Climate change impacts on mesophotic regions of the Great Barrier Reef

Climate change projections for coral reefs are founded exclusively on sea surface temperatures (SST). While SST projections are relevant for the shallowest reefs, neglecting ocean stratification overlooks the striking differences in temperature experienced by deeper reefs for all or part of the year. Density stratification creates a buoyancy barrier partitioning the upper and lower parts of the water column. Here, we mechanistically downscale climate models and quantify patterns of thermal stratification above mesophotic corals (depth 30 to 50 m) of the Great Barrier Reef (GBR). Stratification insulates many offshore regions of the GBR from heatwaves at the surface. However, this protection is lost once global average temperatures exceed ~3 °C above preindustrial, after which mesophotic temperatures surpass a recognized threshold of 30 °C for coral mortality. Bottom temperatures on the GBR (30 to 50 m) from 2050 to 2060 are estimated to increase by ~0.5 to 1 °C under lower climate emissions (SSP1-1.9) and ~1.2 to 1.7 °C under higher climate emissions (SSP5-8.5). In short, mesophotic coral reefs are also threatened by climate change and research might prioritize the sensitivity of such corals to stress.

A diverse portfolio of marine protected areas can better advance global conservation and equity

Marine protected areas (MPAs) are widely used for ocean conservation, yet the relative impacts of various types of MPAs are poorly understood. We estimated impacts on fish biomass from no-take and multiple-use (fished) MPAs, employing a rigorous matched counterfactual design with a global dataset of >14,000 surveys in and around 216 MPAs. Both no-take and multiple-use MPAs generated positive conservation outcomes relative to no protection (58.2% and 12.6% fish biomass increases, respectively), with smaller estimated differences between the two MPA types when controlling for additional confounding factors (8.3% increase). Relative performance depended on context and management: no-take MPAs performed better in areas of high human pressure but similar to multiple-use in remote locations. Multiple-use MPA performance was low in high-pressure areas but improved significantly with better management, producing similar outcomes to no-take MPAs when adequately staffed and appropriate use regulations were applied. For priority conservation areas where no-take restrictions are not possible or ethical, our findings show that a portfolio of well-designed and well-managed multiple-use MPAs represents a viable and potentially equitable pathway to advance local and global conservation.

Mapping the susceptibility of reefs to rubble accumulation across the Great Barrier Reef

Disturbance-induced rubble accumulations are described as "killing fields" on coral reefs as coral recruits suffer high post-settlement mortality, creating a bottleneck for reef recovery. The increasing frequency of coral bleaching events, that can generate rubble once coral dies, has heightened concerns that rubble beds will become more widespread and persistent. But we currently lack the tools to predict where rubble is most likely to accumulate. Here, we developed a modelling framework to identify areas that are likely to accumulate rubble on forereef slopes across the Great Barrier Reef. The algorithm uses new high-resolution bathymetric and geomorphic datasets from satellite remote sensing. We found that 47 km of reef slope (3% of the entire reef surveyed), primarily in the southern region, could potentially reach 50% rubble cover. Despite being statistically significant (p < 0.001), the effects of depth and aspect on rubble cover were minimal, with a 0.2% difference in rubble cover between deeper and shallower regions, as well as a maximum difference of 0.8% among slopes facing various directions. Therefore, we conclude that the effects of depth and aspect were insufficient to influence ecological processes such as larval recruitment and recovery in different coral communities. Maps of potential rubble accumulation can be used to prioritise surveys and potential restoration, particularly after major disturbances have occurred.

Emergent increase in coral thermal tolerance reduces mass bleaching under climate change

Recurrent mass bleaching events threaten the future of coral reefs. To persist under climate change, corals will need to endure progressively more intense and frequent marine heatwaves, yet it remains unknown whether their thermal tolerance can keep pace with warming. Here, we reveal an emergent increase in the thermal tolerance of coral assemblages at a rate of 0.1 °C/decade for a remote Pacific coral reef system. This led to less severe bleaching impacts than would have been predicted otherwise, indicating adaptation, acclimatisation or shifts in community structure. Using future climate projections, we show that if thermal tolerance continues to rise over the coming century at the most-likely historic rate, substantial reductions in bleaching trajectories are possible. High-frequency bleaching can be fully mitigated at some reefs under low-to-middle emissions scenarios, yet can only be delayed under high emissions scenarios. Collectively, our results indicate a potential ecological resilience to climate change, but still highlight the need for reducing carbon emissions in line with Paris Agreement commitments to preserve coral reefs.

Demographic resilience may sustain significant coral populations in a 2°C-warmer world

Projections of coral reefs under climate change have important policy implications, but most analyses have focused on the intensification of climate-related physical stress rather than explicitly modelling how coral populations respond to stressors. Here, we analyse the future of the Great Barrier Reef (GBR) under multiple, spatially realistic drivers which allows less impacted sites to facilitate recovery. Under a Representative Concentration Pathway (RCP) 2.6 CMIP5 climate ensemble, where warming is capped at ~2°C, GBR mean coral cover declined mid-century but approached present-day levels towards 2100. This is considerably more optimistic than most analyses. However, under RCP4.5, mean coral cover declined by >80% by late-century, and reached near zero under RCP ≥6.0. While these models do not allow for adaptation, they significantly extend past studies by revealing demographic resilience of coral populations to low levels of additional warming, though more pessimistic outcomes might be expected under CMIP6. Substantive coral populations under RCP2.6 would facilitate long-term genetic adaptation, adding value to ambitious greenhouse emissions mitigation.

Moving beyond heritability in the search for coral adaptive potential

Global environmental change is happening at unprecedented rates. Coral reefs are among the ecosystems most threatened by global change. For wild populations to persist, they must adapt. Knowledge shortfalls about corals' complex ecological and evolutionary dynamics, however, stymie predictions about potential adaptation to future conditions. Here, we review adaptation through the lens of quantitative genetics. We argue that coral adaptation studies can benefit greatly from "wild" quantitative genetic methods, where traits are studied in wild populations undergoing natural selection, genomic relationship matrices can replace breeding experiments, and analyses can be extended to examine genetic constraints among traits. In addition, individuals with advantageous genotypes for anticipated future conditions can be identified. Finally, genomic genotyping supports simultaneous consideration of how genetic diversity is arrayed across geographic and environmental distances, providing greater context for predictions of phenotypic evolution at a metapopulation scale.

No apparent trade-offs associated with heat tolerance in a reef-building coral

As marine species adapt to climate change, their heat tolerance will likely be under strong selection. Yet trade-offs between heat tolerance and other life history traits could compromise natural adaptation or assisted evolution. This is particularly important for ecosystem engineers, such as reef-building corals, which support biodiversity yet are vulnerable to heatwave-induced mass bleaching and mortality. Here, we exposed 70 colonies of the reef-building coral Acropora digitifera to a long-term marine heatwave emulation experiment. We tested for trade-offs between heat tolerance and three traits measured from the colonies in situ - colony growth, fecundity, and symbiont community composition. Despite observing remarkable within-population variability in heat tolerance, all colonies were dominated by Cladocopium C40 symbionts. We found no evidence for trade-offs between heat tolerance and fecundity or growth. Contrary to expectations, positive associations emerged with growth, such that faster-growing colonies tended to bleach and die at higher levels of heat stress. Collectively, our results suggest that these corals exist on an energetic continuum where some high-performing individuals excel across multiple traits. Within populations, trade-offs between heat tolerance and growth or fecundity may not be major barriers to natural adaptation or the success of assisted evolution interventions.

Setting sustainable limits on anchoring to improve the resilience of coral reefs

Boat anchoring is common at coral reefs that have high economic or social value, but anchoring has received relatively little attention in reef resilience studies. We developed an individual-based model of coral populations and simulated the effects of anchor damage over time. The model allowed us to estimate the carrying capacity of anchoring for four different coral assemblages and different starting levels of coral cover. The carrying capacity of small to medium-sized recreational vessels across these four assemblages was between 0 and 3.1 anchor strikes ha^-1 day^-1. In a case study of two Great Barrier Reef archipelagos, we modelled the benefits of anchoring mitigation under bleaching regimes expected for four climate scenarios. The partial mitigation of even a very mild anchoring incidence (1.17 strikes ha^-1 day^-1) resulted in median coral gains of 2.6-7.7 % absolute cover under RCP2.6, though benefits varied temporally and depended on the Atmosphere-Ocean General Circulation Model used.

Emigration patterns of motile cryptofauna and their implications for trophic functioning in coral reefs

Patterns of movement of marine species can reflect strategies of reproduction and dispersal, species' interactions, trophodynamics, and susceptibility to change, and thus critically inform how we manage populations and ecosystems. On coral reefs, the density and diversity of metazoan taxa are greatest in dead coral and rubble, which are suggested to fuel food webs from the bottom up. Yet, biomass and secondary productivity in rubble is predominantly available in some of the smallest individuals, limiting how accessible this energy is to higher trophic levels. We address the bioavailability of motile coral reef cryptofauna based on small-scale patterns of emigration in rubble. We deployed modified RUbble Biodiversity Samplers (RUBS) and emergence traps in a shallow rubble patch at Heron Island, Great Barrier Reef, to detect community-level differences in the directional influx of motile cryptofauna under five habitat accessibility regimes. The mean density (0.13-4.5 ind cm^-3) and biomass (0.14-5.2 mg cm^-3) of cryptofauna were high and varied depending on microhabitat accessibility. Emergent zooplankton represented a distinct community (dominated by the Appendicularia and Calanoida) with the lowest density and biomass, indicating constraints on nocturnal resource availability. Mean cryptofauna density and biomass were greatest when interstitial access within rubble was blocked, driven by the rapid proliferation of small harpacticoid copepods from the rubble surface, leading to trophic simplification. Individuals with high biomass (e.g., decapods, gobies, and echinoderms) were greatest when interstitial access within rubble was unrestricted. Treatments with a closed rubble surface did not differ from those completely open, suggesting that top-down predation does not diminish rubble-derived resources. Our results show that conspecific cues and species' interactions (e.g., competition and predation) within rubble are most critical in shaping ecological outcomes within the cryptobiome. These findings have implications for prey accessibility through trophic and community size structuring in rubble, which may become increasingly relevant as benthic reef complexity shifts in the Anthropocene.

Research priorities for the sustainability of coral-rich western Pacific seascapes

Nearly a billion people depend on tropical seascapes. The need to ensure sustainable use of these vital areas is recognised, as one of 17 policy commitments made by world leaders, in Sustainable Development Goal (SDG) 14 ('Life below Water') of the United Nations. SDG 14 seeks to secure marine sustainability by 2030. In a time of increasing social-ecological unpredictability and risk, scientists and policymakers working towards SDG 14 in the Asia-Pacific region need to know: (1) How are seascapes changing? (2) What can global society do about these changes? and (3) How can science and society together achieve sustainable seascape futures? Through a horizon scan, we identified nine emerging research priorities that clarify potential research contributions to marine sustainability in locations with high coral reef abundance. They include research on seascape geological and biological evolution and adaptation; elucidating drivers and mechanisms of change; understanding how seascape functions and services are produced, and how people depend on them; costs, benefits, and trade-offs to people in changing seascapes; improving seascape technologies and practices; learning to govern and manage seascapes for all; sustainable use, justice, and human well-being; bridging communities and epistemologies for innovative, equitable, and scale-crossing solutions; and informing resilient seascape futures through modelling and synthesis. Researchers can contribute to the sustainability of tropical seascapes by co-developing transdisciplinary understandings of people and ecosystems, emphasising the importance of equity and justice, and improving knowledge of key cross-scale and cross-level processes, feedbacks, and thresholds.

Functional and phylogenetic responses of motile cryptofauna to habitat degradation

Biodiversity of terrestrial and marine ecosystems, including coral reefs, is dominated by small, often cryptic, invertebrate taxa that play important roles in ecosystem structure and functioning. While cryptofauna community structure is determined by strong small-scale microhabitat associations, the extent to which ecological and environmental factors shape these communities are largely unknown, as is the relative importance of particular microhabitats in supporting reef trophodynamics from the bottom up. The goal of this study was to address these knowledge gaps, provided coral reefs are increasingly exposed to multiple disturbances and environmental gradients that influence habitat complexity, condition and ecosystem functioning. We compared the density, biomass, size range, phylogenetic diversity and functional roles of motile cryptofauna in Palau, Western Micronesia, among four coral-derived microhabitats representing various states of degradation (live coral [Acropora and Pocillopora], dead coral and coral rubble) from reefs along a gradient of effluent exposure. In total, 122 families across ten phyla were identified, dominated by the Arthropoda (Crustacea) and Mollusca. Cryptofauna biomass was greatest in live Pocillopora, while coral rubble contained the greatest density and diversity. Size ranges were broader in live corals than both dead coral and rubble. From a bottom-up perspective, effluent exposure had mixed effects on cryptic communities including a decline in total biomass in rubble. From a top-down perspective, cryptofauna were generally unaffected by predator biomass. Our data show that, as coral reef ecosystems continue to decline in response to more frequent and severe disturbances, habitats other than live coral may become increasingly important in supporting coral reef biodiversity and food webs.

Drivers of global mangrove loss and gain in social-ecological systems

Mangrove forests store high amounts of carbon, protect communities from storms, and support fisheries. Mangroves exist in complex social-ecological systems, hence identifying socioeconomic conditions associated with decreasing losses and increasing gains remains challenging albeit important. The impact of national governance and conservation policies on mangrove conservation at the landscape-scale has not been assessed to date, nor have the interactions with local economic pressures and biophysical drivers. Here, we assess the relationship between socioeconomic and biophysical variables and mangrove change across coastal geomorphic units worldwide from 1996 to 2016. Globally, we find that drivers of loss can also be drivers of gain, and that drivers have changed over 20 years. The association with economic growth appears to have reversed, shifting from negatively impacting mangroves in the first decade to enabling mangrove expansion in the second decade. Importantly, we find that community forestry is promoting mangrove expansion, whereas conversion to agriculture and aquaculture, often occurring in protected areas, results in high loss. Sustainable development, community forestry, and co-management of protected areas are promising strategies to reverse mangrove losses, increasing the capacity of mangroves to support human-livelihoods and combat climate change.

Climate refugia on the Great Barrier Reef fail when global warming exceeds 3°C

Increases in the magnitude, frequency, and duration of warm seawater temperatures are causing mass coral mortality events across the globe. Although, even during the most extensive bleaching events, some reefs escape exposure to severe stress, constituting potential refugia. Here, we identify present-day climate refugia on the Great Barrier Reef (GBR) and project their persistence into the future. To do this, we apply semi-dynamic downscaling to an ensemble of climate projections released for the IPCC's recent sixth Assessment Report. We find that GBR locations experiencing the least thermal stress over the past 20 years have done so because of their oceanographic circumstance, which implies that longer-term persistence of climate refugia is feasible. Specifically, tidal and wind mixing of warm water away from the sea surface appears to provide relief from warming. However, on average this relative advantage only persists until global warming exceeds ~3°C.

A roadmap to integrating resilience into the practice of coral reef restoration

Recent warm temperatures driven by climate change have caused mass coral bleaching and mortality across the world, prompting managers, policymakers, and conservation practitioners to embrace restoration as a strategy to sustain coral reefs. Despite a proliferation of new coral reef restoration efforts globally and increasing scientific recognition and research on interventions aimed at supporting reef resilience to climate impacts, few restoration programs are currently incorporating climate change and resilience in project design. As climate change will continue to degrade coral reefs for decades to come, guidance is needed to support managers and restoration practitioners to conduct restoration that promotes resilience through enhanced coral reef recovery, resistance, and adaptation. Here, we address this critical implementation gap by providing recommendations that integrate resilience principles into restoration design and practice, including for project planning and design, coral selection, site selection, and broader ecosystem context. We also discuss future opportunities to improve restoration methods to support enhanced outcomes for coral reefs in response to climate change. As coral reefs are one of the most vulnerable ecosystems to climate change, interventions that enhance reef resilience will help to ensure restoration efforts have a greater chance of success in a warming world. They are also more likely to provide essential contributions to global targets to protect natural biodiversity and the human communities that rely on reefs.

Projecting coral responses to intensifying marine heatwaves under ocean acidification

Over this century, coral reefs will run the gauntlet of climate change, as marine heatwaves (MHWs) become more intense and frequent, and ocean acidification (OA) progresses. However, we still lack a quantitative assessment of how, and to what degree, OA will moderate the responses of corals to MHWs as they intensify throughout this century. Here, we first projected future MHW intensities for tropical regions under three future greenhouse gas emissions scenario (representative concentration pathways, RCP2.6, RCP4.5 and RCP8.5) for the near-term (2021-2040), mid-century (2041-2060) and late-century (2081-2100). We then combined these MHW intensity projections with a global data set of 1,788 experiments to assess coral attribute performance and survival under the three emissions scenarios for the near-term, mid-century and late-century in the presence and absence of OA. Although warming and OA had predominately additive impacts on the coral responses, the contribution of OA in affecting most coral attributes was minor relative to the dominant role of intensifying MHWs. However, the addition of OA led to greater decreases in photosynthesis and survival under intermediate and unrestricted emissions scenario for the mid- and late-century than if intensifying MHWs were considered as the only driver. These results show that role of OA in modulating coral responses to intensifying MHWs depended on the focal coral attribute and extremity of the scenario examined. Specifically, intensifying MHWs and OA will cause increasing instances of coral bleaching and substantial declines in coral productivity, calcification and survival within the next two decades under the low and intermediate emissions scenario. These projections suggest that corals must rapidly adapt or acclimatize to projected ocean conditions to persist, which is far more likely under a low emissions scenario and with increasing efforts to manage reefs to enhance resilience.

The importance of 1.5°C warming for the Great Barrier Reef

Tropical coral reefs are among the most sensitive ecosystems to climate change and will benefit from the more ambitious aims of the United Nations Framework Convention on Climate Change's Paris Agreement, which proposed to limit global warming to 1.5° rather than 2°C above pre-industrial levels. Only in the latest Intergovernmental Panel on Climate Change focussed assessment, the Coupled Model Intercomparison Project phase 6 (CMIP6), have climate models been used to investigate the 1.5° warming scenario directly. Here, we combine the most recent model updates from CMIP6 with a semi-dynamic downscaling to evaluate the difference between the 1.5 and 2°C global warming targets on coral thermal stress metrics for the Great Barrier Reef (GBR). By ~2080, severe bleaching events are expected to occur annually under intensifying emissions (shared socioeconomic pathway SSP5-8.5). Adherence to 2° warming (SSP1-2.6) halves this frequency but the main benefit of confining warming to 1.5° (SSP1-1.9) is that bleaching events are reduced further to 3 events per decade. Attaining low emissions of 1.5° is also paramount to prevent the mean magnitude of thermal stress from stabilizing close to a critical thermal threshold (8 Degree Heating Weeks). Thermal stress under the more pessimistic pathways SSP3-7.0 and SSP5-8.5 is three to fourfold higher than the present day, with grave implications for future reef ecosystem health. As global warming continues, our projections also indicate more regional warming in the central and southern GBR than the far north and northern GBR.

Knowledge Gaps in the Biology, Ecology, and Management of the Pacific Crown-of-Thorns Sea Star Acanthaster sp. on Australia's Great Barrier Reef

AbstractCrown-of-thorns sea stars (Acanthaster sp.) are among the most studied coral reef organisms, owing to their propensity to undergo major population irruptions, which contribute to significant coral loss and reef degradation throughout the Indo-Pacific. However, there are still important knowledge gaps pertaining to the biology, ecology, and management of Acanthaster sp. Renewed efforts to advance understanding and management of Pacific crown-of-thorns sea stars (Acanthaster sp.) on Australia's Great Barrier Reef require explicit consideration of relevant and tractable knowledge gaps. Drawing on established horizon scanning methodologies, this study identified contemporary knowledge gaps by asking active and/or established crown-of-thorns sea star researchers to pose critical research questions that they believe should be addressed to improve the understanding and management of crown-of-thorns sea stars on the Great Barrier Reef. A total of 38 participants proposed 246 independent research questions, organized into 7 themes: feeding ecology, demography, distribution and abundance, predation, settlement, management, and environmental change. Questions were further assigned to 48 specific topics nested within the 7 themes. During this process, redundant questions were removed, which reduced the total number of distinct research questions to 172. Research questions posed were mostly related to themes of demography (46 questions) and management (48 questions). The dominant topics, meanwhile, were the incidence of population irruptions (16 questions), feeding ecology of larval sea stars (15 questions), effects of elevated water temperature on crown-of-thorns sea stars (13 questions), and predation on juveniles (12 questions). While the breadth of questions suggests that there is considerable research needed to improve understanding and management of crown-of-thorns sea stars on the Great Barrier Reef, the predominance of certain themes and topics suggests a major focus for new research while also providing a roadmap to guide future research efforts.

Integrating environmental variability to broaden the research on coral responses to future ocean conditions

Our understanding of the response of reef-building corals to changes in their physical environment is largely based on laboratory experiments, analysis of long-term field data, and model projections. Experimental data provide unique insights into how organisms respond to variation of environmental drivers. However, an assessment of how well experimental conditions cover the breadth of environmental conditions and variability where corals live successfully is missing. Here, we compiled and analyzed a globally distributed dataset of in-situ seasonal and diurnal variability of key environmental drivers (temperature, pCO₂ , and O₂ ) critical for the growth and livelihood of reef-building corals. Using a meta-analysis approach, we compared the variability of environmental conditions assayed in coral experimental studies to current and projected conditions in their natural habitats. We found that annual temperature profiles projected for the end of the 21st century were characterized by distributional shifts in temperatures with warmer winters and longer warm periods in the summer, not just peak temperatures. Furthermore, short-term hourly fluctuations of temperature and pCO₂ may regularly expose corals to conditions beyond the projected average increases for the end of the 21st century. Coral reef sites varied in the degree of coupling between temperature, pCO₂ , and dissolved O₂ , which warrants site-specific, differentiated experimental approaches depending on the local hydrography and influence of biological processes on the carbonate system and O₂ availability. Our analysis highlights that a large portion of the natural environmental variability at short and long timescales is underexplored in experimental designs, which may provide a path to extend our understanding on the response of corals to global climate change.

Cumulative bleaching undermines systemic resilience of the Great Barrier Reef

Climate change and ENSO have triggered five mass coral bleaching events on Australia's Great Barrier Reef (GBR), three of which occurred in the last 5 years.^1-5 Here, we explore the cumulative nature of recent impacts and how they fragment the reef's connectivity. The coverage and intensity of thermal stress have increased steadily over time. Cumulative bleaching in 2016, 2017, and 2020 is predicted to have reduced systemic larval supply by 26%, 50%, and 71%, respectively. Larval disruption is patchy and can guide interventions. The majority of severely bleached reefs (75%) are predicted to have experienced an 80%-100% loss of larval supply. Yet restoration would not be cost-effective in the 2% of such reefs (∼30) that still experience high larval supply. Managing such climate change impacts will benefit from emerging theory on the facilitation of genetic adaptation,^6,7 which requires the existence of regions with predictably high or low thermal stress. We find that a third of reefs constitute warm spots that have consistently experienced bleaching stress. Moreover, 13% of the GBR are potential refugia that avoid significant warming more than expected by chance, with a modest proportion (14%) within highly protected areas. Coral connectivity is likely to become increasingly disrupted given the predicted escalation of climate-driven disturbances,⁸ but the existence of thermal refugia, potentially capable of delivering larvae to 58% of the GBR, may provide pockets of systemic resilience in the near-term. Theories of conservation planning for climate change will need to consider a shifting portfolio of thermal environments over time.

Marine reserves, fisheries ban, and 20 years of positive change in a coral reef ecosystem

By 2004, Belize was exhibiting classic fishing down of the food web. Groupers (Serranidae) and snappers (Lutjanidae) were scarce and fisheries turned to parrotfishes (Scarinae), leading to a 41% decline in their biomass. Several policies were enacted in 2009-2010, including a moratorium on fishing parrotfish and a new marine park with no-take areas. Using a 20-year time series on reef fish and benthos, we evaluated the impact of these policies approximately 10 years after their implementation. Establishment of the Southwater Caye Marine Reserve led to a recovery of snapper at 2 out of 3 sites, but there was no evidence of recovery outside the reserve. Snapper populations in an older reserve continued to increase, implying that at least 9 years is required for their recovery. Despite concerns over the feasibility of banning parrotfish harvest once it has become a dominant fin fishery, parrotfishes returned and exceeded biomass levels prior to the fishery. The majority of these changes involved an increase in parrotfish density; species composition and adult body size generally exhibited little change. Recovery occurred equally well in reserves and areas open to other forms of fishing, implying strong compliance. Temporal trends in parrotfish grazing intensity were strongly negatively associated with the cover of macroalgae, which by 2018 had fallen to the lowest levels observed since measurements began in 1998. Coral populations remained resilient and continued to exhibit periods of net recovery after disturbance. We found that a moratorium on parrotfish harvesting is feasible and appears to help constrain macroalgae, which can otherwise impede coral resilience.

Benthic micro- and macro-community succession and coral recruitment under overfishing and nutrient enrichment

Herbivory and nutrient availability are fundamental drivers of benthic community succession in shallow marine systems, including coral reefs. Despite the importance of early community succession for coral recruitment and recovery, studies characterizing the impact of top-down and bottom-up drivers on micro- and macrobenthic communities at scales relevant to coral recruitment are lacking. Here, a combination of tank and field experiments were used to assess the effects of herbivore exclusion and nutrient enrichment on micro- to macrobenthic community succession and subsequent coral recruitment success. Herbivore exclusion had the strongest effect on micro- and macrobenthic community succession, including a community shift toward copiotrophic and potentially opportunistic/pathogenic microorganisms, an increased cover of turf and macroalgae, and decreased cover of crustose coralline algae. Yet, when corals settled prior to the development of a macrobenthic community, rates of post-settlement survival increased when herbivores were excluded, benefiting from the predation refugia provided by cages during their vulnerable early post-settlement stage. Interestingly, survival on open tiles was negatively correlated with the relative abundance of the bacterial order Rhodobacterales, an opportunistic microbial group previously associated with stressed and diseased corals. Development of micro- and macrobenthic communities in the absence of herbivory, however, led to reduced coral settlement. In turn, there were no differences in post-settlement survival between open and caged treatments for corals settled on tiles with established benthic communities. As a result, open tiles experienced marginally higher recruitment rates, driven primarily by the higher initial number of settlers on open tiles compared to caged tiles. Overall, we reveal that the primary interaction driving coral recruitment is the positive effect of herbivory in creating crustose coralline algae (CCA)-dominated habitats, free of fleshy algae and associated opportunistic microbes, to enhance coral settlement. The negative direct and indirect impact of fish predation on newly settled corals was outweighed by the positive effect of herbivory on the initial rate of coral settlement. In turn, the addition of nutrients further altered benthic community succession in the absence of herbivory, reducing coral post-settlement survival. However, the overall impact of nutrients on coral recruitment dynamics was minor relative to herbivory.

Evolution and connectivity influence the persistence and recovery of coral reefs under climate change in the Caribbean, Southwest Pacific, and Coral Triangle

Corals are experiencing unprecedented decline from climate change-induced mass bleaching events. Dispersal not only contributes to coral reef persistence through demographic rescue but can also hinder or facilitate evolutionary adaptation. Locations of reefs that are likely to survive future warming therefore remain largely unknown, particularly within the context of both ecological and evolutionary processes across complex seascapes that differ in temperature range, strength of connectivity, network size, and other characteristics. Here, we used eco-evolutionary simulations to examine coral adaptation to warming across reef networks in the Caribbean, the Southwest Pacific, and the Coral Triangle. We assessed the factors associated with coral persistence in multiple reef systems to understand which results are general and which are sensitive to particular geographic contexts. We found that evolution can be critical in preventing extinction and facilitating the long-term recovery of coral communities in all regions. Furthermore, the strength of immigration to a reef (destination strength) and current sea surface temperature robustly predicted reef persistence across all reef networks and across temperature projections. However, we found higher initial coral cover, slower recovery, and more evolutionary lag in the Coral Triangle, which has a greater number of reefs and more larval settlement than the other regions. We also found the lowest projected future coral cover in the Caribbean. These findings suggest that coral reef persistence depends on ecology, evolution, and habitat network characteristics, and that, under an emissions stabilization scenario (RCP 4.5), recovery may be possible over multiple centuries.

Scaling the effects of ocean acidification on coral growth and coral-coral competition on coral community recovery

Ocean acidification (OA) is negatively affecting calcification in a wide variety of marine organisms. These effects are acute for many tropical scleractinian corals under short-term experimental conditions, but it is unclear how these effects interact with ecological processes, such as competition for space, to impact coral communities over multiple years. This study sought to test the use of individual-based models (IBMs) as a tool to scale up the effects of OA recorded in short-term studies to community-scale impacts, combining data from field surveys and mesocosm experiments to parameterize an IBM of coral community recovery on the fore reef of Moorea, French Polynesia. Focusing on the dominant coral genera from the fore reef, Pocillopora, Acropora, Montipora and Porites, model efficacy first was evaluated through the comparison of simulated and empirical dynamics from 2010-2016, when the reef was recovering from sequential acute disturbances (a crown-of-thorns seastar outbreak followed by a cyclone) that reduced coral cover to ~0% by 2010. The model then was used to evaluate how the effects of OA (1,100-1,200 µatm pCO₂) on coral growth and competition among corals affected recovery rates (as assessed by changes in % cover y^-1) of each coral population between 2010-2016. The model indicated that recovery rates for the fore reef community was halved by OA over 7 years, with cover increasing at 11% y^-1 under ambient conditions and 4.8% y^-1 under OA conditions. However, when OA was implemented to affect coral growth and not competition among corals, coral community recovery increased to 7.2% y^-1, highlighting mechanisms other than growth suppression (i.e., competition), through which OA can impact recovery. Our study reveals the potential for IBMs to assess the impacts of OA on coral communities at temporal and spatial scales beyond the capabilities of experimental studies, but this potential will not be realized unless empirical analyses address a wider variety of response variables representing ecological, physiological and functional domains.

Evolution reverses the effect of network structure on metapopulation persistence

Global environmental change is challenging species with novel conditions, such that demographic and evolutionary trajectories of populations are often shaped by the exchange of organisms and alleles across landscapes. Current ecological theory predicts that random networks with dispersal shortcuts connecting distant sites can promote persistence when there is no capacity for evolution. Here, we show with an eco‐evolutionary model that dispersal shortcuts across environmental gradients instead hinder persistence for populations that can evolve because long‐distance migrants bring extreme trait values that are often maladaptive, short‐circuiting the adaptive response of populations to directional change. Our results demonstrate that incorporating evolution and environmental heterogeneity fundamentally alters theoretical predictions regarding persistence in ecological networks.

Revisiting the paradigm of shark-driven trophic cascades in coral reef ecosystems

Global overfishing of higher-level predators has caused cascading effects to lower trophic levels in many marine ecosystems. On coral reefs, which support highly diverse food webs, the degree to which top-down trophic cascades can occur remains equivocal. Using extensive survey data from coral reefs across the relatively unfished northern Great Barrier Reef (nGBR), we quantified the role of reef sharks in structuring coral reef fish assemblages. Using a structural equation modeling (SEM) approach, we explored the interactions between shark abundance and teleost mesopredator and prey functional group density and biomass, while explicitly accounting for the potentially confounding influence of environmental variation across sites. Although a fourfold difference in reef shark density was observed across our survey sites, this had no impact on either the density or biomass of teleost mesopredators or prey, providing evidence for a lack of trophic cascading across nGBR systems. Instead, many functional groups, including sharks, responded positively to environmental drivers. We found reef sharks to be positively associated with habitat complexity. In turn, physical processes such as wave exposure and current velocity were both correlated well with multiple functional groups, reflecting how changes to energetic conditions and food availability, or modification of habitat affect fish distribution. The diversity of species within coral reef food webs and their associations with bottom-up drivers likely buffers against trophic cascading across GBR functional guilds when reef shark assemblages are depleted, as has been demonstrated in other complex ecosystems.

Ecology: Returning Caribbean Coral Reefs to Their Former Glory

Once spectacular coral reefs have often become overrun by persistent seaweed. A new study reveals that elevating the density of herbivorous spider crabs to unnatural levels can reduce seaweed and help corals recover.

Coupled beta diversity patterns among coral reef benthic taxa

Unraveling the processes that drive diversity patterns remains a central challenge for ecology, and an increased understanding is especially urgent to address and mitigate escalating diversity loss. Studies have primarily focused on singular taxonomic groups, but recent research has begun evaluating spatial diversity patterns across multiple taxonomic groups and suggests taxa may have congruence in their diversity patterns. Here, we use surveys of the coral reef benthic groups: scleractinian corals, macroalgae, sponges and gorgonians conducted in the Bahamian Archipelago across 27 sites to determine if there is congruence between taxonomic groups in their site-level diversity patterns (i.e. alpha diversity: number of species, and beta diversity: differences in species composition) while accounting for environmental predictors (i.e. depth, wave exposure, market gravity (i.e. human population size and distance to market), primary productivity, and grazing). Overall, we found that the beta diversities of these benthic groups were significant predictors of each other. The most consistent relationships existed with algae and coral, as their beta diversity was a significant predictor of every other taxa's beta diversity, potentially due to their strong biotic interactions and dominance on the reef. Conversely, we found no congruence patterns in the alpha diversity of the taxa. Market gravity and exposure showed the most prevalent correlation with both alpha and beta diversity for the taxa. Overall, our results suggest that coral reef benthic taxa can have spatial congruence in species composition, but not number of species, and that future research on biodiversity trends should consider that taxa may have non-independent patterns.

Sedimentation and overfishing drive changes in early succession and coral recruitment

Sedimentation and overfishing are important local stressors on coral reefs that can independently result in declines in coral recruitment and shifts to algal-dominated states. However, the role of herbivory in driving recovery across environmental gradients is often unclear. Here we investigate early successional benthic communities and coral recruitment across a sediment gradient in Palau, Micronesia over a 12-month period. Total sedimentation rates measured by 'TurfPods' varied from 0.03 ± 0.1 SE mg cm^-2 d^-1 at offshore sites to 1.32 ± 0.2 mg cm^-2 d^-1 at inshore sites. To assess benthic succession, three-dimensional settlement tiles were deployed at sites with experimental cages used to exclude tile access to larger herbivorous fish. Benthic assemblages exhibited rapid transitions across the sediment gradient within three months of deployment. At low levels of sedimentation (less than 0.6 mg cm^-2 d^-1), herbivory resulted in communities dominated by coral recruitment inducers (short turf algae and crustose coralline algae), whereas exclusion of herbivores resulted in the overgrowth of coral inhibitors (encrusting and upright foliose macroalgae). An 'inducer threshold' was found under increasing levels of sedimentation (greater than 0.6 mg cm^-2 d^-1), with coral inducers having limited to no presence in communities, and herbivore access to tiles resulted in sediment-laden turf algal assemblages, while exclusion of herbivores resulted in invertebrates (sponges, ascidians) and terrestrial sediment accumulation. A 'coral recruitment threshold' was found at 0.8 mg cm^-2 d^-1, below which net coral recruitment was reduced by 50% in the absence of herbivores, while recruitment was minimal above the threshold. Our results highlight nonlinear trajectories of benthic succession across sediment gradients and identify strong interactions between sediment and herbivory that have cascading effects on coral recruitment. Local management strategies that aim to reduce sedimentation and turbidity and manage herbivore fisheries can have measurable effects on benthic community succession and coral recruitment, enhancing reef resilience and driving coral recovery.

Interventions to help coral reefs under global change-A complex decision challenge

Climate change is impacting coral reefs now. Recent pan-tropical bleaching events driven by unprecedented global heat waves have shifted the playing field for coral reef management and policy. While best-practice conventional management remains essential, it may no longer be enough to sustain coral reefs under continued climate change. Nor will climate change mitigation be sufficient on its own. Committed warming and projected reef decline means solutions must involve a portfolio of mitigation, best-practice conventional management and coordinated restoration and adaptation measures involving new and perhaps radical interventions, including local and regional cooling and shading, assisted coral evolution, assisted gene flow, and measures to support and enhance coral recruitment. We propose that proactive research and development to expand the reef management toolbox fast but safely, combined with expedient trialling of promising interventions is now urgently needed, whatever emissions trajectory the world follows. We discuss the challenges and opportunities of embracing new interventions in a race against time, including their risks and uncertainties. Ultimately, solutions to the climate challenge for coral reefs will require consideration of what society wants, what can be achieved technically and economically, and what opportunities we have for action in a rapidly closing window. Finding solutions that work for coral reefs and people will require exceptional levels of coordination of science, management and policy, and open engagement with society. It will also require compromise, because reefs will change under climate change despite our best interventions. We argue that being clear about society's priorities, and understanding both the opportunities and risks that come with an expanded toolset, can help us make the most of a challenging situation. We offer a conceptual model to help reef managers frame decision problems and objectives, and to guide effective strategy choices in the face of complexity and uncertainty.

The commercially important shoemaker spinefoot, Siganus sutor, connects coral reefs to neighbouring seagrass meadows

Spatial management of fish populations can potentially be optimized by determining the area of influence of a particular species. We performed an acoustic tagging study implemented on Denis Island in the Seychelles to assess the area of influence of the heavily targeted shoemaker spinefoot, Siganus sutor. We investigated whether this species acts as a mobile link between coral patches and seagrass meadows, and whether their movements differed between day and night. The study incorporated an array of 22 acoustic stations deployed within dense coral patches, seagrass meadows and mixed habitats of both seagrass and coral. Fifteen S. sutor carrying internal acoustic tags were monitored from November 2016 until May 2017. Detection patterns revealed them to be diurnal herbivores, with only rare nocturnal movements. Home-range estimates showed that individuals differed in their spatial range extents and habitats used, covering ~15% of the total shallow subtidal coastline of the island. However, they displayed very small daily movements (<200 m), concentrated mainly around sites within mixed coral and seagrass habitats. An optimal number of detections was recorded when the coral to seagrass area ratio was approximately 1.6:1. This ratio was confirmed through statistical prediction modelling. Identification of such links of commercially important species between networked habitats may help authorities consider incorporating seagrass meadows of the Seychelles into management discussions, which are currently lacking.

Growth responses of branching versus massive corals to ocean warming on the Great Barrier Reef, Australia

As oceans continue to warm under climate change, understanding the differential growth responses of corals is increasingly important. Scleractinian corals exhibit a broad range of life-history strategies, yet few studies have explored interspecific variation in long-term growth rates under a changing climate. Here we studied growth records of two coral species with different growth forms, namely branching Isopora palifera and massive Porites spp. at an offshore reef (Myrmidon Reef) of the central Great Barrier Reef (GBR), Australia. Skeletal growth chronologies were constructed using a combination of X-radiographs, gamma densitometry, and trace element (Sr/Ca) analysis. General additive mixed-effect models (GAMMs) revealed that skeletal density of I. palifera declined linearly and significantly at a rate of 1.2% yr^-1 between 2002 and 2012. Calcification was stable between 2002 and 2009, yet declined significantly at a rate of 12% yr^-1 between 2009 and 2012 following anomalously high sea surface temperatures (SST). Skeletal density of massive Porites exhibited a significant non-linear response over the 11-year study period (2002-2012) in that density was temporarily reduced during the 2009-2010 anomalously hot years, while linear extension and calcification showed no significant trends. Linear extension, density and calcification rates of I. palifera increased to maximum growth of 26.7-26.9 °C, beyond which they declined. In contrast, calcification and linear extension of Porites exhibited no response to SST, but exhibited a significant linear decline in skeletal density with increasing SST. Our results reveal significant differences in coral growth patterns among coral growth forms, and highlight both the resistant nature of massive Porites and sensitivity of branching I. palifera. Future research should target a broad range of coral taxa within similar environments to provide a community-level response of ocean warming on coral reef communities.

Benthic-based contributions to climate change mitigation and adaptation

Innovative solutions to improve the condition and resilience of ecosystems are needed to address societal challenges and pave the way towards a climate-resilient future. Nature-based solutions offer the potential to protect, sustainably manage and restore natural or modified ecosystems while providing multiple other benefits for health, the economy, society and the environment. However, the implementation of nature-based solutions stems from a discourse that is almost exclusively derived from a terrestrial and urban context and assumes that risk reduction is resolved locally. We argue that this position ignores the importance of complex ecological interactions across a range of temporal and spatial scales and misses the substantive contribution from marine ecosystems, which are notably absent from most climate mitigation and adaptation strategies that extend beyond coastal disaster management. Here, we consider the potential of sediment-dwelling fauna and flora to inform and support nature-based solutions, and how the ecology of benthic environments can enhance adaptation plans. We illustrate our thesis with examples of practice that are generating, or have the potential to deliver, transformative change and discuss where further innovation might be applied. Finally, we take a reflective look at the realized and potential capacity of benthic-based solutions to contribute to adaptation plans and offer our perspectives on the suitability and shortcomings of past achievements and the prospective rewards from sensible prioritization of future research. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.

Mangroves reduce the vulnerability of coral reef fisheries to habitat degradation

Despite general and wide-ranging negative effects of coral reef degradation on reef communities, hope might exist for reef-associated predators that use nursery habitats. When reef structural complexity is lost, refuge density declines and prey vulnerability increases. Here, we explore whether the presence of nursery habitats can promote high predator productivity on degraded reefs by mitigating the costs of increased vulnerability in early life, whilst allowing for the benefits of increased food availability in adulthood. We apply size-based ecosystem models of coral reefs with high and low structural complexity to predict fish biomass and productivity in the presence and absence of mangrove nurseries. Our scenarios allow us to elucidate the interacting effects of refuge availability and ontogenetic habitat shifts for fisheries productivity. We find that low complexity, degraded reefs with nurseries can support fisheries productivity that is equal to or greater than that in complex reefs that lack nurseries. We compare and validate model predictions with field data from Belize. Our results should inform reef fisheries management strategies and protected areas now and into the future.

Despite wide-ranging negative effects of coral reef degradation on reef communities, hope might exist for reef-associated predators that use nursery habitats. This study uses size-based ecosystem models of coral reefs to assess the effects of the presence and absence of mangrove nurseries.

Preferences and perceptions of the recreational spearfishery of the Great Barrier Reef

Recreational fishing practices can have significant impacts on marine ecosystems but their catch dynamics are often difficult to quantify, particularly for spearfishing. On coral reefs, the impacts of recreational spearfishing are often considered to be negligible compared to other practices, but the highly selective method adopted by spearfishers can result in locally distinct ecological consequences. Here we investigated the spatial patterns and catch composition of recreational spearfishers on the Great Barrier Reef using an online survey (n = 141 participants) targeted at spearfishers active along the coastline of Queensland. Observations from within the Queensland spearfishing community were also used to explore perceived changes in catches of three functionally distinct spearing targets. Preferred reef regions (coastal, inshore, offshore) differed among spearfishers from Bundaberg (south) to Cooktown (north). The piscivorous coral trout, Plectropomus leopardus, was suggested to be the preferred target comprising 34% (±1.5 SE) of spearfishers' reported catch composition. Spearfishers also noted a variety of changes in their catch composition over time, particularly regarding parrotfishes (decreased landings) and tuskfishes (increased landings). How this relates to the relative abundance and population biology of these taxa on the Great Barrier Reef requires attention. Spearfishers can provide important information regarding the status of their fishery through direct observations, which can inform legislation when acknowledged.

Split spawning increases robustness of coral larval supply and inter-reef connectivity

Many habitat-building corals undergo mass synchronous spawning events. Yet, despite the enormous amounts of larvae produced, larval dispersal from a single spawning event and the reliability of larval supply are highly dependent on vagaries of ocean currents. However, colonies from the same population will occasionally spawn over successive months. These split spawning events likely help to realign reproduction events to favourable environmental conditions. Here, we show that split spawning may benefit corals by increasing the reliability of larval supply. By modelling the dispersal of coral larvae across Australia's Great Barrier Reef, we find that split spawning increased the diversity of sources and reliability of larval supply the reefs could receive, especially in regions with low and intrinsically variable connectivity. Such increased larval supply might help counteract the expected declines in reproductive success associated with split spawning events.

Survival of a grey reef shark Carcharhinus amblyrhynchos without a dorsal fin

An adult, female grey reef shark Carcharhinus amblyrhnchos was observed missing its first dorsal fin in 2014. The same individual was re-photographed 4 years later indicating that this numerically dominant reef shark can survive total loss of its first dorsal fin. While this disability may impair the shark's ability to undertake pursuit predation, the species has a diversity of foraging modes that probably facilitates survival.