Coral Loss Research Articles

Monthly progression rates of the coral-killing sponge Terpios hoshinota in Sesoko Island, Okinawa, Japan

Terpios hoshinota is a black, encrusting sponge that hosts numerous symbiotic cyanobacteria in its tissues. The sponge often overgrows live scleractinian corals, thereby causing widespread and accelerated coral loss. However, our understanding of how subtropical waters influence the seasonal change in the progression rate of the sponge is poor. To elucidate the progression dynamics over a year, we obtained monthly measurements of linear progression of T. hoshinota (mm month−1) on the branching coral Montipora digitata in the subtropical waters of Sesoko Island, Okinawa, Japan from 2019 to 2020. The progression rate of T. hoshinota varied monthly throughout the year. The progression rates were the highest in summer (July, August, and October) with a mean value of 66 mm month−1, except in September, when strong typhoons came close to Okinawa and the progression rate decreased temporarily. The progression rates were the lowest in February 2020, at 9 mm month−1. The mean annual linear progression rate was estimated to be 28 mm month−1. Therefore, the progression dynamics of T. hoshinota could be linked to their growth mechanisms in relation to the seasonal changes in subtropical waters.

The rapid decline of an Endangered temperate soft coral species

Dendronephthya australis, an Endangered soft coral primarily found in the central coast region of New South Wales (NSW), Australia, is in rapid decline. The largest known aggregations are located within the Port Stephens estuary, where the populations have been mapped in detail via towed video at four-year intervals since 2011. Since the distribution was initially mapped in 2011, the areal extent decreased by 67.5%, and is now 9300 m2 compared to 28,600 m2 in 2011. Forest-based Species Distribution Models (SDMs) determined that the primary variables associated with the presence of D. australis (including depth, current velocity, slope, and the distance from the estuary mouth and the shoreline) remained largely unchanged suggesting an alternative cause of declines. General Linear Models (GLMs) identified that losses were associated with distance from shore, and change in depth and current velocity from 2007 to 2018. As these three variables are linked to sediment mobilisation within the estuary, this process is likely to be a key contributing factor to the decline of D. australis – this is supported by in situ observations over the same time period. As Port Stephens is a hydrodynamically complex environment, and has a wide variety of both commercial and recreational uses, other, as yet unidentified, processes may also be contributing to soft coral loss. The only remaining aggregations are found in depths of 4–11 m, which makes them highly vulnerable to fishing and boating impacts, as well as severe storm events.

Parameter sharing adversarial domain adaptation networks for fault transfer diagnosis of planetary gearboxes

The domain adaptation (DA) model, aiming to solve the task of unlabeled or less-labeled target domain fault classification through the training of labeled source domain fault data, is widely used in the transfer diagnosis task of mechanical faults by unsupervised learning. However, traditional transfer learning models such as deep domain confusion (DDC) and RevGrad still have the problems of high training cost and low classification accuracy. Therefore, this paper proposes a parameter sharing adversarial domain adaptation network (PSADAN). The proposed approach constructs a shared classifier to unify fault classifiers and domain classifiers to reduce the complexity of network structure (i.e. the number of hyperparameters), and adds the CORAL loss for adversarial training to enhance the domain confusion. Meanwhile, an unbalanced adversarial training strategy is proposed for improving the domain confusion ability of the feature extractor, so as to improve the accuracy of transfer diagnosis. The effectiveness and advantage of the proposed method is verified by the planetary gearbox fault transfer diagnosis experiments including several transfer tasks of load condition, speed condition, and measurement point.

Screening a Spliced Leader-Based Symbiodinium microadriaticum cDNA Library Using the Yeast-Two Hybrid System Reveals a Hemerythrin-Like Protein as a Putative SmicRACK1 Ligand

The dinoflagellate Symbiodiniaceae family plays a central role in the health of the coral reef ecosystem via the symbiosis that establishes with its inhabiting cnidarians and supports the host metabolism. In the last few decades, coral reefs have been threatened by pollution and rising temperatures which have led to coral loss. These events have raised interest in studying Symbiodiniaceae and their hosts; however, progress in understanding their metabolism, signal transduction pathways, and physiology in general, has been slow because dinoflagellates present peculiar characteristics. We took advantage of one of these peculiarities; namely, the post-transcriptional addition of a Dino Spliced Leader (Dino-SL) to the 5′ end of the nuclear mRNAs, and used it to generate cDNA libraries from Symbiodinium microadriaticum. We compared sequences from two Yeast-Two Hybrid System cDNA Libraries, one based on the Dino-SL sequence, and the other based on the SMART technology (Switching Mechanism at 5′ end of RNA Transcript) which exploits the template switching function of the reverse transcriptase. Upon comparison of the performance of both libraries, we obtained a significantly higher yield, number and length of sequences, number of transcripts, and better 5′ representation from the Dino-SL based library than from the SMART library. In addition, we confirmed that the cDNAs from the Dino-SL library were adequately expressed in the yeast cells used for the Yeast-Two Hybrid System which resulted in successful screening for putative SmicRACK1 ligands, which yielded a putative hemerythrin-like protein.

Mesophotic coral refuges following multiple disturbances

As coral populations on shallow reefs decline globally, mesophotic coral ecosystems (MCE) have been proposed as potential coral refugia from thermal, storm, and anthropogenic disturbances in the face of climate change. The current study assesses the refuge potential of MCEs in the U.S. Virgin Islands (USVI) for Montastraea cavernosa by extrapolating reproductive potential through depth-stratified coral loss following regional storm, disease, and bleaching perturbations. Fecundity of this depth-generalist coral from 4 to 40 m was measured histologically, and polyp, population, and total habitat fecundities were then extrapolated across the species’ depth range. The number of reproductively active female gonads per polyp and oocyte size experienced a significant, though small, decrease with depth, potentially due to energetic limitations. Notably, the population sex ratio was not different from 1:1 on shallow and mid-depth reefs, but it became significantly male-biased (3.6:1) at mesophotic depths. Population-level differences in oocyte production over depth were primarily driven by changes in coral cover and sex ratio. The large spatial extent of mesophotic reefs relative to shallow reefs in the USVI makes MCEs the primary contributor of oocytes, despite the reduced proportion of females at depth. Following Hurricanes Irma and Maria in 2017, the outbreak of Stony Coral Tissue Loss Disease in 2019, and a bleaching event in 2019, shallow and mid-depth M. cavernosa populations experienced severe coral cover declines. Shallow and mid-depth population fecundities were predicted to decline correspondingly. Coral cover in MCEs remained relatively stable following these largely shallow water perturbations, and predicted population and total habitat fecundities remained constant as well. Thus, MCEs in the USVI currently appear to be reproductive refuges for M. cavernosa, but the persistence of that refuge remains in question as disease perturbation begins to affect deeper reefs.

Animal Forest Mortality: Following the Consequences of a Gorgonian Coral Loss on a Mediterranean Coralligenous Assemblage

In this work, the consequences of a local gorgonian coral mortality on the whole coralligenous assemblage were studied. A Before/After-Control/Impact sampling design was used: the structure of the coralligenous assemblage was compared before and after the gorgonian mortality event at the mortality site and two control sites. At the mortality site, a relevant decrease in alpha and beta diversity occurred, with a shift from a stratified assemblage characterized by gorgonians and other invertebrates to an assemblage dominated by algal turfs; conversely, neither significant variations of the structure nor decrease in biodiversity were observed at the control sites. The assemblage shift involved the main taxa in different times: in autumn 2018, a large proportion of the plexaurid coral Paramuricea clavata died, but no significant changes were observed in the structure of the remaining assemblage. Then, in autumn 2019, algal turfs increased significantly and, one year later, the abundance of the gorgonian Eunicella cavolini and bryozoans collapsed. Although the mechanisms of the assemblage shift following gorgonian loss will remain uncertain and a cause-effect relationship cannot be derived, results suggest the need for detecting signs of gorgonian forests stress in monitoring programs, which should be considered early indicators of their condition. in the coralligenous monitoring programs for detecting any sign of gorgonian forests stress which should be considered an early indicator of the assemblage condition.

Different population trajectories of two reef-building corals with similar life-history traits.

Increases in the frequency and intensity of acute and chronic disturbances are causing declines of coral reefs world‐wide. Although quantifying the responses of corals to acute disturbances is well documented, detecting subtle responses of coral populations to chronic disturbances is less common, but can also result in altered population and community structures.We investigated the population dynamics of two key reef‐building Merulinid coral species, Dipsastraea favus and Platygyra lamellina, with similar life‐history traits, in the Gulf of Eilat and Aqaba, Red Sea from 2015 to 2018, to assess potential differences in their population trajectories.Demographic processes, which included rates of survival, growth, reproduction and recruitment were used to parametrize integral projection models and estimate population growth rates and the likely population trajectories of both coral species.The survival and reproduction rates of both D. favus and P. lamellina were positively related to coral colony size, and elasticity analyses showed that large colonies most influenced population dynamics. Although both species have similar life‐history traits and growth morphologies and are generally regarded as ‘stress‐tolerant’, the populations showed contrasting trajectories—D. favus appears to be increasing whereas P. lamellina appears to be decreasing.As many corals have long‐life expectancies, the process of local and regional decline might be subtle and slow. Ecological assessments based on total living coral coverage, morphological groups or functional traits might overlook subtle, species‐specific trends. However, demographic approaches capable of detecting subtle species‐specific population changes can augment ecological studies and provide valuable early warning signs of decline before major coral loss becomes evident.

Failure to respond to a coral disease epizootic in Florida: causes and consequences

Stony coral tissue loss disease (SCTLD) was first observed in September 2014 near Virginia Key, Florida. In roughly six years, the disease spread throughout Florida and into the greater Caribbean basin. The high prevalence of SCTLD and high resulting mortality in coral populations, and the large number of susceptible species affected, suggest that this outbreak is one of the most lethal ever recorded. The initial recognition and management response to this catastrophic disease in Florida was slow, which delayed the start of monitoring programs and prevented coordinated research programs by at least two years. The slow management response was a result of several factors that operated concurrently. First, the Port Miami dredging project was ongoing during the coral disease epidemic and dredging rather than SCTLD was blamed by some managers and local environmental groups for the extreme coral losses reported in the project’s compliance monitoring program. Second, this blame was amplified in the media because dredging projects are intuitively assumed to be bad for coral reefs. Third, during this same time State of Florida policy prohibited government employees to acknowledge global warming in their work. This was problematic because ocean warming is a proximal cause of many coral diseases. As a result, the well-known links between warming and coral disease were ignored. A consequence of this policy was that the dredging project provided an easy target to blame for the coral mortality noted in the monitoring program, despite convincing data that suggested otherwise. Specifically, results from the intensive compliance monitoring program, conducted by trained scientific divers, were clear. SCTLD that was killing massive numbers of corals throughout Florida was also killing corals at the dredge site – and in the same proportions and among the same suite of species. While eradication of the disease was never a possibility, early control measures may have slowed its spread or allowed for the rescue of significant numbers of large colonies of iconic species. This coral disease outbreak has similarities to the COVID-19 pandemic in the United States and there are lessons learned from both that will improve disease response outcomes in the future, to the benefit of coral reefs and human populations.

Direct and indirect effects of heatwaves on a coral reef fishery

Marine heatwaves are increasing in frequency and intensity, and indirectly impacting coral reef fisheries through bleaching-induced degradation of live coral habitats. Marine heatwaves also affect fish metabolism and catchability, but such direct effects of elevated temperatures on reef fisheries are largely unknown. We investigated direct and indirect effects of the devastating 2016 marine heatwave on the largest reef fishery operating along the Great Barrier Reef (GBR). We used a combination of fishery-independent underwater census data on coral trout biomass (Plectropomus and Variola spp.) and catch-per-unit-effort (CPUE) data from the commercial fishery to evaluate changes in the fishery resulting from the 2016 heatwave. The heatwave caused widespread, yet locally patchy, declines in coral cover, but we observed little effect of local coral loss on coral trout biomass. Instead, a pattern of decreasing biomass at northern sites and stable or increasing biomass at southern sites suggested a direct response of populations to the heatwave. Analysis of the fishery-independent data and CPUE found that in-water coral trout biomass estimates were positively related to CPUE, and that coral trout catch rates increased with warmer temperatures. Temperature effects on catch rates were consistent with the thermal affinities of the multiple species contributing to this fishery. Scaling-up the effect of temperature on coral trout catch rates across the region suggests that GBR-wide catches were 18% higher for a given level of effort during the heatwave year relative to catch rates under the mean temperatures in the preceding 6years. These results highlight a potentially large effect of heatwaves on catch rates of reef fishes, independent of changes in reef habitats, that can add substantial uncertainty to estimates of stock trends inferred from fishery-dependent (CPUE) data. Overestimation of CPUE could initiate declines in reef fisheries that are currently fully exploited, and threaten sustainable management of reef stocks.

Enhancing coral recruitment through assisted mass settlement of cultured coral larvae.

The escalating rate at which coral communities are declining globally requires urgent intervention and new approaches to reef management to reduce and halt further coral loss. For reef systems with limited natural larval supply, the introduction of large numbers of competent coral larvae directly to natural reef substrata provides a potentially useful approach to replenish adult coral populations. While few experiments have tested this approach, only one experiment has demonstrated its long-term success to date. Given the differences in life-history traits among corals, and different sensitivities of larvae to abiotic and biotic factors, coupled with the dynamic nature of post-settlement survivorship and recruitment processes, trials of the larval enhancement technique with larvae of different coral species are needed to test the broader applicability and viability of this approach. Accordingly, in this paper we examine the applicability of the larval enhancement technique to restore a population of Acropora loripes in the Bolinao-Anda Reef Complex, Pangasinan, northwestern Philippines. Larvae were cultured ex situ following spawning of collected A. loripes colonies in June 2014. Competent larvae were transported to degraded reef areas and approximately 300,000 larvae were introduced in each of three 6 × 4 m plots directly on the reef. Fine mesh enclosures retained the larvae inside each treatment plot for five days. Three adjacent 6 × 4 m plots that served as controls were also covered with mesh enclosures, but no larvae were introduced. Each plot contained ten 10 × 10 cm conditioned settlement tiles cut from dead tabulate Acropora that were used to quantify initial larval settlement. After allowing larval settlement for five days, mean settlement on tiles from the larval enhancement plots that were monitored under stereomicroscopes was significantly higher (27.8 ± 6.7 spat per tile) than in control plots, in which not a single recruit was recorded. Post-settlement survivorship and growth of spat and coral recruits on tiles and reef substrata inside the experimental plots were monitored periodically for 35 months. After 35 months, the mean size of each of the remaining 47 A. loripes coral colonies surviving on the reef substrata was 438.1 ± 5.4 cm3, with a mean diameter of 7.9 ± 0.6 cm. The average production cost for each of the surviving A. loripes colonies at 35 months was USD 35.20. These colonies are expected to spawn and contribute to the natural larval pool when they become reproductively mature, thereby enhancing natural coral recovery in the area. This study demonstrates that mass coral larval enhancement can be successfully used for restoring populations of coral species with different life-history traits, and the techniques can rapidly increase larval recruitment rates on degraded reef areas, hence catalysing the regeneration of declining coral populations.

Limited genetic signal from potential cloning and selfing within wild populations of coral-eating crown-of-thorns seastars (Acanthaster cf. solaris)

Population outbreaks of crown-of-thorns seastars (CoTS; Acanthaster spp.) are contributing to extensive coral loss and reef degradation throughout the Indo-Pacific, but the causes and underlying mechanisms of population maintenance and outbreaks are equivocal. Two recent publications suggest that, in addition to outbreeding sexual reproduction, asexual reproduction through larval fission and selfing may contribute to rapid increases in the local abundance of Acanthaster spp. We re-analysed two large microsatellite datasets (collectively representing 3714 individuals) that investigated connectivity in the Great Barrier Reef and wider Pacific region to investigate if potential cloning or selfing can be evidenced in the population genetic structure. Within this dataset, we identified only a small number (18, < 0.5%) of putative clones (repeated multilocus genotypes). We argue that several of these are due to sampling and processing errors rather than direct evidence of cloning. Analysis of the population genetic structure (i.e. pairwise genetic differences between individuals, deviations from Hardy–Weinberg Equilibrium, and linkage disequilibrium) also yielded no genetic evidence for asexual reproduction. There was a tendency towards slight heterozygote deficits, so we cannot refute that selfing does occur, but observed patterns are most likely attributable to sampling artefacts. Although we cannot exclude that asexual reproduction occurs to some extent in Acanthaster populations, we find no evidence that these processes make a contribution to population structure or directly enhance larval supply.

Size-specific recolonization success by coral-dwelling damselfishes moderates resilience to habitat loss

Increasing degradation of coral reef ecosystems and specifically, loss of corals is causing significant and widespread declines in the abundance of coral reef fishes, but the proximate cause(s) of these declines are largely unknown. Here, we examine specific responses to host coral mortality for three species of coral-dwelling damselfishes (Dascyllus aruanus, D. reticulatus, and Pomacentrus moluccensis), explicitly testing whether these fishes can successfully move and recolonize nearby coral hosts. Responses of fishes to localized coral loss was studied during population irruptions of coral feeding crown-of-thorns starfish, where starfish consumed 29 (34%) out of 85 coral colonies, of which 25 (86%) were occupied by coral-dwelling damselfishes. Damselfishes were not tagged or individually recognizable, but changes in the colonization of different coral hosts was assessed by carefully assessing the number and size of fishes on every available coral colony. Most damselfishes (> 90%) vacated dead coral hosts within 5 days, and either disappeared entirely (presumed dead) or relocated to nearby coral hosts. Displaced fishes only ever colonized corals already occupied by other coral-dwelling damselfishes (mostly conspecifics) and colonization success was strongly size-dependent. Despite movement of damselfishes to surviving corals, the local abundance of coral-dependent damselfishes declined in approximate accordance with the proportional loss of coral habitat. These results suggest that even if alternative coral hosts are locally abundant, there are significant biological constraints on movement of coral-dwelling damselfishes and recolonization of alternative coral habitats, such that localized persistence of habitat patches during moderate or patchy disturbances do not necessarily provide resilience against overall habitat loss.

Early trajectories of benthic coral reef communities following the 2015/16 coral bleaching event at remote Aldabra Atoll, Seychelles

Documenting post-bleaching trajectories of coral reef communities is crucial to understand their resilience to climate change. We investigated reef community changes following the 2015/16 bleaching event at Aldabra Atoll, where direct human impact is minimal. We combined benthic data collected pre- (2014) and post-bleaching (2016–2019) at 12 sites across three locations (lagoon, 2 m depth; seaward west and east, 5 and 15 m depth) with water temperature measurements. While seaward reefs experienced relative hard coral reductions of 51–62%, lagoonal coral loss was lower (− 34%), probably due to three-fold higher daily water temperature variability there. Between 2016 and 2019, hard coral cover did not change on deep reefs which remained dominated by turf algae and Halimeda, but absolute cover on shallow reefs increased annually by 1.3% (east), 2.3% (west) and 3.0% (lagoon), reaching, respectively, 54%, 68% and 93% of the pre-bleaching cover in 2019. Full recovery at the shallow seaward locations may take at least five more years, but remains uncertain for the deeper reefs. The expected increase in frequency and severity of coral bleaching events is likely to make even rapid recovery as observed in Aldabra’s lagoon too slow to prevent long-term reef degradation, even at remote sites.

Long-term ecological changes in fishes and macro-invertebrates in the world's warmest coral reefs

The Arabian Gulf is a natural laboratory for examining the consequences of large-scale disturbances due to global warming on coral reef ecosystems because of its extreme temperature regime. Using a coral reef monitoring time series extending from 1985 to 2015, we examined the long-term ecological changes in fish and macro-invertebrate communities as these habitats suffered heat shocks. We used a GLMM modelling framework to obtain clean annual signals in community indicators from noisy data. We also visualized temporal change in the taxonomic composition of fishes and macro-invertebrates. A phase shift from predominantly reef-building corals to barren grounds occurred between 1996 and 2000. Macro-invertebrates responded rapidly, and most of associated indicators recovered to pre-shift levels in 15 years. Fishes generally had lagged responses to the phase shift and had shifted to a new state with lower abundance, as well as different species composition. Increased levels of herbivory first by macro-invertebrates, mostly sea urchins, and then fishes, could have suppressed macro-algae expansion and consequently led to the dominance of barren ground. When the phase shift occurred, most of the 14 fish families declined in abundance while macro-invertebrate groups increased. Fish families able to utilize non-coral habitats appeared more resilient to the disturbances and subsequent coral degradation. Unlike other regions, we observed high resilience of the coral-dependent butterflyfishes to coral loss, possibly due to local migration from other less-impacted coral reefs. We hypothesized a top-down control mechanism mediated by predation by fishes has contributed to shaping the temporal and spatial patterns of the macro-invertebrates. Our results also revealed differences in spatial preferences among fishes and macro-invertebrate groups, which could be used to set priorities and develop effective conservation and management strategies.

Uranium-thorium dating of coral mortality and community shift in a highly disturbed inshore reef (Weizhou Island, northern South China Sea)

Inshore coral habitats are at high risk of loss due to a combination of climate warming and regional-scale human impacts. As a result, they have undergone significant declines. Direct evidence of acute and chronic disturbance on most inshore coral assemblages is limited. Long-term, periodical surveys and historical baseline data essential for effective management are lacking. Using high-precision uranium-thorium (UTh) dating, we reconstruct a ~100-year-long history of extensive coral loss, changes in coral community structure, and a shifting baseline. The data were collected at Weizhou Island, northern South China Sea (SCS), which has highly disturbed inshore coral habitats that are typical globally. According to our UTh dates, major coral mortalities around Weizhou Island have occurred since the 1950s, with increasing frequency and severity since the 1980s. The extensive loss of branching Acropora and collapse of coral communities with peaks around 1960, 1984, and 1998 are accompanied by a shift toward low coral cover and noncoral-dominated assemblages. Prior to this collapse, the local coral community structure sustained remarkable long-term stability over millennia. The timing of the Acropora loss and massive coral mortalities coincides with multiple acute and chronic, natural and anthropogenic disturbance events. We suggest that priority should be given to directly addressing the causes of degradation and effectively controlling chronic disturbances before attempting to restore reef ecosystems. This is probably the only way to solve the “wicked problem” of sustaining the key functions and ecosystem services of inshore coral habitats such as those of Weizhou Island, northern SCS.

Extreme environmental conditions reduce coral reef fish biodiversity and productivity

Tropical ectotherms are hypothesized to be vulnerable to environmental changes, but cascading effects of organismal tolerances on the assembly and functioning of reef fish communities are largely unknown. Here, we examine differences in organismal traits, assemblage structure, and productivity of cryptobenthic reef fishes between the world’s hottest, most extreme coral reefs in the southern Arabian Gulf and the nearby, but more environmentally benign, Gulf of Oman. We show that assemblages in the Arabian Gulf are half as diverse and less than 25% as abundant as in the Gulf of Oman, despite comparable benthic composition and live coral cover. This pattern appears to be driven by energetic deficiencies caused by responses to environmental extremes and distinct prey resource availability rather than absolute thermal tolerances. As a consequence, production, transfer, and replenishment of biomass through cryptobenthic fish assemblages is greatly reduced on Earth’s hottest coral reefs. Extreme environmental conditions, as predicted for the end of the 21st century, could thus disrupt the community structure and productivity of a critical functional group, independent of live coral loss.

Relative efficacy of three approaches to mitigate Crown-of-Thorns Starfish outbreaks on Australia’s Great Barrier Reef

Population outbreaks of Crown-of-Thorns Starfish (COTS; Acanthaster spp.) are a major contributor to loss of hard coral throughout the Indo-Pacific. On Australia’s Great Barrier Reef (GBR), management interventions have evolved over four COTS outbreaks to include: (1) manual COTS control, (2) Marine Protected Area (MPA) zoning, and, (3) water quality improvement. Here we evaluate the contribution of these three approaches to managing population outbreaks of COTS to minimize coral loss. Strategic manual control at sites reduced COTS numbers, including larger, more fecund and damaging individuals. Sustained reduction in COTS densities and improvements in hard coral cover at a site were achieved through repeated control visits. MPAs influenced initial COTS densities but only marginally influenced final hard coral cover following COTS control. Water quality improvement programs have achieved only marginal reductions in river nutrient loads delivered to the GBR and the study region. This, a subsequent COTS outbreak, and declining coral cover across the region suggest their contributions are negligible. These findings support manual control as the most direct, and only effective, means of reducing COTS densities and improving hard coral cover currently available at a site. We provide recommendations for improving control program effectiveness with application to supporting reef resilience across the Indo-Pacific.

Rewiring coral: Anthropogenic nutrients shift diverse coral–symbiont nutrient and carbon interactions toward symbiotic algal dominance

Improving coral reef conservation requires heightened understanding of the mechanisms by which coral cope with changing environmental conditions to maintain optimal health. We used a long-term (10month) in situ experiment with two phylogenetically diverse scleractinians (Acropora palmata and Porites porites) to test how coral-symbiotic algal interactions changed under real-world conditions that were a priori expected to be beneficial (fish-mediated nutrients) and to be harmful, but non-lethal, for coral (fish+anthropogenic nutrients). Analyzing nine response variables of nutrient stoichiometry and stable isotopes per coral fragment, we found that nutrients from fish positively affected coral growth, and moderate doses of anthropogenic nutrients had no additional effects. While growing, coral maintained homeostasis in their nutrient pools, showing tolerance to the different nutrient regimes. Nonetheless, structural equation models revealed more nuanced relationships, showing that anthropogenic nutrients reduced the diversity of coral-symbiotic algal interactions and caused nutrient and carbon flow to be dominated by the symbiont. Our findings show that nutrient and carbon pathways are fundamentally "rewired" under anthropogenic nutrient regimes in ways that could increase corals' susceptibility to further stressors. We hypothesize that our experiment captured coral in a previously unrecognized transition state between mutualism and antagonism. These findings highlight a notable parallel between how anthropogenic nutrients promote symbiont dominance with the holobiont, and how they promote macroalgal dominance at the coral reef scale. Our findings suggest more realistic experimental conditions, including studies across gradients of anthropogenic nutrient enrichment as well as the incorporation of varied nutrient and energy pathways, may facilitate conservation efforts to mitigate coral loss.

High rates of carbon and dinitrogen fixation suggest a critical role of benthic pioneer communities in the energy and nutrient dynamics of coral reefs

Abstract Following coral mortality in tropical reefs, pioneer communities dominated by filamentous and crustose algae efficiently colonize substrates previously occupied by coral tissue. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves. Pioneer communities play an important role for the biological succession and reorganization of reefs after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon (C) and nitrogen (N) within the reef, remains uncertain. We used 96 carbonate tiles to simulate the occurrence of bare substrates after disturbance in a coral reef of the central Red Sea. We measured rates of C and dinitrogen (N2) fixation of pioneer communities on these tiles monthly over an entire year. Coupled with elemental and stable isotope analyses, these measurements provide insights into macronutrient acquisition, export and the influence of seasonality. Pioneer communities exhibited high rates of C and N2 fixation within 4–8 weeks after the introduction of experimental bare substrates. Ranging from 13 to 25 μmol C cm−2 day−1 and 8 to 54 nmol N cm−2 day−1, respectively, C and N2 fixation rates were comparable to reported values for established Red Sea coral reefs. This similarity indicates that pioneer communities may quickly compensate for the loss of benthic productivity by corals. Notably, between 40% and 85% of fixed organic C was exported into the environment, constituting a vital source of energy for the coral reef food web. Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality. While not substituting other critical ecosystem functions provided by corals (e.g. structural habitat complexity and coastal protection), pioneer communities likely contribute to maintaining coral reef nutrient cycling through the accumulation of biomass and import of macronutrients following coral loss. A free Plain Language Summary can be found within the Supporting Information of this article.

Caribbean reefs of the Anthropocene: Variance in ecosystem metrics indicates bright spots on coral depauperate reefs.

Dramatic coral loss has significantly altered many Caribbean reefs, with potentially important consequences for the ecological functions and ecosystem services provided by reef systems. Many studies examine coral loss and its causes—and often presume a universal decline of ecosystem services with coral loss—rather than evaluating the range of possible outcomes for a diversity of ecosystem functions and services at reefs varying in coral cover. We evaluate 10 key ecosystem metrics, relating to a variety of different reef ecosystem functions and services, on 328 Caribbean reefs varying in coral cover. We focus on the range and variability of these metrics rather than on mean responses. In contrast to a prevailing paradigm, we document high variability for a variety of metrics, and for many the range of outcomes is not related to coral cover. We find numerous “bright spots,” where herbivorous fish biomass, density of large fishes, fishery value, and/or fish species richness are high, despite low coral cover. Although it remains critical to protect and restore corals, understanding variability in ecosystem metrics among low‐coral reefs can facilitate the maintenance of reefs with sustained functions and services as we work to restore degraded systems. This framework can be applied to other ecosystems in the Anthropocene to better understand variance in ecosystem service outcomes and identify where and why bright spots exist.