The Great Barrier Reef is part of a global centre of coral diversity located in the Indo-Pacific and possessing more than 70 hard coral genera. The Great Barrier Reef has some 350 individual species compared with a global maximum of about 450 species in Indonesian and Philippine waters. Most of the hard coral species on the Great Barrier Reef are also found in other reef areas, but 10 species are considered endemic, being found only on the Great Barrier Reef. The following account deals primarily with hard corals since this group has been the most comprehensively studied. Soft corals are also an important component of many reefs. Their taxonomy is not well documented and even less is known about their ecology and current status.
Spatial Distribution Patterns
Extensive surveys over the last two decades have demonstrated that corals (like fishes, sponges and macroalgae) show a marked change in species composition as one moves from sheltered inshore fringing reefs to the exposed shelf-edge reefs of the outer barrier in clear nutrient-poor water. Inshore reefs are often characterised by the relatively high abundance of non-Acropora corals such as Galaxea, Montipora and Goniopora, compared to mid-shelf reefs that have more plate-forming Acropora species, and outer-shelf reefs that are frequently dominated by digitate or sub-massive Acropora species.
The cross-shelf pattern is correlated with an increase in wave exposure and light availability from inshore to offshore reefs. In terms of species diversity, the inner-most mainland fringing reefs or platform reefs within a few kilometres of the coast have the lowest diversity (100-150 species), but this rises rapidly away from the coast so that fringing reefs around high island groups such as the Palm Islands have over 300 species. Platform reefs further offshore in mid- and outer-shelf areas have high cover but somewhat lower species counts. Coral cover is extremely variable between reefs but surprisingly, highest cover is often found on nearshore reefs. However, this high cover can be set back to nearly zero by disturbances.
North-south variations in hard corals also exist but are less conspicuous than the cross-shelf differences. In particular, species diversity tends to decrease from north to south along the eastern Australian coast, although most of this variation occurs south of the World Heritage Area. Approximately 350 species exist in the northern and central Great Barrier Reef but only 244 are recorded in the Capricorn-Bunker reefs at the southern end of the Great Barrier Reef. The most recent large-scale monitoring survey of the Great Barrier Reef also indicates that the reefs sampled in the Capricorn-Bunker group had generally lower coral cover than other areas. This is possibly due to a disturbance event in the past, and there is some indication that corals on these reefs are undergoing recovery.
Coral reefs are one of the most
diverse ecosystems in the world.
Although some systematic cross-shelf and north-south trends can be found among the reefs of the Great Barrier Reef, a striking feature is the natural variation which can exist between nearby reefs, and the variation which can occur over time on a single reef. Three separate long-term studies of corals have emphasised this variability through time. In one 30-year study on Heron Island in the southern Great Barrier Reef, coral cover was found to vary between 0 and 80% in different patterns depending on the site.
Changes in Coral Cover at Heron Island
In another study covering six reefs in the central and northern Great Barrier Reef between 1980 and 1995, substantial changes occurred in terms of the numbers of areas dominated by corals, bare substrate or other organisms, but by the end of the study period, the proportions were similar to those at the beginning. However, this overall similarity in average conditions masked the fact that some reefs had improved while others had degraded during the period. At one inshore reef, an area once dominated by macroalgae was replaced by a coral dominated community.
A third, different type of long-term study has involved the analysis of coral density bands, which provide estimates of growth similar to that obtained from tree rings. This study has shown that while the short-term (5-10 year) record might lead one to believe that there has been a general decline in coral growth, a full analysis of the last 231 years indicates a series of repeated declines and recoveries of similar or even greater magnitude.
Changes in Coral Reef Substrate Types
Overall, the long-term studies indicate that coral cover, coral growth and coral degradation can vary considerably over time in the absence of any direct human impacts. There is no clear evidence in any of these studies that there has been a major decline in coral status with increasing European influence in the region. Indeed two more recent monitoring programs (which cover a shorter period) suggest that most reefs have exhibited an increase in coral cover over the last three to ten years.
While these results are generally encouraging, it is important to note that there have been many anecdotal reports that corals in many areas of the Great Barrier Reef have declined in recent memory. It is difficult to verify such claims, but given the variety of human-induced pressures to which many reefs are or have been subjected, it is important not to be complacent.
One way to investigate possible cases of reef degradation over long time periods is through the comparison of historical photographs of the reef with contemporary scenes from the same location. A substantial survey of historical photographs undertaken by the Great Barrier Reef Marine Park Authority has indicated that out of 14 reefs investigated, six showed no obvious changes, four showed decreases in hard coral cover and four showed no obvious changes in some areas, but decreases in coral cover at others. While this study has demonstrated that some sites have undergone significant decline, the cause of this decline is not certain, and the mixture of results does not support the idea that there has been a systematic major decline in most reefs in the Great Barrier Reef.
Nonetheless, the monitoring studies reported here cover barely a fraction of the reefs in the Great Barrier Reef, and it is quite possible that serious but undocumented declines have indeed occurred in some areas.
Changes in Coral Cover on Selected Reefs
Cyclones are one of the most common sources of natural impact on coral communities. For instance they account for virtually all of the declines in coral cover documented during a 30-year study of Heron Island corals. A recent compilation of cyclone data indicates that over the last 28 years there have been 135 cyclones in Queensland waters and that all areas of the Great Barrier Reef have been affected by at least one cyclone in this period. The reefs off Townsville have been particularly hard hit, having been within about 100 km of a cyclone 11 or more times.
This reef-flat at Daydream Island photographed around 1950 (a) and in 1995 (b) shows no significant change in coral cover; while the reef-flat at Stone Island photographed around 1890 (c) and in 1994 (d) shows substantial decline.
The effects of cyclones are extremely variable, depending on the severity of the cyclone, the time spent in the vicinity of a reef, the orientation of the reef with respect to the wind and waves, and the depth of the corals. In extreme cases the reef can be denuded of all living corals and other benthic organisms, while in mild cases only the most fragile shallow corals are broken. On the same reef, one end can be denuded while the other is virtually undamaged.
If there are no subsequent disturbances, even severely affected reef areas (reduced to nearly zero per cent cover) are able to regain their original cover in about 20 years after the cyclone. However, if the original community possessed very old or slow growing corals, the time required to return to the same species and size composition could be substantially longer. Since many parts of the Reef experience more than one cyclone in 10-20 years, the reef can be considered as a mosaic of patches at different stages of recovery. Physical disturbance and recovery are therefore natural phenomena on the Great Barrier Reef and play an important part in determining the abundance and species composition of coral communities.
Great Barrier Reef World Heritage Area - Cyclone Tracks (1969-1997)
The Great Barrier Reef is located in the monsoonal tropics, and experiences distinct wet and dry seasons. During the summer wet season major rainfall events (some of which are associated with cyclones) can lead to extensive flooding of rivers and the discharge of millions of litres of sediment-laden freshwater into the coastal area. Depending on the wave, current and wind patterns, and the volume of discharge, the river plumes during flood events can extend for many kilometres offshore and impact nearshore and mid-shelf reefs.
Ongoing research at the Cooperative Research Centre for Ecologically Sustainable Development of the Great Barrier Reef is examining the probability of different reefs being affected by river plumes at various intervals. In general, most nearshore reefs north of major river mouths experience lowered salinity and increased turbidity during flood events. Lowered salinity has been recorded up to 40 km off the coast in the central Great Barrier Reef, while visible flood plumes have been observed out to the Capricorn-Bunker reefs following the 1991 Fitzroy River flood.
The response of corals to flood plumes varies with the salinity and turbidity levels of the plume, and the duration of exposure. Substantial mortality can occur during extreme flood events. There was 85% mortality of shallow-water corals in the Keppel Islands following the Fitzroy flood.
Terrestrial influences from catchment use
While river flooding and flood plumes with low salinity and high turbidity are a natural feature of the coastal Great Barrier Reef region, the sediment load and associated pollutant load in these plumes may have increased in recent years as a result of adjacent land use. A desktop study has found that sediment and nutrient inputs to the Great Barrier Reef have increased fourfold since European settlement of the adjacent coast. A more detailed account of the nature of terrestrial run-off can be found in Environmental Status - Water Quality.
Both low salinity and high turbidity are known to stress corals and cause mortality in severe cases. In general, terrestrial run-off is considered to be one of the greatest potential threats to the Great Barrier Reef, however there are very little data to demonstrate that coral communities have been directly affected by this impact. As mentioned above many inshore coral reefs show consistently high levels of coral cover, and experience high frequencies of natural flood events. Nevertheless, some scientists feel that the Great Barrier Reef lagoon is showing signs of eutrophication and that decreases in coral cover on some reef areas are caused by this degradation. Macroalgal growth does increase when it is exposed to pulses of higher nutrients similar to those experienced during floods.
Anchoring by boats in coral reef areas is a cause of potentially major coral damage in heavily used areas. Anchors dropped onto areas of branching coral inevitably break at least some corals. Depending on the length and weight of anchor line used, and the strength and variability of the wind, the anchor chain can cause even more damage over a considerable area. While broken fragments are capable of regenerating, and recovery from a single anchoring event could occur within a year or two, repeated anchoring in the same area is clearly unsustainable.
Reports from local divers in the Whitsunday region have suggested that some popular anchorages have been severely affected. High-use anchorages are clearly most likely to have suffered, but the absence of data on coral cover before and after the commencement of frequent anchoring makes it impossible to determine the extent and severity of this impact over the Great Barrier Reef. The use of moorings in all such sites over the next few years is expected to substantially reduce the problem (see Response).
River plumes during flood events can extend for many
kilometres offshore and can impact nearshore and mid-shelf reefs.
Diving and snorkelling
Divers, snorkellers and reef walkers can break or abrade corals through intentional or unintentional contact. These impacts are likely to be highest at major tourist destinations such as Heron Island (reef walking) or around tourist pontoons (diving and snorkelling). Recent studies indicate that diver and snorkeller damage can be detected in high-use sites, but that the level of impact is generally low and the area of reef affected is small in proportion to the surrounding reef.
Construction and operation of tourist facilities
Corals can be damaged during activities associated with the construction of tourist facilities such as marinas and breakwaters, and the installation of piles for jetties. Although quite substantial areas have been affected in the past (e.g. construction of the Hamilton Island runway) most facilities are now preferentially located away from areas of good coral cover. For those structures which have been monitored for ongoing impacts after construction (breakwaters, jetties) no major adverse impacts on adjacent coral areas have been detected. In the case of tourist pontoons, the area directly under the pontoon is usually shaded to some extent and this has resulted in some coral death. Generally, tourist structures have a localised effect during construction, but are often subsequently colonised by corals during ongoing operation.
Stormwater run-off containing rubbish and pollutants, and sewage effluent are also potential impacts on coral reefs adjacent to tourist facilities. However, management regulations usually minimise these effects, and no major problems have been documented during monitoring of such facilities.
Pollution and shipping
Shipping can impact on corals through direct grounding of ships on reefs and through the loss of toxic cargo and fuel. Although there have been no major oil spills in the Great Barrier Reef World Heritage Area (and only one major spill in Torres Strait), 25 groundings and 19 collisions of ships have occurred in the last 22 years. Two recent groundings have been investigated in some detail and in both cases there has been localised mortality of corals crushed under the hull, but no obvious effects on surrounding areas. Information on management of shipping-related issues in the Great Barrier Reef World Heritage Area can be found in Management Status - Shipping and Oil Spills.
Dredging of harbours and boat channels creates highly turbid sediment plumes that can kill corals up to hundreds of metres away. Most ports along the Queensland coast require periodic dredging after initial construction. Maintenance dredging at ports with nearby coral reefs is a potentially significant impact for these corals. In addition, construction of new marinas and boat channels is also occurring along the Great Barrier Reef coast.
There have been two major monitoring studies associated with dredging activities within the World Heritage Area. Both the Magnetic Quays monitoring program and the Townsville Port Authority Capital Dredging monitoring program indicated that, if the appropriate management protocols are put in place, it is possible for major dredging to take place without causing widespread coral mortality on adjacent coral reefs. While these results are encouraging it is important to note that each dredging event is likely to be different in the nature and pattern of potential impact. Therefore careful individual assessment, monitoring and management are required for all future projects.
Events of Uncertain Origin
Crown-of-thorns starfish outbreaks have caused significant and extensive mortality of corals in the Great Barrier Reef on two previous occasions, and outbreaks are currently affecting reefs in the Lizard Island to Innisfail region. For more information on the crown-of-thorns starfish and its effects on corals, see Environmental Status - Crown-of-thorns Starfish.
There has been much debate about the cause of crown-of-thorns starfish outbreaks, and several theories have invoked human activities as an indirect cause. A recent survey of scientists however, suggests that most of them believe that crown-of-thorns starfish outbreaks are a natural phenomenon, although it is possible that the frequency of outbreaks has increased due to some human influence. This possible increased frequency would cause affected reefs to be in a low coral, high algal state for longer periods.
Coral bleaching occurs when corals become stressed and eject the brownish coloured algae which 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, many corals die during some bleaching events.
Bleaching has been formally documented on six occasions on the Great Barrier Reef with the earliest report in 1980 and the most recent event in 1998. Anecdotal reports suggest it may have occurred even earlier than 1980. During the 1998 event over 88% of inshore reefs exhibited some coral bleaching, with 25% of all inshore reefs having more than half of the corals affected. Although the level of mortality associated with the 1998 bleaching event has not yet been determined, up to 50% of the corals died on some reefs in a previous severe episode in 1982.
Generally coral bleaching affects inshore reefs most severely and is most common in shallow waters. In this respect it differs from the effects of crown-of-thorns starfish outbreaks, which tend to affect mid- and outer-shelf reefs and occur over a wide range of depths.
The principal cause of mass coral bleaching (involving a high proportion of corals on reefs spread over hundreds of kilometres) is believed to be elevated summer water temperatures. In addition, high levels of sunlight and lowered salinity are known to contribute to and exacerbate bleaching. Although there has been speculation regarding possible links between increased incidents of coral bleaching and greenhouse gas induced climate change, there is no clear scientific evidence for a link at this stage. Similarly, while convincing links between El Ni–o and coral bleaching have been made for bleaching events in some other countries, no clear relationship exists for bleaching on the Great Barrier Reef.
Although diseases are frequently reported as an important threat to corals in the Caribbean, only isolated reports of disease exist for the Great Barrier Reef. A variety of diseases and abnormalities are recorded as occurring on the Great Barrier Reef (e.g. black band disease, white band disease, coral tumours) but these do not seem to be affecting large areas of coral.
The main cause of mass coral bleaching is believed to
be elevated summer water temperatures.
The Great Barrier Reef Marine Park Authority uses a variety of measures to either eliminate or substantially reduce the magnitude and likelihood of impacts on corals in instances where there is a known potential for impacts as a result of human activities. These measures include the establishment of zones or special areas prohibiting certain activities, the imposition of permit conditions associated with specific activities, and the establishment of guidelines and codes of conduct.
Great Barrier Reef World Heritage Area - 1998 Coral Bleaching
Permits for Commercial Activities and Research
All commercial and most research activities in the Great Barrier Reef Marine Park require a permit from the Great Barrier Reef Marine Park Authority. During the assessment of each permit, potential impacts on corals are considered and, if necessary, permit conditions are imposed to eliminate or minimise these impacts.
Moorings and 'no anchoring areas' are used to reduce coral damage from boat anchors. Many commercial reef operators are required to install and maintain their own mooring, while public moorings are being progressively installed in sensitive, high-use anchorages throughout the Marine Park. In the Whitsunday Islands, where anchor damage is a particular concern, 60 public moorings and 12 'no anchoring areas' have been installed.
Coral Monitoring Programs
An important management response to potential pressures on the environment is the establishment of a monitoring program to document the level of any human-induced or natural impact, and to determine whether any action taken by the relevant management agencies is effective at eliminating or lessening the impact. A major long-term background monitoring program being conducted by the Australian Institute of Marine Science provides feedback on the general condition of the reef, and any long-term trends in coral cover and fish abundance. For more information on the scope and extent of various monitoring programs on the Great Barrier Reef, see Management Status - Monitoring.
Monitoring is often required as a condition of a permit to conduct commercial activities on the Great Barrier Reef, and can include a 'reactive monitoring' component. This ensures that any indications of adverse impacts during the construction or installation of a structure on the reef are detected at an early stage and action is taken to minimise impacts.
Tourist Guidelines for Diving, Snorkelling and Reef Walking
Tourists on most trips to the reef are given an introductory lecture which stresses the importance of avoiding direct contact with coral by snorkellers and divers. Floats are provided at many reef sites to allow snorkellers to rest without standing on the reef. Reef walkers are also briefed to avoid walking directly on corals and are often accompanied by a guide who encourages appropriate behaviour.
Shipping Regulations and Guidelines
In order to reduce the risk of ships grounding within the Great Barrier Reef it is compulsory for all vessels over 70 m or any ships with a cargo of oil, chemicals or liquefied gas to carry a pilot whilst transiting the inner shipping route. As a response to political and community sensitivity, oil companies tend to direct crude oil tankers to travel outside the Reef when travelling along the Queensland coast. Further details on the management of shipping can be found in Management Status - Shipping and Oil Spills.
The Great Barrier Reef World Heritage Area has a highly diverse coral fauna which varies in composition from inshore to offshore. Natural disturbances such as cyclones and river floods create a high level of natural variation in coral communities through time and between reefs. Impacts from crown-of-thorns starfish and coral bleaching also cause major changes to portions of the Great Barrier Reef at irregular intervals, but the link between these perturbations and human activities is uncertain. Although important human impacts such as increased terrestrial impacts from urban and agricultural activities, and tourism have been identified, no major widespread declines in coral status have been documented in relation to these impacts. A variety of management measures have been put in place to address the known impacts.