Climate ChangeThe facts

Coral reef bleaching

According to the Australian Academy of Science:

Imagine you are on a beautiful tropical island on the Great Barrier Reef. You put on your mask, fins and snorkel and jump into the water. But instead of the famous brightly coloured coral, brittle white branches spread as far as the eye can see. The coral is dead.

As the world's oceans become warmer, this scenario is taking place more frequently, on the Great Barrier Reef and on coral reefs all over the world. Coral bleaching has never been more common or widespread, prompting the question: are rising sea temperatures killing the reefs?

Corals are animals

The ancient Greeks mistakenly believed that corals were plants. Corals are actually animals, related to anemones and jellyfish. Corals consist of a limestone structure filled with thousands of small animals called polyps. Each polyp has a skeleton cup, tentacles with stinging cells, a mouth and a stomach. The tiny tentacles snatch at passing plankton for food, but for their main course, reef-building corals have devised a much more ingenious method to get fed.

Algae called zooxanthellae live within each coral. In return for a safe sunny home, the zooxanthellae eat the nitrogen waste that the coral produces (nitrogen is very good for algal growth) and, like all plants, algae turn sunlight into sugars by the process of photosynthesis. The sugars produced by the zooxanthellae make up 98 per cent of the coral's food. So, without having to do any work at all, the coral is kept clean and well fed, and the zooxanthellae with their brilliant reds, oranges and browns give corals their colour.

Importance of coral reefs

Coral reefs are important for many different reasons. Apart from protecting the shoreline from the damaging effects of the ocean, they provide habitats and shelter for many organisms and are the source of nitrogen and other nutrients essential for the food chain. This is why hundreds of thousands of marine species live in reefs. Many fisheries depend on the fish that spend the first part of their life in coral reefs, before making their way out to the open ocean.

The Great Barrier Reef is especially important to the Australian economy and generates 1.5 billion dollars every year from fishing and tourism. In addition, the study of coral reefs can provide a history of past climates.

Rising sea temperatures and coral bleaching

Over the past one hundred years, the temperature of sea water in many tropical areas has been rising. For example, the Australian Institute of Marine Science has collated data showing that 2002 was the warmest year for water temperatures off northeast Australia since 1870. And there are predictions of a sea temperature rise of up to 1ºC within 50 years.

Rising water temperatures block the photosynthetic reaction that converts carbon dioxide into sugar. This results in a build-up of products that poison the zooxanthellae. To save itself, the coral spits out the zooxanthellae and some of its own tissue, leaving the coral a bleached white. The bleached coral can recover, but only if cooler water temperatures return and the algae are able to grow again. Without the zooxanthellae, the coral slowly starves to death.

Other causes of coral bleaching

Apart from heat stress, other causes of coral bleaching may include:

increased exposure to ultraviolet (UV) radiation;
large amounts of storm water from heavy rains flooding the reef;
the exposure of coral to certain chemicals or diseases;
sediments such as sand or dirt covering the coral;
excess nutrients such as ammonia and nitrate from fertilisers and household products entering the reef ecosystem. (The nutrients might increase the number of zooxanthellae in the coral, but it is possible that the nutrient overload increases the susceptibility of coral to diseases.)
Often coral reefs are exposed to a combination of these factors.

Rate of coral bleaching

Coral bleaching is a natural process. For thousands of years, fishermen have noticed mysterious whitening of the reefs. What is not natural is the rate at which coral bleaching is occurring. In 1998, the worst coral bleaching in 700 years struck the Great Barrier Reef, followed by an even worse bleaching only 4 years later. Massive areas of corals were affected all over the world. In Australia alone, the 2002 bleaching saw nearly 60 per cent of the reef suffer bleaching and, in the worst areas, 90 per cent of the coral was bleached.

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