natural limits on growth rates and modified by coral recruitment and nutrient enrichment. Ultimately, climate-impacted reefs can recover to moderate coral cover levels, but, if bleaching causes repeated high coral mortality, short recovery windows will prevent a return to historic levels of coral dominance.

Heat attenuation and nutrient delivery by localized upwelling avoided coral bleaching mortality in northern Galapagos during 2015/2016 ENSO

Abstract: Despite a very strong El Nino Southern Oscillation in 2015/2016, no coral mortality associated with bleaching was observed at the northern Galapagos (Ecuador) Islands of Darwin and Wolf. From March 2016 to March 2018, coral cover and health as well as water chemistry (NO3− and PO43−) and temperature were recorded. A marked heat anomaly reached 30 °C at Wolf in February 2016, but peak temperatures were attenuated after 2 d by a 4 °C drop. Temperature patterns at three depths (10, 15, and 20 m) and a subsequent and persistent phytoplankton bloom suggest topographically driven upwelling as the source of colder water and dissolved inorganic nutrients—both of which helped corals endure the heating episode. Consequently, no mortality and only partial bleaching were recorded in March 2016. Partially bleached corals contained numerous healthy zooxanthellae in deeper tissue layers. A continuous temperature record from 2012 to 2014 suggests that such upwelling events are common, inducing temperature fluctuations of up to 6 °C within 24 h during the observation period. Events at Wolf in 2016 suggest local upwelling reduced coral stress by relieving heat and by delivering nutrients required by corals to retain their regular temperature tolerance.