Algal contact as a trigger for coral disease

Diseases are causing alarming declines in reef‐building coral species, the foundation blocks of coral reefs. The emergence of these diseases has occurred simultaneously with large increases in the abundance of benthic macroalgae. Here, we show that physical contact with the macroalga Halimeda opuntia can trigger a virulent disease known as white plague type II that has caused widespread mortality in most Caribbean coral species. Colonies of the dominant coral Montastraea faveolata exposed to algal transplants developed the disease whereas unexposed colonies did not. The bacterium Aurantimonas coralicida, causative agent of the disease, was present on H. opuntia sampled close to, and away from diseased corals, indicating that the alga serves as a reservoir for this pathogen. Our results suggest that the spread of macroalgae on coral reefs could account for the elevated incidence of coral diseases over past decades and that reduction of macroalgal abundance could help control coral epizootics.     

The Changing Health of Coral Reefs

Throughout their entire global range coral reefs are in decline. Coral bleaching, macroalgal overgrowth and coral diseases — responses signaling the declining health of coral reefs — have occurred with increasing frequency and intensity in recent decades. Decreased calcification may also be affecting coral reefs over longer time scales. Declines in coral reef health have been attributed to various natural and anthropogenic processes, but assignment of causality has proved problematic. Coral bleaching has been observed during extreme climate events such as El Niño; furthermore, there are indications that exposure to UV radiation, air, infectious microbes, and elevated temperature plays a role in the dramatic increase of coral bleaching since the mid-1970s. Macroalgal overgrowth is usually ascribed to eutrophi-cation and coral diseases to weakening of the coral host resistance by anthropogenic pollution. An issue precluding a strict anthropogenic cause of coral reef decline is that both overgrowth and coral diseases are known to occur, although less frequently, on reefs remote from human development. While its causes are still being unraveled, the overall decline in coral reef health sends an unambiguous signal that the coral reef system is losing its ability to withstand sudden or persistent environmental changes.     

Suitable Environmental Ranges for Potential Coral Reef Habitats in the Tropical Ocean

Coral reefs are found within a limited range of environmental conditions or tolerance limits. Estimating these limits is a critical prerequisite for understanding the impacts of climate change on the biogeography of coral reefs. Here we used the diagnostic model ReefHab to determine the current environmental tolerance limits for coral reefs and the global distribution of potential coral reef habitats as a function of six factors: temperature, salinity, nitrate, phosphate, aragonite saturation state, and light. To determine these tolerance limits, we extracted maximum and minimum values of all environmental variables in corresponding locations where coral reefs are present. We found that the global, annually averaged tolerance limits for coral reefs are 21.7—29.6 °C for temperature, 28.7—40.4 psu for salinity, 4.51 μmol L^-1 for nitrate, 0.63 μmol L^-1 for phosphate, and 2.82 for aragonite saturation state. The averaged minimum light intensity in coral reefs is 450 μmol photons m^-2 s^-1. The global area of potential reef habitats calculated by the model is 330.5 × 10³ km². Compared with previous studies, the tolerance limits for temperature, salinity, and nutrients have not changed much, whereas the minimum value of aragonite saturation in coral reef waters has decreased from 3.28 to 2.82. The potential reef habitat area calculated with ReefHab is about 121×10³ km² larger than the area estimated from the charted reefs, suggesting that the growth potential of coral reefs is higher than currently observed.     

Coral reefs: sources or sinks of atmospheric CO2?

Because the precipitation of calcium carbonate results in the sequestering of carbon, it frequently has been thought that coral reefs functions as sinks of global atmospheric CO₂. However, the precipitation of calcium carbonate is accompanied by a shift of pH that results in the release of CO₂. This release of CO₂ is less in buffered sea water than fresh water systems; nevertheless, coral reefs are sources, not sinks, of atmospheric carbon. Using estimated rates of coral reef carbonate production, we compute that coral reefs release 0.02 to 0.08 Gt C as CO₂ annually. This is approximately 0.4% to 1.4% of the current anthropogenic CO₂ production due to fossil fuel combustion.     

Predictive Modeling of Coral Disease Distribution within a Reef System

Diseases often display complex and distinct associations with their environment due to differences in etiology, modes of transmission between hosts, and the shifting balance between pathogen virulence and host resistance. Statistical modeling has been underutilized in coral disease research to explore the spatial patterns that result from this triad of interactions. We tested the hypotheses that: 1) coral diseases show distinct associations with multiple environmental factors, 2) incorporating interactions (synergistic collinearities) among environmental variables is important when predicting coral disease spatial patterns, and 3) modeling overall coral disease prevalence (the prevalence of multiple diseases as a single proportion value) will increase predictive error relative to modeling the same diseases independently. Four coral diseases: Porites growth anomalies (PorGA), Porites tissue loss (PorTL), Porites trematodiasis (PorTrem), and Montipora white syndrome (MWS), and their interactions with 17 predictor variables were modeled using boosted regression trees (BRT) within a reef system in Hawaii. Each disease showed distinct associations with the predictors. Environmental predictors showing the strongest overall associations with the coral diseases were both biotic and abiotic. PorGA was optimally predicted by a negative association with turbidity, PorTL and MWS by declines in butterflyfish and juvenile parrotfish abundance respectively, and PorTrem by a modal relationship with Porites host cover. Incorporating interactions among predictor variables contributed to the predictive power of our models, particularly for PorTrem. Combining diseases (using overall disease prevalence as the model response), led to an average six-fold increase in cross-validation predictive deviance over modeling the diseases individually. We therefore recommend coral diseases to be modeled separately, unless known to have etiologies that respond in a similar manner to particular environmental conditions. Predictive statistical modeling can help to increase our understanding of coral disease ecology worldwide.     

Complex Environmental Forcing across the Biogeographical Range of Coral Populations

Although there is a substantial body of work on how temperature shapes coastal marine ecosystems, the spatiotemporal variability of seawater pH and corresponding in situ biological responses remain largely unknown across biogeographic ranges of tropical coral species. Environmental variability is important to characterize because it can amplify or dampen the biological consequences of global change, depending on the functional relationship between mean temperature or pH and organismal traits. Here, we characterize the spatiotemporal variability of pH, temperature, and salinity at fringing reefs in Moorea, French Polynesia and Nanwan Bay, Taiwan using advanced time series analysis, including wavelet analysis, and infer their potential impact on the persistence and stability of coral populations. Our results demonstrate that both the mean and variance of pH and temperature differed significantly between sites in Moorea and Taiwan. Seawater temperature at the Moorea site passed the local bleaching threshold several times within the ~45 day deployment while aragonite saturation state at the Taiwan site was often below commonly observed levels for coral reefs. Our results showcase how a better understanding of the differences in environmental conditions between sites can (1) provide an important frame of reference for designing laboratory experiments to study the effects of environmental variability, (2) identify the proximity of current environmental conditions to predicted biological thresholds for the coral reef, and (3) help predict when the temporal variability and mean of environmental conditions will interact synergistically or antagonistically to alter the abundance and stability of marine populations experiencing climate change.     

Diel pCO2 variation among coral reefs and microhabitats at Lizard Island,Great Barrier Reef

Most laboratory experiments examining the effect of ocean acidification on marine organisms use stable pH/pCO₂ treatments based on average projections for the open ocean. However, pH/pCO₂ levels vary spatially and temporally in marine environments, and this variation can affect organism responses to pH/pCO₂. On coral reefs, diel pH/pCO₂ variability at the individual reef scale has been reported in a few studies, but variation among microhabitats within a reef remains poorly understood. This study determined the pH/pCO₂ variability of three different reefs, and three contrasting coral reef microhabitats (dominated by hard coral, soft coral, or open substrate) within each reef. Three SeaFET pH loggers were deployed simultaneously at the three microhabitats within a reef over a 9-day period. This was repeated at three different reefs around the Lizard Island lagoon. The loggers recorded pH_T and temperature every 5 min. Water samples were collected from each microhabitat during four points of the tidal cycle (high, low, rising, and falling) and analysed for total alkalinity and dissolved inorganic carbon. The data show a clear diel pCO₂ cycle, increasing overnight and decreasing during the day, in association with photosynthesis and respiration cycles. Diel pCO₂ differed more between reefs than between microhabitats within reefs. Variation between reefs was most likely influenced by water flow, with the more protected (low flow) reefs experiencing a greater range in pCO₂ (Δ 250 μatm) than the exposed (high flow) reefs (Δ 116 μatm). These results add to a growing body of the literature on the diel variation of pCO₂ of shallow, nearshore environments and suggest that when projecting future pCO₂ levels, it is important to consider reef metabolism as well as physical and hydrodynamic factors.

     

Indirect Effects of Human Development Along the Coast on Coral Health

Accelerated human development in coastal areas is causing declines in coral reefs worldwide, but the mechanisms by which development leads to reef degradation are often difficult to identify. Here, we use Akaike information criterion model averaging in combination with path analysis to test for the direct and indirect effects of potential environmental stressors on two coral diseases on the west coast of Hawai'i. We quantified human‐altered land cover and human population density at two spatial scales: inland area of 1‐km radius and watershed. We then tested for the effects of these land cover variables, as well as seawater chlorophyll α concentrations, depth, host coral cover, and proximity to surface water discharge on the density of growth anomalies (GA) and prevalence of trematodiasis (TRE) affecting Porites lobata, the locally dominant reef‐building coral species. Our analyses showed that human‐altered land cover measured at 1‐km scale was a strong indirect predictor of GA. Specifically, we found that human interference adjacent to the coast predicted higher chlorophyll α concentrations, which in turn predicted higher GA density. We also found that chlorophyll α and depth were strong negative predictors of TRE, and host coral cover a positive predictor. Our results indicated that GA are likely regulated by indirect land‐based anthropogenic impacts, whereas TRE is mostly affected by host density‐dependent forces. Path analysis can serve as a useful tool to rapidly identify the scale and indirect effects of anthropogenic stressors related to coral diseases, allowing for accurate conservation planning in the face of limited resources for tropical conservation.     

Ocean acidification and warming will lower coral reef resilience

Ocean warming and acidification from increasing levels of atmospheric CO₂ represent major global threats to coral reefs, and are in many regions exacerbated by local‐scale disturbances such as overfishing and nutrient enrichment. Our understanding of global threats and local‐scale disturbances on reefs is growing, but their relative contribution to reef resilience and vulnerability in the future is unclear. Here, we analyse quantitatively how different combinations of CO₂ and fishing pressure on herbivores will affect the ecological resilience of a simplified benthic reef community, as defined by its capacity to maintain and recover to coral‐dominated states. We use a dynamic community model integrated with the growth and mortality responses for branching corals (Acropora) and fleshy macroalgae (Lobophora). We operationalize the resilience framework by parameterizing the response function for coral growth (calcification) by ocean acidification and warming, coral bleaching and mortality by warming, macroalgal mortality by herbivore grazing and macroalgal growth via nutrient loading. The model was run for changes in sea surface temperature and water chemistry predicted by the rise in atmospheric CO₂ projected from the IPCC's fossil‐fuel intensive A1FI scenario during this century. Results demonstrated that severe acidification and warming alone can lower reef resilience (via impairment of coral growth and increased coral mortality) even under high grazing intensity and low nutrients. Further, the threshold at which herbivore overfishing (reduced grazing) leads to a coral–algal phase shift was lowered by acidification and warming. These analyses support two important conclusions: Firstly, reefs already subjected to herbivore overfishing and nutrification are likely to be more vulnerable to increasing CO₂. Secondly, under CO₂ regimes above 450–500 ppm, management of local‐scale disturbances will become critical to keeping reefs within an Acropora‐rich domain.     

Coral population dynamics across consecutive mass mortality events

Annual coral mortality events due to increased atmospheric heat may occur regularly from the middle of the century and are considered apocalyptic for coral reefs. In the Arabian/Persian Gulf, this situation has already occurred and population dynamics of four widespread corals (Acropora downingi, Porites harrisoni, Dipsastrea pallida, Cyphastrea micropthalma) were examined across the first‐ever occurrence of four back‐to‐back mass mortality events (2009–2012). Mortality was driven by diseases in 2009, bleaching and subsequent diseases in 2010/2011/2012. 2009 reduced P. harrisoni cover and size, the other events increasingly reduced overall cover (2009: ?10%; 2010: ?20%; 2011: ?20%; 2012: ?15%) and affected all examined species. Regeneration was only observed after the first disturbance. P. harrisoni and A. downingi severely declined from 2010 due to bleaching and subsequent white syndromes, while D. pallida and P. daedalea declined from 2011 due to bleaching and black‐band disease. C. microphthalma cover was not affected. In all species, most large corals were lost while fission due to partial tissue mortality bolstered small size classes. This general shrinkage led to a decrease of coral cover and a dramatic reduction of fecundity. Transition matrices for disturbed and undisturbed conditions were evaluated as Life Table Response Experiment and showed that C. microphthalma changed the least in size‐class dynamics and fecundity, suggesting they were ‘winners’. In an ordered ‘degradation cascade’, impacts decreased from the most common to the least common species, leading to step‐wise removal of previously dominant species. A potentially permanent shift from high‐ to low‐coral cover with different coral community and size structure can be expected due to the demographic dynamics resultant from the disturbances. Similarities to degradation of other Caribbean and Pacific reefs are discussed. As comparable environmental conditions and mortality patterns must be expected worldwide, demographic collapse of many other coral populations may soon be widespread.     

Protection of Coral Larvae from Thermally Induced Oxidative Stress by Redox Nanoparticles

Coral reefs are one of the most biologically diverse and economically important ecosystems on earth. However, the destruction of coral reefs has been reported worldwide owing to rising seawater temperature associated with global warming. In this study, we investigated the potential of a redox nanoparticle (RNP^O) to scavenge reactive oxygen species (ROS), which are overproduced under heat stress and play a crucial role in causing coral mortality. When reef-building coral (Acropora tenuis) larvae, without algal symbionts, were exposed to thermal stress at 33 °C, RNP^O treatment significantly increased the survival rate. Proteome analysis of coral larvae was performed using nano-liquid chromatography-tandem mass spectrometry for the first time. The results revealed that several proteins related to ROS-induced oxidative stress were specifically identified in A. tenuis larvae without RNP^O treatment, whereas these proteins were absent in RNP^O-treated larvae, which suggested that RNP^O effectively scavenged ROS from A. tenuis larvae. Results from this study indicate that RNP^O treatment can reduce ROS in aposymbiotic coral larvae and would be a promising approach for protecting corals from thermal stress.     

Coral reefs modify their seawater carbon chemistry – implications for impacts of ocean acidification

Reviews suggest that that the biogeochemical threshold for sustained coral reef growth will be reached during this century due to ocean acidification caused by increased uptake of atmospheric CO₂. Projections of ocean acidification, however, are based on air‐sea fluxes in the open ocean, and not for shallow‐water systems such as coral reefs. Like the open ocean, reef waters are subject to the chemical forcing of increasing atmospheric pCO₂. However, for reefs with long water residence times, we illustrate that benthic carbon fluxes can drive spatial variation in pH, pCO₂ and aragonite saturation state (Ω_a) that can mask the effects of ocean acidification in some downstream habitats. We use a carbon flux model for photosynthesis, respiration, calcification and dissolution coupled with Lagrangian transport to examine how key groups of calcifiers (zooxanthellate corals) and primary producers (macroalgae) on coral reefs contribute to changes in the seawater carbonate system as a function of water residence time. Analyses based on flume data showed that the carbon fluxes of corals and macroalgae drive Ω_ain opposing directions. Areas dominated by corals elevate pCO₂ and reduce Ω_a, thereby compounding ocean acidification effects in downstream habitats, whereas algal beds draw CO₂ down and elevate Ω_a, potentially offsetting ocean acidification impacts at the local scale. Simulations for two CO₂ scenarios (600 and 900 ppm CO₂) suggested that a potential shift from coral to algal abundance under ocean acidification can lead to improved conditions for calcification in downstream habitats, depending on reef size, water residence time and circulation patterns. Although the carbon fluxes of benthic reef communities cannot significantly counter changes in carbon chemistry at the scale of oceans, they provide a significant mechanism of buffering ocean acidification impacts at the scale of habitat to reef.     

Coral and sea urchin assemblage structure and interrelationships in Kenyan reef lagoons

Patterns of hard coral and sea urchin assemblage structure (species richness, diversity, and abundance) were studied in Kenyan coral reef lagoons which experienced different types of human resource use. Two protected reefs (Malindi and Watamu Marine National Parks) were protected from fishing and coral collection, but exposed to heavy tourist use. One reef (Mombasa MNP) received protection from fishermen for one year and was exploited for fish and corals prior to protection and was defined as a transitional reef. Three reefs (Vipingo, Kanamai, and Diani) were unprotected and experienced heavy fishing and some coral collection. Protected and unprotected reefs were distinct in terms of their assemblage structure with the transitional reef grouping with unprotected reefs based on relative and absolute abundance of coral genera. Protected reefs had slightly higher (p<0.01) coral cover (23.6 ± 8.3 % ± S.D.) than unprotected reefs (16.7 ± 8.5), but the transitional reef had the highest coral cover (30.8 ± 6.4) which increased by 250% since measured in 1987: largely attributable to a large increase inPorites nigrescens cover. Protected reefs had higher coral species richness and diversity and a greater relative abundance ofAcropora, Montipora andGalaxea than unprotected reefs. The transitional reef had high species richness, but lower diversity due to the high dominance ofPorites. Sea urchins showed the opposite pattern with highest diversity in most unprotected reefs. Coral cover, species richness, and diversity were negatively associated with sea urchin abundance, but the relative abundance ofPorites increased with sea urchin abundance to the point wherePorites composed >90% of the coral cover at sites with the highest sea urchin abundance. Effects of coral overcollection was only likely for the genusAcropora (staghorn corals). A combination of direct and indirect effects of human resource use may reduce diversity, species richness, and abundance of corals while increasing the absolute abundance of sea urchins and the relative cover ofPorites.     

Coral reefs studies and threats in Malaysia: a mini review

Coral reefs in Malaysia are about 4,006 km² with over 550 species contributed to nation’s economy. Coral reefs studies and threats in Malaysia have been reviewed briefly. Perspectives are addressed as coral reefs studies, threats, gaps and future studies. Coral reefs in Malaysia are being damaged at an increasing rate where it faces natural and anthropogenic stresses. Excellent summaries are available in terms of coral reefs cover throughout Malaysia however scarce in terms of qualitative, quantitative and biogeographical data. There are also limited studies on heavy metals concentration in corals skeleton studies. Poor to fair conditions of coral reefs in Peninsular Malaysia is due to increases of sedimentation and tourism impacts. Overfishing and fish blasting were main threats of coral reefs damage in Sabah. In Sarawak, coral reefs are threatened by high sedimentation and sand mining. The 1998–1999 bleaching event also affected coral reefs in Malaysia due to climate change. Gaps in coral reefs studies can be completed by continuous collaborations between local and international researchers as well as research by local universities. Economic valuation, policy analysis and community participation are directions in future coral reefs studies in Malaysia. Future studies are to understand effects of management on coral reefs health and impact of pollution on coral reefs growth with a standard coral reefs methodology. Established legal systems to reduce threats received by coral reefs are also need to be introduced. Role of science-driven management with community participation and media mass are also gaps to be highlighted in future studies.     

Coral diversity across a disturbance gradient in the Pulau Seribu reef complex off Jakarta, Indonesia

Very few coral reefs are located close enough to metropolitan cities to study the influence of large urban populations on reef communities. Here, we compare the impact of a large-scale disturbance gradient with local-scale disturbance on coral richness, cover, and composition in the Jakarta Bay and Pulau Seribu reef complex off Jakarta, Indonesia. We found no effect of local land-use type of coral reef islands on richness, composition or cover, nor did taxon richness differ among zones at the large-scale. There was, however, a pronounced difference in composition and coral cover among zones. Cover was very low and composition differed markedly in the near-shore zone 1 (Jakarta Bay) where human-induced disturbance is most intense. Cover was highest in the outlying reefs of zone 3. The highly perturbed zone 1 reefs were, furthermore, distinguished by the virtual absence of otherwise abundant coral taxa such as Acropora hyacinthus and Porites rus and the prevalence of taxa such as Oulastrea crispata and Favia maxima. Almost 60% of the spatial variation in composition was related to variation in shelf depth and island size. The importance of shelf depth was related to the prevalence of a strong environmental gradient in reef depth, pollution, and mechanical reef disturbance and salinity from Jakarta Bay to the outlying reefs. Although there was a significant univariate relationship between spatial variation in composition and distance, this did not enter into the multivariate model, except when presence–absence data was used, indicating that environmental processes are the primary structuring forces in determining local coral assemblage composition across the Pulau Seribu complex.     

Coral associations of the Pleistocene Ironshore Formation,Grand Cayman

The 125-ka sea level, which was approximately 6 m above present-day sea level, led to the partial flooding of many Caribbean islands. On Grand. Cayman, this event led to the formation of the large Ironshore Lagoon that covered most of the western half of the island and numerous, small embayments along the south, east, and north coasts. At that time, at least 33 coral species grew in waters around Grand Cayman. This fauna, like the modern coral fauna of Grand Cayman, was dominated byMontastrea annularis, Porites porites, Acropora polmata, andA. cervicornis. Scolymia cubensis andMycetophyllia ferox, not previously identified from the Late Pleistocene, are found in the Pleistocene patch reefs.Madracis mirabilis, Colpophyllia breviserialis, Agaricia tenuifolia, A. lamarcki, A. undata, Millepora spp., Mycetophyllia reesi, M. aliciae, andM. danaana, found on modern reefs, have not been identified from the Late Pleistocene reefs. Conversely,Pocillopora sp. cf.P. palmata, which is found in Late Pleistocene reefs, is absent on the modern reefs around Grand Cayman. The corals in the Ironshore Formation of Grand Cayman have been divided into 10 associations according to their dominant species, overall composition, and faunal diversity. Many of these associations are similar to the modern associations around Grand Cayman. Each of the Pleistocene coral associations, which can be accurately located on the known Late Pleistocene paleogeography of Grand Cayman, developed in distinct environmental settings. Overall trends identified in the modern settings are also apparent in the Late Pleistocene faunas. Thus, the diversity of the coral faunas increased from the interior of the Ironshore Lagoon to the reef crest. Similarly, the coral diversity in the Pleistocene patch reefs was related to the size of the reefs and their position relative to breaks in the barrier reef. The barrier reef included corals that are incapable of sediment rejection; whereas the patch reefs lacked such corals.     

Coral Community Adaptability to Environmental Change at the Scales of Regions, Reefs and Reef Zones

SYNOPSIS. Projected global increases in temperature, sea level,storminess and atmospheric carbon dioxide (CO₂) are likely tocause changes in reef coral communities which the present humangeneration will view as deleterious. It is likely coral communitytrajectories will be influenced as much by the reduction inintervals between extreme events as the projected increasesin means of environmental parameters such as temperature, atmosphericCO₂ and sea-level. Depressed calcification rates in corals causedby reduced aragonite saturation state of water may increasevulnerability of corals to storms. Moreover, reduction in intervalsbetween storms and other extreme events causing mass mortalityin corals (coral predators, diseases, bleaching) are likelyto more frequently "set back" reef coral communities to earlysuccessional stages or alternate states characterized by non-calcifyingbenthos (plants, soft corals, sponges). The greater the areaand the longer the duration of dominance of putative "coral/corallinealgae" zones of coral reefs by non-calcifying stages, the lesswill be the reefs capacity to accrete limestone bulk lockedup in the big skeletal units of late successional stages (i.e.,very large old corals). Averaged over decades to centuries,the effects of such changes on the coral community's carryingcapacity for other biota such as fish are unpredictable. A "shiftingsteady-state mosaic" null model may provide a useful conceptualtool for defining a baseline and tracking changes from thatbaseline through time.     

Vulnerability of global coral reef habitat suitability to ocean warming,acidification and eutrophication

Coral reefs are threatened by global and local stressors. Yet, reefs appear to respond differently to different environmental stressors. Using a global dataset of coral reef occurrence as a proxy for the long‐term adaptation of corals to environmental conditions in combination with global environmental data, we show here how global (warming: sea surface temperature; acidification: aragonite saturation state, Ω_arag) and local (eutrophication: nitrate concentration, and phosphate concentration) stressors influence coral reef habitat suitability. We analyse the relative distance of coral communities to their regional environmental optima. In addition, we calculate the expected change of coral reef habitat suitability across the tropics in relation to an increase of 0.1°C in temperature, an increase of 0.02 μmol/L in nitrate, an increase of 0.01 μmol/L in phosphate and a decrease of 0.04 in Ω_arag. Our findings reveal that only 6% of the reefs worldwide will be unaffected by local and global stressors and can thus act as temporary refugia. Local stressors, driven by nutrient increase, will affect 22% of the reefs worldwide, whereas global stressors will affect 11% of these reefs. The remaining 61% of the reefs will be simultaneously affected by local and global stressors. Appropriate wastewater treatments can mitigate local eutrophication and could increase areas of temporary refugia to 28%, allowing us to ‘buy time’, while international agreements are found to abate global stressors.     

Spatial resilience of the Great Barrier Reef under cumulative disturbance impacts

In the face of increasing cumulative effects from human and natural disturbances, sustaining coral reefs will require a deeper understanding of the drivers of coral resilience in space and time. Here we develop a high‐resolution, spatially explicit model of coral dynamics on Australia's Great Barrier Reef (GBR). Our model accounts for biological, ecological and environmental processes, as well as spatial variation in water quality and the cumulative effects of coral diseases, bleaching, outbreaks of crown‐of‐thorns starfish (Acanthaster cf. solaris), and tropical cyclones. Our projections reconstruct coral cover trajectories between 1996 and 2017 over a total reef area of 14,780 km², predicting a mean annual coral loss of ?0.67%/year mostly due to the impact of cyclones, followed by starfish outbreaks and coral bleaching. Coral growth rate was the highest for outer shelf coral communities characterized by digitate and tabulate Acropora spp. and exposed to low seasonal variations in salinity and sea surface temperature, and the lowest for inner‐shelf communities exposed to reduced water quality. We show that coral resilience (defined as the net effect of resistance and recovery following disturbance) was negatively related to the frequency of river plume conditions, and to reef accessibility to a lesser extent. Surprisingly, reef resilience was substantially lower within no‐take marine protected areas, however this difference was mostly driven by the effect of water quality. Our model provides a new validated, spatially explicit platform for identifying the reefs that face the greatest risk of biodiversity loss, and those that have the highest chances to persist under increasing disturbance regimes.     

Strategies for Gardening Denuded Coral Reef Areas: The Applicability of Using Different Types of Coral Material for Reef Restoration

Recreational and other human activities degrade coral reefs worldwide to a point where efficient restoration techniques are needed. Here we tested several strategies for gardening denuded reefs. The gardening concept consists of in situ or ex situ mariculture of coral recruits, followed by their transplantation into degraded reef sites. In situ nurseries were established in Eilat's (Northern Red Sea) shallow waters, sheltering three types of coral materials taken from the branching species Stylophora pistillata (small colonies, branch fragments, and spat) that were monitored for up to two years. Pruning more than 10% of donor colonies' branches increased mortality, and surviving colonies displayed reduced reproductive activity. Maricultured isolated branches, however, exceeded donor colony life span and reproductive activity and added 0.5–45% skeletal mass per year. Forty‐four percent of the small colonies survived after 1.5‐year mariculture, revealing average yearly growth of 75 ± 32%. Three months ex situ maintenance of coral spat (sexual recruits) prior to the in situ nursery phase increased survivorship. Within the next 1.5 years, they developed into colonies of 3–4 cm diameter. Nursery periods of 2 years, 4–5 years, and more than> 5 years have been estimated for small colonies, spat, and isolated branches, respectively. These and other results, including the possible use of nubbins (minute fragments the size of a single or few polyps), are discussed, revealing benefits and drawbacks for each material. In situ coral mariculture is an improved practice to the common but potentially harmful protocol of direct coral transplantation. It is suggested that reef gardening may be used as a key management tool in conservation and restoration of denuded reef areas. The gardening concept may be applicable for coral reefs worldwide through site‐specific considerations and the use of different local coral species.