Coral reef bleaching, the whitening of diverse invertebrate taxa, results from the loss of symbiotic zooxanthellae and/or a reduction in photosynthetic pigment concentrations in zooxanthellae residing within the gastrodermal tissues of host animals. Of particular concern are the consequences of bleaching of large numbers of reef-building scleractinian corals and hydrocorals. Published records of coral reef bleaching events from 1870 to the present suggest that the frequency (60 major events from 1979 to 1990), scale (co-occurrence in many coral reef regions and often over the bathymetric depth range of corals) and severity (>95% mortality in some areas) of recent bleaching disturbances are unprecedented in the scientific literature. The causes of small scale, isolated bleaching events can often be explained by particular stressors (e.g., temperature, salinity, light, sedimentation, aerial exposure and pollutants), but attempts to explain large scale bleaching events in terms of possible global change (e.g., greenhouse warming, increased UV radiation flux, deteriorating ecosystem health, or some combination of the above) have not been convincing. Attempts to relate the severity and extent of large scale coral reef bleaching events to particular causes have been hampered by a lack of (a) standardized methods to assess bleaching and (b) continuous, long-term data bases of environmental conditions over the periods of interest. An effort must be made to understand the impact of bleaching on the remainder of the reef community and the long-term effects on competition, predation, symbioses, bioerosion and substrate condition, all factors that can influence coral recruitment and reef recovery. If projected rates of sea warming are realized by mid to late AD 2000, i.e. a 2°C increase in high latitude coral seas, the upper thermal tolerance limits of many reef-building corals could be exceeded. Present evidence suggests that many corals would be unable to adapt physiologically or genetically to such marked and rapid temperature increases.

Coral reef bleaching: ecological perspectives

Climate is a driver of biotic systems. It affects individual fitness, population dynamics, distribution and abundance of species, and ecosystem structure and function. Regional variation in climatic regimes creates selective pressures for the evolution of locally adapted physiologies, morphological adaptations (e.g., color patterns, surface textures, body shapes and sizes), and behavioral adaptations (e.g., foraging strategies and breeding systems). In the absence of humans, broad-scale, long-term consequences of climatic warming on wild organisms are generally predictable. Evidence from Pleistocene glaciations indicates that most species responded ecologically by shifting their ranges poleward and upward in elevation, rather than evolutionary through local adaptation (e.g., morphological changes). But these broad patterns tell us little about the relative importance of gradual climatic trends as compared to extreme weather events in shaping these processes. Here, evidence is brought forward that extreme ...

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Impacts of Extreme Weather and Climate on Terrestrial Biota

High temperature tolerance experiments performed on Pocillopora damicornis, a major reef-building coral in the tropical eastern Pacific, resulted in loss of zooxanthellae, histopathological abnormalities, and mortality similar to that observed during the severe 1982–83 El Nino-Southern Oscillation (ENSO) event. Coral vitality declined significantly at 30–32°C during a 10-week period, but remained high at normal temperatures (26–28°C). Laboratory time courses to coral morbidity and death were similar to those observed in the field. Experimental high temperatures had a greater negative effect on corals from the Gulf of Panama, which experiences seasonally cool upwellings, than on corals from the nonupwelling Gulf of Chiriqui. The condition of obligate symbiotic crustaceans (Trapezia, Alpheus) associated with experimental corals declined with their host's declining condition. All Gulf of Panama corals subjected to 32°C were dead after 5 weeks, and all of their associated crustacean symbionts were dead after 9 weeks. Gulf of Chiriqui corals at 30°C survived for 9 weeks and 42% of their crustacean symbionts were still alive after 10 weeks. Coral mortality in the Gulf of Panama was significantly higher (68.5%) after El Nino warming than after subsequent episodes of unusually intense cool upwellings (10.4%). Low temperature stress (cool currents and upwelling) has been generally suggested as the critical limiting condition that prevents extensive coral reef development in the eastern Pacific. Our results suggest that infrequent but severe ENSO sea warming events also may limit reef development in this region.

Experimental evidence for high temperature stress as the cause of El Niño-coincident coral mortality

Bleaching and mortality of zooxanthellate corals during the 1997-98 El Nino-Southern Oscillation (ENSO) event are documented for eastern equatorial Pacific localities in Panama (Gulf of Chiriqui and Gulf of Panama) and Ecuador (Galapagos Islands and mainland coast). Overall, the very strong 1997-98 and 1982-83 ENSOs were similar in magnitude and duration, but varied spatially, resulting in different patterns of elevated sea temperature stress and coral responses during the two disturbance events. Two bleaching episodes occurred in the Gulf of Chiriqui, each coincident with high (≥30°C) in situ temperatures and warm water filaments visible in NOAA/NCEP SST fields. Coral mortality was significantly different among localities: Galapagos Islands (26.2%) > Gulf of Chiriqui (13.1%) > coastal Ecuador (7.0%) > Gulf of Panama (0%). Coral mortality was notably higher (52-97%) in the eastern equatorial Pacific in 1982-83 than in 1997-98. Coral mortality among most sites within localities was also significantly different. Highest coral mortalities occurred at offshore compared with nearshore sites in Panama, and at the Galapagos Islands compared with mainland Ecuador. Although species responses varied among localities, tissue death was especially high in Millepora spp., Pavona spp., Pocillopora spp., and Porites spp. Corals present in relatively deep (12-18 m), inter-reef habitats suffered lower rates of bleaching and mortality than similar and different species present in shallow (1-10 m) habitats. Bleached coral tissues in the Gulf of Chiriqui demonstrated a significant increase in zooxanthella density, but only a slight increase in chlorophyll a concentration over a 5 mo respite, from the end of the first bleaching event (October 1997) to the beginning of the second event (March 1998). The use of molecular DNA techniques to compare algal symbionts in bleached and healthy coral colonies revealed a strong correlation between bleaching severity and symbiont genotype, regardless of depth. In particular, one symbiont genotype (commonly found in the scleractinian genus Pocillopora in the Gulf of Chiriqui) was particularly resistant to bleaching, indicating that symbiont diversity can play an important role in explaining spatial and host systematic patterns of bleaching. Extreme reductions in abundance of some species populations in Panama have resulted in local extirpations. It is possible that Millepora boschmai, a Gulf of Chiriqui endemic hydrocoral, is now extinct. Remnant patches of Gardineroseris planulata that survived the 1982-83 ENSO in the Galapagos Islands have also disappeared following the 1997-98 event, causing local extinctions. Marked declines in external bioerosion and corallivore abundances in Panama before and after the 1997-98 ENSO should have less of an effect on surviving corals than in 1982-83 when such effects continued to degrade reefs long after the initial disturbance. Surviving corals in the Galapagos Islands are still subject to heavy grazing pressure by abundant echinoid populations. The response of eastern Pacific coral reefs to the 1997-98 El Nino cannot be fully understood without an appreciation of spatial/temporal variability and the historical stresses to which these reefs have been subject over the past 25+ yrs.

Coral bleaching and mortality in Panama and Ecuador during the 1997-1998 El Niño-Southern Oscillation event : Spatial/temporal patterns and comparisons with the 1982-1983 event

High-precision Sr/Ca ratios are reported for Porites corals from Davies Reef, at ∼19°S on the midshelf of the Great Barrier Reef, Australia. Skeletal growth parameters were also determined from coral slabs using X ray photographs and γ-densitometry measurements. A calibration of the Sr/Ca thermometer has been obtained, using in situ measured sea surface temperatures, which gives Sr/Ca × 10³ = 10.48 (±0.01) − 0.0615 (±0.0004) × T, whereT is in °C. This calibration applies to different species of Porites and a large range of annual extension and calcification rates, provided sampling follows a major growth axis. Biases of 1°–2°C have been observed in the low-density margins where growth is hampered by interfering fans of corallites. The analytical and the long-term reproducibility of Sr/Ca-derived temperatures are both better than ±0.3°C (2σ). Coral growth parameters such as the extension rate or density generally appear to respond to environmental changes such as rainfall but do not affect the Sr/Ca partitioning between coralline aragonite and seawater. No correlation has been found between calcification rate and Sr/Ca variations, consistent with Sr/Ca being mainly controlled by temperature. In the Great Barrier Reef, the interannual variability in SST is primarily related to the El Nino-Southern Oscillation phenomenon and is typically marked by a cold anomaly during the winter preceding the mature warm phase. Instrumental records show that the surface waters of the Coral Sea around 19°S have experienced a significant cooling associated with the El Ninos of 1965, 1972, and 1982-1983 which the Porites corals record faithfully. The coral-derived temperatures for the period from 1965 to 1993 show an increase of 1.3°C, which has occurred mainly since 1979. This warming is larger in magnitude but consistent with the trend for the southern hemisphere land and marine temperature records, which shows an increase of around 0.4°C.

Strontium/calcium ratios in modern Porites corals from the Great Barrier Reef as a proxy for sea surface temperature: calibration of the thermometer and monitoring of ENSO

J.K. Oliver

Papers

Coral Mortality Outside of the Eastern Pacific During 1982-1983: Relationship to El Niño