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> Underwater surveys provided detailed information on impacts of bleaching

> Aerial surveys provided a rapid, broad-scale picture of the spatial extent of bleaching. However, detailed underwater surveys were required to determine the extent of bleaching on the reef slope, measure the amount of coral death on a reef, and therefore assess the ecological implications of this coral-bleaching event.

> GBRMPA conducted detailed underwater surveys of coral reefs to assess the ecological consequences of this year’s coral bleaching event. Corals that recover from bleaching are likely to suffer sub-lethal effects such as reduced growth rates, lower reproduction and possible decreased resistance to disease2-5. Comprehensive, longer-term research programs are required to determine the long-term effects of these sub-lethal impacts of coral bleaching on coral reef communities. However, the most important and immediate ecological impacts arise when bleaching leads to death of corals. The major goal of our underwater survey program is to assess the proportion of corals that die versus those that survive the 2002 coral bleaching event. Further surveys are planned for later in 2002 when the fate of corals that bleached during the Summer will be studied further.

> Few reefs have completely escaped the effects of bleaching

> In total, we completed detailed underwater assessments at 27 reefs during April and May using SCUBA and underwater video. We selected reefs to represent both inshore and offshore reefs along three cross-shelf transects off Cairns, Townsville and Airlie Beach.

> Preliminary analysis of results from this intensive program indicates that very few reefs within the survey area have completely escaped the effects of coral bleaching. Even reefs that were recorded as only lightly affected in the aerial surveys had many corals that had bleached and some that were suffering partial mortality and overgrowth by algae. This discrepancy reflects the different resolutions of the two survey methods. While aerial surveys are effective at detecting bleaching when it affects large coral colonies or corals in high densities, underwater surveys are able to detect bleaching even when corals occur in low densities or bleaching has affected only a small proportion of colonies in an area.

> These photos show the progression of coral bleaching. The first photo shows a coral in the early stages of bleaching, with 'fluorescent' colours visible. The second photo shows a completely bleached colony with partial mortality of lower parts of branches. The third photo shows a coral that has suffered extensive partial mortality and the dead branches have been overgrown by algae.

> In a supplementary survey program, we assessed the impacts of bleaching at an additional 10 inshore reefs that were recorded as extremely bleached during aerial surveys. With these surveys, we aimed to determine the fate of coral communities on reefs that experienced especially severe bleaching.

> Effects of bleaching ranged from negligible to severe

> Our underwater surveys found that the immediate ecological consequences of bleaching varied from negligible to severe. The coral communities at some survey sites, such as the inshore reefs of the Frankland Islands, were almost completely unaffected by bleaching. In contrast, some other inshore reefs were devastated, with the proportion of dead corals ranging from 50% to 90% on reefs around Bowen. In general, however, most reefs appear to be likely to recover from bleaching with minimal mortality. Further surveys are planned to ascertain the final fate of corals that were still bleached at the time of the underwater surveys. Among the offshore reefs in particular, even sites recorded as extensively bleached are likely to suffer very little mortality, such as Thetford Reef on the mid-shelf out from Cairns.

> Bleaching intensity varied between reefs: (a) unaffected reef, (b) moderately bleached offshore reef and (c) severely bleached inshore reef.

> Inshore reefs were most severely affected by bleaching

> Inshore reefs were the most severely affected during this recent bleaching event. Extensive mortality was not seen on any of the offshore reefs surveyed, while several inshore reefs had suffered the loss of over 80% of corals. Interestingly, some of the few reefs surveyed in detail that were completely unaffected by bleaching were also inshore reefs, specifically those between Townsville and Cairns, including Pandora Reef and the Frankland Islands.

> The inshore reefs most damaged in 2002 are different from those affected in 1998. While inshore reefs between Townsville and Cairns were least affected in 2002, they were the most severely affected during 1998. Similarly, the Whitsunday Islands and associated inshore reefs were relatively unaffected by bleaching in 1998, but severely affected in 2002.

> Coral species differ in their susceptibility to bleaching

> Coral species are known to differ substantially in their susceptibility to bleaching, and these patterns of susceptibility were seen in the underwater surveys. At all sites surveyed, Pocilloporid (needle and clumping corals) and Acroporid (staghorn and plate corals) were the most severely affected. Poritid and Faviid (massive, hemispherical and brain) corals were generally the least affected by bleaching, although at some sites even these more resistant corals were bleached, with many showing signs of partial mortality on their upper surfaces. As has been previously documented, the vase coral (Turbinaria) and branching species of Porites (P. cylindrica and P. rus) appeared to be uniformly unaffected by bleaching8,9. Invariably, reefs that were observed to have the most intense bleaching were those that had high abundances of the more susceptible species.

> Some especially sensitive species suffered extensive mortality

> Some specific groups of corals were heavily affected throughout the survey area, even on reefs that did not suffer extensive coral mortality. For example, susceptible corals such as those in the family Pocilloporidae had suffered extensive mortality at most of the reefs surveyed. Almost all colonies of one especially sensitive species, the needle coral Seriatopora hystrix, had been killed by bleaching at all reefs surveyed off Airlie Beach.

> Bleaching severity varied with depth

> The severity of bleaching also showed strong variability with depth at most reefs. Corals in shallower water tend to be exposed to warmer water and more intense sunlight, both of which combine to be the key causative stresses in mass bleaching events6. Predictably, corals in the shallower waters of the reef flat and crest were more likely to be bleached than those in deeper water7. However, bleached corals were seen on the lower reef slope down to 10-15 m at most reefs.

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