Mass Coral Bleaching And Disease

Australia's tropical waters have warmed significantly over the past 150 years, with much of the increase being seen over the past 50 years. Rates of warming are similar to that observed globally for tropical/subtropical waters (+0.73°C from 1951-1990). These changes have brought corals on reefs closer to their thermal limits, with the result that warmer than average years (arising due to natural variability) now push corals beyond their upper thermal thresholds. Corals respond to stress by losing their normal brown colour ('bleaching'). Coral bleaching occurs when the symbiosis between coral and their dinoflagellate symbionts (Fig. 10.3A, B) disintegrates, with the rapid movement of the brown symbionts out of the otherwise translucent tissues of the coral (Fig. 10.3C). As a result, corals change rapidly from brown to white. Bleaching is a generic response that occurs in response to a wide array of stresses, including reduced salinity, high or low irradiance, some toxins like cyanide and many herbicides, microbial infection and high or low temperatures (see Additional reading).

Coral bleaching has been known for over 70 years from reports in which individual colonies or small patches of reefs were observed to have bleached. However, more recently 'mass' coral bleaching events have affected coral reefs over hundreds and even thousands of square kilometres. Mass coral bleaching events have only been reported since 1979. Work done during the 1980s and 1990s revealed that mass bleaching events are triggered by warmer than normal

Figure 10.3 A, reef building coral (Acropora sp.) with normal populations of dinoflagellate symbionts; B, the key dinofla-gellate symbiont of corals, Symbiodinium; C, coral bleached due to rising environmental stress; D, bleached coral reef in January 2006 in Keppel I., on the southern Great Barrier Reef. (Photos: O. Hoegh-Guldberg.)

Figure 10.3 A, reef building coral (Acropora sp.) with normal populations of dinoflagellate symbionts; B, the key dinofla-gellate symbiont of corals, Symbiodinium; C, coral bleached due to rising environmental stress; D, bleached coral reef in January 2006 in Keppel I., on the southern Great Barrier Reef. (Photos: O. Hoegh-Guldberg.)

conditions and can be predicted using sea-surface temperature anomalies measured by satellites. Light is an important co-factor. Corals that are shaded tend not to bleach as severely as those under normal irradiances. Corals also differ in their susceptibility, with some corals such as Porites and Favia being more tolerant of thermal stress than Acropora, Stylophora and Pocillopora. Differences in sensitivity probably relate to host characteristics such as tissue thickness and pigmentation, and possibly genotype of the symbiotic dinoflagellates within coral tissues. Coral bleaching is also affected by water motion, with corals in still, warm, and sunlit conditions showing the greatest impact of thermal stress. The latter is consistent with the first observations of the association of coral bleaching with the doldrum conditions typical of El Niño years in the eastern Pacific.

Mass coral bleaching has affected almost every coral reef worldwide since the 1980s and it occurs on the GBR approximately every 3-5 years at the present time (late 20th and early 21st century). Mass coral bleaching has occurred most often in El Niño years (e.g. 1987-1988; 1997-1998) and less often in the cooler non-El Niño or La Nina years (1988-1989; 1999-2000). Bleached corals tend to recover their dinoflagellate symbiotic populations in the months following an event if the stress involved is mild and short-lived. But mortality (up to 100% of corals over large areas of coral reef) occurs following intense and long-lasting stress. This was seen in many parts of the world in 1998, in which approximately 16% of corals that were surveyed prior to the global cycle of bleaching were estimated to have died by the end of 1998. This particular figure is an average and conceals the fact that in some oceans, for example, the Western Indian Ocean, up to 46% of corals may have died.

Coral reefs in Australia have bleached repeatedly over the past 30 years. Mass bleaching events occurred in Australia in 1983, 1987, 1991, 1998, 2002 and in 2006, with large sections of the GBR bleached in each case. Mortality rates on the GBR have been relatively low compared to coral reefs elsewhere because conditions have been relatively unchanged there so far. By contrast, a very warm core of water sat above Scott Reef in the northwest waters of Australia for several months in 1998, resulting in an almost total bleaching and mortality of corals down to 30 m and 95% of reef-building corals dying in the months that followed. Recent reports indicate that recovery of these reefs has been very slow, primarily because recruitment to these remote reefs is difficult and rare. Coral disease, driven by pathogenic bacteria in addition to coral bleaching, is on the rise and may be connected to warmer than normal conditions. While coral disease affects less than 5% of the population, the incidence of diseases such as 'white syndrome' and other diseases (Fig. 10.4) are on the increase. While coral disease is currently not considered a major threat to coral reefs in Australia and many parts of the Pacific, recent experiences in the Caribbean, where coral disease decimated populations of Acropora corals, suggest that understanding and monitoring coral disease is important.

The strong relationship between coral bleaching and sea temperature provides an opportunity to explore how changes in sea temperature in the future might affect the incidence of mass coral bleaching. Past studies have revealed that corals in a region have particular thermal 'thresholds' for bleaching. These thresholds for triggering bleaching are reliable to the point that they predict which regions will experience coral bleaching based on their sea-surface temperatures measured from satellites. If these thresholds are compared to projections of future sea temperatures trends (produced by the Global Circulation Models, see above), it is possible to estimate how the frequency and intensity of mass coral bleaching and mortality will change over time. These changes show that an increase of 2°C over pre-industrial temperatures in the average sea temperature in tropical and subtropical Australia (expected as a result of a doubling of CO2) will lead to annual bleaching and a major escalation in the number of mass mortality events.

The risk of mass bleaching has been examined for the GBR and revealed that the return time of severe mass coral bleaching in their models for low to moderate changes in the climate increased to the point where the ability of reefs to recover is severely compromised. Deterioration of coral populations is likely in most of the scenarios examined.

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