Coral bleaching – will global warming kill the reefs?

Key text

This topic is sponsored by the Australian Institute of Marine Science.
Coral reefs are sensitive to environmental change. Recently, the frequency and distribution of coral bleaching have increased, and in 2002 the Great Barrier Reef experienced its worst case of coral bleaching on record.

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

Related site: What is a coral?
Provides a labelled diagram of a coral polyp as well as information about corals and related animals. Includes coloured photographs.
(Bishop Museum, USA)

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

Related site: A coral diary of Australian droughts before European settlement
Explains how core samples taken from coral reefs can provide clues to past climate changes.
(On Line Opinion, Australia)

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.

Can corals survive bleaching?

In the past, rapid changes of climate have sometimes led to extinctions. The dinosaurs were wiped out 65 million years ago, and there have been several mass extinctions before and since. Does the coral have any hope?

Australian scientists have found that corals contain a kind of sunblock called fluorescent pigments. These pigments form a kind of shield around the zooxanthellae and protect them from the harmful effects of sunlight at high temperatures. The pigments change the harmful UV and blue wavelengths in sunlight to softer, lower energy wavelengths like green and yellow.

Related site: Photobiological chemistry of coral symbiosis
Explains why the production of natural sunscreens is important to marine organisms that live in shallow-water habitats.
(Australian Institute of Marine Science)

Fluorescent corals survive bleaching events better than non-fluorescent corals, and every species of coral has some varieties that have fluorescent pigments. Over time, more fluorescent pigments may develop to protect the corals. Even though non-fluorescent varieties may die out, there will be some corals that have the fluorescent pigments and therefore may survive.

If a channel of cold water constantly runs through the warm water, thus keeping the zooxanthellae cool, then those corals can survive. Another way in which corals can survive is just by being tougher than others, in the same way that some people are always healthy while other people are always sick.

Protecting coral reefs

Perhaps the best thing we can do is to protect the corals that do survive bleaching better than others. One way we can do this is by creating marine parks: areas protected in some way from souvenir hunters and activities such as fishing and boating.

At the moment almost a third of the Great Barrier reef is protected from fishing. An important consideration is to locate marine parks in areas where the coral is resistant to bleaching. If the coral dies, eventually the fish and other marine species will die too.

Related site: Bleach watch
Explains how to recognise bleached corals and how to report coral bleaching.
(Great Barrier Reef Marine Park Authority, Australia)

The other thing we can do for the reef is to keep an eye on it. The reef has 2.4 million visitors every year, and these people go to places scientists can’t always get to. GBRMPA can use information from visitors about where they saw bleaching. And, just as importantly, where they didn’t see bleaching. This helps scientists to form an overall picture of which areas are vulnerable to bleaching.

Though the reefs may not be doomed, there is definitely cause for concern. Some scientists have predicted that by 2030 massive and devastating coral bleaching events will occur every year. Other scientists believe that there is a future for the reefs, and that though the reefs may change, they will be there in some form or another. We can only hope that they are right.

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Posted April 2003.