Of the total area of the ocean, reefs comprise just 0.2%, yet these areas of great biological wealth contain approximately one-quarter of the planet’s marine fish species, with an estimated total of over a million species. Unfortunately, of these spectacular “underwater gardens”, 11% have already been damaged and 60% are already threatened by human activities.
Great Barrier Reef is the Largest in the World
The Great Barrier Reef is the world’s largest, extending for more than 2000 km and taking in 35 million ha on Australia’s north-eastern continental shelf. About 98% of this enormous reef is nationally protected within the Great Barrier Reef Marine Park, which has separate zones for conservation, tourism and sustainable fishing. In July 2004, 33% of the Great Barrier Reef was zoned as a “no-take” area, supporting reef conservation; however, it is still at risk from increasing water temperatures as a result of global warming. Compared to reef ecosystems elsewhere on the planet, the Great Barrier Reef is in relatively good shape, although coral bleaching in 2002 forced corals to eject the life-giving symbiotic zooxanthellae algae from their polyps, threatening the Reef’s health. This happened when temperatures rose as much as 2 °C above normal. Corals already live just 1–2 °C below their maximum thermal limit, which means even a slight rise in temperature can impact on coral survival.
Global Warming and Coral Bleaching
The Great Barrier Reef is estimated to have warmed by 0.4 °C since the early 19th century, and has experienced at least two major coral bleaching incidents, in 1998 and again in 2002. In 1998, 42% of the shallow-water reef corals bleached and 2% of them died. In 2002, 55% of the corals were bleached and 5% died. Fortunately, cyclone Larry, which hit the Queensland coast in 2006, could have prevented a further major coral bleaching incident, because it stirred up and cooled the water at a time when summer water temperatures were around 1.2 °C higher than the reef’s seasonal average. Although not as major as earlier incidents, some coral bleaching did occur.
Carbon Dioxide in the Ocean
Global warming is also likely to increase the levels of carbon dioxide in the ocean, changing the ocean’s chemistry by making it increasingly acidic and limiting the amount of free carbonate and bicarbonate ions in the water. This could have serious consequences for coral reefs and for all calcifying organisms, making coral grow more slowly and not as strong.
Clearly the world’s reefs are in need of more than just ecotourism to help them survive into the next century.
References
Ayling, T. The Great Barrier Reef, Steve Parish Publishing, 2007
Cox, K. Amazing Facts about Australia, Steve Parish Publishing, 2009
