Pressure

Pressure

The coral reefs of the Great Barrier Reef experience a wide range of pressures from both natural events and human activities. For some pressures such as crown-of-thorns starfish outbreaks, there is no scientific consensus as to whether they are caused by human activity (see Environmental Status – Crown-of-thorns starfish). As such, the section on Pressures is divided into Pressures from natural causes, Pressures from human activities, Pressures of unknown origin and Pressures from global phenomena.

Episodic pressures that result in major disturbances such as cyclones, coral bleaching and crown-of-thorns starfish outbreaks cause the most noticeable changes on coral reefs. Furthermore, coral reefs may be affected by several different pressures at the same time which makes it difficult to accurately detect or predict the impact of any one pressure. As such, while pressures are described as separate entities, the overall pressure on coral reefs in the Great Barrier Reef is a cumulative combination of a number of pressures. For this reason, otherwise insignificant declines in coral condition or increases in the intensity or frequency of major disturbance events, may have disproportionate long-term effects on the condition of corals on the Great Barrier Reef.

Pressure from natural events

Cyclones

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 kilometres of a cyclone 11 or more between 1969 and 1997. Furthermore, evidence from alluvial sediment deposits indicates that every two to three hundred years, the Great Barrier Reef is likely to experience a “super-cyclone” that may have significant and wide ranging effects on the long-term development and dynamics of the Great Barrier Reef.

Great Barrier Reef World Heritage Area Cyclone Tracks (1969-1997)

The effects of cyclones on reef communities are extremely variable, depending on the severity of the cyclone, its duration, its proximity to the reefs, 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 stripped of all living corals and other benthic organisms, while in mild cases only the most fragile shallow corals are damaged. On the same reef, one side can be stripped while the other is virtually undamaged.

While cyclones can have large impacts on reef communities, the overall effect of a cyclone may depend on the types of corals living in the community at that time

In the absence of further disturbances, even severely affected reef areas (reduced to nearly zero per cent) are able to regain their original cover after a cyclone.  Recovery of fast growing corals may occur within 5 years however, in cases where 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.

Floods

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 associated with monsoon troughs or 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 wave current and wind patterns, as well as the volume of discharge, flood plumes can extend for many kilometres offshore and impact mid-shelf and offshore reefs.

Flood plumes do not necessarily affect the reefs closest to the river mouth as they may be directed by wind, currents and topography towards reefs much further away, even reaching mid-shelf reefs

Flood plumes usually migrate north along the coast due to the influence of the south-easterly trade winds, however calm conditions can result in plume migration in an easterly direction away from the coast. Lowered salinity from flood plumes have been recorded up to 40 kilometres off the coast in the central Great Barrier Reef.  Following the 1991 Fitzroy River flood, visible flood plumes were observed out to the Capricorn-Bunker reefs up to 100 kilometres from the river mouth. Recent research has investigated the behaviour of flood plumes and identified the reefs most likely to experience them. Under certain conditions, flood plumes can travel for great distances along the coast, reaching reefs hundreds of kilometres away from the river mouth. The reefs closest to the river mouths are not necessarily affected more frequently than those further away. Flood plumes are subject to a large number of influences such as volume of discharge, coastal topography, continental islands and wind conditions that can direct flood plumes towards reefs far from the river mouth. While many inshore reefs may experience flood plumes every one to three years, many mid-shelf reefs also experience flood plumes quite regularly. An assessment of the reefs most likely to regularly experience flood plumes has been completed.

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 1991 Fitzroy flood and a 90% reduction in coral cover following floods at Snapper Island.

Pressure from human activities

Terrestrial influences from catchment use

Over the last 12 years a wide ranging research and monitoring program has investigated sources of pollution on the Great Barrier Reef Catchment, transport of pollutants from the catchment to the waters of the Great Barrier Reef and the effects of pollutants on various ecosystems. This research has identified land runoff as the most important issue affecting water quality in the Great Barrier Reef. It is now widely accepted that the amount of sediment, nutrients and other pollutant inputs to the Great Barrier Reef have significantly increased since European settlement. The capacity for flood plumes to disperse some of these pollutants out to distant reefs is a further cause for concern.

Low salinity, high nutrient levels and high turbidity can stress many corals and in severe cases, cause mortality. Coupled with changes in land use patterns and the loss of critical coastal habitats that might filter out some of these elements, terrestrial run-off is considered to be one of the greatest potential threats to the Great Barrier Reef. Today, many inshore coral reefs showing high levels of coral cover, are generally found in areas where there has been relatively limited changes to land management practices since European settlement. There is a growing body of evidence suggesting that the Great Barrier Reef lagoon is showing signs of eutrophication and that this has caused decreases in coral cover and diversity on some reefs. This evidence has been subject to extensive review and debate including review by an independent panel of scientists commissioned by the Queensland Government in 2002. There is also scientific consensus on the issue urging the reduction of terrestrial pollutants to the Great Barrier Reef as a matter of priority. A more detailed account of the nature of terrestrial runoff can be found in Environmental Status - Water Quality.

Anchoring

Anchoring by boats in coral reef areas is a cause of some major coral damage in heavily used areas. Anchors dropped in areas with more fragile coral species inevitably break at least some corals. Depending on the length and weight of the anchor line used, and the strength and wind conditions, the anchor chain can cause severe physical damage over a considerable area. While broken fragments of some species are capable of regenerating, recovery from a single anchoring event could take from one to ten years and repeated anchoring in the same area can result in persistent decrease in coral cover and species diversity that is ecologically unsustainable.

Reports from scientists, Marine Parks staff, tourist operators and locals in the Cairns and Whitsunday regions have suggested that severe localised effects have occurred at some popular anchorages. High-use anchorages are clearly the most likely areas suffering from this type of damage, 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 on the Great Barrier Reef. While the damage caused by anchoring is small compared to other pressures such as cyclones and crown-of-thorns starfish outbreaks, the impacts observed were sufficient to warrant a management response to address the issue including no-anchoring zones, a mooring policy and education in best environmental practices (see Response).

Properly managed diving and snorkelling activities can have relatively little impact compared to natural disturbances or other pressures

Diving and snorkelling

Divers, snorkellers and reef walkers can break or abrade corals through physical contact. These impacts are likely to be highest at major tourist destinations or around tourist pontoons. The impact of divers on coral reefs has been extensively studied. Generally, only a small percentage of divers damage corals, mostly through contact with their fins. Interestingly, male divers appear to be more likely to damage corals than female divers and dedicated photographers with specialist equipment were more likely to break corals than non-photographers or divers with small disposable cameras. 

Research has shown that snorkellers using snorkelling trails can cause considerably more damage than divers with six times more broken corals recorded on trails compared to that of adjacent sites after two months of use. Snorkelling trails concentrate use in clearly defined areas where snorkellers stand on corals or kick them with their fins. Coral breakage is highest around interpretative signs and rapidly accumulates until reaching a relatively stable state under continued use. The use of general snorkelling areas does not appear to result in as much coral breakage, however opportunities for education and interpretation in general snorkelling areas may be diminished.

While diver and snorkeller damage can be observed in high-use sites, the level of impact is relatively low and the area of reef affected is small in proportion to the surrounding reef. When compared to the impact natural disturbances such as cyclones have on coral reefs, diver and snorkeller damage is not considered a major cause of concern. Furthermore, studies of pontoon based dive sites demonstrated that the repeated use of dive sites has not necessarily lead to their degradation with no cumulative effects detected. Nevertheless, intense snorkelling and diving activities can cause declines in the aesthetic quality of high use sites and should be taken into account  (see Response). 

Pontoons are installed over sand to reduce the impact to corals of shading and reduce the risk of damage from the mooring chains and anchors

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 localised impacts from these activities have occurred in the past, careful site selection, planning and environmental control of construction can minimise impacts. Most facilities are now preferentially located away from areas of high coral cover. For those structures that have been monitored for ongoing impacts after construction (breakwaters, jetties) no major adverse impacts on adjacent coral areas have been detected once construction is completed. 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. Nevertheless, careful site selection and use of appropriate technology and design can significantly reduce the risk of damage to corals from the movement or failure of pontoon anchoring systems and reduce the impact on corals from shading. Generally, tourist structures have a localised effect during construction that is superseded by colonisation by corals after construction or installation is complete.

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.

Ship groundings pose a significant risk to coral reefs due to the potential for oil spills, physical damage and from toxic residues from anti-foulant paint

Pollution and shipping

Shipping can impact on corals through direct grounding of ships on reefs and through spills 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), two hundred and sixty shipping and boating incidents were recorded in the waters of the Great Barrier Reef World Heritage Area between 1987 and 1997. Since 1995 there have been six major ship groundings. Site assessments show that in general, groundings result in severe but localised physical damage. Corals directly under the grounding scar are pulverised and compacted and adjacent corals toppled over and pushed into ridges. Residues of toxic Tributyl tin (TBT) and copper used in ships anti-foulant paint are scrapped off the hull and can later be resuspended and distributed, affecting corals some distance from the grounding site. Recovery from physical damage is likely to take decades but the long term effects of TBT contamination are poorly understood. More details on the management of shipping-related issues in the Great Barrier Reef World Heritage Area can be found in  Management Status - Shipping and Oil Spills.

Dredging

Dredging near coral reefs poses a high risk that requires careful management and intense monitoring

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 several major monitoring studies associated with dredging activities within the GBRWHA. The Magnetic Quays monitoring program, the Townsville Port Authority Capital Dredging monitoring program and Nelly Bay Harbour Development Impact Monitoring Program have shown that, if the appropriate management systems are put in place, it is possible for major dredging works to take place without causing widespread coral mortality on adjacent coral reefs. While these results are encouraging, it is important to note that these reefs are still likely to be subjected to some degree of sub-lethal stress, which would decrease their ability to cope with additional pressures. Furthermore, each dredging operation is likely to be unique in the nature and pattern of potential impact and environmental risk and as such, assessment of proposed dredging activities needs to be undertaken on a case by case basis.  If dredging is approved, it should be implemented in accordance with a reactive management and monitoring regime specifically tailored to the operation which may require significant financial and institutional investment and commitment to environmental management and monitoring.

Pressures of uncertain origin

Crown-of-thorns starfish

Crown-of-thorns starfish outbreaks have caused significant and extensive mortality of corals in the Great Barrier Reef on two previous occasions, and a third outbreak is currently affecting reefs in the Townsville and Cape Upstart regions. The Swains reefs have experienced recurring outbreaks of crown-of-thorns starfish at intermittent periods. 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. While there is no consensus on what causes outbreaks, it is possible that the frequency of outbreaks has increased due to human influences, particularly those associated with increased run-off from coastal areas. This possible increased frequency would cause affected reefs to be in a low coral, high algal state for longer periods and potentially impact biodiversity across the entire Great Barrier Reef.

Coral disease

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. While a variety of diseases and abnormalities are recorded as occurring on the Great Barrier Reef (e.g. black band disease, white syndrome, coral tumours), these do not seem to be affecting large areas of coral. Surveys by the AIMS LTMP show that while disease is distributed widely, the number of infected colonies on each reef tends to be low. Furthermore, surveys have shown that unlike the Caribbean, it is the offshore reefs that tend to have higher incidence of disease suggesting that pollution from coastal sources is not the primary cause of these coral diseases. Many of these offshore reefs have recently had high levels of coral cover which may aid the transmission of disease, and the incidence of coral disease has been closely linked with coral bleaching events and warm sea water temperatures, both of which have recently occurred (see below). Many disease agents are known to increase under unusually warm conditions, suggesting that greater incidence of coral disease may occur under predicted climate warming scenarios. Nevertheless, little is known about the incidence, causes and consequences of coral disease on the Great Barrier Reef and targeted research into these areas is only just beginning. 

Pressures from global phenomena

Climate change and coral bleaching

• an increase in global average surface temperature of about 0.6⁰C over the 20^th 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 1⁰C and 2⁰C 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% of inshore reefs surveyed were bleached to some extent while bleaching affected 28% of surveyed mid-shelf and offshore reefs. Of the bleached reefs, the inshore reefs were the most severely affected, 67% with high levels of bleaching and 25% with extreme bleaching levels. In comparison, mid-shelf and offshore reefs were less affected with only 14% 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% 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% 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% 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% and 90% 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 Acroporidand 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 World Heritage Area 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.

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