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Threats - Bleaching

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Search Result: 4 records

Coral Bleaching

1 . Torres Strait & Great Barrier Reef (2003)     Torres Strait & Great Barrier Reef
In 2001 the Intergovernmental Panel on Climate Change (IPCC) released the report “Climate Change 2001, the Third Assessment Report of the IPCC” and it is clear that the overwhelming consensus of scientific opinion is that the Earth’s climate is changing. Some of the major findings of the 2001 report include:
• an increase in global average surface temperature of about 0.6°C over the 20th century;
• a decrease in snow cover and ice extent;
• increases in global average sea level and ocean heat content;
• concentrations and effects of greenhouse gases have continued to increase as a result of human activities;
• new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities; and
• global average temperature and sea level are projected to rise even under the most conservative of the IPCC scenarios.

In terms of the Great Barrier Reef, climate change is predicted to increase the frequency and intensity of cyclones and severe storms, cause sea level change and has probably already increased the frequency of major coral bleaching events.

Bleached corals are not necessarily dead. In time they may regain their symbiotic algae and recover, thus returning to their normal colour
Coral bleaching occurs when corals become stressed and eject the brownish coloured algae that live within their tissues. When this happens the white coral skeleton is visible through the clear coral tissue and the corals appear bleached white. Bleached corals are not dead and, if they are not severely stressed, they can regain their original algal densities and make a full recovery. However, bleached corals will die if stresses are extreme or persistent and bleaching has led to mass mortality in many places around the world. Even corals that survive bleaching however are likely to suffer reduced growth rates, decreased reproductive outputs and increased susceptibility to other factors such as disease.

The principal cause of mass coral bleaching (involving a high proportion of corals on reefs spread over hundreds of kilometres) is elevated summer water temperatures. In addition, high levels of sunlight and lowered salinity are known to contribute to and exacerbate bleaching. Bleaching has been formally documented on many occasions on the Great Barrier Reef with the earliest report in 1980, however, bleaching appears to have increased in both frequency and geographic scale in recent years. In 1998 and 2002, the Great Barrier Reef experienced major, reef wide bleaching events associated with high sea temperatures.

In 1998, sea temperatures in some parts of the Great Barrier Reef were between 1°C and 2°C above normal temperatures for that period. Globally, the temperatures reached and the extents of bleaching at this time were the highest ever recorded. Aerial surveys of the Great Barrier Reef showed that 87 percent of inshore reefs surveyed were bleached to some extent while bleaching affected 28 percent of surveyed mid-shelf and offshore reefs. Of the bleached reefs, the inshore reefs were the most severely affected, 67 percent with high levels of bleaching and 25 percent with extreme bleaching levels. In comparison, mid-shelf and offshore reefs were less affected with only 14 percent highly bleached and none bleached to extreme levels. Subsequent research found that different coral species were affected to different extents. Corals such as the Acroporids, branching Porites and Pocilloporids suffered extreme bleaching and mortality while Turbinaria corals tended to be unaffected or only slightly bleached. Researchers also found that the effect on any one reef depended on the coral community composition, but also varied according to water depth with the shallowest regions of reefs being the most affected. Post bleaching surveys revealed that inshore reefs suffered the highest mortality rates while mid-shelf and offshore reef corals had generally escaped with minimal mortality.

The Great Barrier Reef experienced mass coral bleaching events in 1998 and 2002. The frequency of coral bleaching episodes appears to be increasing and most evidence points to a positive link between global warming and increased coral bleaching
The 2002 mass coral bleaching event was the largest on record for the Great Barrier Reef. Two periods of several weeks of hot weather resulted in seawater temperatures several degrees centigrade higher than long-term seasonal averages. Aerial surveys conducted in March and April revealed that 60 percent of reefs surveyed were bleached. While inshore reefs were again the most affected, a greater proportion of mid-shelf and offshore reefs bleached in 2002 than in 1998. In terms of bleaching severity, 69 percent of inshore reefs had moderate to high levels of bleaching but mid-shelf and offshore reefs were much more affected than in 1998 with 51 percent showing moderate to high levels of bleaching. Surveys conducted by divers later showed that although few reefs escaped bleaching, the impacts varied between reefs. Some inshore reefs such as those off Bowen suffered between 50 percent and 90 percent mortality while inshore reefs of the Frankland Islands were almost completely unaffected. As in 1998, different species were affected to varying extents with the more susceptible Acroporid and Pocilloporid species most affected. Furthermore, corals in shallow regions of the reefs were again the most extensively bleached, however some bleached corals were also found at depths between ten to fifteen metres. While data on mortality of bleached corals is still being analysed, the 2002 event was more severe than that of 1998 with bleaching spread across a much larger area of the Great Barrier Reef.

The Great Barrier Reef, as a whole, was less affected by both bleaching events than many other reefs around the world. Relatively few reefs within the Great Barrier Reef suffered extensive mortality and in the absence of further pressures, most are expected to recover. Many other reefs around the world have been completely decimated by coral bleaching. The 1998 bleaching event saw catastrophic bleaching with massive mortality occurring on reefs in Bahrain, Maldives, Sri Lanka, Singapore, and parts of Tanzania. While some reefs are showing encouraging signs of recovery, there are also many reefs where recovery is barely evident. Increases in the frequency and severity of coral bleaching will have significant and cumulative effects on the corals of the Great Barrier Reef. Considering its potential to directly affect the entire Great Barrier Reef at one time, and the potential to exacerbate the effects of other pressures, climate change and coral bleaching are likely to pose the greatest long-term risks to the Great Barrier Reef. While the long-term effects of climate change and increased coral bleaching on the Great Barrier Reef are unknown, most scientists agree that the link between climate change and coral bleaching is well established. It is widely accepted that climate change will cause coral reefs to change, the challenge now is to determine what changes are likely to occur, how these changes will occur and what effects these changes will have on reef ecosystems and on people interacting with them.
Source: Chin, A. , 2003 , Corals. . in Chin, A, (ed) The State of the Great Barrier Reef On-line, Great Barrier Reef Marine Park Authority, Townsville. http://www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/corals/index.html (See Document)

Coral Bleaching

2 . Torres Strait & Great Barrier Reef (2002)     Torres Strait & Great Barrier Reef
North Queensland had unusually hot and still weather during the summer of 2001-02, which resulted in increased sea temperatures and stress to the Great Barrier Reef (GBR). A mass bleaching event followed in the Great Barrier Reef Marine Park, similar in scale to the 1998 event, but this one affected a much larger area than in 1998, and the inshore reefs were once again the most severely affected. When the first warning signals came in December 2001, the Great Barrier Reef Marine Park Authority (GBRMPA) implemented the ‘Bleaching Response Strategy’ in collaboration with the Australian Institute of Marine Science (AIMS), the CRC Reef Research Centre (CRC Reef) and the U.S. National Oceanic and Atmospheric Administration (NOAA). This Strategy had 4 components: monitoring of sea temperature and levels of stress on the reef; early warning of coral bleaching via a public reporting program; broad-scale aerial surveys of the extent and intensity of bleaching on the GBR; and detailed underwater
surveys of the impacts of the bleaching. An important component was to provide
regular reports of reef conditions and survey results to the community via Internet updates and press releases.

The first aerial surveys showed that the coral bleaching affected almost 60% of the total GBR reef area. The inshore reefs were more severely affected by bleaching (similar to 1998), however, many more offshore reefs were affected in 2002, than in 1998. The aerial surveys were followed by more detailed, underwater surveys that confirmed that few reefs had escaped the effects of the bleaching. There was extensive mortality on a few inshore reefs, with up to 90% of corals dead at the worst affected sites, however, it now appears that the majority of reefs will survive the bleaching event with only minimal coral mortality. A number of reefs in the Coral Sea adjacent to the GBR (e.g. Flinders and Holmes Reef) also suffered extensive bleaching mortality with up to 95% of corals dead at some sites. The overall pattern was complex and highly variable (from negligible to severe), even between reefs at similar distance from the shore. Bleaching was generally most severe in shallow water and strong patterns of species susceptibility were seen at all sites. The GBR was fortunate to escape with only a few reefs suffering extensive coral mortality in the 1998 and 2002 bleaching events. However, the extent of bleaching on the GBR in 2002 indicates that few reefs are immune from increased sea surface temperatures. The area affected by bleaching in 2002, combined with the potential for mass coral death at the worst affected sites, provides a vivid warning of the potential for widespread and severe ecological damage should warm water events increase in severity, duration or frequency in the future.
Source: Wilkinson, C. , 2002 , Coral Bleaching and Mortality – the 1998 Event 4 Years Later and Bleaching to 2002. . In: C.R. Wilkinson (ed.), Status of coral reefs of the world:2002. GCRMN Report, Australian Institute of Marine Science, Townsville. Chapter 1, pp 33-44 (See Document)

Coral Bleaching

3 . Torres Strait & Great Barrier Reef (2000)     Torres Strait & Great Barrier Reef
Overview
There was some bleaching along the Australian Great Barrier Reef (GBR) in early 1998 with extensive coral bleaching and mortality on some inshore reefs, while bleaching was relatively minor on middle- and outer-shelf reefs. Aerial surveys of 654 reefs between March and April showed that 87% of inshore reefs (25% of reef with severe bleaching, >60%; 30% high, 30–60% bleached; 12% mild, 10–30% bleached) had some bleaching, compared to 28% of offshore reefs (no severe bleaching; only 5% more than 30% bleached corals). Impacts were patchy: on Orpheus Island (an inshore reef of the Central GBR), 84-87% of corals bleached, but 5 weeks later, mortality was 2.5-17%; 10km away on Pandora there was almost 100% coral mortality, including some large Porites colonies that
were centuries old. Soft corals which often dominate inner-shelf reefs were extensively bleached, with almost 100% mortality in some areas, whereas in other areas there was major recovery.
Source: Wilkinson, C.R. (Ed) , 2000 , Status of coral reefs of the world: 2000. . Australian Institue of Marine Science, Cape Ferguson, Queensland, 363 p. (See Document)

Coral Bleaching

4 . Torres Strait & Great Barrier Reef (2000)     Torres Strait & Great Barrier Reef
GBR Bleaching Update - September 2000
Around 28% of offshore reefs, which make up the bulk of GBR reefs, were bleached in early 1998, but the majority showed full recovery by the end of 1998. The inshore reefs bleached more extensively (87% of the reefs with some bleaching), with 55% showing high to extreme bleaching and recovery was patchy. Reefs in the Keppel Island group (inshore southern GBR) recovered well, while reefs on the Palm Island group (inshore central GBR) suffered high mortality. Overall, hard coral cover at Orpheus and Pelorus Islands (northern Palm group) declined from an average of 13.1% to 5% at 4 sites, while soft coral cover declined only marginally from 18.4 to 16%. The biggest impact was on two hard coral families: the Acroporidae, which suffered a 91% relative decline and the Milleporidae, which suffered >99.99% decline. Attempts to find living
colonies of Millepora tenella (a previously dominant species), failed to find any living colonies around Orpheus Island, however, there was an anecdotal report of a single small colony in deep water. Two years after the bleaching, recovery of Acropora species is slow in the Palm Islands. At Northeast Reef on Orpheus Island, staghorn coral patches (A. nobilis), which were initially thought dead, are showing signs of recovery with many new branches (10-15cm) growing out of the dead skeleton. These are not new recruits, and they presumably derive from residual live tissue within the skeleton or
underneath the colony. Other staghorn species (A. formosa and A. grandis) do not show this recovery pattern. Plating Acropora species (particularly A. hyacinthus and A. cytherea) suffered near total mortality and have not recovered, and recruitement of all Acropora species is very low. Thick tissued and massive coral species (poritids, favids,
mussids, fungiids) generally recovered well, and pocilloporid recovery is intermediate and patchy.

Source: Wilkinson, C.R. (Ed) , 2000 , Status of coral reefs of the world: 2000. . Australian Institue of Marine Science, Cape Ferguson, Queensland, 363 p. (See Document)

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