Mass Coral Mortality Research Articles

Predator crown-of-thorns starfish (Acanthaster planci) outbreak, mass mortality of corals, and cascading effects on reef fish and benthic communities.

Outbreaks of the coral-killing seastar Acanthaster planci are intense disturbances that can decimate coral reefs. These events consist of the emergence of large swarms of the predatory seastar that feed on reef-building corals, often leading to widespread devastation of coral populations. While cyclic occurrences of such outbreaks are reported from many tropical reefs throughout the Indo-Pacific, their causes are hotly debated, and the spatio-temporal dynamics of the outbreaks and impacts to reef communities remain unclear. Based on observations of a recent event around the island of Moorea, French Polynesia, we show that Acanthaster outbreaks are methodic, slow-paced, and diffusive biological disturbances. Acanthaster outbreaks on insular reef systems like Moorea's appear to originate from restricted areas confined to the ocean-exposed base of reefs. Elevated Acanthaster densities then progressively spread to adjacent and shallower locations by migrations of seastars in aggregative waves that eventually affect the entire reef system. The directional migration across reefs appears to be a search for prey as reef portions affected by dense seastar aggregations are rapidly depleted of living corals and subsequently left behind. Coral decline on impacted reefs occurs by the sequential consumption of species in the order of Acanthaster feeding preferences. Acanthaster outbreaks thus result in predictable alteration of the coral community structure. The outbreak we report here is among the most intense and devastating ever reported. Using a hierarchical, multi-scale approach, we also show how sessile benthic communities and resident coral-feeding fish assemblages were subsequently affected by the decline of corals. By elucidating the processes involved in an Acanthaster outbreak, our study contributes to comprehending this widespread disturbance and should thus benefit targeted management actions for coral reef ecosystems.

Geographic expansion of hermatypic and ahermatypic corals in the Gulf of Mexico, and implications for dispersal and recruitment

The >3000 oil/gas platforms in the northern Gulf of Mexico (GOM) provide shallow hard-substratum in a soft-bottom dominated ecosystem. Excepting the Flower Garden Banks (FGB) and deeper mesophotic banks, no shallow hard substrate has been available offshore since the Holocene. Platforms have facilitated coral geographic expansion in this region. We determined the distribution, abundance, and species diversity patterns on 42 platforms in this region, at ≤37m depth, along four cross-shelf transects: 1) S-SE from Corpus Christi, Texas; 2) S from Lake Sabine, Texas; 3) S from Terrebonne Bay, Louisiana; and 4) S-SW from Mobile, Alabama. Nine hermatypic, two ahermatypic, and one hydrozoan coral were found: hermatypes—Madracis decactis, Diploria strigosa, Montastraea cavernosa, Porites astreoides, Madracis formosa, Colpophyllia natans, Stephanocoenia intercepta, Stephanocoenia michelinii, and Millepora alcicornis (Hydrozoa); ahermatypes—Tubastraea coccinea, Oculina diffusa, and Phyllangia americana. Pattern-seeking analyses identified four community types: one with no hermatypic corals (inner and mid-shelf); one high diversity set of platforms, dominated by abundant Madracis decactis, occurring with four other species per platform (outer shelf); and a third dominated by Madracis decactis, in low abundance, co-dominated by D. strigosa and Millepora alcicornis (outer shelf). Species diversity was highest around the Flower Garden Banks. Hermatypic density and D. strigosa (broadcaster) in particular, was maximum around the FGB, extending northwards. Madracis decactis (brooder) densities peaked at the shelf edge, off Terrebonne Bay, Louisiana. Montastraea cavernosa densities were bimodal, peaking near the FGB and east of the Mississippi River. The FGB appear to be the source of hermatypic corals for most platform populations. Differences between the distribution patterns of Madracis decactis and D. strigosa, Montastraea cavernosa, and O. diffusa suggest that the brooder effects longer recruitment distances by averaging larval dispersal over a broad range of hydrographic conditions throughout the year, while the broadcaster utilizes only one set of conditions over the same period. Averaging respective dispersal strategies over decades may have produced the observed disparate distribution patterns. Ahermatypic coral density exceeded hermatypic density by 10,000 fold, peaking south of Terrebonne Bay, Louisiana and Mobile, Alabama, due to T. coccinea. Ahermatypic diversity was highest off Matagorda Island, Texas, decreasing from west to east. Multi-variate pattern-seeking analysis (PATN) identified four ahermatypic community types: one dominated by O. diffusa and T. coccinea in low abundances (shelf-wide distribution); one characterized by the absence of ahermatypic corals (generally near-shore); one heavily dominated by T. coccinea, and also by O. diffusa in low abundances (mid-shelf to shelf edge); and one dominated by T. coccinea, co-dominated by both P. americana and O. diffusa in low abundances (shelf-edge). T. coccinea and P. americana appear to be derived from the southern GOM off Mexico or the Caribbean. Unlike shrinking coral populations in other parts of the world, corals have expanded their range substantially in the northern Gulf of Mexico, in areas previously devoid or near devoid of corals, facilitated by thousands of oil/gas platforms deployed for decades throughout the region. It is also possible that coral populations on these platforms may act as potential larval sources for the FGB in the event of a mass coral mortality there.

IMAGE FUSION APPLIED TO SATELLITE IMAGERY FOR THE IMPROVED MAPPING AND MONITORING OF CORAL REEFS: A PROPOSAL

Abstract. A coral reef ecosystem, one of the most complex marine environmental systems on the planet, is defined as biologically diverse and immense. It plays an important role in maintaining a vast biological diversity for future generations and functions as an essential spawning, nursery, breeding and feeding ground for many kinds of marine species. In addition, coral reef ecosystems provide valuable benefits such as fisheries, ecological goods and services and recreational activities to many communities. However, this valuable resource is highly threatened by a number of environmental changes and anthropogenic impacts that can lead to reduced coral growth and production, mass coral mortality and loss of coral diversity. With the growth of these threats on coral reef ecosystems, there is a strong management need for mapping and monitoring of coral reef ecosystems. Remote sensing technology can be a valuable tool for mapping and monitoring of these ecosystems. However, the diversity and complexity of coral reef ecosystems, the resolution capabilities of satellite sensors and the low reflectivity of shallow water increases the difficulties to identify and classify its features. This paper reviews the methods used in mapping and monitoring coral reef ecosystems. In addition, this paper proposes improved methods for mapping and monitoring coral reef ecosystems based on image fusion techniques. This image fusion techniques will be applied to satellite images exhibiting high spatial and low to medium spectral resolution with images exhibiting low spatial and high spectral resolution. Furthermore, a new method will be developed to fuse hyperspectral imagery with multispectral imagery. The fused image will have a large number of spectral bands and it will have all pairs of corresponding spatial objects. This will potentially help to accurately classify the image data. Accuracy assessment use ground truth will be performed for the selected methods to determine the quality of the information derived from image classification. The research will be applied to the Kuwait's southern coral reefs: Kubbar and Um Al-Maradim.

Macrobioerosion of dead branching Porites, 4 and 6 years after coral mass mortality

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 458:103-122 (2012) - DOI: https://doi.org/10.3354/meps09726 Macrobioerosion of dead branching Porites, 4 and 6 years after coral mass mortality M. Carreiro-Silva1,*, T. R. McClanahan2 1Center of IMAR of the University of Azores, Department of Oceanography and Fisheries & LarSyS – Associated Laboratory, Horta 9901-862, Portugal 2Marine Programs, Wildlife Conservation Society, Bronx, New York 10460, USA *Email: mcsilva@uac.pt ABSTRACT: Internal bioerosion by macroborers (polychaetes, sipunculans, bivalves, and sponges) was investigated in dead Porites branches collected from 8 coral reefs along the Kenyan coast, 4 and 6 yr after the 1998 mass mortality of corals. Levels of nutrients, benthic cover, and numbers of grazing and invertebrate-eating fish and sea urchins were measured and evaluated for their influence on macrobioerosion. The macroboring community composition was influenced by the grazer composition on each reef; worms were the major macroboring agent where sea urchin biomass was high, and sponges were the dominant agent where herbivorous fish biomass was high. Bivalves accounted for a small proportion of the internal bioerosion and were not measurably influenced by consumers or water quality. The total macrobioerosion rates in Porites branches ranged from 534 ± 70 to 1134 ± 44 g CaCO3 m−2 (4 yr after the coral death) and 837 ± 111 to 2149 ± 314 g CaCO3 m−2 (6 yr after the coral death). The macrobioerosion rates were linearly and positively correlated with chlorophyll a concentrations (chl a) in the water column 4 and 6 yr after the coral death. Sponge boring rates were also positively correlated to chl a 6 yr after coral death but not after the initial 4 yr. Consequently, the macrobioerosion rates responded to nutrient status, but the community of borers changed with the dominant grazers, which in turn were influenced by fisheries management. KEY WORDS: Calcium carbonate · Climate disturbance · Kenya · Marine protected areas · Monitoring · Pollution · Reef framework · Thermal anomaly Full text in pdf format PreviousNextCite this article as: Carreiro-Silva M, McClanahan TR (2012) Macrobioerosion of dead branching Porites, 4 and 6 years after coral mass mortality. Mar Ecol Prog Ser 458:103-122. https://doi.org/10.3354/meps09726 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 458. Online publication date: July 03, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Challenges for Managing Fisheries on Diverse Coral Reefs

Widespread coral reef decline has included the decline of reef fish populations, and the subsistence and artisanal fisheries that depend on them. Overfishing and destructive fishing have been identified as the greatest local threats to coral reefs, but the greatest future threats are acidification and increases in mass coral bleaching caused by global warming. Some reefs have shifted from dominance by corals to macroalgae, in what are called “phase shifts”. Depletion of herbivores including fishes has been identified as a contributor to such phase shifts, though nutrients are also involved in complex interactions with herbivory and competition. The depletion of herbivorous fishes implies a reduction of the resilience of coral reefs to the looming threat of mass coral mortality from bleaching, since mass coral deaths are likely to be followed by mass macroalgal blooms on the newly exposed dead substrates. Conventional stock assessment of each fish species would be the preferred option for understanding the status of the reef fishes, but this is far too expensive to be practical because of the high diversity of the fishery and poverty where most reefs are located. In addition, stock assessment models and fisheries in general assume density dependent populations, but a key prediction that stocks recover from fishing is not always confirmed. Catch Per Unit Effort (CPUE) has far too many weaknesses to be a useful method. The ratio of catch to stock and the proportion of catch that is mature depend on fish catch data, and are heavily biased toward stocks that are in good condition and incapable of finding species that are in the worst condition. Near-pristine reefs give us a reality check about just how much we have lost. Common fisheries management tools that control effort or catch are often prohibitively difficult to enforce for most coral reefs except in developed countries. Ecosystem-based management requires management of impacts of fishing on the ecosystem, but also vice versa. Marine Protected Areas (MPAs) have been a favorite management tool, since they require little information. MPAs are excellent conservation and precautionary tools, but address only fishing threats, and may be modest fisheries management tools, which are often chosen because they appear to be the only feasible alternative. “Dataless management” is based on qualitative information from traditional ecological knowledge and/or science, is sufficient for successful reef fisheries management, and is very inexpensive and practical, but requires either customary marine tenure or strong governmental leadership. Customary marine tenure has high social acceptance and compliance and may work fairly well for fisheries management and conservation where it is still strong.

Detecting coral bleaching using high-resolution satellite data analysis and 2-dimensional thermal model simulation in the Ishigaki fringing reef, Japan

In 2007, high-temperature-induced mass coral mortality was observed in a well-developed fringing reef area on the southeastern coast of Ishigaki Island, Japan. To analyze the response of the corals to thermal stress, the coral cover was examined using Quickbird data, taken across the reef flat just before and after the bleaching event and performing a reef scale horizontal 2-dimensional thermal model simulation. The Quickbird data consisted of multispectral (MSS) imagery, which had a spatial resolution of 2.4 m, and panchromatic (PAN)-fused multispectral imagery, which had a 0.6-m spatial resolution. The observed changes in coral cover implied that the delineation of partially bleached coral was more precise with PAN + MSS. The classification accuracy achieved using PAN + MSS (93%) was superior to that obtained using MSS (88%). The in situ water temperature observations and 2-dimensional thermal model simulation results indicated that the water temperature fluctuated greatly in the inner reef area in late July 2007. Different thermal stress indices, including daily average temperature, daily maximum excess temperature, and daily accumulated temperature, were examined to define a suitable index that represented the severity of the thermal stress on coral cover. The results suggested that the daily accumulated temperature that occurred during the maximum sea surface temperature period of the bleaching season provided the best predictor of bleaching. The changes in water temperature, bathymetry, and coral patch size affected the severity of bleaching; therefore, the spatial dependence of these variables was examined using Moran’s I and Lagrange multiplier tests. An investigation of the effect of coral patch sizes on coral bleaching indicated that large coral patches were less affected than the small patches, which were more likely to suffer bleaching and coral mortality.

Assessment of coral bleaching using symbiotic zooxanthellae density and satellite remote sensing data in the Nansha Islands, South China Sea

Coral bleaching, characterized by a significant loss of symbiotic zooxanthellae, is the primary cause of mass coral mortality and reef degradation throughout the world. The characteristics, processes, and resistance of corals to bleaching varies significantly and is dependent on environmental conditions. We documented a mass coral bleaching event in June 2007 at the Meiji and Zhubi Reefs, Nansha Islands (NS), South China Sea using ecological surveys and measurement of coral zooxanthellae density and sea surface temperatures (SST). More than 35 species of corals (between 0–20 m in depth) were bleached. These bleached corals accounted for 15.6% of total corals in the investigated quadrats. The branching corals Pocillopora and Acropora were the most vulnerable species whereas the massive corals Porites and Favia were more tolerant of the high SSTs. Surprisingly, we found no evidence of bleaching in Agariciidae corals suggesting that this family is resistant to thermal stresses. The bleached corals had lost 72%–90% of their symbiotic zooxanthellae. Furthermore, corals that had no visual signs of bleaching had also lost 31%–53% of their zooxanthellae suggesting that most corals were experiencing the early stage of bleaching. The monthly mean SST during June 2007 was 30.8°C, the highest since 1998. Based on measurements of SST and the Hotspots and DHW data (NOAA), we conclude that it the extremely high SSTs triggered this coral bleaching event. Our results suggest that the previously accepted temperature thresholds used to predict coral bleaching based on satellite data are likely to underestimate the extent and intensity of coral bleaching, at least in the NS.

Coral mortality associated with thermal fluctuations in the Phoenix Islands, 2002–2005

The Phoenix Islands (Republic of Kiribati, 172–170°W and 2.5–5°S) experience intra- and inter-annual sea surface temperature variability of ≈2°C and have few local anthropogenic impacts. From July 2002, a thermal stress event occurred, which peaked at 21 Degree Heating Weeks (DHW) in January 2003 and persisted for 4 years. Such thermal stress was greater than any thermal event reported in the coral reef literature. Reef surveys were conducted in July 2000, June 2002, and May 2005, for six of the eight islands. Sampling was stratified by exposure (windward, leeward, and lagoon) and depth (5, 10, 15, and 25 m). The thermal stress event caused mass coral mortality, and coral cover declined by approximately 60% between 2002 and 2005. However, mortality varied among sites (12–100%) and among islands (42–79%) and varied in accordance with the presence of a lagoon, island size, and windward vs. leeward exposure. Leeward reefs experienced the highest and most consistent decline in coral cover. Island size and the presence of a lagoon showed positive correlations with coral mortality, most likely because of the longer water residence time enhancing heating. Windward reefs showed cooler conditions than leeward reefs. Recently dead corals were observed at depths >35 m on windward and >45 m on leeward reefs. Between-island variation in temperature had no effect on between-island variation in coral mortality. Mortality levels reported here were comparable to those reported for the most extreme thermal stress events of 9–10 DHW in other regions. These results highlight the high degree of acclimation and/or adaptation of the corals in the Phoenix Islands to their local temperature regime, and their consequent vulnerability to anomalous events. Moreover, the results suggest the need to adjust thermal stress calculations to reflect local temperature variation.

Coral-killing cyanobacteriosponge (Terpios hoshinota) on the Great Barrier Reef

[Extract] The encrusting cyanobacteriosponge Terpios hoshinota was originally described from Guam (Rutzler and Muzik 1993) and is expanding its range in coral reefs of the northwestern Pacific (Liao et al. 2007). Terpios hoshinota encrusts many hard substrates, including live coral, and occasionally undergoes massive outbreaks that can cover huge areas, which can result in the mass mortality of corals and other resident organisms (Bryan 1973; Rutzler and Muzik 1993). This sponge presents a potentially serious threat to coral reefs, and data are needed on its current distribution and abundance to assess its geographical expansion.

How much time can herbivore protection buy for coral reefs under realistic regimes of hurricanes and coral bleaching?

Coral reefs have been more severely impacted by recent climate instability than any other ecosystem on Earth. Corals tolerate a narrow range of physical environmental stress, and increases in sea temperature of just 1 °C over several weeks can result in mass coral mortality, often exceeding 95% of individuals over hundreds of square kilometres. Even conservative climate models predict that mass coral bleaching events could occur annually by 2050. Unfortunately, managers of coral-reef resources have few options available to meet this challenge. Here, we investigate the role that fisheries conservation tools, including the designation of marine reserves, can play in altering future trajectories of Caribbean coral reefs. We use an individual-based model of the ecological dynamics to test the influence of spatially realistic regimes of disturbance on coral populations. Two major sources of disturbance, hurricanes and coral bleaching, are simulated in contrasting regions of the Caribbean: Belize, Bonaire, and the Bahamas. Simulations are extended to 2099 using the HadGEM1 climate model. We find that coral populations can maintain themselves under all levels of hurricane disturbance providing that grazing levels are high. Regional differences in hurricane frequency are found to cause strikingly different spatial patterns of reef health with greater patchiness occurring in Belize, which has less frequent disturbance, than the Bahamas. The addition of coral bleaching led to a much more homogenous reef state over the seascape. Moreover, in the presence of bleaching, all reefs exhibited a decline in health over time, though with substantial variation among regions. Although the protection of herbivores does not prevent reef degradation it does delay rates of coral loss even under the most severe thermal and hurricane regimes. Thus, we can estimate the degree to which local conservation can help buy time for reefs with values ranging between 18 years in the Bahamas and over 50 years in Bonaire, compared with heavily fished systems. Ultimately, we demonstrate that local conservation measures can benefit reef ecosystem services but that their impact will vary spatially and temporally. Recognizing where such management interventions will either help or fail is an important step towards both achieving sustainable use of coral-reef resources and maximizing resource management investments.

Caribbean Corals in Crisis: Record Thermal Stress, Bleaching, and Mortality in 2005

BackgroundThe rising temperature of the world's oceans has become a major threat to coral reefs globally as the severity and frequency of mass coral bleaching and mortality events increase. In 2005, high ocean temperatures in the tropical Atlantic and Caribbean resulted in the most severe bleaching event ever recorded in the basin.Methodology/Principal FindingsSatellite-based tools provided warnings for coral reef managers and scientists, guiding both the timing and location of researchers' field observations as anomalously warm conditions developed and spread across the greater Caribbean region from June to October 2005. Field surveys of bleaching and mortality exceeded prior efforts in detail and extent, and provided a new standard for documenting the effects of bleaching and for testing nowcast and forecast products. Collaborators from 22 countries undertook the most comprehensive documentation of basin-scale bleaching to date and found that over 80% of corals bleached and over 40% died at many sites. The most severe bleaching coincided with waters nearest a western Atlantic warm pool that was centered off the northern end of the Lesser Antilles.Conclusions/SignificanceThermal stress during the 2005 event exceeded any observed from the Caribbean in the prior 20 years, and regionally-averaged temperatures were the warmest in over 150 years. Comparison of satellite data against field surveys demonstrated a significant predictive relationship between accumulated heat stress (measured using NOAA Coral Reef Watch's Degree Heating Weeks) and bleaching intensity. This severe, widespread bleaching and mortality will undoubtedly have long-term consequences for reef ecosystems and suggests a troubled future for tropical marine ecosystems under a warming climate.

Do corals select zooxanthellae by alternative discharge?

Loss of zooxanthellae (dinoflagellate Symbiodinium) from corals will sometimes lead to mass mortality of corals. To detect and quantify Symbiodinium released from corals, we developed a zooxanthellae “trap” and a quantitative PCR (qPCR) system with Symbiodinium clades A–F-specific primer sets. The trap was attached to a branch or the surface of several wild stony corals, and the water samples within the traps, including released Symbiodinium, were subjected to qPCR. All tested corals released clade C Symbiodinium at estimates of ~5,900 cells h−1 cm−2 of coral surface. Although all tested Pocillopora eydouxi harboured both clades C and D, some of these colonies released only clade C or released a lesser amount of clade D than that in the tissues. Our Symbiodinium quantification system revealed that wild hermatypic corals constantly release Symbiodinium to the environment. Our result suggests that some corals may discharge certain clades of Symbiodinium alternatively.

Combined effects of two stressors on Kenyan coral reefs are additive or antagonistic, not synergistic

AbstractA challenge for conservation science is predicting the impacts of co‐occurring human activities on ecological systems. Multiple anthropogenic and natural stressors impact ecosystems globally and are expected to jeopardize their ecological functions and the success of conservation and management initiatives. The possibility that two or more stressors interact synergistically is of particular concern, but such nonadditive effects remain largely unidentified in nature. A long‐term data set of hard coral cover from Kenyan reefs was used to examine the independent and interactive effects of two stressors: fishing and a temperature anomaly in 1998 that caused mass coral bleaching and mortality. While both stressors decreased coral cover, fishing by 51% and bleaching by 74%, they did not interact synergistically. Instead, their combined effect was antagonistic or weakly additive. The observed nonsynergistic response may be caused by the presence of one dominant stressor, bleaching, and cotolerance of coral taxa to both bleaching and fishing stressors. Consequently, coral bleaching has been the dominant driver of coral loss on Kenyan reefs and while marine reserves offer many benefits to reef ecosystems, they may not provide corals with a refuge from climate change.

Impacts of artisanal fishing on key functional groups and the potential vulnerability of coral reefs

SUMMARYFishing can have major impacts on the structure of coral reef ecosystems. Overfishing of herbivores is particularly detrimental, as it makes the coral system more likely to undergo shifts to macroalgal dominance in the event of coral mass mortality. Knowing when important processes, such as herbivory, are becoming brittle is important because it can provide an opportunity for managers to avoid undesirable ecosystem-level changes. This study investigates the impact of artisanal fishing on three important functional groups of herbivores (grazers, scrapers and excavators) on five coral-dominated reefs outside Zanzibar (Tanzania). There was a negative correlation between fishing pressure and fish biomass, abundance, diversity and species richness. Moreover, fishing had a negative influence on the demographic structure of functional groups, particularly excavators, manifesting itself as a skewness towards smaller individuals within populations. Artisanal fishing can have significant impacts on key functional groups of herbivorous reef fishes which may increase the vulnerability of coral reefs to undesirable ecosystem shifts.

Ecological stages of Maldivian reefs after the coral mass mortality of 1998

The bleaching event of 1998 caused widespread mortality on coral reefs in the Maldives. Nearly 10 years after the coral mass mortality, the state of Maldivian reefs was evaluated paying specific attention to three ecological stages, linked to the 3D structure of the coral community: young (high living hard coral cover), mature (a balance between living coral cover and loose sediment), and regressive (high amount of rubble and sand). The relative importance of three biogeomorphological descriptors (living hard corals; rubble and sand; coralline algae on rock) in the reef flat (2–7 m depth) and in the slope (7–18 m) of three reefscapes related to wave exposure was assessed. The role of wave energy in shaping ecological stages is different in the reef flat (early stages are found in low energy conditions) and in the slope (early stages are associated with high energy sites).

Effects of Climate Change/Global Warming on Coral Reefs: Adaptation/Exaptation in Corals, Evolution in Zooxanthellae, and Biogeographic Shifts

Increased sea-surface temperatures (SSTs) associated with climate change/global warming have caused bleaching in scleractinian corals (the loss of obligate symbiotic zooxanthellae) on a global basis, resulting in mass mortality of corals and decimation of reefs. This symbiotic relationship makes these corals an excellent bioindicator of climate change. It has been hypothesized that bleaching is a mechanism by which corals can adapt to changing environmental conditions via the “shuffling” of symbiont clades and acquisition of better-adapted symbionts. Experimental research has confirmed that zooxanthellae are sensitive to increases in seawater temperatures, exhibiting apoptosis (a form of programmed cell death) at temperatures of ≥30οC while in situ. The coral hosts, however, tolerate experimental temperatures up to 34οC, not showing signs of apoptosis and necrosis until 36οC. Thus, zooxanthellae currently appear to be poorly adapted to temperature increases, while the corals are resistant to higher temperatures, indicating that they are either already adapted or exapted (previously termed preadapted) to such. Coral hosts are experiencing no mortality from increased temperatures but a great deal of mortality from the death of their zooxanthellar symbionts. Symbiodinium sp. has a short generation time of one to several days, vs. that of its host coral—one to several years, affording the symbionts a distinct advantage for adaptation. Mutation rates are low in micro-organisms and higher in larger organisms. Smaller organisms have a disproportionately higher mutation rate than larger organisms, however, when compared on a per-generation basis. Thus, zooxanthellae are positioned better to adapt more rapidly than their coral hosts. Zooxanthellae have numerous clades, some of which are suspected to be better-adapted to higher seawater temperatures than others. The discovery of new Symbiodinium clades may be due to our ability to detect them via better technology or due to mutation, adaptation, and directional selection. Changes in the frequency of occurrence of different clades better-adapted to higher temperatures may be due to the emergence of newly mutated strains (i.e., sub-clades or sub-types) or re-distribution of existing clades. Predictions for the future of coral reefs include the following: a) Some tropical corals may be lost to local or global extinction, based on species-specific susceptibility to temperature increases; b) it is unlikely that other symbionts such as cyanobacteria will be able to replace Symbiodinium in the host-symbiont relationship; c) the tropics and sub-tropics will expand poleward and the other climatic zones will shift poleward, at the expense of the polar and sub-polar zones; d) a new “hyper-tropical zone” may appear near the equator, within which corals may be less species diverse, ill represented, or absent; and e) the depth distribution of corals may be extended, but probably only nominally so.

Energetics approach to predicting mortality risk from environmental stress: a case study of coral bleaching

1. Coral bleaching events, predicted to increase in frequency and severity as a result of climate change, are a threat to tropical coral-reef ecosystems worldwide. Although the onset of spatially extensive, or 'mass', bleaching events can be predicted using simple temperature stress metrics, no models are available for predicting coral mortality risk or sub-lethal stress associated with bleaching. Here, we develop a model that links the functional response of colony energy balance and energy store dynamics to coral mortality risk and recovery during and following bleaching events. 2. In a series of simulations using response functions and parameter values derived from experimental studies for two Indo-Pacific coral species (Acropora intermedia and Montipora monasteriata), we demonstrate that prior energy-costly disturbances and alternative energy sources are both important determinants of coral mortality risk during and following bleaching. 3. The timing of the onset of coral mass mortality is determined by a combination of bleaching severity (loss rate of photopigments), duration of the bleaching event, heterotrophy and the size of energy reserves (as lipid stores) before bleaching occurs. 4. Depending on initial energy reserves, model results showed that high rates of heterotrophy could delay the onset of coral mortality by up to three weeks. Survival following bleaching was also strongly influenced by remaining lipid reserves, rates of heterotrophy, and rates of photopigment (or symbiont) recovery. 5. Our results indicate that energy-costly disturbances and low availability of food, before and during bleaching events, respectively, work to increase bleaching-induced coral mortality risk for acroporid corals on Indo-Pacific reefs.

Status of Maldivian reefs eight years after the 1998 coral mass mortality

Episodes of coral bleaching and mortality have occurred with increasing frequency over the past two decades, often followed by proliferation of non-constructional organisms leading to a ‘phase shift’ in reef ecosystems. As the Maldives were severely affected by the 1998 episode, concern raised about the possibility of a phase shift there. The status of Maldivian reefs was surveyed eight years after the mass mortality of 1998; the percentages of biotic and abiotic benthic categories were estimated in both lagoon and ocean reefs. Hard coral cover did not exceed 37%, but no threat was recognized of a phase shift toward the dominance of soft corals, algae or sponges (7% on average). The high percentage of bare coral rock (up to almost 50%) and the local variability of living benthic cover suggest that Maldivian reefs were still in a re-colonization phase. Monitoring of their evolution is in progress.

Regional Decline of Coral Cover in the Indo-Pacific: Timing, Extent, and Subregional Comparisons

BackgroundA number of factors have recently caused mass coral mortality events in all of the world's tropical oceans. However, little is known about the timing, rate or spatial variability of the loss of reef-building corals, especially in the Indo-Pacific, which contains 75% of the world's coral reefs.Methodology/Principle FindingsWe compiled and analyzed a coral cover database of 6001 quantitative surveys of 2667 Indo-Pacific coral reefs performed between 1968 and 2004. Surveys conducted during 2003 indicated that coral cover averaged only 22.1% (95% CI: 20.7, 23.4) and just 7 of 390 reefs surveyed that year had coral cover >60%. Estimated yearly coral cover loss based on annually pooled survey data was approximately 1% over the last twenty years and 2% between 1997 and 2003 (or 3,168 km2 per year). The annual loss based on repeated measures regression analysis of a subset of reefs that were monitored for multiple years from 1997 to 2004 was 0.72 % (n = 476 reefs, 95% CI: 0.36, 1.08).Conclusions/SignificanceThe rate and extent of coral loss in the Indo-Pacific are greater than expected. Coral cover was also surprisingly uniform among subregions and declined decades earlier than previously assumed, even on some of the Pacific's most intensely managed reefs. These results have significant implications for policy makers and resource managers as they search for successful models to reverse coral loss.

Species composition and distribution of algae on the fringing coral reef of Sesoko Island (Ryukyu Archipelago) before and after the natural catastrophe of 1998

The species composition and distribution of marine plants on the fringing reef of Secoko Island were studied before and after the mass coral mortality in 1998. The study showed that changes in the bottom communities that occurred after bleaching of corals were caused by the presumed development of marine plants substituting reef-building corals on the bottom. The number of algal species grew from 211 to 345. The projective cover (PC) of hard substrate with macroalgae increased: in 1998, it was 1–10% in the subtidal zone and 20–50% in the intertidal zone, while in 2002 through 2005, the PC reached 71% in the subtidal and 40–85% in the intertidal zone. It is assumed that the phase of the “plant reef” on Sesoko Island is a temporary event, and that the coral reef can recover within several decades, unless a natural catastrophe occurs again.