The phenomenon of extensive coral bleaching was first described by Glynn (1984), after reefs along the Pacific coast of Panama bleached in response to the El Niño-Southern Oscillation event of 1982-83 (see also Glynn 1988), although more localised events had been reported earlier. Only isolated bleachings had been reported previously since monitoring began there in the 1950s (Sebens 1994). There have been a series of global bleaching events since the early 1980s whose frequency, scale and severity are unprecedented in modern times (for reviews see Berkelmans and Oliver 1999; Hoegh-Guldberg 1999). Bleaching events similar to that associated with the 1982-83 El Niño were observed throughout the Caribbean in 1987, 1989 and 1990 (Williams and Bunkley-Williams 1990).

Bleaching occurs when the corals lose the pigments or cells of their symbiotic zooxanthellae (photosynthetic algae), leaving the corals pale to white in appearance. This may occur, for example, when the thermal tolerance of corals or their zooxanthellae is exceeded, resulting in the mass movement of the zooxanthellae from the coral tissues. Although many corals can recover from this, by repopulation from other or remaining zooxanthellae, their loss makes the coral tissue far more sensitive to light damage, with extreme or prolonged stress resulting in death. Several studies now show that even with recovery, sublethal chronic effects on growth and reproduction may be apparent (Hoegh-Guldberg 1999). The vulnerability of corals to bleaching varies: some massive taxa such as Porites are more resistant than the branching Acropora species, the former having a thicker layer of coral tissue covering the skeleton. In addition, pigmented species, such as some of the pocilloporids, also appear more resistant to bleaching (Wilkinson 1998b; Hoegh-Guldberg 1999). Extensive bleaching occurred during early 2002 on the GBR, especially on inshore reefs[128].

A wide variety of factors provoke this stress response, including altered seawater temperatures, increased irradiance (including UV), decreased salinity, and bacterial and other infections (Table 6.3). However, it is important to distinguish between factors that cause extensive (“mass”) bleaching as opposed to localised events (e.g., over a few hundred metres, in intertidal areas, river mouths etc.), and between primary causes versus compounding stresses. Most large-scale bleaching has mostly been attributed to a rise in sea temperature; in particular when sea-surface temperatures (SST) exceed their summer maximum, or through some combination of temperature and irradiance[129] (Brown 1997). The apparently increased episodes of coral bleaching over the last two decades are well correlated with increased SST (e.g., reviews by Williams and Bunkley-Williams 1990; Glynn 1991; Brown 1997; Hoegh-Guldberg 1999; Reaser et al. 2000; Fitt et al. 2001; Woesik 2001). Decreased SST and reduced salinity generally cause much more localised bleaching. Bacterial infections have been cited as a cause of bleaching, but it is difficult to attribute this unequivocally as a causal agent (Brown 1997). 

Coral reefs around the world are subject to a range of chronic stresses, including pollution, disease, predation by Acanthaster, etc., and it has been suggested that these may lower the corals’ resistance to the bleaching process (e.g., Williams and Bunkley-Williams 1990), as well as hampering recovery. In general, most evidence points to increased global temperatures as the primary cause of global bleaching, with irradiance/UV as an important secondary variable (O. Hoegh-Guldberg pers. comm.).

Table 6.3: A summary of selected papers which have identified bleaching-related responses to various stressors (after Brown 1997).

STRESSOR

FIELD

LABORATORY

Elevated sea water temperature

Glynn (1993) for reviews see Hoegh-Guldberg (1999) and Berkelmans (1999).

Hoegh-Guldberg and Smith (1989); Glynn and D’Croz (1990); Lesser et al. (1990); Iglesias Prieto et al. (1992); Fitt and Warner (1995); Warner et al. (1996); Jones et al. (1998). 

Decreased sea water temperature

Coles and Fadlallah (1991); Kobluk and Lysenko (1994)

Muscatine et al. (1991); Gates et al. (1992)

Increased irradiance (including UV)

Fisk and Done (1985); Gleason and Wellington (1993); Brown et al. (1994); Gleason and Wellington (1995)

Hoegh-Guldberg and Smith (1989); Lesser (1989); Lesser and Shick (1989); Lesser et al. (1990)

Combination of elevated temperature and irradiance

Harriott (1985); Brown and Suharsono (1990); Williams and Bunkley-Williams (1990) for review; Glynn (1993) for review; Brown et al. (1995)

Lesser et al. (1990); Glynn et al. (1992)

Reduced salinity

Goreau (1964); Van Woesik et al. (1995); De Vantier et al. (1996)

Fang et al. (1995); Kushmaro et al. (1996)

Bacterial and other infections

Upton and Peters (1986); Kushmaro et al. (1996)

Kushmaro et al. (1996)

While SST is the most common factor believed to be responsible for extensive bleaching, there has been some controversy in attributing the cause to global warming (Glynn 1993). In part, this has been due to an apparent lack of correlation between bleaching and surface temperature anomalies (Brown 1997). However, there are problems with using satellite data alone to determine SST, the most significant of which are lack of resolution and difficulties in ground-truthing. In addition, satellite data only relate to the top few millimetres of surface waters, which may correspond more closely to solar radiation than bulk sea temperature (Brown 1997). Nonetheless, during the recent 1997-98 El Niño-induced global bleaching event, the US National Oceanic and Atmospheric Administration were able to accurately predict where bleaching would occur using satellite SST data (Baird and Marshall 1998; Hogarth 1999). There is now an on-line service that accurately shows ocean hot spots (Coral Reef Bleaching Indices: - NOAA’s National Environmental Satellite, Data, and Information Service (NESDIS), inaugurated 22 February 2000).

Another problem in explaining mass bleaching events has been the lack of continuous long-term data (Glynn 1993). In Phuket, Thailand, long-term continuous environmental monitoring has been in place since 1942 (Brown 1997). At this site, elevated seawater temperatures are thought to be primarily responsible for observed coral bleaching. The highest seawater temperatures on record occurred in 1991 and 1995 and were associated with bleaching events (Brown et al. 1996). The monthly mean SST for Phuket during the last 50 years showed a long-term and significant (p<0.001) decadal increase of 0.126ºC. This is consistent with positive trends in the Indian Ocean area and tropical seas as a whole (Bottomley et al. 1990;  cited in Brown 1997).

The Great Barrier Reef appeared to be largely immune from mass coral bleaching prior to 1998, although more localised bleaching certainly has occurred at several locations. For instance, recurrent bleaching was reported on the fringing reefs of Magnetic Island during the summers of 1979/80, 1981/82, 1986/87, 1991/92 and 1993/94 (Jones et al. 1997) and again during 1998 and 2002 (Berkelmans and Oliver 1999; and GBRMPA website ). Continuous in situ water temperature recordings suggest a close correlation between bleaching and periods of average daily seawater temperatures approaching 32ºC. Each of the bleaching events has occurred during periods of unusually high air temperatures and there has been a significant increase in annual summer and winter air temperatures in this area since the middle of last century. Differential coral survival may be explained, at least in part, by differences in local currents, not only providing different temperature regimes but also differences in diffusion boundaries between the coral and the water (Nakamura and Woesik 2001).

Intensive and extensive coral bleaching on the GBR occurred during 1997-98, (although extensive bleaching has also occurred in early 2002- see GBRMPA web site) when it was reported along the length of the GBR, particularly on inshore fringing reefs in the central GBR.  Aerial surveys of 654 reefs (approximately 23% of all GBR reefs) showed that 87% of inshore reefs were bleached at least to some extent – 67% had >10% loss of coral cover and 25% had >60% loss of coral cover. This compared to 28% of offshore (mid- and outer-shelf) reefs with at least some bleaching, 14% with >10% loss and none with >60% loss (Berkelmans and Oliver 1999). Ground surveys showed that the aerial survey data underestimated the extent and intensity of bleaching (Berkelmans and Oliver 1999). Around Orpheus Is, where 103 scleractinian species were recorded from transects, colonies from 101 species were found to be bleached, as well as those of three common Alcyonacae genera and a hydrocoral. Acroporid corals were the worst affected; for example, all colonies of Acropora hyacinthus and A. gemmifera were bleached and 70-80% were dead within five weeks after the initial bleaching reports. Such high mortality of the dominant acroporids suggests this event was more severe than bleaching in 1982 (Baird and Marshall 1998). After the 1998 bleaching there was no coordinated reef-wide follow up to determine the rate of recovery and mortality of bleached coral colonies. With the onset of the 2002 bleaching, GBRMPA has established video transects on 30 affected reefs scattered along the reef and will continue to monitor these over the next few months to determine overall rates of mortality and recovery. On some of these reefs, it appears that between 50-90% of the bleached corals have died (P. Marshall, pers. comm.).

Given the finding that coral bleaching can occur in response to increased sea temperature, global warming has potentially serious consequences for coral reefs worldwide. Presently it is a matter of debate whether this would lead to catastrophe, including, potentially, “the complete loss of coral reefs on a global scale” (Hoegh-Guldberg 1999), or some form of adaptive response by corals. The report released by Hoegh-Guldberg (1999) predicted a grim future for coral reefs, in Australia and worldwide, if global warming continues unchecked, including severe annual bleaching events in most tropical oceans within 30-50 years (Hoegh-Guldberg 1999). Other researchers disagree, arguing that this fails to take into account natural selection or the potential for adaptive evolutionary change. For instance, it has been hypothesised (e.g., Buddemeier and Fautin 1993; Ware et al. 1996) that bleaching is an adaptive mechanism allowing the coral to be repopulated with a different, possibly more stress resistant, type of alga. The fact that some coral types do survive bleaching events suggests that some genotypes could take over if the seas do warm, while others have argued that the distribution of corals and coral reefs may shift to suit new conditions. Given the rapidity of the changes currently occurring it is unlikely, however, that such adaptive mechanisms can occur in time (Glynn 1993; Brown 1997; Berkelmans and Oliver 1999; Hoegh-Guldberg 1999) to prevent a decline in the distribution and extent of coral reefs. Corals clearly can adapt over evolutionary time scales, but such changes are expected to take many hundreds, if not thousands of years (Hoegh-Guldberg 1999) and any change in distribution would be limited to the availability of suitable environments.

Reefs worldwide are increasingly being subjected to a variety of anthropogenic impacts (e.g., eutrophication, increased sedimentation, mining, physical destruction, destructive fishing methods); with about 50-70% of all coral reefs being directly impacted by human activities (Wilkinson 1998; 2000 and references therein). Thus, climate-induced stresses cannot be considered in isolation and the interaction of anthropogenic and natural influences must be a major factor determining not only the mortality and recovery of reefs from bleaching, but also their ability to adapt to future change. While Australia’s reefs are considered to be in relatively good condition (Wilkinson and Buddemeier 1994; Wilkinson 1998, 2000), with the exception of some inshore reefs, there is no room for complacency and active management of this increasingly used resource must be maintained.

Bleaching in other invertebrates


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