By Francesca Davis
From space, big coral reefs like the Great Barrier Reef are the only visible evidence of life on earth besides human cities and structures. Hundreds of different coral species create reefs, in which a quarter of all marine plants and animals live. Australia alone accounts for 350 known species of reef-forming coral. Although they cover less than 1% of the ocean floor, around 950,000 square kilometres, the reefs support close to 1 million known species.
Coral reefs are essential to the 150 million people in the Third World whose livelihoods depend on local fishing. Fish is often their only source of protein. Reefs have other roles too, such as providing protection for swamps, marshes and beaches against the steady pressure of the sea and storms.
Coral play an important role in maintaining marine ecosystems. Coral is a type of animal called a polyp which eats drifting zooplankton. They filter water and process calcium and other minerals from sea water. Their enormous colonies can last for thousands of years and form reef structures that are hundreds of kilometres long. The largest reef in the world, the Great Barrier Reef, stretches for over 2500 kilometres. Reefs are among the most complex, diverse and productive ecosystems in the world.
It is estimated that in 20-40 years, 70% of all the world's coral will be dead. Already 10% have died. There are reefs off the coast of 93 countries. In some cases, such as the coasts of Florida, Cuba and Jamaica, more than 90% of the reefs are dead or dying. The loss of reefs would devastate marine ecology and seriously reduce the world's food resources.
Causes of reef destruction range from water pollution and fishing practices to climate change and tourism.
Overfishing and other injurious practices destroy thousands of hectares of coral reefs every year. New studies show that fish larvae spawned in one reef migrate to live in other reefs as adults. Consequently, protection of an individual reef cannot guarantee survival of fish species.
Dr Eric Wolanski, from the Australian Institute of Marine Science, argues that the nearly 3000 individual reefs forming the Great Barrier Reef must be treated as an integrated biological system. He says the reef is already dangerously overfished. Often reef plants and animals are co-dependent, which is why the loss of even one species can have a huge impact.
Reefs in countries such as Indonesia and the Philippines are particularly threatened by the trade in live reef fish that supplies restaurants in places like Hong Kong. Recently the use of dynamite to catch reef fish has been replaced by cyanide which stuns the fish so they can be sold live. Larger fish survive cyanide, but smaller fish and invertebrates in the reefs do not. Cyanide also kills corals at concentrations hundreds of thousands of times lower than the typical concentrations used by divers.
Crucial to coral survival are zooxanthellae — microscopic, single-cell algae that live inside the coral and keep them alive by using sunlight to produce oxygen. Unfortunately, algae can also suffocate the coral if they bloom too much — known as eutrophication. The main cause of coral death is eutrophication.
Chemicals like nitrogen, phosphorus from raw sewage, agricultural pesticides, liquid effluent from mining and fertiliser run-off from sugar plantations and resort hotels have caused usually nutrient-poor waters, where coral thrive, to become nutrient-rich, allowing algae to thrive. Pollution can come from kilometres away. The Great Barrier Reef is already getting four to five times more nitrogen and phosphorus than it did 100 years ago.
As coral immune systems break down, disease is also increasing. Between 1996 and 1997, the incidence of disease in the Florida Keys increased by 292% from as many as six distinct diseases.
A more recent threat is "bleaching" caused by global warming. An increase in the water temperature causes coral to expel their coloured zooxanthellae, exposing the corals' skeletal secretions of white limestone, and leaving them without food. Many of the polyps then die. East Africa, Samoa, Western Australia and the Galapagos Islands have all experienced bleaching this year.
Bleaching is killing reefs along 1000 kilometres of the Queensland coast, from Cooktown to Bundaberg, according to the Great Barrier Reef Marine Park Authority. In some cases, 80% of corals have been bleached.
It appears that as little as a one degree temperature rise can cause bleaching. A recent study by the Southern Cross University showed that up to 20% of bleached corals on Heron Island in the Great Barrier Reef have died. As global warming increases, more coral deaths will follow.
The cumulative effects of fishing, tourism, water pollution and climate change are wiping out coral reefs that have survived for 30 million years. The dark dead coral that haunt the bays of Jamaica and Cuba replaced their colourful predecessors in only the last two or three decades.
The world's remaining coral reefs face annihilation over the next few decades. Unless we act now to preserve the reefs, they will be lost forever.
[Much of the information for this article was taken from a more comprehensive article by Bradford Matsen that appeared in the US magazine Mother Jones.]