Environmental threats to the Great Barrier Reef

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The Great Barrier Reef is the world's largest coral reef system,^[1][2] composed of roughly 3,000 individual reefs and 900 islands that stretch for 2,600 kilometres (1,616 mi) and cover an area of approximately 344,400 km².^[3][4] The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia. A large part of the reef is protected by the Great Barrier Reef Marine Park.

The Great Barrier Reef's environmental pressures include water quality from runoff, climate change and mass coral bleaching, cyclic outbreaks of the crown-of-thorns starfish, overfishing, and shipping accidents.

Contents 1 Environmental threats 1.1 Water quality 1.2 Climate change 1.3 Crown-of-thorns starfish 1.4 Overfishing 1.5 Shipping 1.6 Oil 1.7 Tropical cyclones 2 References

[edit] Environmental threats

[edit] Water quality

The coastline of north eastern Australia has no major rivers, (except during tropical flood events caused by tropical cyclones). It also has several major urban centres including Cairns, Townsville, Mackay, Rockhampton and the industrial city of Gladstone.

Cairns and Townsville are the largest of these coastal cities, with populations of approximately 150,000 each.^[5] Unlike most reef environments, the Great Barrier Reef is the only one worldwide where the water catchment area is home to industrialised urban areas and where extensive areas of coastal lands and rangelands have been used for agricultural and pastoral purposes.

Due to the range of human uses made of the water catchment area adjacent to the Great Barrier Reef, some 400 of the 3000 reefs are within a risk zone where water quality has declined owing to the naturally acidic sediment and chemical runoff from farming, and to loss of coastal wetlands which are a natural filter.^[6] Principal agricultural activity is sugar cane farming in the wet tropics and cattle grazing in the dry tropics regions. Both are considered significant factors affecting water quality.^[7] In February 2007, due to a monsoonal climate system, plumes of sediment runoff have been observed reaching to the outmost regions of the reef.^[8]

It is thought that the mechanism behind poor water quality affecting the reefs is due to increased light and oxygen competition from algae,^[9] but it has also been suggested that poor water quality encourages the spread of infectious diseases among corals.^[10] In general, the Great Barrier Reef is considered to have low incidences of coral diseases.^[11] The long-term monitoring program has found an increase in incidences of coral disease in the period 1999-2002, although they dispute the claim that on the Great Barrier Reef, coral diseases are caused by anthropogenic pollution.^[12]

Copper, a common industrial pollutant in the waters of the Great Barrier Reef, has been shown to interfere with the development of coral polyps.^[13]

[edit] Climate change

Most people believe that the most significant threat to the status of the Great Barrier Reef and of the planet's other tropical reef ecosystems is climate change, comprising chiefly of global warming and the El Niño effect.^[15] Many of the corals of the Great Barrier Reef are currently living at the upper edge of their temperature tolerance, as demonstrated in the mass coral bleaching events of the summers of 1998, 2002 and 2006.^[16] In February 2007, the current threat of mass coral bleaching was assessed as being "low" due to a monsoonal climate system, although sites displaying some coral bleaching were monitored.^[17]

As demonstrated in 1998, 2002 and 2006, corals expel their photosynthesising zooxanthellae (which provide up to 90% of the coral’s energy requirements)^[18] and turn colourless, revealing their white calcium carbonate skeletons, under the stress of waters that remain too warm for too long. At this stage the coral is still alive, and if the water cools, the coral can regain its zoozanthellae.^[18] If the water does not cool within about a month, the coral will die of starvation. Australia experienced its warmest year on record in 2005. Abnormally high sea temperatures during the summer of 2005-2006 have caused massive coral bleaching in the Keppel Island group. A draft report by the UN Intergovernmental Panel on Climate Change states that the Great Barrier Reef is at grave risk and will be "functionally extinct" by 2030, warning that coral bleaching will likely become an annual occurrence.^[19]

Global warming may have triggered the collapse of reef ecosystems throughout the tropics. Increased global temperatures are thought by some to bring more violent tropical storms, but reef systems are naturally resilient and recover from storm battering. Most people agree that an upward trend in temperature will cause much more coral bleaching;^[15][20] others suggest that while reefs may die in certain areas, other areas will become habitable for corals, and new reefs will form.^[21] However, the rate at which the mass bleaching events occur is estimated to be much faster than reefs can recover from, or adjust to.^[18]

However, Kleypas et al. in their 2006 report suggest that the trend towards ocean acidification indicates that as the sea's pH decreases, corals will become less able to secrete calcium carbonate.^[22]

Climate change and global warming are one of the greatest threats to the reef.^[15] A temperature rise of between two and three degrees Celsius would result in 97% of the Great Barrier Reef being bleached every year.^[23] Reef scientist Terry Done has predicted that a one-degree rise in global temperature would result in 82% of the reef bleached, two degrees resulting in 97% and three degrees resulting in "total devastation".^[24]

Climate change has implications for other forms of life on the Great Barrier Reef as well - some fish's preferred temperature range lead them to seek new areas to live, thus causing chick mortality in seabirds that prey on the fish. Also, in sea turtles, higher temperatures mean that the sex ratio of their populations will change, as the sex of sea turtles is determined by temperature. The habitat of sea turtles will also shrink.^[15]

[edit] Crown-of-thorns starfish

The crown-of-thorns starfish is a coral reef predator which preys on coral polyps by climbing onto them, extruding its stomach over them, and releasing digestive enzymes to absorb the liquified tissue. An individual adult of this species can wipe out up to six square metres of living reef in a single year.^[25]

Although large outbreaks of these starfish are believed to occur in natural cycles, human activity in and around the Great Barrier Reef can worsen the effects. Reduction of water quality associated with agriculture can cause the crown-of-thorns starfish larvae to thrive. Overfishing of its natural predators, such as the Giant Triton, is also considered to contribute to an increase in the number of crown-of-thorns starfish.^[26] The CRC Reef Research Centre defines an outbreak as when there are more than 30 adult starfish in an area of one hectare.^[27]

[edit] Overfishing

The unsustainable overfishing of keystone species, such as the Giant Triton, can cause disruption to food chains vital to life on the reef. Fishing also impacts the reef through increased pollution from boats, by-catch of unwanted species (such as dolphins and turtles) and reef habitat destruction from trawling, anchors and nets.^[28] As of the middle of 2004, approximately one-third of the Great Barrier Reef Marine Park is protected from species removal of any kind, including fishing, without written permission.^[29]

[edit] Shipping

Shipping accidents are also a pressing concern, as several commercial shipping routes pass through the Great Barrier Reef. The GBRMPA estimates that about 6000 vessels greater than 50 metres in length use the Great Barrier Reef as a route.^[30] From 1985-2001, there were 11 collisions and 20 groundings on the inner Great Barrier Reef shipping route. The leading cause of shipping accidents in the Great Barrier Reef is human error.^[31]

Although the route through the Great Barrier Reef is not easy, reef pilots consider it safer than outside the reef in the event of mechanical failure, since a ship can sit safely while being repaired.^[32] 75% of all ships that use the Great Barrier Reef as a route use the inner route.^[30] On the outside, wind and swell will push a ship towards the reef and the water is deep right up to the reef so anchoring is impossible.^[32] Captain Cook in the Endeavour nearly came to grief that way, being utterly becalmed and pushed towards the reef by the swell. Right up to within 80 metres of the Great Barrier Reef, the water was so deep that no ground (to anchor against) could be felt with 220 metres of line.^[33] There have been over 1600 known shipwrecks in the Great Barrier Reef region.^[34]

Waste and foreign species discharged in ballast water from ships (when purging procedures are not followed) are a biological hazard to the Great Barrier Reef.^[35] Tributyltin (TBT) compounds found in some antifouling paint on ship hulls leaches into seawater and is toxic to marine organisms and humans; efforts are underway to restrict its use.^[36]

[edit] Oil

It is suspected that the Great Barrier Reef is the cap to an oil trap. In the 1960s and early 1970s, there was some speculation about drilling for oil and gas there.^[37] In 1970, two Royal Commissions were ordered "into exploratory and production drilling for petroleum in the area of the Great Barrier Reef".^[38][4] Oil drilling is not permitted on the Great Barrier Reef, yet oil spills are still a threat to the reef system, with a total of 282 oil spills between 1987-2002.^[9]

[edit] Tropical cyclones

Tropical cyclones are a cause of ecological disturbance to the Great Barrier Reef. The types of damage caused by tropical cyclones to the Great Barrier Reef is varied, including fragmentation, sediment plumes, and decreasing salinity following heavy rains (Tropical Cyclone Joy). The patterns of reef damage are similarly 'patchy'. From 1910-1999, 170 cyclones' paths came near or through the Great Barrier Reef. Most cyclones pass through the Great Barrier Reef within a day.^[39] In general, compact corals such as Porites fare better than branching corals under cyclone conditions. The major damage caused by the most recent, Tropical Cyclone Larry, was to underlying reef structures, and breakage and displacement of corals, which is overall consistent with previous tropical cyclone events.^[40] Severe tropical cyclones hit the Queensland coast every 200 to 300 years,^[11] however, during the period 1969-1999 most cyclones in the region were very weak - category one or two on the Australian Bureau of Meteorology scale.^[39]

[edit] References