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.

Coloring coral larvae allows tracking of local dispersal and settlement.

Quantifying patterns of dispersal and settlement in marine benthic invertebrates is challenging, largely due the complexity of life history traits, small sizes of larvae (<1 mm), and potential for large-scale dispersal (>100 km) in the marine environment. Here, we develop a novel method that allows for immediate differentiation and visual tracking of large numbers of coral larvae (106 to 109) from dispersal to settlement. Neutral red and Nile blue stains were extremely effective in coloring larvae, with minimal impacts on survival and settlement following optimization of incubation times and stain concentrations. Field validation to wild-captured larvae from the Great Barrier Reef demonstrates the efficacy of staining across diverse taxa. The method provides a simple, rapid (<60 minutes), low-cost (approximately USD$1 per 105 larva) tool to color coral larvae that facilitates a wide range of de novo laboratory and field studies of larval behavior and ecology with potential applications for conservation planning and understanding patterns of connectivity.

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.

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.

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.

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.

Reservas Marinas, Vedas Pesqueras y 20 Años de Cambios Positivos en un Ecosistema de Arrecife de Coral Resumen Para el año 2004, Belice estaba exhibiendo la clásica pesca de los niveles más bajos de las cadenas alimenticias marinas. Los meros (Serranidae) y los pargos (Lutjanidae) eran escasos y las pesquerías comenzaron a consumir a los peces loro (Scarinae), lo que resultó en una declinación del 41% de su biomasa. Entre el 2009 y el 2010 se promulgaron varias políticas, incluyendo una moratoria para la pesca del pez loro y un nuevo parque marino con zonas de no consumo. Mediante una serie temporal de 20 años para los peces de arrecifes y el bentos, evaluamos el impacto de estas políticas aproximadamente diez años después de su implementación. La creación de la Reserva Marina del Cayo Southwater resultó en la recuperación del pargo en dos de tres sitios, pero no hubo evidencias de la recuperación fuera de la reserva. Las poblaciones de pargos en una reserva más vieja continuaron su incremento, lo que implica que se requieren al menos nueve años para su recuperación. A pesar de la preocupación por la viabilidad de la veda para el pez loro una vez que se haya convertido en una pesquería dominante, los peces loro regresaron al sitio de pesca y excedieron los niveles de biomasa previos a la pesquería. La mayoría de estos cambios involucró un incremento en la densidad de los peces loro; la composición de especies y la talla corporal adulta generalmente exhibieron pocos cambios. La recuperación ocurrió equitativamente bien en las reservas y en las áreas abiertas a otras formas de pesca, lo que implica un estricto cumplimiento de las restricciones. Las tendencias temporales en la intensidad de pastoreo de los peces loro estuvieron fuertemente asociadas de manera negativa con la cobertura de macroalgas, la cual para el 2018 había caído a los niveles más bajos observados desde que se comenzó a medir en 1998. Las poblaciones coralinas permanecieron resilientes y continuaron exhibiendo periodos de recuperación neta después de la perturbación. Descubrimos que una moratoria para la pesca de pez loro es viable y parece ayudar a restringir las macroalgas, las cuales de otra forma pueden impedir la resiliencia del coral.

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.

U-Th dating reveals regional-scale decline of branching Acropora corals on the Great Barrier Reef over the past century.

Hard coral cover on the Great Barrier Reef (GBR) is on a trajectory of decline. However, little is known about past coral mortality before the advent of long-term monitoring (circa 1980s). Using paleoecological analysis and high-precision uranium-thorium (U-Th) dating, we reveal an extensive loss of branching Acropora corals and changes in coral community structure in the Palm Islands region of the central GBR over the past century. In 2008, dead coral assemblages were dominated by large, branching Acropora and living coral assemblages by genera typically found in turbid inshore environments. The timing of Acropora mortality was found to be occasionally synchronous among reefs and frequently linked to discrete disturbance events, occurring in the 1920s to 1960s and again in the 1980s to 1990s. Surveys conducted in 2014 revealed low Acropora cover (<5%) across all sites, with very little evidence of change for up to 60 y at some sites. Collectively, our results suggest a loss of resilience of this formerly dominant key framework builder at a regional scale, with recovery severely lagging behind predictions. Our study implies that the management of these reefs may be predicated on a shifted baseline.

Broadening the taxonomic scope of coral reef palaeoecological studies using ancient DNA.

Marine environments face acute pressures from human impacts, often resulting in substantial changes in community structure. On the inshore Great Barrier Reef (GBR), palaeoecological studies show the collapse of the previously dominant coral Acropora from the impacts of degraded water quality associated with European colonization. Even more dramatic impacts can result in the replacement of corals by fleshy macroalgae on modern reefs, but their past distribution is unknown because they leave no fossil record. Here, we apply DNA metabarcoding and high-throughput sequencing of the 18S rDNA gene on palaeoenvironmental DNA (aeDNA) derived from sediment cores at two sites on Pandora Reef (GBR), to enhance palaeoecological studies by incorporating key soft-bodied taxa, including macroalgae. We compared temporal trends in this aeDNA record with those of coral genera derived from macrofossils. Multivariate analysis of 12 eukaryotic groups from the aeDNA community showed wide variability over the past 750 years. The occurrence of brown macroalgae was negatively correlated only with the dominant coral at both sites. The occurrence of coralline and green macroalgae was positively correlated with only the dominant coral at one of the sites, where we also observed a significant association between the whole coral community and the occurrence of each of the three macroalgae groups. Our results demonstrate that reef sediments can provide a valuable archive for understanding the past distribution and occurrence of important soft-bodied reef dwellers. Combining information from fossils and aeDNA provides an enhanced understanding of temporal changes of reefs ecosystems at decadal to millennial timescales.

Sensitivity of coral recruitment to subtle shifts in early community succession.

Community succession following disturbance depends on positive and negative interactions, the strength of which change along environmental gradients. To investigate how early succession affects coral reef recovery, we conducted an 18-month experiment in Palau, using recruitment tiles and herbivore exclusion cages. One set of reefs has higher wave exposure and had previously undergone a phase shift to macroalgae following a major typhoon, whereas the other set of reefs have lower wave exposure and did not undergo a macroalgal phase shift. Similar successional trajectories were observed at all sites when herbivores were excluded: turf algae dominated early succession, followed by shifts to foliose macroalgae and heterotrophic invertebrates. However, trajectories differed in the presence of herbivores. At low wave exposure reefs, herbivores promoted coralline algae and limited turf and encrusting fleshy algae in crevice microhabitats, facilitating optimal coral recruitment. Under medium wave exposure, relatively higher but still low coverage of turf and encrusting fleshy algae (15-25%) found in crevice microhabitats inhibited coral recruitment, persisting throughout multiple recruitment events. Our results indicate that altered interaction strength in different wave environments following disturbance can drive subtle changes in early succession that cascade to alter secondary succession to coral recruitment and system recovery.

Decadal-scale rates of reef erosion following El Niño-related mass coral mortality.

As the frequency and intensity of coral mortality events increase under climate change, understanding how declines in coral cover may affect the bioerosion of reef frameworks is of increasing importance. Here, we explore decadal-scale rates of bioerosion of the framework building coral Orbicella annularis by grazing parrotfish following the 1997/1998 El Niño-related mass mortality event at Long Cay, Belize. Using high-precision U-Th dating and CT scan analysis, we quantified in situ rates of external bioerosion over a 13-year period (1998-2011). Based upon the error-weighted average U-Th age of dead O. annularis skeletons, we estimate the average external bioerosion between 1998 and 2011 as 0.92 ± 0.55 cm depth. Empirical observations of herbivore foraging, and a nonlinear numerical response of parrotfish to an increase in food availability, were used to create a model of external bioerosion at Long Cay. Model estimates of external bioerosion were in close agreement with U-Th estimates (0.85 ± 0.09 cm). The model was then used to quantify how rates of external bioerosion changed across a gradient of coral mortality (i.e., from few corals experiencing mortality following coral bleaching to complete mortality). Our results indicate that external bioerosion is remarkably robust to declines in coral cover, with no significant relationship predicted between the rate of external bioerosion and the proportion of O. annularis that died in the 1998 bleaching event. The outcome was robust because the reduction in grazing intensity that follows coral mortality was compensated for by a positive numerical response of parrotfish to an increase in food availability. Our model estimates further indicate that for an O. annularis-dominated reef to maintain a positive state of reef accretion, a necessity for sustained ecosystem function, live cover of O. annularis must not drop below a ~5-10% threshold of cover.

Anticipative management for coral reef ecosystem services in the 21st century.

Under projections of global climate change and other stressors, significant changes in the ecology, structure and function of coral reefs are predicted. Current management strategies tend to look to the past to set goals, focusing on halting declines and restoring baseline conditions. Here, we explore a complementary approach to decision making that is based on the anticipation of future changes in ecosystem state, function and services. Reviewing the existing literature and utilizing a scenario planning approach, we explore how the structure of coral reef communities might change in the future in response to global climate change and overfishing. We incorporate uncertainties in our predictions by considering heterogeneity in reef types in relation to structural complexity and primary productivity. We examine 14 ecosystem services provided by reefs, and rate their sensitivity to a range of future scenarios and management options. Our predictions suggest that the efficacy of management is highly dependent on biophysical characteristics and reef state. Reserves are currently widely used and are predicted to remain effective for reefs with high structural complexity. However, when complexity is lost, maximizing service provision requires a broader portfolio of management approaches, including the provision of artificial complexity, coral restoration, fish aggregation devices and herbivore management. Increased use of such management tools will require capacity building and technique refinement and we therefore conclude that diversification of our management toolbox should be considered urgently to prepare for the challenges of managing reefs into the 21st century.

Phase shift facilitation following cyclone disturbance on coral reefs.

While positive interactions have been observed to influence patterns of recruitment and succession in marine and terrestrial plant communities, the role of facilitation in macroalgal phase shifts is relatively unknown. In December 2012, typhoon Bopha caused catastrophic losses of corals on the eastern reefs of Palau. Within weeks of the typhoon, an ephemeral bloom of monospecific macroalgae (Liagora sp.) was observed, reaching a peak of 38.6% cover in February 2013. At this peak, we observed a proliferation of a second macroalgal species, Lobophora variegata. Lobophora was distributed non-randomly, with higher abundances occurring within the shelter of Liagora canopies than on exposed substrates. Bite rates of two common herbivorous fish (Chlorurus sordidus and Ctenochaetus striatus) were significantly higher outside canopies (2.5- and sixfold, respectively), and cage exclusion resulted in a significant increase in Lobophora cover. Experimental removal of Liagora canopies resulted in a 53.1% decline in the surface area of Lobophora after 12 days, compared to a 51.7% increase within canopies. Collectively, these results indicate that Liagora canopies act as ecological facilitators, providing a 'nursery' exclusion zone from the impact of herbivorous fish, allowing for the establishment of understory Lobophora. While the ephemeral Liagora bloom had disappeared entirely 9 months post-typhoon, the facilitated shift to Lobophora has persisted for over 18 months, dominating ~40% of the reef substrate. While acute disturbance events such as typhoons have been suggested as a mechanism to reverse algal phase shifts, our results suggest that typhoons may also trigger, rather than just reverse, phase shifts.

Palaeoecological evidence of a historical collapse of corals at Pelorus Island, inshore Great Barrier Reef, following European settlement.

The inshore reefs of the Great Barrier Reef (GBR) have undergone significant declines in water quality following European settlement (approx. 1870 AD). However, direct evidence of impacts on coral assemblages is limited by a lack of historical baselines prior to the onset of modern monitoring programmes in the early 1980s. Through palaeoecological reconstructions, we report a previously undocumented historical collapse of Acropora assemblages at Pelorus Island (central GBR). High-precision U-series dating of dead Acropora fragments indicates that this collapse occurred between 1920 and 1955, with few dates obtained after 1980. Prior to this event, our results indicate remarkable long-term stability in coral community structure over centennial scales. We suggest that chronic increases in sediment flux and nutrient loading following European settlement acted as the ultimate cause for the lack of recovery of Acropora assemblages following a series of acute disturbance events (SST anomalies, cyclones and flood events). Evidence for major degradation in reef condition owing to human impacts prior to modern ecological surveys indicates that current monitoring of inshore reefs on the GBR may be predicated on a significantly shifted baseline.

Regulation of bacterial communities through antimicrobial activity by the coral holobiont.

Interactions between corals and associated bacteria and amongst these bacterial groups are likely to play a key role in coral health. However, the complexity of these interactions is poorly understood. We investigated the functional role of specific coral-associated bacteria in maintaining microbial communities on the coral Acropora millepora (Ehrenberg 1834) and the ability of coral mucus to support or inhibit bacterial growth. Culture-independent techniques were used to assess bacterial community structures whilst bacterial culture was employed to assess intra- and inter-specific antimicrobial activities of bacteria. Members of Pseudoalteromonas and ribotypes closely related to Vibrio coralliilyticus displayed potent antimicrobial activity against a range of other cultured isolates and grew readily on detached coral mucus. Although such bacterial ribotypes would be expected to have a competitive advantage, they were rare or absent on intact and healthy coral colonies growing in situ (analysed using denaturing gradient gel electrophoresis and 16S rRNA gene sequencing). The most abundant bacterial ribotypes found on healthy corals were Gammaproteobacteria, previously defined as type A coral associates. Our results indicate that this group of bacteria and specific members of the Alphaproteobacteria described here as 'type B associates' may be important functional groups for coral health. We suggest that bacterial communities on coral are kept in check by a combination of host-derived and microbial interactions and that the type A associates in particular may play a key role in maintaining stability of microbial communities on healthy coral colonies.

Reef accumulation is decoupled from recent degradation in the central and southern Red Sea.

Reefs are biogenic structures that result in three-dimensional accumulations of calcium carbonate. Over geological timescales, a positive balance between the production and accumulation of calcium carbonate versus erosional and off-reef transport processes maintains positive net accretion on reefs. Yet, how ecological processes occurring over decadal timescales translate to the accumulation of geological structures is poorly understood, in part due to a lack of studies with detailed time-constrained chronologies of reef accretion over decades to centuries. Here, we combined ecological surveys of living reefs with palaeoecological reconstructions and high-precision radiometric (U-Th) age-dating of fossil reefs represented in both reef sediment cores and surficial dead in situ corals, to reconstruct the history of community composition and carbonate accumulation across the central and southern Saudi Arabian Red Sea throughout the late Holocene. We found that reefs were primarily comprised of thermally tolerant massive Porites colonies, creating a consolidated coral framework, with unconsolidated branching coral rubble accumulating among massive corals on shallow (5-8 m depth) exposed (windward), and gently sloping reef slopes. These unconsolidated reef rubble fields were formed primarily from ex situ Acropora and Pocillopora coral fragments, infilled post deposition within a sedimentary matrix. Bayesian age-depth models revealed a process of punctuated deposition of post-mortem coral fragments transported from adjacent reef environments. That a large portion of Saudi Arabian Red Sea reef slopes is driven by allochthonous deposition (transportation) has important implications for modeling carbonate budgets and reef growth. In addition, a multi-decadal lag exists between the time of death for branching in situ coral and incorporation into the unconsolidated reef rubble. This indicates that recent climate related degradation in the 21st century has not had an immediately negative effect on reef building processes affecting a large portion of the reef area in the Saudi Arabian Red Sea.

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.

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.

Successive marine heatwaves cause disproportionate coral bleaching during a fast phase transition from El Niño to La Niña.

The frequency and intensity of marine heatwaves that result in coral bleaching events have increased over recent decades and led to catastrophic losses of reef-building corals in many regions. The high-latitude coral assemblages at Lord Howe Island, which is a UNESCO listed site is the world southernmost coral community, were exposed to successive thermal anomalies following a fast phase-transition of the record-breaking 2009 to 2010 warm pool El Niño in the Central Pacific to a strong La Niña event in late 2010. The coral community experienced severe and unprecedented consecutive bleaching in both 2010 and 2011. Coral health surveys completed between March 2010 and September 2012 quantified the response and recovery of approximately 43,700 coral colonies to these successive marine heatwaves. In March 2010, coral bleaching ranged from severe, with 99% of colonies bleached at some shallow lagoon sites, to mild at deeper reef slope sites, with only 17% of individuals affected. Significant immediate mortality from thermal stress was evident during the peak of the bleaching event. Overall, species in the genera Pocillopora, Stylophora, Seriatopora and Porites were the most affected, while minimal bleaching and mortality was recorded among members of other coral families (e.g. Acroporidae, Dendrophyllidae & Merulinidae). Surviving corals underwent a subsequent, but much less intense, thermal anomaly in 2011 that led to a disproportionate bleaching response among susceptible taxa. While this observation indicates that the capacity of thermally susceptible high-latitude corals to acclimatize to future ocean warming may be limited, particularly if bleaching events occur annually, our long-term survey data shows that coral cover at most sites recovered to pre-bleaching levels within three years in the absence of further thermal anomalies.

Decline of coastal apex shark populations over the past half century.

Overexploitation of large apex marine predators is widespread in the world's oceans, yet the timing and extent of declines are poorly understood. Here we reconstruct a unique fisheries-independent dataset from a shark control programme spanning 1760 km of the Australian coastline over the past 55 years. We report substantial declines (74-92%) of catch per unit effort of hammerhead (Sphyrnidae), whaler (Carcharhinidae), tiger shark (Galeocerdo cuvier) and white sharks (Carcharodon carcharias). Following onset of the program in the 1960s, catch rates in new installations in subsequent decades occurred at a substantially lower rate, indicating regional depletion of shark populations over the past half a century. Concurrent declines in body size and the probability of encountering mature individuals suggests that apex shark populations are more vulnerable to exploitation than previously thought. Ongoing declines and lack of recovery of vulnerable and protected shark species are a cause for concern.