Forecasting decadal changes in sea surface temperatures and coral bleaching within a Caribbean coral reef

Elevated sea surface temperature (SST) caused by global warming is one of the major threats to coral reefs. While increased SST has been shown to negatively affect the health of coral reefs by increasing rates of coral bleaching, how changes to atmospheric heating impact SST distributions, modified by local flow environments, has been less understood. This study aimed to simulate future water flow patterns and water surface heating in response to increased air temperature within a coral reef system in Bocas del Toro, Panama, located within the Caribbean Sea. Water flow and SST were modeled using the Delft3D-FLOW© computer simulation package. Locally measured physical parameters, including bathymetry, astronomic tidal forcing, and coral habitat distribution were input into the model and water flow, and SST was simulated over a four-month period under present day, as well as projected warming scenarios in 2020s, 2050s, and 2080s. Changes in SST, and hence the thermal stress to corals, were quantified by degree heating weeks. Results showed that present-day reported bleaching sites were consistent with localized regions of continuous high SST. Regions with highest SST were located within shallow coastal sites adjacent to the mainland or within the interior of the bay, and characterized by low currents with high water retention times. Under projected increases in SSTs, shallow reef areas in low flow regions were found to be hot spots for future bleaching.     

Coral bleaching, reef fish community phase shifts and the resilience of coral reefs

The 1998 global coral bleaching event was the largest recorded historical disturbance of coral reefs and resulted in extensive habitat loss. Annual censuses of reef fish community structure over a 12-year period spanning the bleaching event revealed a marked phase shift from a prebleach to postbleach assemblage. Surprisingly, we found that the bleaching event had no detectable effect on the abundance, diversity or species richness of a local cryptobenthic reef fish community. Furthermore, there is no evidence of regeneration even after 5–35 generations of these short-lived species. These results have significant implications for our understanding of the response of coral reef ecosystems to global warming and highlight the importance of selecting appropriate criteria for evaluating reef resilience.     

Effects of modeled tropical sea surface temperature variability on coral reef bleaching predictions

Future widespread coral bleaching and subsequent mortality has been projected using sea surface temperature (SST) data derived from global, coupled ocean–atmosphere general circulation models (GCMs). While these models possess fidelity in reproducing many aspects of climate, they vary in their ability to correctly capture such parameters as the tropical ocean seasonal cycle and El Ni?o Southern Oscillation (ENSO) variability. Such weaknesses most likely reduce the accuracy of predicting coral bleaching, but little attention has been paid to the important issue of understanding potential errors and biases, the interaction of these biases with trends, and their propagation in predictions. To analyze the relative importance of various types of model errors and biases in predicting coral bleaching, various intra- and inter-annual frequency bands of observed SSTs were replaced with those frequencies from 24 GCMs 20th century simulations included in the Intergovernmental Panel on Climate Change (IPCC) 4th assessment report. Subsequent thermal stress was calculated and predictions of bleaching were made. These predictions were compared with observations of coral bleaching in the period 1982–2007 to calculate accuracy using an objective measure of forecast quality, the Peirce skill score (PSS). Major findings are that: (1) predictions are most sensitive to the seasonal cycle and inter-annual variability in the ENSO 24–60 months frequency band and (2) because models tend to understate the seasonal cycle at reef locations, they systematically underestimate future bleaching. The methodology we describe can be used to improve the accuracy of bleaching predictions by characterizing the errors and uncertainties involved in the predictions.     

Elevated temperatures and bleaching on a high latitude coral reef: the 1988 Bermuda event

Sea temperatures were normal in Bermuda during 1987, when Bermuda escaped the episodes of coral bleaching which were prevalent throughout the Caribbean region. Survey transecs in 1988 on 4–6 m reefs located on the rim margin and on a lagoonal patch reef revealed bleaching only of zoanthids between May and July. Transect and tow surveys in August and September revealed bleaching of several coral species;Millepora alcicornis on rim reefs was the most extensively affected. The frequency of bleaching in this species,Montastrea annularis and perhapsDiploria labyrinthiformis was significantly higher on outer reefs than on inshore reefs. This bleaching period coincided with the longest period of elevated sea temperatures in Bermuda in 38 years (28.9–30.9°C inshore, >28° offshore). By December, when temperatures had returned to normal, bleaching of seleractinians continued, but bleaching ofM. alcicornis on the outer reefs was greatly reduced. Our observations suggest that corals which normally experience wide temperature ranges are less sensitive to thermal stress, and that high-latitude reef corals are sensitive to elevated temperatures which are within the normal thermal range of corals at lower latitudes.     

Bioindication in coral reef ecosystems

The concept of bioindication in the sense of the use of organisms for detecting environmental stress has been employed in coral reef conservation and management for the past several years. Important tools are coral growth rates and various community parameters, notably hard coral cover. The present need is the optimal coordination of international efforts for the earliest possible institution of an effective monitoring system.     

Histological observations in the Hawaiian reef coral, Porites compressa, affected by Porites bleaching with tissue loss

The scleractinian finger coral Porites compressa is affected by the coral disease Porites bleaching with tissue loss (PBTL). This disease initially manifests as bleaching of the coenenchyme (tissue between polyps) while the polyps remain brown with eventual tissue loss and subsequent algal overgrowth of the bare skeleton. Histopathological investigation showed a loss of symbiont and melanin-containing granular cells which was more pronounced in the coenenchyme than the polyps. Cell counts confirmed a 65% reduction in symbiont density. Tissue loss was due to tissue fragmentation and necrosis in affected areas. In addition, a reduction in putative bacterial aggregate densities was found in diseased samples but no potential pathogens were observed.     

Revisiting coral reef connectivity

A large river plume generated by anomalous precipitation and oceanic circulation associated with Hurricane Mitch was detected off Honduras in October 1998 using SeaWiFS ocean color images. This event provides the background for analyzing connectivity between coral reefs and land in the Meso-American reef system. We discuss the potential implications of such short-term events for disease propagation and nutrification, and their potential significance in evolutionary processes.     

Correction to: Skeletal records of bleaching reveal different thermal thresholds of Pacific coral reef assemblages

Coral Reefs - Figure 2 in the original article has been updated with this figure 2 due to discrepancies related to incorrect mapping with one of the islands.     

Microscopic observations of recovery in the reef-building scleractinian coral,Montastrea annularis,after bleaching on a Cayman reef

Living tissues of a single massive colony of the reef-building scleractinian coral, M. annularis, were sampled at one month intervals following the observation that areas of marked discoloration (bleaching) had developed by the beginning of the summer of 1988. Histological analysis of replicate plugs of tissue from bleached and unbleached regions of that colony allowed comparison of time-matched samples as well as zone-matched samples over a period of six months. During that interval, the entire colony was restored to a uniform and normal degree of pigmentation. Histology of the bleached zones documents the gradual accumulation of zooxanthellae and the thickening of the gastrodermal epithelium of the coral. Once reconstituted, the previously bleached gastrodermis, repopulated with algal symbionts, is indistinguishable from unbleached tissue in the coral colony.     

The use of coral nubbins in coral reef ecotoxicology testing

While there is an urgent demand to establish reliable ecotoxicological assays for reef corals, there has not been yet an available source material that can supply the high number of colony replicates needed for reliable tests. In past experiments, the major obstacle to obtaining as many fragments as possible had been the damage inflicted to donor colonies by pruning. In this paper, we present the application of coral nubbins, a novel source material for coral ecotoxicology assays. Nubbins from the branching Red Sea coral Stylophora pistillata (n>450) were used for evaluating the impacts of water soluble fractions from a crude oil, an oil dispersant and dispersed oil. Coral nubbins (minute coral fragments in the size of one to several polyps) harvested from a single colony are genetically identical to each other, may be obtained in any quantity needed and whenever research activities demand their use. Several dozens of nubbins can be obtained from a single small branch in branching coral species, a procedure that has minimal impact on donor genotypes. Nubbins production is a low cost procedure and requires limited maintenance space. Results of short and long-term acute ecotoxicological tests are revealed and discussed here, indicating the advantageous use of nubbins as ubiquitous coral material for toxicology assays and physiological studies.     

A novel reef coral symbiosis

Reef building corals form close associations with unicellular microalgae, fungi, bacteria and archaea, some of which are symbiotic and which together form the coral holobiont. Associations with multicellular eukaryotes such as polychaete worms, bivalves and sponges are not generally considered to be symbiotic as the host responds to their presence by forming physical barriers with an active growth edge in the exoskeleton isolating the invader and, at a subcellular level, activating innate immune responses such as melanin deposition. This study describes a novel symbiosis between a newly described hydrozoan (Zanclea margaritae sp. nov.) and the reef building coral Acropora muricata (=A. formosa), with the hydrozoan hydrorhiza ramifying throughout the coral tissues with no evidence of isolation or activation of the immune systems of the host. The hydrorhiza lacks a perisarc, which is typical of symbiotic species of this and related genera, including species that associate with other cnidarians such as octocorals. The symbiosis was observed at all sites investigated from two distant locations on the Great Barrier Reef, Australia, and appears to be host species specific, being found only in A. muricata and in none of 30 other species investigated at these sites. Not all colonies of A. muricata host the hydrozoans and both the prevalence within the coral population (mean = 66%) and density of emergent hydrozoan hydranths on the surface of the coral (mean = 4.3 cm⁻², but up to 52 cm⁻²) vary between sites. The form of the symbiosis in terms of the mutualism–parasitism continuum is not known, although the hydrozoan possesses large stenotele nematocysts, which may be important for defence from predators and protozoan pathogens. This finding expands the known A. muricata holobiont and the association must be taken into account in future when determining the corals’ abilities to defend against predators and withstand stress.     

The abundance of herbivorous fish on an inshore Red Sea reef following a mass coral bleaching event

A healthy herbivore community is critical for the ability of a reef to resist and recover from severe disturbances and to regain lost coral cover (i.e., resilience). The densities of the two major herbivorous fish groups (the family Acanthuridae and scarine labrids) were comparatively studied for an inshore reef that was severely impacted by a mass coral bleaching event in 2010 and an unaffected reef within the same region. Densities were found to be significantly higher on the affected reef, most likely due to the high algal densities on that reef. However, densities of herbivores on both reefs were found to be on average about 1–2 orders of magnitude lower than previously published reports from some Pacific reefs and from Red Sea reefs in the Gulf of Aqaba and only slightly higher than Caribbean reefs. Thus, it is predicted that recovery for this reef and similarly affected reefs may be very slow. The protection of herbivores from overfishing and the introduction of other management strategies that maximize reef resilience in Saudi Arabian waters are highly recommended.     

Quantifying the quality of coral bleaching predictions

Techniques that utilize sea surface temperature (SST) observations to predict coral reef bleaching are in common use and form the foundation for predicted global coral reef ecosystem demise within this century. Yet, quality assessments of these methods are typically qualitative or anecdotal. Quality is the correspondence of forecasts with observations and has standard quantitative measures. Here a forecast verification method, commonly used in meteorology, is presented and used to measure the quality of the degree heating weeks (DHW) technique as an exploration of insights that can be gleaned from this methodology. DHW values were calculated from NOAA Optimum Interpolation SST version 2 data and compared to a database of bleaching observations from 1990–2007. Quality is expressed with an objective measure, the Peirce Skill Score (PSS). The quality at varying DHW thresholds above which bleaching was projected to occur is calculated. By selecting the thresholds that maximize quality, the predictive technique is objectively optimized. This results in optimal threshold maps, showing reefs more prone and more resistant to bleaching. Optimization increases the quality of DHW as a predictor of bleaching from PSS = 0.55 to PSS = 0.83, in global average, but the optimal PSS and corresponding DHW values vary significantly from location to location. The coral reef research and management community are urged to adopt the simple, but rigorous tools of forecast verification routinely used in other disciplines so that bleaching forecasts can be quantitatively compared and their quality improved.     

Coral and algal changes after the 1998 coral bleaching: interaction with reef management and herbivores on Kenyan reefs

Interaction between the El Niño and Indian Ocean dipole ocean-atmosphere quasi-periodic oscillations produced one of the warmest seawater temperatures on record in 1998. During the warm northeast monsoon in March and April, Kenya's shallow coral reefs experienced water temperatures between 30 and 31 °C and low winds. This caused large-scale bleaching of hard and soft corals at the end of March, which extended into the cooler months of May and June. Direct observations of coloration in the Mombasa Marine National Park found that the coral genera Acropora, Millepora, Pocillopora, branching Porites and Stylophora showed rapid bleaching and high mortality by the end of May 1998. Other hard coral genera that bleached significantly included Echinopora, Favia, Favites, Galaxea, Hydnophora, Goniopora, Leptoria, Montipora, Platygyra and massive Porites, but mortality was variable among these genera. Astreopora, Coscinarea, Cyphastrea and Pavona were the least responsive genera, with some paling, but little evidence of full bleaching or significant mortality. We compared changes in reef ecology in four national parks (protected from fishing) with four non-park areas (heavy fishing) to determine how coral mortality and herbivory interact under the two management regimes. Benthic studies using line transects in 16 sites spread across ~150 km of coastline were completed before and 6 to 13 months after the bleaching event and found that the cover of nine hard coral genera including Acropora, Alveopora, Favites, Goniopora, Platygyra, Pocillopora, branching Porites, Stylophora and Tubipora decreased significantly (p<0.04) after the event, usually by >85%, and soft coral cover decreased by ~75%. One year after the bleaching, sites in the national parks experienced 88 and 115% increases in turf and fleshy algal cover, respectively, while reefs outside the parks had a 220% increase in fleshy algal cover with no appreciable change in turf-forming algal cover. There was, however, high spatial variation and no statistically significant difference in the change in fleshy algal cover between sites in and out of the national parks. Estimates of herbivory by both fish and sea urchins at each site were a good predictor of the change in fleshy algal cover over the 1-year post-bleaching period in about half of the sites. The other half of the sites did not exhibit large changes in fleshy algal cover, and algal cover at these sites was not clearly influenced by herbivory. The number of coral genera per transect at each site decreased significantly by 31 and 44% in and out of the national parks respectively. Larger-scale search sampling to determine the presence/absence of genera in study sites found consistent losses of coral genera from sites, but produced smaller differences than the line-transect method, suggesting that some genera persisted, but at very low population densities and small colony sizes.     

Species-specific habitat distribution of coral reef fish assemblages in relation to habitat characteristics in an Okinawan coral reef

We determined the species-specific habitat associations of coral reef fishes and environmental characteristics in an Okinawan coral reef in Japan. We focused on three families (Pomacentridae, Gobiidae and Labridae) and attempted to determine differences in habitat utilization. We selected six sites along the coast of Amitori Bay, from the entrance to the innermost part, in order to cover a wide range of habitat characteristics (exposed habitat, semi-exposed habitat and sheltered habitat). The species diversity of coral assemblages was greater at the exposed and semi-exposed habitats, whereas branching coral mostly covered the sheltered habitat. The environmental factors that determine the species-specific spatial association in fishes differed among families. Both biological characteristics (coral morphology and coral species diversity) and physical characteristics (water depth and wave exposure) affected the spatial association of pomacentrids and gobiids. In contrast, physical characteristics such as substrate complexity and water depth affected the species-specific spatial association of labrid species. Further study is needed to determine the ecological factors that regulate the species-specific habitat preference in Okinawan coral reefs.